Evolving Theories on the Origin of the Moon [1st ed. 2019] 978-3-030-29118-1, 978-3-030-29119-8

Table of contents :
Front Matter ....Pages i-xix
Gaping at the Moon—18th and 19th Century Ideas on the Formation of Lunar Craters and Other Surface Features (Warren D. Cummings)....Pages 1-17
Lunar Observations and Speculations—From Gilbert to the Apollo Explorations (Warren D. Cummings)....Pages 19-49
Pre-Apollo Theories About the Origin of the Moon (Warren D. Cummings)....Pages 51-75
Exploring the Moon—The Apollo Investigations (Warren D. Cummings)....Pages 77-109
Post-Apollo Synthesis and Debate (Warren D. Cummings)....Pages 111-140
Widening the Research Front (Warren D. Cummings)....Pages 141-156
The Kona Conference—Day 1 (Warren D. Cummings)....Pages 157-192
The Kona Conference—Day 2 (Warren D. Cummings)....Pages 193-242
The Kona Conference—Day 3 (Warren D. Cummings)....Pages 243-272
Assessments (Warren D. Cummings)....Pages 273-281
Epilogue (Warren D. Cummings)....Pages 283-287
Back Matter ....Pages 289-311

Citation preview

Historical & Cultural Astronomy Series Editor: Wayne Orchiston

Warren D. Cummings

Evolving Theories on the Origin of the Moon

Historical & Cultural Astronomy

Series Editor:

WAYNE ORCHISTON, Adjunct Professor, Astrophysics Group, University of Southern Queensland, Toowoomba, QLD, Australia

Editorial Board:

JAMES EVANS, University of Puget Sound, Tacoma, WA, USA MILLER GOSS, National Radio Astronomy Observatory, Charlottesville, USA DUANE HAMACHER, Monash University, Melbourne, Australia JAMES LEQUEUX, Observatoire de Paris, Paris, France SIMON MITTON, St. Edmund’s College Cambridge University, Cambridge, UK MARC ROTHENBERG, Smithsonian Institution Archives, North Bethesda, MD, USA CLIVE RUGGLES, University of Leicester, Leicester, UK XIAOCHUN SUN, Institute of History of Natural Science, Beijing, China VIRGINIA TRIMBLE, University of California Irvine, Irvine, CA, USA GUDRUN WOLFSCHMIDT, Institute for History of Science and Technology, University of Hamburg, Hamburg, Germany

More information about this series at http://www.springer.com/series/15156

Warren D. Cummings

Evolving Theories on the Origin of the Moon

123

Warren D. Cummings USRA Columbia, MD, USA

ISSN 2509-310X ISSN 2509-3118 (electronic) Historical & Cultural Astronomy ISBN 978-3-030-29118-1 ISBN 978-3-030-29119-8 (eBook) https://doi.org/10.1007/978-3-030-29119-8 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Cover image: David A. Hardy/Science Photo Library This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Foreword

The Moon has been Earth’s constant companion since shortly after its formation over 4.5 billion years ago. Arguably, it has provided stability to Earth’s rotation axis and may have at least partially enabled the relatively long-lived benign environment that allowed the development of life. Although tidal forces have caused the Moon to slowly distance itself from its home planet, it has presented an essentially constant perspective to humans since they first evolved on Earth, other than its monthly cycle of waxing and waning from the new Moon to the full Moon and back again. As such a prominent feature in the night sky, the Moon has remained a subject of great curiosity to humans: What is it? What is it made of? Why is it there? How did it form? What can it tell us about our place in the universe? In the last thousand years, several major technological advances resulted in significant new insight into the Moon: firstly with the invention of the telescope and secondly with the advent of the space age and robotic and human exploration missions to lunar orbit and the surface of the Moon. These latter missions enabled observations of both the near- and far-side of the Moon, geophysical, geochemical and space physics measurements, and ultimately sample return. It is with the most recent history of lunar science that this book is concerned, focusing on how our understanding of the Moon evolved from early telescopic observations to the paradigm-changing results from the Russian and American missions to (and from) the Moon. A third renaissance is currently underway, with major international activities at the Moon over the past decade and likely to continue as more nations join those already exploring space with robots and humans. While these recent observations and measurements have certainly increased our understanding of our nearest long-term neighbor in space and of the early evolution of the Earth-Moon system and the Solar System, the rate of increase in fundamental scientific understanding of the Moon pales in comparison to that during the early days of the space age. This book chronicles major questions pertaining to the processes that formed the major features on the lunar near-side and how understanding evolved as better observations were obtained, and ultimately as lunar samples provided ground truth. While the early robotic lunar missions during the latter part of the twentieth century vii

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were initially focused on technology demonstration and preparation for human exploration, lunar science was clearly a beneficiary, and the lunar science community grew as new and better data were returned from these missions. As NASA was still a fledgling agency at the time of Apollo and consumed with the challenges of manned space flight, a mechanism was required to engage this newly energized lunar science community and connect them to the science and samples returning from the missions. This need was also recognized by the US National Academy of Sciences (NAS) who, together with NASA, sought to establish an organization in Houston, adjacent to the NASA Manned Spacecraft Center (now the Johnson Space Center) where “scientists—and their students—from all over the world” could “set down new patterns of scientific cooperation which will have profound effects on man’s knowledge of his universe. This new Lunar Science Institute will provide a new means of communication and research for the world’s scientific community.”1 Shortly after the establishment of the LSI, NASA, and the NAS formed the Universities Space Research Association (USRA) to manage the Institute. The Lunar Science Institute (LSI; now the Lunar and Planetary Institute) was established as an organization to provide scientists with an academic-style environment to work with the data and access the samples brought back by the robotic and human missions. LSI also had a broader role in drawing the lunar science community together for conferences and workshops to discuss the latest results and models for the formation and evolution of the Moon and the Earth-Moon system. It was in these meetings that often conflicting ideas were presented by different groups, resulting in lively discussion and ultimately in building some level of consensus. This book details the key points of discussion and how they evolved through these LSI-sponsored meetings, leading up to the understanding of the Moon at the time of the Kona Conference on the Origin of the Moon in Kona, Hawaii, on October 14–16, 1984. Although this book focuses on the evolution in the knowledge of the Moon during a period of rapid change in our ability to measure its structure and properties, it provides an additional perspective on how science evolves as technology advances and enables major new insight into the physical universe. While such paradigm-shifting events have occurred throughout history, rarely have we been provided with such a clear demonstration on how new data infuse the scientific establishment, are debated, and ultimately generate fundamental new understanding. Stephen J. Mackwell American Institute of Physics College Park, USA

1

President Lyndon B. Johnson, Houston, Texas, March 1, 1968.

Acknowledgements

I am grateful for the support of the President of USRA, Jeffrey A. Isaacson, and the Vice President for Corporate Affairs and Communications, Kevin Schmadel, during the writing of this book. I am also indebted to Stephen Mackwell, the former Director of the Lunar and Planetary Institute, for his support and guidance. I acknowledge with tremendous gratitude the assistance provided to me by the LPI Library staff. The Library Manager, Mary Ann Hager (now retired), guided me to resources available online and gave me a number of valuable suggestions as the manuscript progressed. Linda Chappell, an LPI Library Technical Assistant, was of enormous help in finding and providing me with copies of research articles, and she thoroughly reviewed and edited the list of references for the book. Renee Dotson, Heidi Lavelle, and Linda Chappell of the LPI helped me obtain permissions for the use of material that I incorporated into the book. Finally, I acknowledge the encouragement and support of my wife, Sue.

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Gaping at the Moon—18th and 19th Century Ideas on the Formation of Lunar Craters and Other Surface Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Hooke and Herschel . . . . . . . . . . . . . . . . . . . . . . . 1.3 Schröter and Gruithuisen . . . . . . . . . . . . . . . . . . . . 1.4 Beer, Mädler and Althans . . . . . . . . . . . . . . . . . . . 1.5 Dana and Proctor . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Nasmyth, Carpenter, and Neison . . . . . . . . . . . . . . 1.7 Meydenbauer and the Thiersches . . . . . . . . . . . . . . 1.8 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lunar Observations and Speculations—From Gilbert to the Apollo Explorations . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Gilbert’s Analysis . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Immediate Reactions to Gilbert’s Ideas—Davis and Althans . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Meteor Crater in Arizona . . . . . . . . . . . . . . . . . . 2.5 Shaler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Morozov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Günther . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 Öpik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 Wegener . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10 Delmotte and Darney . . . . . . . . . . . . . . . . . . . . . 2.11 Spurr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12 Baldwin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13 Dietz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.14 Kuiper and Urey . . . . . . . . . . . . . . . . . . . . . . . . .

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2.15 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Pre-Apollo Theories About the Origin of the Moon . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Pre-Apollo Estimates for the Age of the Moon . . . . 3.3 Pre-Apollo Theories for the Formation of the Moon 3.3.1 Rotational Fission . . . . . . . . . . . . . . . . . . . 3.3.2 Collisional Fission . . . . . . . . . . . . . . . . . . 3.3.3 Co-accretion . . . . . . . . . . . . . . . . . . . . . . . 3.3.4 Capture . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Post-Apollo Synthesis and Debate . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 O’Keefe’s Arguments for Fission . . . . . . . . . . . . . . . . 5.3 The Fourth Lunar Science Conference . . . . . . . . . . . . 5.4 Harold Urey’s 80th Birthday . . . . . . . . . . . . . . . . . . . 5.5 The Lunar Petrology Conference . . . . . . . . . . . . . . . . 5.6 The Fifth Lunar Science Conference . . . . . . . . . . . . . 5.7 The Conference on Satellites of the Solar System . . . . 5.8 The Sixth Lunar Science Conference . . . . . . . . . . . . . 5.9 The Seventh Lunar Science Conference . . . . . . . . . . . 5.10 The Eighth Lunar Science Conference . . . . . . . . . . . . 5.11 The Ninth Lunar and Planetary Science Conference . . 5.12 The Tenth Lunar and Planetary Science Conference . . 5.13 Basaltic Volcanism on the Terrestrial Planets . . . . . . . 5.14 The Eleventh Lunar and Planetary Science Conference 5.15 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Widening the Research Front . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Meteorites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Workshop on Early Crustal Genesis: Implications from Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 The Snowbird Conferences . . . . . . . . . . . . . . . . . . . . . . . 6.5 Discussions on the Formation of the Moon in a Planetary Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Planning for a Conference on the Origin of the Moon . . . . 6.7 The Fourteenth Lunar and Planetary Science Conference . 6.8 Conferences and Workshops on Planetary Issues—The Impact of Lunar Research . . . . . . . . . . . . . . . . . . . . . . . . 6.9 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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The Kona Conference—Day 1 . . 7.1 Introduction . . . . . . . . . . . . 7.2 Invited Reviews . . . . . . . . . 7.2.1 Larimer . . . . . . . . . 7.2.2 Wetherill . . . . . . . . 7.2.3 Drake . . . . . . . . . . 7.2.4 (Jeffrey) Taylor . . . 7.2.5 Hood . . . . . . . . . . . 7.2.6 Burns . . . . . . . . . . 7.3 Geophysical Constraints . . . 7.3.1 Turcotte . . . . . . . . . 7.3.2 Matsui and Abe . . . 7.3.3 Binder . . . . . . . . . . 7.3.4 Solomon . . . . . . . . 7.3.5 Yoder . . . . . . . . . . 7.3.6 Russell . . . . . . . . . 7.3.7 Cisowski and Fuller 7.3.8 Banerjee . . . . . . . . 7.3.9 Runcorn . . . . . . . . . 7.4 Concluding Remarks . . . . . . References . . . . . . . . . . . . . . . . . .

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The Kona Conference—Day 2 . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . 8.2 Chemical and Petrological Constraints 8.2.1 Newsom . . . . . . . . . . . . . . . 8.2.2 Ringwood and Seifert . . . . . . 8.2.3 Delano . . . . . . . . . . . . . . . . . 8.2.4 Dickinson and Newsom . . . .

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8.2.5 Goodrich and Barnes . . . . . . . . 8.2.6 Warren and Rasmussen . . . . . . . 8.2.7 Warren . . . . . . . . . . . . . . . . . . 8.2.8 Goettel . . . . . . . . . . . . . . . . . . . 8.2.9 Shervais and (Lawrence) Taylor 8.2.10 Kreutzberger, Drake, and Jones . 8.2.11 Koeberl . . . . . . . . . . . . . . . . . . 8.2.12 Swindle, Caffee, and Hohenberg 8.2.13 (Ross) Taylor . . . . . . . . . . . . . . 8.3 Dynamical Constraints . . . . . . . . . . . . . 8.3.1 Vanyo . . . . . . . . . . . . . . . . . . . 8.3.2 VanArsdale . . . . . . . . . . . . . . . 8.3.3 Yoder . . . . . . . . . . . . . . . . . . . 8.3.4 Hartung . . . . . . . . . . . . . . . . . . 8.3.5 Singer . . . . . . . . . . . . . . . . . . . 8.3.6 Conway . . . . . . . . . . . . . . . . . . 8.3.7 McKinnon and Mueller . . . . . . . 8.3.8 Durisen, Gingold, and Scott . . . 8.3.9 Boss and Mizuno . . . . . . . . . . . 8.3.10 Mizuno and Boss . . . . . . . . . . . 8.3.11 Cox . . . . . . . . . . . . . . . . . . . . . 8.3.12 Hartmann . . . . . . . . . . . . . . . . . 8.4 My Model of Lunar Origin I . . . . . . . . . 8.4.1 Malcuit . . . . . . . . . . . . . . . . . . 8.4.2 Cassidy . . . . . . . . . . . . . . . . . . 8.4.3 Ringwood . . . . . . . . . . . . . . . . 8.4.4 Binder . . . . . . . . . . . . . . . . . . . 8.4.5 Wänke and Dreibus . . . . . . . . . 8.5 Concluding Remarks . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

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The Kona Conference—Day 3 . . . . . . . . . . . . . . 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9.2 My Model of Lunar Origin—II . . . . . . . . . . 9.2.1 Greenberg . . . . . . . . . . . . . . . . . . . 9.2.2 Hartmann . . . . . . . . . . . . . . . . . . . . 9.2.3 Hartmann and Vail . . . . . . . . . . . . . 9.2.4 Herbert and Davis . . . . . . . . . . . . . 9.2.5 Weidenschilling . . . . . . . . . . . . . . . 9.2.6 Chapman and Greenberg . . . . . . . . . 9.2.7 Herbert, Davis, and Weidenschilling 9.2.8 Weidenschilling et al. . . . . . . . . . . . 9.2.9 Wasson and Warren . . . . . . . . . . . .

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9.2.10 Cameron . . . . . . . . . . . . . . . 9.2.11 Benz, Slattery, and Cameron . 9.2.12 Kaula and Beachey . . . . . . . . 9.2.13 Stevenson . . . . . . . . . . . . . . 9.2.14 Melosh and Sonett . . . . . . . . 9.2.15 Kipp and Melosh . . . . . . . . . 9.3 Summary and Open Discussion . . . . . 9.4 Concluding Remarks . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . .

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10 Assessments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Looking Back . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Contributing Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1 Early Research . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.2 NASA and the University Research Community 10.3.3 Non-U.S. Researchers . . . . . . . . . . . . . . . . . . . . 10.3.4 The Lunar and Planetary Institute . . . . . . . . . . . 10.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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11 Epilogue . . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . 11.2 Continuing Research Issues . 11.3 Concluding Remarks . . . . . . References . . . . . . . . . . . . . . . . . .

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About the Author

Warren D. Cummings was born in Joy, Texas, in 1941. He grew up in north Texas and graduated from Rice University with a B.A. in Physics in 1963. He received his Ph.D. in Space Science at Rice in 1966; he was the first Ph.D. to graduate from the newly formed Space Science Department there. Dr. Cummings joined the Department of Planetary and Space Science at UCLA as an Assistant Professor in the fall of 1966, where in addition to teaching he was a Co-Investigator for the magnetometer experiments on the first Applications Technology Satellite (ATS-1). His research focus at Rice and UCLA was space plasma physics. After three years at UCLA, Dr. Cummings was appointed Head of the Physics Department at Grambling College in north Louisiana, where he spent seven years in undergraduate teaching, research, and administration. In the spring of 1976, Dr. Cummings was appointed the Executive Director of the Universities Space Research Association (USRA). For most of the next 31 years, Dr. Cummings was the Executive Director, later the Executive Vice President, of USRA, and was heavily involved with the oversight of a wide variety of research and educational activities in the space sciences and technologies, in addition to being responsible for the overall business management of the Association. Dr. Cummings is currently the Senior Advisor and Historian at USRA. He and his wife, Sue, reside in Columbia, Maryland.

xvii

About the Universities Space Research Association

The Universities Space Research Association (USRA) was founded in 1969 under the auspices of the US National Academy of Sciences (NAS), at the request of the National Aeronautics and Space Administration (NASA). USRA operates scientific institutes and facilities and conducts other major research and educational programs, fulfilling its nonprofit charter to advance space-related science and technology. USRA has a storied heritage that traces back to the Apollo era. James Webb, NASA Administrator, and Frederick Seitz, NAS President, recognized the technical challenges that exploration of space would require. Webb supported the universityoriented national R&D policy of the USA, and looked upon universities as the principal vehicles for building a “Space Age America.” In 1968, with the return of the lunar samples imminent, they worked together to create USRA, an organization to serve the university community, operating in parallel with NASA. As its first job, USRA took on the operation of the newly formed Lunar Science Institute, now the Lunar and Planetary Institute. Over the decades, the scope of USRA’s work grew to include aeronautics, astrophysics, computer science, Earth science, heliophysics, microgravity science, and advanced space technology development. USRA’s governance is grounded in its 111 member universities. More information about USRA is available at www.usra.edu.

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Chapter 1

Gaping at the Moon—18th and 19th Century Ideas on the Formation of Lunar Craters and Other Surface Features

1.1

Introduction

In the hot summer of 1892, the U.S. Congress debated the budget for the U.S. Geological Survey. Some congressmen from the Western states were unhappy about the Survey’s interference with the unrestricted use of the public domain and its materials resources in their states. During one of the congressional debates, a congressman denounced the survey by asserting that “So useless has the survey become that one of its most distinguished members has no better way to employ his time than to sit up all night gaping at the moon” (Davis, 1927: 176). The “distinguished member” under discussion during the debate was Grove Karl Gilbert (1843–1918) (Fig. 1.1), the Chief Geologist at the USGS. (The position of Chief Geologist would soon be abolished by congressional action, which also reduced the budget for geological work at the Survey by about 50%.) During August, September, and October of 1892, Gilbert had taken the opportunity to escape the daytime heat of Washington, both thermal and political, by observing the Moon through two lunations with the 26½-inch telescope at the U.S. Naval Observatory. Gilbert was a distinguished field geologist who had made his reputation through the study of the origins of terrestrial topography. He was intrigued by the lunar landscape and wanted to try his hand at its interpretation. Gilbert approached his study of the lunar surface as a judge between the hypotheses of volcanic or impact origin of the lunar craters. The volcanic theory was dominant at the time, despite the various observations and model experiments over many decades that supported the impact hypothesis. A brief review of the dialogue between the proponents of the volcanic and impact hypotheses follows.

© Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_1

1

1 Gaping at the Moon—18th and 19th …

2 Fig. 1.1 Grove Karl Gilbert (Courtesy U.S. Geological Survey)

1.2

Hooke and Herschel

The minority opinion of the impact origin of lunar craters was briefly held by Robert Hooke (1634–1703), who later thought that the craters were burst bubbles on the surface of a boiling, but rapidly cooling Moon (Sheehan & Dobbins, 2001: 25). The majority opinion was held by William Herschel (1738–1822), who had a distinguished career as a British astronomer. In 1781, Herschel discovered a planet, which he called the “Georgian planet” in honor of King George III, but which was later renamed Uranus. In 1787, Herschel gave an account of three volcanoes on the Moon, at least one of which he thought was active, beginning his description with: Thus, when we see, on the surface of the moon, a great number of elevations, from half a mile to a mile and an half in height, we are strictly intitled to call them mountains; but, when we attend to their particular shape, in which many of them resemble the craters of our volcanos, and thence argue, that they owe their origin to the same cause which has modelled many of these, we may be said to see by analogy, or with the eye of reason. … (Herschel & Banks, 1787: 229)

Herschel believed that life forms had probably been able to adapt to the various environments of the planets, the satellites of the planets (several of which he had discovered), and the Sun. In 1795, Herschel had argued for the existence of life on the Sun and other planetary bodies, as follows.

1.2 Hooke and Herschel

3

I shall now endeavor, by analogical reasonings, to support the ideas I have suggested concerning the construction and purposes of the sun; in order to which, it will be necessary to begin with such arguments as the nature of the case will admit, to shew that our moon is probably inhabited. … It [the Moon] is a secondary planet, of a considerable size; the surface of which is diversified, like that of the earth, by mountains and vallies. Its situation, with respect to the sun, is much like that of the earth; and, by a rotation on its axis, it enjoys an agreeable variety of seasons, and of day and night. To the moon, our globe will appear to be a very capital satellite; undergoing the same regular changes of illuminations as the moon does to the earth. The sun, the planets, and the starry constellations of the heavens, will rise and set there as they do on the earth. There seems only to be wanting, in order to complete the analogy, that it should be inhabited like the earth. To this it may be objected, that we perceive no large seas in the moon; that its atmosphere (the existence of which has even been doubted by many) is extremely rare, and unfit for the purposes of animal life; that its climates, its seasons, and the length of its days, totally differ from ours; that without dense clouds (which the moon has not), there can be no rain; perhaps no rivers, no lakes. In short, that, notwithstanding the similarity which has been pointed out, there seems to be a decided difference in the two planets we have compared. My answer to this will be, that that very difference which is now objected, will rather strengthen the force of my argument than lessen its value: we find, even upon our globe, that there is the most striking difference in the situation of the creatures that live upon it. While man walks upon the ground, the birds fly in the air, and fishes swim in water; we can certainly not object to the conveniences afforded by the moon, if those that are to inhabit its regions are fitted to their conditions as well as we on this globe are to ours. An absolute, or total sameness, seems rather to denote imperfections, such as nature never exposes to our view; and, on this account, I believe that analogies that have been mentioned fully sufficient to establish the high probability of the moon’s being inhabited like the earth. (Herschel, 1795: 65–66)

Herschel further argued by analogy that “… the sun is richly stored with inhabitants,” and “… if stars are suns, and suns are inhabitable, we see at once what an extensive field for animation opens itself to our view.” (ibid.: 68).

1.3

Schröter and Gruithuisen

Johann Hieronymous Schröter (1745–1816), who is regarded by some as the father of modern selenography, spent thirty years observing the Moon’s surface with his own telescopes, one of which was identical to the one that William Herschel had used for the discovery of Uranus. He published the results of his observations in two large volumes, titled Selenotopographische Fragment. Through his observations, Schröter made a number of discoveries, including the existence of lunar domes and wrinkle ridges, and he made many accurate measurements of the heights of lunar mountains and the (shallow) depths of lunar craters. He followed Herschel’s line of thinking, as was shared among many subsequent lunar observers —namely, a belief in the existence of advanced forms of life on the Moon (Sheehan & Dobbins, 2001:59–72).

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1 Gaping at the Moon—18th and 19th …

A Bavarian physician cum astronomer, Franz von Paula Gruithuisen (1774– 1852) (Fig. 1.2), championed the impact theory in the early part of the 19th century. In 1812, Gruithuisen was among the first to purchase small refracting telescopes from Joseph Fraunhofer (ibid.: 76–77). The largest of his three instruments had an aperture of 4 inches (102 mm) and a focal length of five feet (1.5 m). He set up his telescopes in a small observatory at his house in Munich. Gruithuisen and others developed an “aggregation” hypothesis to explain the formation of planetary bodies, including the Moon and the Earth. Spherical bodies of various sizes, with layered, concentric shells came together to form the planets, and the craters on the Moon were the signatures of the final influx of these bolides. Gruithuisen had in mind gentle collisions, so that the bolides sank into a viscous Moon, leaving the outer layers of their shells to form ringed mountains around their entry point.

Fig. 1.2 Franz von Paula Gruithuisen (The lithograph of Gruithuisen is by Roman Leiter (1805– 1834) from an original drawing by Joseph Anton Rhomberg (1786–1855). It is reproduced from a Google digitization of Gruithuisen, Fr. V. P. (1830) Analekten für Erd-und Himmelskund, Munich, Germany: Joh Palm’schen. It was used as the frontispiece for the book. Gruithuisen began his medical career at the age of 14, when he joined the Austrian Army as a volunteer assistant to a field surgeon. He obtained his medical degree with a dissertation “On the existence of sensation in the heads and trunks of those who have been decapitated, and the ways in which one could learn about this.” His intent was to show that death via the guillotine was one of the cruelest ways to die (Zajaczkowski, Zamann, & Rathert, 2003). Gruithuisen’s medical reputation rests mainly upon his pioneering work in urology and in particular his development of instruments for the transurethral destruction of bladder stones (ibid.: 369). His passion, however, was planetary astronomy, with a focus on the Moon

1.3 Schröter and Gruithuisen

5

That which we cannot see on Earth, because we are too close to it, we often see on other celestial bodies, and it is only after the fact that we find that things are the same on Earth. For many years, I have been diligently observing the skies with the best telescopes. Almost daily I found the following: the circular mountains of the Moon (not old [volcanic] craters —what nonsense!—which often must have had a diameter of 50 German miles) could not be anything other than the tail ends of the alien celestial bodies that sank into the lunar mass, as a result of which the Moon was aggregated. We see all of the circular walls better the greater and younger they are, the more clearly stratified they are, exactly like our mountains; the mound of the buried body often protruded quite clearly out of their center, and running away from it when it is deeper, when it perhaps has sunken faster, there are bright, hazy rays as traces of the riftings, for example, in the case of Tycho. (Gruithuisen, 1825: 6–7, German translation by Gerald L. Geiger)

Gruithuisen did not have the correct geophysics, but he was perhaps the first to recognize that the lunar Apennine Mountains, which lie in the form of an arc on the border of Mare Imbrium, are the result of the impact of a bolide on the Moon. … even the Apennines of the Moon are still a definite piece of an old circular mountain that was destroyed elsewhere by more recent events, in other words, a circular mountain that was the outer shell of a huge celestial body that had sunk into the Moon. (ibid.: 8)

Gruithuisen (wrongly) concluded that surface features on the Earth, such as island chains in the shape of an arc, could be explained by his aggregation theory and be accepted by the scientific community: … the moment one can be completely sure that the ring-shaped mountains of the celestial bodies are the sole result of the aggregation of the latter. But it would be too easy here for the doubting Thomas to become gradually and laboriously familiar with all of the major world building conditions before the establishment of the already developed theory because one thing is certain, and that is most people could not do anything but gradually grasp how little primeval nature was able to provide in terms of space, time, substance, and events. (Gruithuisen, 1821: 638–639, German translation by Gerald L. Geiger)

In line with the thinking of predecessors such as Herschel and Schröter, and foreshadowing an inclination of lunar scientists, institutions, and federal agencies one hundred and fifty years later, Gruithuisen (ibid.: 653) admitted to the motivation of the search for life beyond the Earth. “… but we do love the beautiful Moon and are able to spend more time on dry reports of observations if we can only conceive of the possibility of Selenites.” Gruithuisen became infamous for his observations that purported to find evidence for “Selenites,” and in fact for a city of lunar residents. His discoveries in this direction began on the 12th of July, 1822, when he found “Colossal construction on the Moon, not dissimilar to our cities.” (Gruithuisen, 1824: 34). Figure 1.3 is taken from Fig. 6 in his 1824 book with the translated title The Discovery of Many Distinct Evidences of Lunar Inhabitants, in Particular a Colossol Artifical Structure by the Same (Sheehan & Dobbins, 2001: 82). Gruithuisen’s figure caption reads, in part: This figure is the image of the great artificial structure in the Schröter moon spots, … . The circle in the south [In the figure, north is at the top, and west, by convention at the time, is to the right.], the two high mountains in north and northwest, and the two hills in the

6

1 Gaping at the Moon—18th and 19th …

Fig. 1.3 Gruithuisen’s sketch from his lunar observations, showing a structure that Gruithuisen thought served as a dwelling for intelligent beings on the Moon. Reproduced from a Google digitization of Fig. 6 following page 114 in Gruithuisen, F. V. P. (1824) Entdeckung Vieler Deutlichen Spuren der Mondbewohner, Besonders eines Collossalen Kunstgebäudes Derselben. Munich, Germany: Bayerische Staatsbibliothek

southwest are natural objects; the circle to the west seems to be have been constructed, everything else in this figure is obviously the product of Selenistic diligence. Here, the walls are usually over a mile wide, they may surpass in height even the tallest building in the world, and they are several miles long. It is highly likely that they serve as dwellings for intelligent beings on the moon. (Gruithuisen, 1824: 110, German translation by Gereld L. Geiger)

Gruithuisen conjectured that the star-shaped structure to the northeast had religious significance: Our star-like structure seems a kind of temple, and because it is star shaped, it is perhaps dedicated to star services, all the more likely as one can easily see the stars even in daylight, because of the purity of the air. (ibid.: 42)

Many of his contemporaries poked fun at Gruithuisen’s overactive imagination, but we should not lose sight of the contributions Gruithuisen made toward understanding the formation of the Moon. For example, it would take another sixty years before someone else (Albrecht Meydenbauer) recognized that the lunar Apennine mountains were caused by the impact of a bolide. It could be argued that Gruithuisen actually suffered from a failure of imagination. He was unable to recognize, as Gilbert did, that the various sculptural features on the Moon’s surface could be explained without recourse to Selenites.

1.4 Beer, Mädler and Althans

1.4

7

Beer, Mädler and Althans

Beginning in 1830, Johann Heinrich Mädler (1794–1874) began to make observations of the Moon using a 3.75-inch refractor telescope that had been built in Berlin with funding from Wilhelm Beer (1794–1850). Beer was a wealthy banker and amateur astronomer. Beer and Mädler followed up on initial efforts by Wilhelm Gotthelf Lohrmann (1796–1840) to map the Moon (Sheehan & Dobbins, 2001: Chap. 8). In 1837, Beer and Mädler published (in German) Der Mond, which quickly became a classic—the standard reference for a description of the Moon’s surface. Der Mond, however, was primarily descriptive. As noted by Sheehan and Dobbins (ibid.: 109): Mädler offered no firm opinion of the lunar craters, but he did emphasize the almost complete lack of analogy with all terrestrial crater-forms. ‘The vastness of our volcanic cones,’ he remarked, ‘would hardly compare with the smallest pits visible on the lunar surface.’

In the early 1840s, Carl Ludwig Althans (1788–1864), a German architect, performed perhaps the first model experiment on the formation of lunar craters. Fifty years later, Althans’ son Ernst described the experiment, in which he participated (Fig. 1.4). In a wooden cubic box containing about ¾ m3, a fast rigidifying, but still liquid, mortar pulp consisting of milk of lime, cement, and gypsum was mixed and chosen to be the substitute for the lunar surface that was still imagined to be viscous. I was still a schoolboy when, on my father’s urging, I let one cartridge ball each drop perpendicularly down into the mortar pulp at a height of about 8 m at intervals one after the other. The jet that erupted centrally from the impact point and the ring-shaped wave that it produced on the surface

Fig. 1.4 Photograph showing the result of an experiment performed by Carl Ludwig Althans, in which his son, Ernst Friedrich Althans dropped cartridge balls into mortar pulp from a height of 8 m. Reproduced from Fig. 1 in a Google digitization of Althans, E. (1895) “Über Versuche, die eigentümliche Gestalt der Mondoberfläche zu erklären.” Gaea, volume 27, page 8

1 Gaping at the Moon—18th and 19th …

8

were completely dissolved upon the first impacts in the still-liquid pulp without leaving a surface image behind. Only the third cartridge ball in the now stiffer and more malleable mortar pulp produced the deceptively similar replication of a lunar crater with a circular wall, an internal mountain cone plus appendix, and lateral depressions. … The central mountain is the lower part of the ray that protrudes out of the impact canal of the ball. A small secondary mount is formed by a separated part of the ray that rose to a low height. A chunk, which had flown higher and laterally from the crater ring, knocked a deep hole into the mortar pulp, whose stiffness permitted only the development of a wave ring. And so, as a result of the impact of a comparatively small body weighing about ½ kg, there developed three different mount forms so characteristic of the Moon: the 11-cm-wide crater ring, the central mount plus companion, the simple whole crater as secondary formation. The typical depression inside the ring wall against the surrounding surface corresponds to the volume content of the masses that had risen up. (Althans, 1895: 7, German translation by Gerald L. Geiger)

Carl Althans subscribed to Laplace’s hypotheses for the formation of the solar system, namely that the proto Sun/solar system was a large lens-shaped rotating gaseous mass from which planetary materials were sequentially spun off to form rings of gas and matter around the Sun and subsequently to coalesce into planetary systems. Althans thought that the building blocks of the Earth’s crust were formed within the ring of material that orbited the Earth and that these masses would spiral into the Earth owing to the drag on them by the extended atmosphere of the proto-Earth (Althans, 1839: 26–44). Althans assumed that the Moon, as the Earth’s companion, would receive its share of planetesimal impacts, and he attempted to explain various lunar surface features accordingly. Among the masses that fell on the Moon, some larger ones … fell into the soft lunar mass and submerged in it. If the latter contained water formations in their constituents, then the waters were converted in the hot lunar mass into mighty water vapors which, with tremendous force and in the form of big bubbles, again rose to the surface, where they burst and were able to bring about magnificent annular formations in the viscous-igneous lunar surface. The mere fall of the masses, thus submerged without any rising water vapors, could also produce similar figures in the soft lunar surface. All you have to do is let a stone that you have flung upward fall as perpendicularly as possible in a stiff but sufficiently deep mass of mud; then you will see the formation of such figures. The mud must be stiffened up so that it will not again turn into liquid but instead will retain its assumed shape. After the lunar surface, as a result of the previously mentioned heat radiation, had gotten an already thin, rigidified crust, the falling force of such masses caused big cracks in the rigid crust, which often had to extend very far and which also were often bound to emanate in a radial form from the hole that had been punched; this compares to the case when we allow a stone almost perpendicularly to fall into a thin ice cover on the water, of course from an adequate height. … (Althans, 1839: 47–48, German translation by Gerald L. Geiger)

1.5 Dana and Proctor

1.5

9

Dana and Proctor

In 1846, about the time of Carl Althans’ model experiments in Germany, James Dwight Dana (1813–1895) of Yale University published a paper in the American Journal of Science and Arts titled “On the volcanoes of the Moon.” Dana had recently returned from an exploring expedition in the Pacific, and he argued that the lunar craters were volcanoes of the Kilauea type, rather than Vesuvian. Such are the general facts [about the lunar surface], which call for explanation, to wit: the existence of circular pit craters, 5 to 150 miles in diameter, and five to twenty-four thousand feet in depth; - the great number of these pit craters, and their peculiar features; - the depressions of a similar character of still larger area; - and the various degrees of illumination of the craters. Well may the Vesuvian vulcanist look with doubt upon such vast gulfs; for he finds in his well known volcano, nothing parallel in kind or degree. The little dark hole at the top of his mountain, has scarcely a single point of resemblance to the open walled areas of the moon. But the case is different with Kilauea, to which we now direct our attention. We observe that the facts this crater presents are precisely the same in kind as those of the moon. 1. The crater is a large open pit, exceeding three miles in its longer diameter, and nearly a thousand feet deep. 2. It has clear bluff walls through a greater part of its circuit, with an inner ledge or plain at their base, raised 340 feet above the bottom. 3. The bottom is a plain of solid lavas, entirely open to day, which may be traversed with safety; over it there are pools of boiling lava in active ebullition, and one is more than a thousand feet in diameter. … (Dana, 1846: 341–342)

Dana continued his exposition of the similarities of lunar craters with the Kilauea volcano, concluding with “… A map of the moon, if there is any truth in these views, should be in every geological lecture room; for no where can we have a more complete or magnificent illustration of volcanic operations. …” (ibid.: 347). Nearer to the time of Gilbert’s observations, the idea of impacts as the origin of lunar craters was advanced, somewhat tentatively, by a British astronomer in 1873. Richard Anthony Proctor (1837–1888) was a popular lecturer and writer whose premise was that “… the solar system formed by the gathering in from outer space of materials once widely scattered.” (Proctor, 1873: 342). According to this view, the moon, formed at a comparatively distant epoch in the history of the solar system, would have not merely had its heat originally generated for the most part by meteoric impact, but while still plastic would have been exposed to meteoric downfalls, compared with which all that we know, in the present day, of meteor-showers, aerolitic masses, and so on, must be regarded as altogether insignificant … … If we attempt to picture the condition of the moon in that era of her history when first the process of downfall became so far reduced in activity as to permit of her cooling down, we shall be tempted, I believe, to consider that some of the more remarkable features of her globe had its origin in that period. It may seem, indeed, at a first view, too wild and fanciful an idea to suggest that the multitudinous craters on the moon, and especially the smaller craters revealed in countless numbers when telescopes of high power are employed, have been caused by the plash of meteoric rain, - and I should certainly not care to maintain that

10

1 Gaping at the Moon—18th and 19th … as the true theory of their origin; yet it must be remembered that no plausible theory has yet been urged respecting this remarkable feature of the moon’s surface. It is impossible to recognize a real resemblance between any terrestrial feature and the crateriferous surface of the moon. … If we consider the explanation advanced by Hooke, that these numerous craters were produced in the same way that small cup-shaped depressions are formed when thick calcareous solutions are boiled and left to cool, we see that it is inadequate to account for lunar craters, the least of which … are at least half a mile in diameter. The rings obtained by Hooke were formed by the breaking of surface bubbles or blisters, and it is impossible for such bubbles to be formed on the scale of the lunar craters. Now so far as the smaller craters are concerned, there is nothing incredible in the supposition that they were due to meteoric rain falling when the moon was in a plastic condition. Indeed, it is somewhat remarkable how strikingly certain parts of the moon resemble a surface which has been rained upon while sufficiently plastic to receive the impressions, but not too soft to retain them. Nor is it any valid objection to this supposition, that the rings left by meteoric downfall would only be circular when the falling matter chanced to strike the moon’s surface squarely; for it is far more probable that even when the surface was struck very obliquely and the opening first formed by the meteoric mass or cloud of bodies was therefore markedly elliptic, the plastic surface would close in round the place of impact until the impression actually formed had assumed a nearly circular shape. (ibid.: 343–346)

Proctor subscribed to the theory of the Irish geophysicist and engineer, Robert Mallet (1810–1881), in which the contraction of the Earth’s crust was the source of earthquakes and volcanism. Mallet believed that the same process would have occurred in other planets and the Moon. Proctor was a master of the use of caveats, but he presumably believed that the great lunar craters were volcanic in origin, caused by the contraction of the Moon’s crust. He argued that: Again, the aspect of the regions surrounding the great lunar craters—and especially the well-studied Copernicus—accords closely, when sufficient telescopic power is employed, with the theory that there has been a general contraction of the outer crust of the moon, resulting in foldings and cross-foldings, wrinkles, corrugations, and nodules. But the multiplicity of smaller craters does not seem to be explained at all satisfactorily; while the present absence of water, as well as the want of any positive or direct evidence that water ever existed upon the moon, compels us to regard even the general condition of the moon’s surface as a problem which has still to be explained. If, however, it be admitted that the processes of contraction proceeded with sufficient activity to produce fusion in the central part of a great region of the contracting crust, and that the heat under the crust sufficed for the vaporization of a considerable portion of the underlying parts of the moon’s substance, we might find an explanation of the great craters like Copernicus, as caused by true volcanic action. The masses of vapour which, according to that view, sought outlet at craters like Copernicus must have been enormous however. Almost immediately after their escape they would be liquefied, and flow down outside the raised mouth of the crater. According to this view we should see, in the floor of the crater, the surface of what had formerly been the glowing nucleus of the moon: the masses near the centre of the floor (in so many cases) might be regarded as, in some instances, the débris left after the great outburst, and in others as the signs of a fresh outburst proceeding from a yet lower level; while the glistening matter which lies all round many of the monster craters would be regarded as the matter which had been poured out during the outburst. (ibid.: 370–371)

1.5 Dana and Proctor

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After his death, Proctor’s daughter suggested that her father changed his mind on the impact origin of any of the lunar craters. In her book, Romance of the Moon, she pointed out that in the 1878 edition of his book, her father omitted any reference to the “meteoric hypothesis” (Proctor, 1928: 83).

1.6

Nasmyth, Carpenter, and Neison

In 1874, James Nasmyth and James Carpenter began to publish editions of a book titled The Moon, Considered as a Planet, a World, and a Satellite. The first three editions of the book had been published by 1875, and they represented a culmination of some 30 years of observations with a telescope, followed by detailed drawings of surface features, followed by the creation of models based on the drawings, followed by photographs of the models in sun light. The result was a book that contained stunning illustrations (See Fig. 1.5, taken from Plate XIV in the 4th edition of the book). Fig. 1.5 Plate XIV from The Moon by Nasmyth and Carpenter, showing the craters Aristarchus on the left, with a diameter of 40 km, and Herodotus, to its right, with a diameter of 35 km. Reproduced from a Nabu Public Domain Reprint of Nasmyth, J., & Carpenter, J. (1903) The Moon, Considered as a Planet, a World, and a Satellite (4th ed.). London, England: John Murray, following page 144

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1 Gaping at the Moon—18th and 19th …

Fig. 1.6 An illustration by Nasmyth and Carpenter showing their concept of how underlying magma would burst through cracks in the lunar crust to form volcanoes and other features of the lunar surface. Reproduced from a Nabu Public Domain Reprint of Nasmyth, J., & Carpenter, J. (1903) The Moon, Considered as a Planet, a World, and a Satellite (4th ed.). London, England: John Murray, following page 46

Nasmyth and Carpenter assumed that the Moon was initially completely molten, owing to the transfer of kinetic energy to heat from in-falling material during its aggregation. The surface began to cool through the radiation of heat into space. A thin crust formed, and as it further cooled, it contracted and so developed cracks. An underlying still liquid magma expanded as it cooled and burst through the cracks to form lunar volcanic craters (See Fig. 1.6, taken from Fig. 5 in The Moon). Nasmyth and Carpenter argued that other features of the Moon, e.g., mountain ridges, troughs, and domes could be explained by “… the alternate expansion and contraction of successive strata of the lunar sphere, when in a state of transition from an igneous and molten to a cooled and solidified condition … .” (Nasmyth & Carpenter, 1903: 49). They acknowledged that the size of the lunar craters vastly exceeded the size of terrestrial volcanic craters, but: … there is a feature in the majority of the ring-mountains that, as we conceive, demonstrates completely the fact of volcanic force having been in full action, and that seems to stamp the volcanic character upon the crater-forms. This special feature is the central cone, so well known as a characteristic of terrestrial volcanoes, accepted as the result of the last expiring effort of the eruptive force, and formed by the deposit, immediately around the volcanic orifice, of matter which there was not force enough to project to a greater distance. … (ibid.: 156)

In 1876, an English astronomer, Edmund Neville Nevill, who took the name Edmund Neison (1849–1940), published The Moon and the Condition and Configuration of its Surface. As Neison explained in the preface of his book: This work was undertaken with the view of promoting the study of Selenography, by supplying what has long been much wanted—namely, a work on the Moon which should treat of the present condition of the surface and deal with the configurations of the lunar crust with some degree of comprehensiveness. English selenographers have long felt the want of such a treatise on the Moon, and its absence has been often urged as a main cause of the slow progress that has been made in the study of the phenomena presented by the

1.6 Nasmyth, Carpenter, and Neison

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Moon. Hitherto, the only work on this subject has been Beer and Mädler’s grand ‘Der Mond’—published forty years back, and, being in German, practically inaccessible to most English astronomers, besides standing in much need of revision and extension to bring it up to date. (Neison, 1876: v)

As with Beer and Mädler, Neison confined himself to descriptions of the Moon’s surface features of the Moon, whereas Nasmyth and Carpenter had argued for a volcanic interpretation.

1.7

Meydenbauer and the Thiersches

At about this same time, a champion for the impact hypothesis came forward in the person of Albrecht Meydenbauer (1834–1921), a well-known German architect, who began to try to reproduce the surface features of the Moon by dropping pinches of powdered substances, such as dextrin, starch, sulfur, and road dust, on surfaces of the same powder (Meydenbauer, 1877: 180). Meydenbauer thought that the Moon was probably an agglomeration of powdered meteor masses and that the last meteors that fell on the dust-like layered surface of the Moon left their marks in what we now see (Meydenbauer, 1882: 60–61). Meydenbauer could reproduce a variety of lunar surface features, including the “walled planes,” craters, grooves, and Mare areas (Fig. 1.7). He recognized that lunar mountain ranges such as the Alps and Apennines were remnants of walled planes. He thought the great walled basins were caused by the impacts of small moonlets that had been independently orbiting the Earth (ibid.: 61–62). In 1879, another German architect, August Thiersch, and his father, Heinrich Thiersch, who was a professor of theology, published a paper titled “Die Physiognomie Des Mondes,” using the pseudonym Asterios. The Thiersches were familiar with Richard Proctor’s papers and book, and they knew that while he explained the small lunar craters and pits as being caused by a “meteor rain,” he stuck with volcanism to explain the large craters and walled planes. The Thiersches, on the other hand, forcefully argued that the impact hypothesis could explain all the various lunar surface features. Those walled plains, those annular mountains, those cylindrical gorges and abysses, those smaller crater openings and pits, all of them, along with their secondary phenomena, resulted from the fall of cosmic bodies. Spherical celestial bodies having dimensions smaller than those of the Moon and that collided with it gave the Moon the shape of its surface. (Thiersch & Thiersch, 1879: 10, German translation by Gerald L. Geiger)

The Thiersches envisaged the early Moon as being completely molten and being struck by celestial bodies as it solidified from the outer shell inward. They viewed the Schickard crater [a crater that is filled with lava to its rim], for example, as having been caused by an impact that occurred after a thin lunar shell had been formed.

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1 Gaping at the Moon—18th and 19th …

Fig. 1.7 A figure showing several experiments by Albrecht Meydenbauer, who dropped pinches of powdery substances on surfaces of the same powder to reproduce features on the Moon. Reproduced from a Google digitization of Althans, E. (1895) “Über Versuche, die eigentümliche Gestalt der Mondoberfläche zu erklären.” Gaea, volume 27, the frontispiece

1.7 Meydenbauer and the Thiersches

15

This, in other words, is a place where a spherical body broke in and sank. Accordingly, in this case, the surrounding wall would not consist of material of that maverick, but rather of constituents of the lunar shell that had been melted and pushed aside and that had again solidified. Presumably, on top of the sunken body, the bubbling liquid mass of the lunar interior again closed up, and during its rigidification, it formed the smooth, circular surface that is now bordered by the solidified surf. (ibid.: 13)

As we will see, this picture is only partially correct, since it assumes a low velocity of impact for the collision between the celestial body and the Moon. The Thiersches realized, however, that higher velocity impacts would cause explosions because of the sudden heat generated by the impact. This would be particularly true after the Moon had further cooled and formed a thicker shell that an impacting body could not penetrate. Depending on the degree of its intensity, percussion caused heating, specifically both in the falling body and in the part of the lunar surface that it touched. Sudden heating, which probably in some cases increased up to red heat and melting, was bound to promote dissolution and disintegration. More than that: not only heating and melting alone, but rather explosion is to be assumed with a degree of probability in the case of many of these bodies as a consequence of their impact. This is how we might interpret those huge steep and almost cylindrical crater walls, which we cannot explain in terms of an eruption from below toward the top but rather in terms of a lateral pressure. It was the effect of the explosion toward the side that piled up the exploding body in the shape of an abrupt annular mountain, said body perhaps also consisting of previously present loose material into which the body rammed its way. (ibid.: 16–17)

The explanation for lunar ray systems given by Nasmyth and Carpenter was that eruptions caused radial surface cracks that were filled by igneous material that welled up from the Moon’s interior. The Thiersches found flaws in this hypothesis and argued instead that the various surface effects were related to the different substances that might make up the impacting celestial body. The viscous ones will be deposited nearby in the form of mountains and terraces; on the other hand the easily liquefiable ones, for example, sulfur, will be flung far away by the explosion and would form a precipitate on the lunar surface, which is noticed not by an elevation of the terrain but rather by an odd coloration. (ibid.: 20)

Finally, the Thiersches turned to an explanation of the lunar mare. We would be wrong if we were to consider these plains as the remnants of the original lunar surface, such as it might have taken shape as a result of normal cooling without any action from the outside. Most probably, little or nothing is to be seen of that primitive formation. … The igneous substances that were forced upward by a powerful thrust and pressure from above out of the lunar interior through the blasted openings spread out and formed these surfaces. (ibid.: 21)

The Thiersches applied the same argument, not only to the lava-filled craters, but to the great lunar seas as well. It is probably not too daring if we were to extend this interpretation to all of the seas and if we were to consider them as grandiose floodings that filled and covered the depressions on the Moon. We should not be astonished at their huge volume if we visualize the weight of the cosmic bodies and the effect of their plunge upon the lunar crust and the fluid interior.

1 Gaping at the Moon—18th and 19th …

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The term “sea” and “ocean bight” accordingly would not seem to be entirely wrong. Water masses, of course, they are not, but they do resemble frozen seas of lava. Those bays that did not suffer any disfigurement on their surface would probably be the result of the latest floodings; they belong to a period when the great hailstorms were over with and when the cosmic ammunition supply, so to speak, was exhausted. (ibid.)

It is not clear if the Thiersches assumed that lunar lava filled some of craters and the seas in conjunction with the impact that created the craters. Certainly many others who followed them made that assumption— incorrectly, as it turned out. It would be seventeen years after the publication of “Die Physiognomie Des Mondes” before Henri Becquerel would discover radioactivity, which was an important source of heating of the Moon’s interior that was unknown to the Thiersches. In any case, the Thiersches were among the first to recognize that Mare Imbrium, for example, was a lava-filled basin of huge dimensions. This surface is bordered by the most astonishing mountains; in the south along its edge rise the Carpathians and Apennines, in the west the Caucasus and the Alps. The Apennines and the Alps … exceed everything that we have in the way of mountains on Earth in terms of abruptness and wildness. These mountain chains have abrupt slopes facing toward the Mare Imbrium, whereas toward the outside, they blend into a high plateau. They form parts of an elliptical ring. … Gruithuisen was right in recognizing these mountains as segments of the shoreline that encloses the Sea of Rains. The boundary line can still be recognized where there are no mountains. … So here again, we have a mainly normal wall plain. A huge piece of lunar shell has collapsed. Proportional to the latter’s size are these chains of huge mountains and rubble walls that extend along the rim. … The elliptical shape of the Mare Imbrium and the analogy with the hitherto considered cases entitles us to presume that here again the collapse, with its violent and colossal effects, was caused by a thrust from above. As a plunging body with big dimensions happened to hit, a flood wave rose which ruptured the lunar crust in a vast area and which threw the rubble on the remaining flaps of the rim. The same happens on a small scale in an experiment. If a stone is thrown through thin ice with great force, then the stirred-up water makes the opening wider, it floods the surroundings, and covers them with the wreckage of the ice. (ibid.: 22)

1.8

Concluding Remarks

Prior to the 20th century, most of the dialog about the surface features of the Moon took place among European observers. So many of the conclusions of the those favoring an impact origin for lunar surface features turned out to be essentially correct that it is surprising that their view did not carry the day. As we will see, this phenomenon was replicated in the United States. Acknowledgements We gratefully acknowledge Mr. Gerald L. Geiger for his translations of sections of papers by Gruithuisen, Althans, and the Thiersches.

References

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References Althans, C. L. (1839). Grundzüge zur Gänzlichen Umgestaltung der Bisherigen Geologie, oder Kurze Darstellung der Weltkörper- und Erdrindenbildung. Koblenz, Germany: Karl Bädeker. Althans, K. L. (1895). Über Versuche, die eigentümliche Gestalt der Mondoberfläche zu erklären. Gaea, 27(7), 87. Dana, J. D. (1846). On the volcanoes of the Moon. American Journal of Science and Arts, 2(6), 335–353. Davis, W. M. (1927). Grove Karl Gilbert, 1843–1918. National Academy of Sciences Biographical Memoirs, 1–303. Gruithuisen, Fr. V. P. (1821). Selenognostische Fragmente. Nova Acta Physico-Medica Academiae Caesareae Leopoldino-Carolinae Naturae Curiosorum, 10, 635–693. Gruithuisen, Fr. V. P. (1824). Entdeckung Vieler Deutlichen Spuren der Mondbewohner, Besonders eines Collossalen Kunstgebäudes Derselben. Munich, Germany: Bayerische Staatsbibliothek. Gruithuisen, Fr. V. P. (1825). Gedanken und Ansichten über die Ursachen der Erdbeben nach der Aggregations Theorie der Erde. Nuremburg, Germany: J. L. Schrag. Gruithuisen, Fr. V. P. (1830). Analekten für Erd-und Himmelskunde. Munich, Germany: Joh. Palm’schen Buchhandlung. Herschel, W. (1795). On the nature and construction of the Sun and fixed stars. Philosophical Transactions of the Royal Society London, 85, 46–72. Hershel, W., & Banks, J. (1787). An account of three volcanos in the Moon. Philosophical Transactions of the Royal Society London, 77, 229–232. Meydenbauer, A. (1877). Über die bildung der Mondoberfläche. Sirius, 10(N.F. 5), 180. Meydenbauer, A. (1882). Die gebilde der Mondoberfläche. Sirius, 15(N.F. 10), 59–64. Nasmyth, J., & Carpenter, J. (1903). The Moon, considered as a planet, a world, and a satellite (4th ed.). London, England: John Murray. Neison, E. (1876). The Moon and the condition and configurations of its surface. London, England: Longmans, Green, and Co. Proctor, M. (1928). Romance of the Moon. New York and London: Harper & Brothers. Proctor, R. A. (1873). The Moon: Her motions, aspect, scenery, and physical condition. London, England: Longmans, Green, and Co. Sheehan, W. P., & Dobbins, T. A. (2001). Epic Moon: A history of lunar exploration in the age of the telescope. Richmond, VA: Willmann-Bell. Thiersch, H. W. J., & Thiersch, A. (1879). Die Physiognomie des Mondes. Nördlingen, Germany: C. H. Bech’sche Buchhandlung. Zajaczkowski, T., Zamann, A. M., & Rathert, P. (2003). Franz von Paula Gruithuisen (1774– 1852): Lithotrity pioneer and astronomer. On the 150th anniversary of his death. World Journal of Urology, 20(6), 367–373.

Chapter 2

Lunar Observations and Speculations— From Gilbert to the Apollo Explorations

2.1

Introduction

As the 19th century was coming to a close, the scientific dialogue about the lunar surface continued and was eventually joined by scientists in the United States. The discussions now had the benefit of observations made by more powerful telescopes, such as the one used by Grove Karl Gilbert in 1893. In this chapter, we follow the arguments for and against the impact hypothesis as the cause of most of the lunar surface features.

2.2

Gilbert’s Analysis

As Grove Karl Gilbert stepped to the podium to deliver the conclusions from his telescopic observations in his address as Retiring President of the Philosophical Society of Washington on 10 December 1892, he was aware of some, but not all, of the “moon gaping” and model experimentation that had preceded his. Gilbert had read Proctor’s work, as well as the papers by Meydenbauer, and he knew of Die Physiognomie des Mondes by “Asterios” (Heinrich and August Thiersch) but had not read it. He mentioned the point made by James Dwight Dana that lunar craters resembled the Hawaiian type of terrestrial volcanoes much more so than the Vesuvian type. He referenced Der Mond by Beer and Mädler and The Moon by Neison, but he made no reference to the works of Franz von Paula Gruithuisen or Carl Ludwig Althans. Gilbert told his audience that he had concluded that the great majority of the Moon’s craters were created by impact of other solar system bodies, rather than by volcanic activity.

© Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_2

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2 Lunar Observations and Speculations …

20 Fig. 2.1 Trends of lunar sculpture sketched by Gilbert. General sculpture is represented by shading; great furrows by heavy lines. Irregular lines show crests of uplands surrounding Mare Imbrium. Adapted from Fig. 14 in a Google digitization of Gilbert, G. K. (1893) “The Moon’s face: A study of the origin of its features.” Bulletin of the Philosophical Society of Washington, volume 12, page 277

Ninety-nine times in one hundred the bottom of the lunar crater lies lower than the outer plain; ninety-nine times in a hundred the bottom of the Vesuvian crater lies higher than the outer plain. Ordinarily the inner height of the lunar crater rim is more than double its outer height; ordinarily the outer height of the Vesuvian crater rim is more than double its inner height. The lunar crater is sunk in the lunar plain; the Vesuvian is perched on a mountain top. The rim of the Vesuvian crater is not developed, like the lunar, into a complex wreath, but slopes outward and inward from a simple crest-line. If the Vesuvian crater has a central hill, that hill bears a crater at summit and is a miniature reproduction of the outer cone; the central hill of the lunar crater is entire, and is distinct in topographic character from the circling rim. The inner cone of a Vesuvian volcano may rise far higher than the outer; the central hill of the lunar crater never rises to the height of the rim and rarely to the level of the outer plain. The smooth inner plain characteristic of so many lunar craters is either rare or unknown in craters of Vesuvian type. Thus, through the expression of every feature the lunar crater emphatically denies kinship with the ordinary volcanoes of the earth. If it was once nourished by a vital fluid, that fluid was not the steam-gorged lava of Vesuvius and Etna. (Gilbert, 1893: 250)

Gilbert had carefully examined the Moon’s surface, and he believed he was the first to correctly attribute the “features of sculpture,” e.g., aligned hills, groves in ridges, and furrows in plains radiated from the large craters, particularly the Imbrium crater, as being caused by the ejecta (Fig. 2.1). … Tracing out these sculptured areas and platting the trend lines on a chart of the moon, I was soon able to recognize a system in their arrangement, and this led to the detection of fainter evidences of sculpture in yet other tracts. The trend lines converge toward a point near the middle of the plain called Mare Imbrium, although none of them enter that plain. Associated with the sculpture lines is a peculiar softening of the minute surface configurations, as though a layer of semi-liquid matter had been overspread, and such I believe to be the fact; the deposit has obliterated the smaller craters and partially filled some of the

2.2 Gilbert’s Analysis

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larger. These and allied facts, taken together, indicate that a collision of exceptional importance occurred in the Mare Imbrium, and that one of its results was the violent dispersion in all directions of a deluge of material—solid, pasty, and liquid. … … … The whole topography may be classified as antediluvial and post diluvial. The only small craters are those of the later series, as the older have been filled and buried. Craters of the older series have lost in accentuation not only through the paring of their rims but also through the partial filling of their valleys, and their rims no longer exhibit the fine details of inner terracing and outer furrowing. … … Adjoining this district on the south and extending thence to the south pole is a broad area, known as the honeycomb district, to which the flood did not extend and with which the characters of the flooded district may be compared. In the honeycomb district distinctions of age may, indeed, be recognized, but there is gradation instead of sharp demarcations between old and new. Those parts of the surface which have been longest exempt from the downfall of large bodies are profusely pitted with minor craters, and it is these which dim the outlines of larger formations of ancient date. Thus, by the outrush from the Mare Imbrium were introduced the elements necessary to a broad classification of the lunar surface. A part was buried by liquid matter whose congelation produced smooth plains. Another part was overrun by a flood of solid and pasty matter which sculptured and disguised its former details. The remainder was untouched, and probably represents the general condition of the surface previous to the Imbrium event. … … It thus appears possible, if not probable, that they [the sculptured features] were produced simultaneously with the Imbrium deluge, and the implication of power is thereby rendered even more impressive. What must have been the violence of a collision whose scattered fragments, after a trajectory of more than a thousand miles, scored valleys comparable in magnitude with the Grand Canyon of the Colorado! (ibid.: 275–280)

Ursula Marvin (1921–2018) of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, pointed out that through his brief study of the Moon’s surface, Gilbert developed the discipline of lunar stratigraphy as he worked out the chronology of various lunar features based on crater counts, overlapping formations, etc. (Marvin, 2002: 29). Gilbert developed a “Moonlet Theory” for the formation of the Moon. The cratered surface, particularly the very large impacts, suggested to him that the Earth must have been ringed with moonlets. … It is my hypothesis that before our moon came into existence the earth was surrounded by a ring similar to the Saturnian ring; that the small bodies constituting this ring afterward gradually coalesced, gathering first around a large number of nuclei, and finally all uniting in a single sphere, the moon. Under this hypothesis the lunar craters are the scars produced by the collision of those minor aggregations, or moonlets, which last surrendered their individuality. (Gilbert, 1893: 262)

Gilbert thought that the collision of the moonlets with the Moon would adjust the rotation rate of the Moon so as to account for the circular form of almost all the lunar craters and thereby relieve the meteoric theory of “its most formidable difficulty.”

2 Lunar Observations and Speculations …

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The motion of each moonlet at the instant of collision may be conceived as resolved into two components, one normal to the moon’s surface and the other tangential. If the tangential component coincided in direction and velocity with the rotational motion of the moon’s surface, the collision would not affect the moon’s rotation; but if the tangential component had a velocity greater or less than the rotational motion, the moon’s rotation would be accelerated or retarded. The aggregate result of all collisions would be such a rotation of the moon that its surface speed would equal the average of the tangential components of the velocities of the moonlet impact. It is evident that if the tangential component of a moonlet’s motion coincided exactly with the motion of the moon’s surface, the impact phenomena would be the same as though the moonlet fell vertically on a motionless surface; and the harmonious adjustment of moon rotation to the motions of a system of moonlets would reduce to a minimum the ellipticity of craters. (ibid.: 269)

Gilbert also argued that the collisions of the moonlets with the Moon would change its axis of rotation, and he thus explained the widespread distribution of craters on the lunar surface. … If, as I have assumed, the moonlets approached the moon approximately in the plane of its equator, the fact is not attested by the groupings of the craters in a medial zone, and so it is necessary to assume further that the axis of rotation was not constant. This assumption need occasion no difficulty, for unless the approaching moonlets moved precisely in the plane of the moon’s equator, their collisions would disturb its axis of rotation, and there is no reason to suppose that these disturbances would be compensatory rather than cumulative. Under the successive impulses thus given the moon’s equator may have occupied successively all parts of its surface, without ever departing widely from the plane of the moon’s orbit. (ibid.: 275; Gilbert’s original italics)

Since the moonlets would all have been travelling in similar orbits around the Earth, their coalescence into the Moon would not have generated an excessive amount of heat. Gilbert concluded that “During the whole period of growth the body of the moon was cold.” (ibid.: 290). This sketch of the life of our nearest neighbor has but little in common with the accounts of other biographers. To her has been ascribed a fiery youth, after the manner of the sun, a middle life of dissipation, like Jupiter and Saturn, a hardening and wrinkling old age, toward which the earth is tending, and finally, the end of change, death. If the record of her scarred face has now been read aright, all that remains of the old narrative is its denouement: the moon is dead. (ibid.)

2.3

Immediate Reactions to Gilbert’s Ideas—Davis and Althans

William Morris Davis (1850–1934) was among the first to criticize Gilbert’s ideas, particularly his moonlet theory. The weakest point [of Gilbert’s theory] seems to us to lie in the assumption that the moonlets all moved in a single plane; for it is hardly possible to imagine that the internal and external perturbations to which these numerous little bodies would be subject, could have allowed them always to move in so orderly a fashion. The wide departure of the

2.3 Immediate Reactions to Gilbert’s Ideas—Davis and Althans

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asteroids from a common orbital plane is well known; and if even only a moderate departure from a plane be admitted for the moonlets, their average angle of incidence on the lunar surface might be decidedly altered from the favorable values determined by the assumed conditions of simpler movement. (Davis, 1893: 343)

Ernst Friedrich Althans (1828–1899), the son of Carl Ludwig Althans, soon had excerpts of Gilbert’s paper, with which he was mostly in agreement, as it was consistent with the work of his father and himself. About three years after Gilbert’s address, Ernst reported on experiments to “explain the peculiar configuration of the lunar surface.” (Althans, 1895). Like his father, Ernst was Privy Councillor of the Mining Board, and he had access to experiments on the effect of artillery projectiles against armored turrets. The impressions left by the hard, sugar-cone-shaped projectiles on the armor, which is brittle in the case of hard castings and which is tough in the case of steel, display great similarity to the areas of the Moon. (ibid.: 12, translated from the German by Gerald L. Geiger)

Ernst Althans further developed his father’s hypothesis for the impact origin of the Moon’s surface features in two ways. Firstly, Althans argued that because of the differences in surface gravity, density, and area of the Moon, compared to the Earth, the tangential stress in the Moon’s crust would be less than that of the Earth’s crust by a factor of more than 100. Assuming an always constant rigidification thickness, it follows from this, regarding identical rock formations, that the rigidifying lunar crust was 100 times more resistant to compression via horizontal thrust than was the Earth’s crust. In addition, there is the very much faster cooling of the lunar surface, which is not protected by any atmospheric sheath. Well into most recent times, the mountain ranges of our Earth appear as the folded and pressed edges of continental faults of the crushed crust. The few mountain ranges on the half of the Moon that we can observe point to an entirely different origin. The lunar surface does not reveal any such continental crushing action and horizontally acting forcible pressing. Considering the weak horizontal thrust of the crust, it remained intact during the hardening of the crust. The related necessary dwindling of the mass accordingly must have caused the formation of bubbles and cavities in the lunar crust. (ibid.: 11)

Secondly, Ernst Althans was writing after the discovery by Robert Mayer (and only slightly later by James Prescott Joule) of the mechanical equivalence of heat. He recognized that his father’s assumption of a fluid surface for the Moon was not necessary. In case of very considerable impact masses and larger impact velocities, there are bound to be comprehensive melting actions so that this must also create the conditions for the formation of the central mountains in many of the annular mounts of the Moon. The presence of a viscous lunar surface, such as my father presumed to exist in his experiments, where from the mortar path he obtained the above-discussed typical lunar mountain model, accordingly was not required here for the formation of the lunar crater depressions and ring-shaped mountains. The compacted surfaces became in themselves liquid.

2 Lunar Observations and Speculations …

24

The impact energy was projected radially and mechanically all around the crater depression, crushing flat and raising the rocks also in the unmolten lunar mass of the crater area over wide distances. … (ibid.: 16)

Like William Morris Davis, Ernst Althans agreed with many of the observations made by Gilbert, but disagreed with Gilbert’s moonlet theory. … [Gilbert] arrives at the assumption of the impression of large meteors as they hit the Moon, and he arrives at the theory already established by my father … . Gilbert shares our view to the effect that Mare Imbrium, with its surrounding area, might have originated as a result of a particularly powerful impact. Noteworthy and new is his reference to numerous lines that converge in the surface upon Mare Imbrium without entering the latter; furthermore, he notes the rounded surface in its surrounding area, which would probably come from the fact that during the impact of spraying material, liquid parts were collected in the lower regions, thus concealing their surface shapes. In the process, the larger solid clumps scratched those deep grooves (comparable in terms of size to the Grand Canyon of the Colorado) of which more than half are pointed to the Mare Imbrium. According to the excerpt, Gilbert also relates other features of the lunar surface, in other words, presumably not only the grooves—as just remarked—but also the radial formations to the mass impacts. His explanation of the irregular distribution of the craters over the lunar surface resulting from continuing changes in the rotation axis of the Moon due to collisions with meteors that become noticeable along the equatorial plane the author, of course, cannot go along with. (ibid.: 95–96)

2.4

Meteor Crater in Arizona

In general, Gilbert’s ideas seem to have been accepted more readily in Europe than in the United States. There were a few exceptions to U.S. reticence, however. In 1905, Daniel Moreau Barringer and Benjamin Chew Tilghman published papers in which they argued that a large circular depression in northern Arizona was caused by the impact of an iron meteor (Barringer, 1905; Tilghman, 1905). Barringer and Tilghman had formed a company to try to find and mine the iron and other elements, such as nickel, iridium, and platinum, from the meteor that they thought must be buried beneath the floor of the crater. Their papers laid out the arguments for an impact origin of the crater, as opposed to the origin that had been suggested by Gilbert, ironically, namely that the crater was formed by a volcanically driven steam explosion (Gilbert, 1896). In 1911 the astronomer Elihu Thomson visited the site, which at the time was known as Coon Butte, and he concurred with the analyses of Barringer and Tilghman. The circular hill or rim of this huge hole is composed of material evidently uplifted, crushed and pulverized in part and cast out from the crater. Much of it is finely pulverized white silica from a white sandstone which underlies an upper stratum of limestone. Fragments of the limestone layer of all sizes are seen; some of them very large. The slope of the rim extends for about a quarter to a half a mile but there is scattered material three miles or more away. Blocks of limestone and sandstone cap the ridge, ranging from up to 30 feet in

2.4 Meteor Crater in Arizona

25

diameter, while one block 10 feet through, is half a mile distant. The most significant constituent of the upcast ridge, is, however, meteoric iron in pieces of varying size, and so-called shale ball iron or oxidized meteoric iron; - oxidized because in its composition there was chlorine enough to cause rapid rusting after it entered the earth’s atmosphere and became subject to moisture. The plain around the crater has yielded numerous masses of the iron, some of them several hundred pounds in weight, and one discovered during the past summer weighs about 1700 lb. … The attempts to explain this crater as the result of volcanic action, or as produced by a steam explosion from below, are certainly to be regarded as rather far fetched in view of the presence and mode of occurrence of the meteoric irons, and the many evidences presented which seem to me to lead inevitably to the conclusion that here indeed we have a huge impact crater and that only. … (Thomson, 1912: 722–723)

Thomson informed Barringer of Gilbert’s 1893 paper, and Barringer then became an advocate for the idea that lunar craters were formed by impacts from other planetary bodies (Hoyt, 1987: 157). The saga of Barringer’s attempts to recover the iron meteorite that caused Meteor Crater and the decades-long controversy about the nature of the crater is well told by William Hoyte Graves in his book Coon Mountain Controversies: Meteor Crater and the Development of Impact Theory (Hoyt, 1987).

2.5

Shaler

A decade after the publication of The Moon’s Face, an eminent Harvard geologist, Nathaniel Southgate Shaler, published A Comparison of the Features of the Earth and the Moon, in which he presented a number of arguments against the impact hypothesis for the formation of craters on the Moon. Shaler first issued an admonition about the danger of interpreting features of the Moon in terms of processes that were familiar on Earth. At every stage in the advance of selenography we note the curious persistency of the endeavor not only to interpret the lunar features by the terrestrial, but to warp the observed facts into accord with those seen on earth. (Shaler, 1903: 2)

And then, inadvertently, he proceeded to illustrate his point. The foregoing account of the pits on the lunar surface suggests to the reader that these features are volcanoes. That view of their nature was taken by the astronomers who first saw them with the telescope and has been generally held by their successors. That they are in some way, and rather nearly, related to the volcanic vents of the earth appears certain. … (ibid.: 7–8)

Shaler was probably not familiar with the ideas of Barringer and Tilghman regarding the Meteor Crater in Arizona. One of his arguments against the impact hypothesis for lunar craters was based on the absence of evidence of impact-related

26

2 Lunar Observations and Speculations …

features on the Earth, and on the observation that if large bolides had impacted the Earth, the result would have been the destruction of all life. It is also to be noted that since the earth’s surface came to its present state there is good reason to believe that no such falls of large bodies as are supposed by the bolide hypotheses to have fallen upon the satellite have ever come to the planet. There are no traces of like craters, for even the greatest of calderas, such as that which holds Lago Bolsena or Kilauea, are evidently volcanic and in no way related to meteoric action. Moreover, the fall of a bolide of even ten miles in diameter would, by the inevitable development of heat due to its arrest, have been sufficient to destroy the organic life of the earth, yet this life has evidently been continued without interruption since before the Cambrian time. (ibid.: 14)

Shaler could not have known that later discoveries would link near extinctions of life to the impacts of bolides on Earth. In 1980, Luis W. Alvarez and his colleagues at the University of California, Berkeley proposed the impact-extinction hypothesis on the basis of the discovery of a layer of clay in the geological record at the end of the Cretacious period (65 million years ago) that contained high concentrations of the element iridium. They proposed that the iridium’s was contained in an asteroid that collided with the Earth, and they estimated that the asteroid’s size was between 6 and 14 km in diameter (Alvarez, Alvarez, Asaro, & Michel, 1980: 1095). Scientists at the Lunar and Planetary Institute, including Virgil “Buck” Sharpton, Peter Schultz, and David Kring, helped to demonstrate that the impact at the end of the Cretacious period created a crater at the edge of the Mexico’s Yucatan Peninsula, now called the Chicxulub crater. Shaler understood the implications of the conservation of energy, but he nevertheless seems to have shared with Gilbert a misunderstanding about why the outline of craters (or vulcanoids, as he termed them) were almost always circular. The point to be last noted is that so far as I have been able to determine from an extended inspection of lunar craters, including several hundred of the more determinable, they all have the axes of their pits at right angles to the surface. Now if these pits had been formed by bolides encountering the moon in their movement, that movement necessarily being at planetary velocity, it does not seem possible that they could all have come upon the sphere in a path normal to its surface. … (Shaler, 1903: 14)

Despite his previous argument about the destruction of life caused by the impact of large bolides on Earth, Shaler did “… hold with Gilbert and with other inquirers that the maria were formed by molten rock produced by the impact of large bodies falling upon the surface of the moon. (ibid.: 16–17). As we will see, both Gilbert and Shaler were wrong about the origin of the maria, which did not immediately fill the basins that were created by giant impacts. Gilbert and Shaler both thought, wrongly, that a perpendicular impact was necessary to create a crater with a circular outline. (Proctor’s idea about the formation of circular craters by the elasticity of the lunar surface material was wrong, as well.)

2.6 Morozov

2.6

27

Morozov

A Russian natural scientist Nikolai Alexandrovich Morozov (1854–1946) (Fig. 2.2) realized that the heat generated when a high-speed mass impacts the Moon would result in an explosion that would form a circular crater, and that this would occur regardless of the angle of impact. Morozov thus independently arrived at the same conclusion as the Thiersches and Ernst Althans. Morozov was apparently unaware of the work of Gilbert, perhaps because he had been held in solitary confinement by Tsar Alexander III and his successor, Tsar Nicholas II, from 1882 to 1905. In 1909, a year before Morozov re-entered prison (for the crime of publishing a book of poetry), he published Mysteries of the Moon.

Fig. 2.2 A photograph of Nikolai A. Morozov (Obtained from Wikimedia Commons). Morozov was a revolutionist and had been a leader of the terrorist organization The People’s Will, which had assassinated the father of Alexander III. While in prison, Morozov spent his time studying chemistry, physics, astronomy, mathematics, and history. Following his release from prison, he published The Periodic Systems of the Structure of Matter (1907) and D. I. Mendeleev and the Importance of His Periodic System for the Chemistry of the Future (1907). Among other things, Morozov predicted the existence of inert gases, calculated their atomic weights, and indicated their exact place in Mendeleev’s periodic table—all this before the first inert gas, argon, was isolated by John William Strutt (Lord Rayleigh) and William Ramsay in 1894 (Glinka, 1965: 85)

2 Lunar Observations and Speculations …

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Fig. 2.3 Crater profiles, a sketch by Philipp Fauth. Fauth’s sketch of crater profiles is reproduced from a Nabu Public Domain Reprint of Fauth, P. (1909) The Moon in Modern Astronomy: Summary of Twenty Years Selenographic Work, and a Study of Recent Problems. [Translated from the German by Joseph McCabe] New York, NY: Van Nostrand, Fig. 17, page 66

Not having access to a telescope, Morozov used the next best thing—the photographs of the Moon from a lunar atlas produced by Maurice Loewy and Pierre Henri Puiseux of the Paris Observatory. Editions of L’Atlas Photographique de La Lune were produced between 1896 and 1910, and Morozov was able to obtain a copy of the 1899 edition. During his twenty-three years in prison, Morozov had plenty of time to gaze at the Moon and the photographs of Loewy and Puiseux. In Mysteries of the Moon, he inserted a sketch that had been produced by a German astronomer, Philipp Johann Heinrich Fauth (1867–1941), which intended to show “The ratio of the width and depth of the lunar cirques.” The sketch (Fig. 2.3) included the profile of Mt Etna to demonstrate the vastly different ratios of these dimensions for craters formed by Vesuvian volcanic action on the Earth (Morozov, 1909: 1). One can hear poetry in Morozov’s prose: After a brief rain there was a dark cool evening. Cumuli rushed through the multi-star southern sky. The Moon in its first quarter rose high over the horizon. Dark spots of seas stood out brightly against its background, which raised several thoughts in my head. What is represented in this near world, this faithful satellite of the inhabited planet where we live? I imagined the route of the Moon in the observed hemisphere of space against the background of the Zodiac constellations and reevaluated everything that had been said about this planet. A lot of valuable information was known about it, however many of the already widely-accepted ideas seemed to me to be groundless. First of all, what are the multiple circular formations called lunar craters that are absent on the Earth or Mars? … It is clear that it is absolutely impossible to ascribe these flat-bottom formations to volcanic causes. The only possible explanation of their origin can be based on the assumption that the so-called continents of the lunar surface facing the Earth are lifeless deserts covered with quick fine dust like fine sand or clay of several tens or even hundreds of meters deep. It is clear that an impact of any small dust or gas comet onto this dust thickness will produce a dust vortex in all directions like a gulp of wind does and will deposit this dust on

2.6 Morozov

29

the edges of the sphere around the impact point in the shape of circular dunes. Insignificant lunar gravity will facilitate manifold this work, … At the present time the origin of lunar craters can be explained in this way only. The most difficult is to explain their almost ideal circular shape and the absence of oval or elliptical formations that are supposed to take place, as it seems at first sight, when a bolide hits the Moon. However, the lunar photos show us only round figures and a few furrows. The round figures include first of all the craters of Tycho, Copernicus …, Kepler, Aristarchus, Plato, Anaxagoras and some smaller ones, from which the band-shaped splashes of white dust stretch as rays bright as snow. They cover the quick lunar soil above the majority of neighbor craters, which shows clearly that the Tycho crater …, for instance, is one of the newest ones by its origin. There is not the slightest doubt that these radial bands belong not to the lunar surface but to the fallen meteorite or comet matter and represent a formation similar to splashes of the fallen liquid mass knocking out, on the one hand, a hollow in the quick soil, and on the other hand, splashing in all directions. … The rareness (of oval craters), as compared to the round objects, indicates, probably, that meteorites at the moment of their fall on the Moon blew up due to self-heating. Therefore, the surrounding dust was thrown away in all directions independent of their forward motion, like that of artillery grenades falling on a loose soil. (Morozov, 1909: 6–11, translated from the Russian by Nora Tamberg)

Some of Morozov’s conjectures proved to be wrong, e.g., that the lunar seas might have actually contained water at one time, or that there is an absence of impact craters on Earth or Mars, and that this absence is attributable to the shielding effect of the Earth’s atmosphere. Modern lunar scientists, however, would likely agree with Morozov’s closing sentiment: When I thought about all these mysteries of the Moon during that chilly lunar evening, observing how this faithful Earth’s satellite slowly moved among the blinking stars, one thought predominated in my mind: how much longer we should work hard in the field of science in order that we could make clearer at least the life of the world nearest to us! (ibid.: 11)

2.7

Günther

Despite Gilbert’s paper and the work of his predecessors going back to Gruithuisen, the impact theory of the origin of lunar craters was unpersuasive to most geologists and geographers. A German mathematician and geographer, Adam Wilhelm Siegmund Günther (1848–1923) expressed the majority view in a book that he published in 1911. Günther included Gilbert’s paper in his historical review, which was a comparative “planetology” between the Moon and the Earth (Günther, 1911). His primary objection to the impact theory was the failure to find large numbers of impact craters on the Earth. The greatest difficulty felt from the very beginning by the supporters of the [impact] theory to the effect that all formative differences on the face of the Moon depend on external

2 Lunar Observations and Speculations …

30

influences to the point where people asked themselves: How did it happen that a comparatively small secondary planet was hit by so many meteorites and in such a decisive manner, whereas the very much larger main planet, on the other hand, was hit so rarely and so moderately? (ibid.: 180, translated from the German by Gerald L. Geiger)

Had he lived into the late 20th century, Günther likely would have regretted his summary views. The meteoric lunar theory characterizes an intermediate stage in our ken, which even those who think otherwise would not really like to do without because very much intellect and skill went into it to discover entirely new aspects of a problem in astrophysics that had attained a certain measure of stability. But it seems to us that there is little likelihood that this theory could become a permanent part of the treasure trove of science. Too many obstacles stand in the way of a satisfactory explanation of its main principles that are rooted both in the cosmic and the purely lunar part of the entire complex of hypotheses. (ibid.: 183)

and We hope that we have shown that it is possible to account for the vast majority of the things which we see on the Moon by transferring the norms, which are recognized as valid for the volcanic and tectonic phenomena on Earth, also to the neighboring planet with proper caveats. (ibid.: 187)

2.8

Öpik

An Estonian, Ernst Julius Öpik (1893–1985) (Fig. 2.4), was undeterred by the majority opinion. He followed instead the work with which he was familiar, namely that of Nikolai Morozov. At about the time Öpik completed his undergraduate work at Moscow University in 1916, he wrote a paper that took Morozov’s “Meteoric Theory” as its starting point. The meteoric theory of the formation of lunar craters that was proposed by Nikolai Morozov is briefly as follows: a space body, moving with high velocity, hits the surface of our satellite, breaks apart and digs into the surface rocks; due to heat, which is released during the impact, part of the meteor’s mass and surrounding lunar rocks are converted into gas, which has enormous force because of its high temperature; an explosion occurs that scatters around lunar rocks and forms a crater—a circular crater; the meteor mass itself forms a hill in the center. The absence of such formations on the Earth is explained by the stopping effect of our atmosphere, which prevents the impact. (Öpik, 1916: 125, translated from the Russian by Yury Vikhliaev)

Öpik considered it a weakness of Morozov’s theory that he had not calculated the mass of the hypothetical body that would impact the Moon to form a typical crater. Öpik calculated the probable size and velocity of the bodies that would form craters if they impacted the Moon. He made his estimates of the minimum value of the mass of such bodies by assuming that all of the kinetic energy of the impacting bolide was used to move lunar material from the cavity of the crater to the ring

2.8 Öpik

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Fig. 2.4 A photograph of Ernst J. Öpik, courtesy of the Armagh Observatory. Öpik did not like the political system that began to form following the 1917 Russian Revolution, and he soon moved to his native Estonia, where he obtained his doctorate in 1923. However, his life in Estonia became unsettled by the Second World War, and he and his family migrated from Estonia to Germany via horse and cart. Öpik taught briefly in Germany after the war until 1948, when he found a position at the Armagh Observatory in Ireland (Wayman & Mullan, 1986). From that post, Öpik continued his research in astronomy, including lunar science, for more than three decades

around the crater on the Moon’s surface. Using a model geometry for a crater, he produced the accompanying Table 2.1. Öpik’s estimated minimal mass of a bolide that could create a crater with a diameter of 60 km, for example, was 30 million tons, assuming an impact velocity of 30 km/sec. Such a bolide would have had a diameter of at least 250 m. Öpik calculated the effect of the Earth’s atmosphere on the velocity of such a large mass and showed that it would be negligible, in contradiction to his earlier assumption. Thus, it is clear that fire balls with diameters of several dozen meters should reach the Earth’s surface with the same velocity as if atmosphere were absent; and in this case the explanation of the absence of circular hills by the stopping effect of the atmosphere is invalid, let alone, if such a huge meteor mass was falling in large amounts, they would be commonly found in rocks. If one follows the meteoric theory of lunar craters, than the absence of similar craters on the Earth should be explained not by the resistance of the atmosphere to meteor flight, but by erosion and denudation processes, by corrosive effect of precipitation and by eolation,

2 Lunar Observations and Speculations …

32

Table 2.1 Estimates of the minimal mass and diameter of a bolide with an impact velocity of 2, 30, or 60 km/sec that would form a lunar crater with a diameter of 2, 30, or 60 km Meteor velocity v km/sec

Diameter of crater d km

X0 Minimal mass of meteor in tons

a Minimal diameter in kilometers of meteor of density = 6

2 km/sec

2 km 8000 0.02 30 4
108 0.6 6
109 60 1.5 0.003 30 km/sec 2 km 40 0.1 30 2
106 3
107 0.25 60 0.002 60 km/sec 2 km 9 0.06 30 4
105 7
106 0.15 60 Adapted from a table in Öpik, E. (1916) “Замечание по поводу метеорной теории пуных цирков” (“Note on the meteoric theory of lunar craters”). Bulletin de la Societe Russe des Amis de l’Etude de l’Univers, volume 5, page 127 which always flatten the Earth terrain, such that if no tectonic processes were taking place, all mountain ridges would disappear in several dozen million years. However, with such explanation, we need to assume that the cause of formation of circular hills disappeared during recent geological periods—otherwise footprints of such hills would be present on the Earth today. … (ibid.: 129–130)

Öpik had found the right answer to Günther’s objection, but then he lost his way into the volcanologist camp. … Does it not make more sense to seek this cause not in space bodies, but in tectonic activity of the planet—the Earth or the Moon? I think, from this consideration, it is possible to explain the formation of lunar craters and some other geological structures using a combination of meteor and eruption theories. Let Fig. 45 (Fig. 2.5) represent the Moon in an early stage of development, and let its thin crust be broken by volcanic explosion at location A; fragments of crust and volcanic ash fly apart with velocities which have different magnitudes and directions, such velocities are represented as arrows in Fig. 45 (Fig. 2.5); trajectories are represented with dotted lines; due to the absence of atmosphere, these trajectories will depend only on the magnitude and direction of the velocity, but not on the size of fragment. Since the lunar gravity is small, fragments can fly off far away from the location of explosion; if the velocity of a fragment is close to the velocity of a body falling from infinity—for the Moon this velocity is only 2.4 km/sec—fragments will reach any point of the lunar surface. Thus, the impact of fragments of a volcanic explosion with large mass can result in formation of circular hills the same way as the impact of falling space bodies; however, since the velocities of volcanic fragments are less than velocities of meteorites and cannot exceed 2.4 km/sec on the moon, since otherwise the fragment will fly to infinity, the mass of the fragment should be larger to form a crater. For example, it follows from the table above, that to form a crater similar to Tycho, a fragment with velocity 2 km/sec should weight 6 billion tons, and if it has the same density as lunar rocks, it should have a diameter of 2 km: this quantity is of the same order as the diameter of the central hill of Tycho; but if we assume a space origin of the impact body and a corresponding impact velocity, the diameter of the body should be significantly less than the diameter of Tycho.

2.8 Öpik

33

Fig. 2.5 Illustration of Öpik’s idea of the formation of impact craters as the result of lunar volcanic eruptions. Adapted from Fig. 45 in Öpik, E. (1916) “Зaмeчaниe пo пoвoдy мeтeopнoй тeopии пyныx циpкoв” (“Note on the meteoric theory of lunar craters”). Bulletin de la Societe Russe des Amis de l’Etude de l’Univers, volume 5, page 130

Thus, everywhere on the lunar surface, where more or less massive fragments fall—and they should fall everywhere if the strength of the eruption is the same as for Earth volcanoes —circular hills of different sizes will be formed; and at the location of an eruption a crater will be formed because some fragment thrown vertically or at small angles will fall back. (ibid.: 130–131)

It appears that Öpik held open the possibility of the impact origin of the larger craters, such as Tycho, but that he favored the hypothesis that most of the small craters were caused by the impact of fragments from volcanic eruptions. Secondary craters of the kind suggested by Öpik do exist on the Moon, but the vast majority of craters of all sizes are due to primary, not secondary impact. The secondary craters that do exist on the Moon are mostly secondary to an impact-caused explosion on the lunar surface, not to a volcanic eruption. Öpik, as well as Morozov, Gruithuisen, and others were wrong to associate the central hill in some craters with the remnant of an impacting body. Öpik’s paper was published in a Russian journal and went unnoticed among lunar scientists in the U.S. for at least 40 years.

2.9

Wegener

Sometimes unaware of the ideas and work of Gruithuisen, Meydenbauer, Gilbert, Morozov, Öpik and others, later geologists and astronomers repeated their discoveries. As an example, in the winter of 1918/1919, a German geophysicist, Alfred Wegener (1880–1930), performed a series of experiments on impact craters, using cement powder both as a target surface and impacting object.

34

2 Lunar Observations and Speculations … … The reason I used such a weakly coherent powder is because the impacts were created by hand power, that is, the force of the impacting object was reduced considerably compared to the lunar processes. If one wants to obtain similar forms in spite of this small force then one must also diminish the molecular force of the rock resistance that is overcome by the impact force up to the crater limits, on the same scale. Hence such a powder with a small degree of cohesion. Therefore, it cannot be concluded from my experiments that the surface of the Moon is covered by powder-like material. The powder in the experiment corresponds to the solid rocks on the Moon which are similar to the ones on Earth. Cement was especially advantageous as it is possible to harden the resulting craters without causing any change in the forms. In order to do that, water is very carefully sprayed onto the surface. The next day when the surface is hardened the whole is saturated with water. (Wegener, 1921b)

Wegener measured crater diameters and depths for the craters (distance from the top of the crater wall to the crater floor) of his experiments, and his experiments gave ratios of diameter to crater depth in good agreement with the smaller lunar craters that are not filled with lava (Fig. 2.6). The ratio of the depth of the crater to the height of the walls was also in satisfactory agreement with lunar craters. Wegener found that when the thickness of the target layer did not exceed a tenth of

Fig. 2.6 Profiles of different craters (true to scale). Reproduced from Fig. 1 in Wegener, A. (1921b) “The origin of lunar craters.” [Translated from the German by A. M. Celâl Ṣengör.] The Moon, volume 14, issue 2, page 216, with the kind permission from Springer Science and Business Media. Copyright 1975 by D. Reidel Publishing Company

2.9 Wegener

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Fig. 2.7 Diameters of the largest craters. Reproduced from Fig. 2 in Wegener, A. (1921b) “The origin of lunar craters.” [Translated from the German by A. M. Celâl Şengör.] The Moon, volume 14, issue 2, page 219, with the kind permission from Springer Science and Business Media. Copyright 1975 by D. Reidel Publishing Company

the diameter of the crater, a central peak always formed. He found that for his experiments, the average ratio of the depth of the crater to the height of the central peak was 3.3, i.e., the central peak rises only to the height of 1/3 the height of the crater wall. For the lunar data with which Wegener was comparing, the average was 2.9. Wegener argued that the size distribution of lunar craters was inconsistent with a volcanic origin for them. He noted that on the Moon: … we see an uninterrupted series of features ranging in size from the limit of resolution to the size of many kilometers (Fig. 2.7) The typical and most striking features have diameters ranging between 50 and 300 km. … With those figures we have already reached the dimensions of Mare Crisium with a diameter of 450–550 km. It is impossible to separate this feature with almost circular peripheral mountains having steep interior slopes and gentle exterior slopes from the above mentioned crater seas and ring mountains. And with this step, we reach the greatest insurmountable difficulty of the volcanic hypotheses. Also, those seas other than Mare Crisium, like Mare Serenitatis with a diameter of 700 km, and Mare Imbrium with a diameter of a little more than 1000 km, which are surrounded by almost circular peripheral mountains, cannot be excluded from the series. Basically they are also lunar craters and therefore must have originated in the same way. (ibid.: 218–219)

36

2 Lunar Observations and Speculations …

Gruithuisen, Proctor and Gilbert had dismissed the “broken bubble” hypothesis that had been proposed by Hooke, but Wegener accompanied the dismissal with a rather withering tutorial on the difference between gravitational forces and molecular forces. It can easily be demonstrated that this bubble hypothesis is based on a false conclusion, although we must go a bit further to show this. In translating experiments into cosmic dimensions, one must consider whether he is dealing with molecular forces or gravitational forces, since the relationship between the two is completely reversed in the translation. In the laboratory, we work principally with molecular forces—aside from the gravity that generally appears as interference. To demonstrate the gravitational force even qualitatively, we must use extremely sensitive instruments. In space it is quite different, for the molecular forces do not increase when a body grows into a cosmic body, whereas the gravitational forces do. In astronomy, therefore, the molecular forces of magnetism, electricity, resistance, etc., are overshadowed by the gravitational force to such an extent that, before the introduction of spectral analysis, space physics consisted essentially of the one Newtonian law of gravity. … The spherical shape of the heavenly bodies thus indicates the victory of the gravitational forces over the molecular forces. Meteorites and even the smallest planetary moons and the asteroids are small enough to resist this leveling gravitational force. The moon and the earth are subject to it, and only the mountains demonstrate to what extent the molecular forces can still assert themselves. The fact that the lunar mountains are steeper and higher than the terrestrial ones is probably due not only to the lack of erosion but particularly to the lunar gravity being only a sixth of that of the earth. These observations indicate that we can translate only those laboratory experiments into cosmic dimensions which are based on gravitational rather than molecular forces. However, bubble formation is essentially a function of the molecular force of the surface tension. Depending on the amount of the latter, it is possible to produce bubbles of various magnitudes within certain limits. On the moon, too, there cannot now be nor could there ever have been larger bubbles. Expressing this idea just once is probably enough to pull the rug out from under the bubble hypotheses once and for all. Anyone who explains the enormous lunar formations of several hundred kilometer diameters as burst bubbles draws as monstrously false a conclusion as he who tries to attribute the floating of ocean steamers to the surface tension of water … . (Wegener, 1921a: 2)

At the time of Wegener’s lunar research, he had been working on his theory of continental drift, for which he became famous. He used his knowledge about continental drift and its relation to the formation of volcanoes on the Earth, to make a final argument against the volcanic origin for lunar craters. … The volcanoes on Earth mainly relate to the areas of tectonic movements. This is, of course, easy to explain, as in these areas such pressures are created as to push the magma inclusions of the lithosphere to the surface. So the volcanoes, on the one hand, accompany the zones of young fold-mountains of the Cordilleras and the Alps-Himalaya system; on the other hand, they follow the fracture zones of the central seas of the West Indies up to Indochina and the continental margin of East Asia. Through growing geologic knowledge concerning the enormous horizontal pressures that created the mountains, and which are expressed in the form of large thrust sheets, and through new geophysical studies revealing the horizontal movements of the continents, new views on the system of those tectonic movements are being developed. According to these ideas the continental plates are the compressed remnants of a lithosphere that once covered the whole Earth. The deep sea floor represents the Sima-sphere on which the continental plates are floating. They are sunk into the Sima 95%. This long process of continental compression of the remnants of the

2.9 Wegener

37

lithosphere causes the eruption of the baryspheric inclusions which accompany the process. As the size of the Moon is much smaller, it is doubtful whether the crust of the Moon also has experienced such processes or if it is still undergoing them. In fact, on the Moon there are no such features which can compare with the continental areas and the fold mountains of the Earth. Though the lunar seas resemble the deep sea basins of the Earth and we can even recognize a small continental drift of the northwest peripheral area of Mare Imbrium toward the interior of the sea, such events play a very insignificant role on the Moon. Therefore, according to our present knowledge on the nature of the volcanoes, it is at least doubtful whether we can expect to see them on the Moon. There is no way, however, that we can explain the incredible abundance of the lunar craters with our knowledge of terrestrial volcanoes. Inasmuch as the face of the Moon is covered with craters to such a degree that it seems doubtful to be able to find a single point which at least once has not been a part of the floor of a crater. The volcanoes on the Earth cover a negligible area of the Earth’s crust and with the present size of volcanic craters it would be either extremely difficult or simply impossible for an astronomer on the Moon to detect those even with the largest telescope. Who knows if such Moon astronomers would know anything about the existence of the terrestrial volcanoes? On the Moon, on the other hand, the craters represented in every size up to the giant dimensions of the seas are not only the most typical and most common surface forms, they are, as a first look would reveal, almost the only forms! I do not understand how one cannot come to the following conclusion when one compares the Moon with the Earth: The forms are fundamentally different; therefore, the origins also should be different. The contradiction is so flagrant that the next generation will only laugh at our desperate experiments with which we try to establish an equality between the Moon and the Earth. (Wegener, 1921b: 220–221)

Wegener’s research convinced him that Gilbert’s impact theory for the formation of lunar craters was right; “… the typical lunar craters can best be interpreted as impact craters.” (ibid.: 230). Both Gilbert and Wegener envisaged relatively low-velocity impacts, and it seems that they did not associate the circular form of craters with the explosive force of the impacts, per the explanations of Nikolai Morozov, Ernst Althans, and August and Heinrich Thiersch.

2.10

Delmotte and Darney

In 1923, French lunar scientist Gabriel Delmotte (1876–1950) produced a book titled Recherches Sélénographiques et Nouvelle Théorie de Cirques Lunaires. Delmotte built a theory based on the assumption that the crater Tycho was at one time the south pole of a rotating Moon. He thought he could see, in addition to the rays of Tycho, other related features, which he ascribed to stresses caused by the Moon’s rotation. In addition to the “Tycheen” system of features, he recognized other “interfering” systems, among those a system of linear ridges and furrows that had a focus within Mare Imbrium (Delmotte, 1923). Some researchers dismissed Delmotte’s observations as delusions. For example, another French astronomer, Max Hoyaux, argued that when a pole is chosen at random, one can find “… alignments as interesting as those of the Tycho system

2 Lunar Observations and Speculations …

38

…” (Hoyaux, 1946: 169). Hoyaux compared the alignments reported by Delmotte with the famous illusory canals of Mars. Another French researcher could see the alignment of lunar features that Delmotte had recognized, at least the ones related to Mare Imbrium. In 1933, Maurice Darney (1882–1958) responded to Delmotte’s ideas in an article titled “Le Système Imbrien” that was published in the Bulletin Société Astronomique. Darney’s drawings of the ridges and furrows emanating from the Imbrium basin are similar to the figure drawn by Gilbert in 1892. Darney certainly understood that the ridges and furrows were related to Mare Imbrium, but only conjectured that a huge increase in the temperature at the center of Imbrium, perhaps caused by the impact of a falling satellite (as had been suggested by Delmotte) could have caused fractures in the lunar crust radiating from Imbrium (Darney, 1933). It does not appear that Delmotte or Darney knew of Gilbert’s The Moon’s Face.

2.11

Spurr

Toward the end of his life, the prominent American geologist Josiah Edward Spurr (1870–1950) wrote four volumes on Geology Applied to Selenology, the first of which had the subtitle of The Imbrian Plain Region of the Moon. One might have thought that Spurr would have followed up on Gilbert’s analysis, as both were attempting to apply their knowledge of geology to the Moon’s surface features, and both had been associated with the U.S. Geological Survey. In the text of the first volume of Geology Applied to Selenology, however, Spurr did not mention Gilbert. He did mention Gilbert in an Addendum, in which he concluded that “Gilbert, in his summing up of the differences between terrestrial craters and lunar craters, makes a generalization which does not accord with the facts.” (Spurr, 1944: 110). The issue was whether or not there were craterlets on the summits of central hills in lunar craters. Gilbert concluded that these craterlets were absent in lunar craters, whereas in Vesuvian-type craters on the Earth, the central hill has a craterlet at its summit. Spurr believed that “The surface of the moon as shown in photographs displays phenomena which are related to phenomena that we describe as volcanicity on the earth …” (ibid.: x), and he disputed Gilbert’s observation that the central hills in large lunar craters do not show crater-like orifices at their summits. Spurr cited the conclusion of a contemporary astronomer, William Wallace Campbell (1862–1938), who was a former director of the Lick Observatory: Must we not agree that the unquestioned existence of craterlets in the summits of central crater peaks is absolutely fatal to the impact theory of the origin of those peaks, and at the same time in full and complete harmony with the hypothesis of the volcanic upbuilding of those peaks? … (Campbell, 1920: 133)

2.12

2.12

Baldwin

39

Baldwin

As we will see, astronomers tended to be skeptical of the impact origin of lunar craters, while geologists were more open to the idea. Spurr was an exception to this rule, as was Ralph Belknap Baldwin (1912–2010) (Fig. 2.8), who was the first American astronomer to follow in the footsteps of Gilbert. Baldwin earned his Ph. D. in astronomy from the University of Michigan in 1937, and by 1938 he was working as an instructor at the Dearborn Observatory of Northwestern University. In 1941, he got an additional part-time job giving public lectures at the Adler Planetarium on Chicago’s Lakeshore Drive. While waiting to lecture, Baldwin would examine pictures of the Moon on the walls of the Adler Planetarium, and he noticed the same structures emanating from the Imbrium basin that Gilbert had observed. Baldwin immediately associated the features with the result of a huge impact that had formed the Imbrium basin. He described to the Director of the Dearborn Observatory, Oliver Lee, the evidence he had found for the impact origin of lunar craters, but was rebuffed by Lee. In a later interview with Ursula B. Marvin (UBM), Baldwin (RBB) described what happened next. RBB: Soon after that, I rode to Lake Geneva, Wisconsin, with Lee to attend the monthly symposium at the Yerkes Observatory. While there, I mentioned my idea of the impact origin of lunar craters to the director, Otto Struve. Struve immediately responded: ‘Ralph, you are giving next month’s symposium.’ UBM: How did you feel about that? RBB: I was delighted. This would give me a chance to introduce my ideas to a group of eminent astronomers. The next month, I went to Yerkes well prepared with photographs and slides and presented what I thought was a good paper, with clear, persuasive arguments for the impact origin of lunar craters. Dr. Lee was there, as were several world-renowned astronomers, including Jesse Greenstein, Philip Keenan, George Van Biesbroek, Gerard Kuiper, and Otto Struve. UBM: How was your talk received? RBB: I struck out completely. They showed not a flicker of interest, except for Keenan who jumped up and said: ‘I just can’t conceive of this impact process being right.’ UBM: But Struve knew you would be talking about lunar impact craters. RBB: Yes he did. He must have thought it would make an interesting program, and he listened politely, but he didn’t believe a word of it. UBM: What did you do next? RBB: I wrote a paper expanding on what I said in my talk. Struve refused to publish it in The Astrophysical Journal; Lee refused to include it in The Annals of the Dearborn Observatory; and the editor of The Astronomical Journal refused to accept it. In 1942, I finally got it accepted by Popular Astronomy, a publication of the Goodsell Observatory at Carleton College in Minnesota. A year later, I wrote a second article that went through the same rigmarole and appeared in Popular Astronomy. (Marvin, 2003: A165)

Fifty years after Gilbert’s address to the Philosophical Society of Washington, in which he amply demonstrated the impact origin of the Moon’s craters, Baldwin published a paper in Popular Astronomy that made the same case in the face of

40

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Fig. 2.8 Ralph B. Baldwin. This photograph of Baldwin was taken by John A. Wood in 1986 at the meeting of the Meteoritical Society, when Baldwin received the Leonard Medal. Used with permission from Professor Wood. Ralph Baldwin led an interesting life, with three different careers. Following his stint at the Dearborn Observatory, he began war-related work in June 1942 at the Johns Hopkins Applied Physics Laboratory, where he focused on the development of the radio proximity fuze. The Johns Hopkins Applied Physics Lab regards the radio proximity fuze as its founding invention and claims that it was one of the three most important technology developments of World War II, along with radar and the atomic bomb. After his war service, Baldwin worked in his family’s business, the Oliver Machine Company, while continuing his lunar and planetary research on a part-time basis

opposition by some of the most distinguished astronomers in the world. Baldwin later admitted that he had not known about Gilbert’s paper of 1893 when he wrote his paper for Popular Astronomy in 1942, which was titled “The Meteoritic Origin of Lunar Craters” (Marvin, 2002: 26). In this first paper, Baldwin noted that the existence of so many features that were oriented toward Mare Imbrium was evidence of a terrific explosion that must have formed the Imbrium basin. He concluded that “… Mare Imbrium is nothing more nor less than the largest crater on the moon” (Baldwin, 1942: 367).

2.12

Baldwin

41

In a following paper in Popular Astronomy, Baldwin elaborated on the evidence that the Imbrium basin was formed by a large body that had impacted the Moon. Additionally, he argued that Sinus Iridum, Mare Crisium, and Mare Serenitatis were also created by the impacts of “supergiant meteorites” (Baldwin, 1943: 119). Baldwin was able to complete his first book on the Moon in 1949. It was titled The Face of the Moon and has been described as “… probably the most influential book ever written in lunar science” (Wilhelms, 1963: 15), because the book was read by Harold Urey, who then became an advocate to NASA for lunar exploration. Among other things, Baldwin showed that a smooth curve would fit the data for the log of the diameter of lunar craters versus the log of their depth. The dimensions for terrestrial meteorite craters lay along the curve, as did those for shell craters and bomb craters (Fig. 2.9) (Baldwin, 1949: 132). Baldwin recognized that the lava plains were not generated by the impacts that created the great basins but came later. He pointed out that one can see smaller craters inside the great basins that have been filled with lava. Thus, the sequence of events was (1) creation of the great basin by the impact of a large meteorite, (2) creation of smaller craters inside the great basin by smaller meteorites, and (3) upward seeping of molten lava inside the great basins to form the lava plains (ibid.: 205).

2.13

Dietz

One would have thought that the arguments provided by Gruithuisen, Meydenbauer, the Thiersches, Gilbert, the Althans, Morozov, and Baldwin would have settled the debate over the origin of lunar craters. In 1946, however, a well-regarded geologist, Robert Sinclair Dietz (1914–1995), asserted that the question was still open. In a paper that Dietz published in the Journal of Geology, he wrote: … there is no general agreement concerning the origin of the features on the moon’s surface. Most interpretations can be classified in two general categories: (1) that the features are the result of volcanism or some related type of internal magmatic activity and (2) that the features are the result of the impact of meteorites and related extra-lunar bodies with the moon’s surface. Nearly all astronomy texts discuss both theories but favor volcanism, inasmuch as it appears to have a large majority of proponents. The present writer has, however, been impressed with the fact that all the major topographical forms on the moon, including the craters, the maria, and most of the mountains, can be explained most satisfactorily by the impact of extra-lunar bodies. … (Dietz, 1946: 359)

Dietz came to many of the same correct conclusions as Baldwin and his predecessors, and also some of the same incorrect ones, e.g., “Maria are probably extensive lava plains generated by the impact of bodies of asteroidal dimensions and were formed relatively late in lunar history” (ibid.). Dietz knew of Gilbert’s paper, which he referenced for the purpose of correcting a mistake:

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Fig. 2.9 Baldwin’s data and curves showing the relationship between depth and diameter for terrestrial explosion craters, terrestrial meteoric craters, and lunar craters. Republished with permission of the University of Chicago Press, from Baldwin, R. B. (1949) The Face of the Moon. Figure 12, page 132. Copyright 1949 by The University of Chicago

2.13

Dietz

43

… G. K. Gilbert supposed that the missiles were part of a Saturn-like ring which revolved around the earth. On this basis he showed how a majority of the missiles would strike at a high angle of incidence. It is, however, unnecessary to postulate any special condition, because an explosion crater, in contrast to a percussion crater, has a circular shape and well-developed radial symmetry, regardless of the angle of incidence. … (ibid.: 367)1

2.14

Kuiper and Urey

In 1953, Gerard Peter Kuiper (1905–1973) (Fig. 2.10), who at the time was an astronomer at the Yerkes Observatory of the University of Chicago, made a systematic study of the lunar surface with the 82-inch telescope at the McDonald Observatory. Kuiper had heard Baldwin’s lecture at Yerkes twelve years earlier, and he cited Baldwin’s book, but he seemed unaware of Gilbert’s observations and repeated many of Gilbert’s discoveries, e.g., the observations that a: … whole area looks as if it has been covered with a thick layer of mud. The coarse features are often aligned to form broad ridges or rows of rounded mountains, which usually have a preferred direction in a given area, as if they were caused by enormous lava splashes. The mountain ranges surrounding Mare Imbrium—the Alps, the Caucasus, and the Apennines —belong to this … class of structure. So do many isolated ridges east of Crater Copernicus and near the center of the lunar disk; both groups look like jets of lava or rather lavacovered bits of old crust (or planetesimal) which have been shot from the same splashing center of Mare Imbrium. Related, but in a different category, are the many grooves cut in the lunar surface, particularly in high regions like old crater walls, and radiating also from Mare Imbrium. They are U-shaped in cross-section and are rarely more than five diameters long … It is natural to assume that they also have been caused by flying fragments. (Kuiper, 1954: 1104)

Kuiper not only repeated Gilbert’s observations, but he also repeated Gilbert’s main conclusion that “… the impact hypothesis is confirmed as the chief cause of lunar topography.” (ibid.: 1097). Still, Kuiper went beyond Gilbert’s conclusions, stating, “… the moon was nearly completely melted by its own radioactivity, some 0.5–1 billion years after its formation, and … the maria were formed during this epoch, and … were not, as has been supposed, primarily the result of melting caused by the impacts themselves.” (ibid.). Kuiper had been working on the origin of meteorites, and his argument that the Moon must have been molten at one time rested on an analogy with asteroids, from which meteorites are derived. The meteorites were derived from asteroids, bodies much smaller than the moon, and yet these bodies were molten, at least in part, and the metal phase separated at least partially

1

Robert Dietz is known today primarily for his pioneering work in marine geology. He coined the term “sea-floor spreading,” and he and, independently, Harry Hess of Princeton, first published that theory. He is also known for his advocacy of the importance of the impacts of meteorites and asteroids on both the Earth and the Moon (Koppes, 1998: 25–27).

44

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Fig. 2.10 Photograph of Gerard P. Kuiper, courtesy of NASA. Kuiper left the Yerkes Observatory and the University of Chicago in 1960, moving to the University of Arizona, where he established the Lunar and Planetary Laboratory. There, he continued a very productive career in astronomy and planetary science. He was instrumental in the development of airborne infrared astronomy, and a 1-m telescope in a C-141 aircraft was named the Kuiper Airborne Observatory in recognition of his pioneering work in this area (Cruikshank, 1993: 272–276)

from the silicate phase. During a recent symposium on the origin of meteorites the writer showed that the melting of the larger asteroids may be interpreted as due to heating of initially cool, accreted masses by their own radioactivity. With the measured ages of the solid earth and solid meteorites 4.5
109 and 4.6
109 years, respectively, the probably age of the solar system may now be put very close to five billion years. At that time the heat production of U and K40 was about 10 times greater than now, and, for a representative mixture of meteorite material (or a more uncertain terrestrial mixture), the spheres would have heated to the melting point in less than 109 years, provided that the heat losses by conduction were not too large. Depending on the precise value adopted for the conductivity of loosely packed, accreted material, one finds that all spheres larger than about 100 km. in diameter will have melted at least close to the center; substantially larger spheres will have melted almost entirely, leaving only an outer shell of a few kilometers of nearly unaltered accreted material. (Kuiper, 1954: 1100–1101)

Kuiper’s observations and ideas about the surface features of the Moon not only reflected those of Gilbert, but they even mirrored in some respects the much earlier views of Nasmyth and Carpenter, e.g.: … One must next consider the cooling of the solid moon. Clearly, the outside cooled well below the melting point first. This caused contraction of the outer shell, which rested on a hot solid interior of unchanged dimensions. Therefore, the outside shell developed tensions, and large impacts might well have set off cracks that subsequently lengthened, widened, and deepened as the cooling progressed inward and the shrinking increased. The cracks

2.14

Kuiper and Urey

45

Fig. 2.11 Photograph of Harold C. Urey, courtesy of NASA. Urey, a professor at the University of Chicago, was awarded the Nobel Prize in chemistry in 1934 for his discovery of deuterium

would have released pressures in the deeper layers and lowered the melting point locally. Thus lavas might have come up through the cracks and partially filled them … . (ibid.: 1103 and 1104)

And even sharing with Nasmyth and Carpenter the erroneous conclusion about the cause of central peaks of craters. In my visual observations with the 82-inch telescope I have paid much attention to the central peaks, and my general impression is that they are igneous masses, pressed up as lavas through the crater bottoms. (ibid.: 1107)

In agreement with Carl Ludwig Althans, the central peaks are now thought to have been created by a “rebound” of lunar material that was compressed by the impact of a bolide. Kuiper’s colleague at the University of Chicago, Harold Clayton Urey (1893– 1981) (Fig. 2.11), was annoyed as he read Kuiper’s paper. The idea that the Moon had melted owing to the radioactivity of elements in its mass was at odds with Urey’s views, and he wrote in a subsequent article in the same journal: Kuiper has made additional observations on the moon with the 82-inch reflector at McDonald Observatory; but, so far as I can deduce from his discussion, he has not observed anything markedly different from what has been previously observed and recorded in the extensive literature on the moon. It may be that he has observed some new details, but the information bearing on his theories has been available before. His discussion, which takes

2 Lunar Observations and Speculations …

46

issue with my ideas, really derives from his view that the moon was completely melted in its early history because of radioactive heating. In the fall of 1953, I remarked to Professor Kuiper in a discussion of the increased age of the solar system as presented by Patterson that it seemed to me that the constants, i.e., the age of the solar system and amounts of the radioactive elements, should be such that the moon would not have melted. From this remark it should have been obvious that, at that time, I had already made Dr. Kuiper’s calculations in regard to the melting of the moon. … … Though the process of the moon’s origin was undoubtedly more intricate than anyone has the courage to imagine, it is my belief today, as it has been for some five years, that very probably the moon was accumulated at low temperatures from a primitive dust cloud of solar composition with the iron in oxidized states and that the concentrations of radioactive substances within the moon are sufficiently low that melting has never occurred. In fact, I believe that present temperatures are so low that the interior of the moon has a high strength and that such low temperatures require the moon to have been formed at low temperatures and never to have been melted at any time. (Urey, 1955: 423 and 427)

Kuiper responded as follows: The main difference of opinion appears to be that Urey assumes the surface lavas of the moon to be due to local melting caused by impacts, while I have concluded that the surface lavas are related both in time and by nature to many other surface phenomena and that therefore the entire moon participated in the heating process. I have assumed the main source of this heating to be radioactivity of the moon itself. (Kuiper, 1955: 820)

As we will see, the analyses of lunar rocks proved Kuiper right on the point that the maria lavas were not an expression of local melting caused by impacts. But in 1955, the only clear thing was that from Earth-based observations and meteorite data alone, two leading scientists could not agree on the thermal history of the Moon, much less its origin. Kuiper argued that the bodies that struck the Moon and formed the craters and the large basins did not come from the asteroid belt, nor were they planetesimals orbiting the Sun. Rather, these bodies: … formed inside protoearth, but outside the original moon’s orbit; this would imply that this part of protoearth was too tenuous to become gravitationally unstable and form a second, outer satellite around the earth … . (Kuiper, 1954: 1109)

In his Face of the Moon, Baldwin had included a curve that related the diameter D (in feet) of an impact crater to the exlosive energy e (in calories) required to produce it. He used actual data from military mines, mortars, and artillery shells to fit the curve, which had the equation: e ¼ 0:1139 D2 þ 2:2504 D þ 4:2240 ðBaldwin; 1949 : 225Þ Kuiper used this equation, along with other relationships of crater dimensions that had been derived by Baldwin, to conclude that the velocity of impact of most of the lunar craters was about the escape velocity for the Moon, 2.4 km/sec. With this velocity, he calculated that the planetesimal that created the Imbrium impact basin must have been about 150 km in diameter, and he argued that:

2.14

Kuiper and Urey

47

… the moon, on its outward journey from the earth resulting from tidal friction, struck a region in space where it was bombarded with a swarm of objects that, from the small impact velocities, must be considered to have been moving around the earth in or near the plane of the ecliptic. … (Kuiper, 1954: 1110)

2.15

Concluding Remarks

Beginning in 1959, scientists began to make measurements of the Moon and its surroundings with space-borne equipment. The Soviet Luna 1 was launched on 2 January 1959 and was the first spacecraft to reach the Moon. It discovered that the Moon has no magnetic field. Luna 3 was the first spacecraft to return images of the Moon’s far side, and it found fewer of the mare-filled craters than exist on the near side. The far side was found to have heavily-cratered terrae (“uplands” or “highlands”) regions similar to those found on the near side of the Moon. The Ranger program of the United States was designed to develop and test equipment and systems that would be needed for the Apollo Moon landings and to begin to acquire detailed images of the lunar surface. After a number of failures in the program, Ranger 7 was launched on 28 July 1964. In the final 17 min of flight before impact, the spacecraft’s six TV cameras took more than 4,300 images of the lunar surface. The Ranger 7 photographs, as well as photographs from similarly successful Rangers 8 and 9, demonstrated that craters could be seen at all size ranges down to as small as 10 inches in diameter, and the lunar craters become more abundant as their sizes decline. Thus by the mid-1960s, Ralph Baldwin felt that at least one lunar argument was over. The vast majority of lunar craters were caused by impacts of extra-lunar bodies upon the surface of the Moon (Baldwin, 1965: 73 and 137). Acknowledgements We gratefully acknowledge Gerald L. Geiger for his translations of text by Ernst Althans, Nora Tamberg for her translations of sections of Nikolai Alexandrovich Morozov’s Mysteries of the Moon, and Yury Vikhliaev for his translation of the 1916 paper by Ernst Julius Öpik.

References Althans, K. L. (1895). Über Versuche, die eigentümliche Gestalt der Mondoberfläche zu erklären. Gaea, 27(7), 87. Alvarez, L. W., Alvarez, W., Asaro, F., & Michel, H. V. (1980). Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, 208(4448), 1095–1108. Baldwin, R. B. (1942). The meteoritic origin of lunar craters. Popular Astronomy, 50, 365–369. Baldwin, R. B. (1943). The meteoritic origin of lunar structures. Popular Astronomy, 51, 117–127. Baldwin, R. B. (1949). The face of the Moon. Chicago, IL: University of Chicago Press. Baldwin, R. B. (1965). A fundamental survey of the Moon. New York, NY: McGraw-Hill.

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Barringer, D. M. (1905). Coon mountain and its crater. In Proceedings of the Academy of Natural Sciences of Philadelphia, pp. 861–886. Campbell, W. W. (1920). Notes on the problem of the origin of the lunar craters. Publications of the Astronomical Society of the Pacific, 32(186), 126–138. Cruikshank, D. P. (1993). Gerard Peter Kuiper, 1905–1973. National Academy of Sciences Biographical Memoirs, 259–295. Darney, M. (1933). Le Systéme Imbrien. Bulletin De La Société Astronomique De France, 47, 452–457. Davis, W. M. (1893, May 11). Lunar craters. The Nation, 56(1454), 342–343. Delmotte, G. (1923). Recherches sélénographiques et nouvelle théorie des cirques lunaires. Paris, France: Librairie Scientifique Albert Blanchard. Dietz, R. S. (1946). The meteoritic impact origin of the Moon’s surface features. Journal of Geology, 54(6), 359–375. Fauth, P. (1909). The Moon in modern astronomy: A summary of twenty years selenographic work, and a study of recent problems [Translated by Joseph McCabe]. London, England: A. Owen & Co. Gilbert, G. K. (1893). The Moon’s face: A study of the origin of its features. Bulletin of the Philosophical Society of Washington, 12, 241–292. Gilbert, G. K. (1896). The origin of hypotheses, illustrated by the discussion of a topographic problem. Science, 3(53), 1–13. Glinka, N. L. (1965). General chemistry. Russian Monographs and Texts on Advanced Mathematics and Physics, 17. (New York, NY: Gordon and Breach Science Publishers). Günther, S. (1911). Vergleichende Mond- und erdkunde. Braunschweig, Germany: Friedr. Vieweg & Sohn. Hoyaux, M. (1946). Contribution à l’ étude de la genèse lunaire. Ciel et Terre, 62, 165–195. Hoyt, W. G. (1987). Coon Mountain controversies: Meteor Crater and the development of impact theory. Tucson, AZ: University of Arizona Press. Koppes, S. (1998). Memorial to Robert Sinclair Dietz: 1914–1995. Geological Society of America Memorials, 29, 25–27. Kuiper, G. P. (1954). On the origin of the lunar surface features. Proceedings of the National Academy of Sciences USA, 40(12), 1096–1112. Kuiper, G. P. (1955). The lunar surface—further comments. Proceedings of the National Academy of Sciences USA, 41(11), 820–823. Marvin, U. B. (2002). Geology: From an Earth to a planetary science in the twentieth century. In D. R. Oldroyd (Ed.), The Earth inside and out: Some major contributions to geology in the twentieth century, Special Publication 192 (pp. 17–57). London, England: Geological Society of London. Marvin, U. B. (2003). Oral histories in meteoritics and planetary science: X. Ralph B. Baldwin. Meteoritics & Planetary Science, 38(S7), A163–A175. Morozov, N. A. (1909). Загадки Луны (Mysteries of the Moon). Russian Academy of Sciences Online Archives, Inventory 1(File 0036), 1–11. Retrieved from http://www.ras.ru/MArchive/ Act.aspx?invid=2&id=40. Öpik, E. (1916). “Зaмeчaниe пo пoвoдy мeтeopнoй тeopии пyныx циpкoв” (“Note on the meteoric theory of lunar craters”). Bulletin de la Societe Russe des Amis de l’Etude de l’Univers, 5, 125–134. Shaler, N. S. (1903). A comparison of the features of the Earth and the Moon. Washington, DC: Smithsonian Institution. Spurr, J. E. (1944). Geology applied to selenology: The Imbrian plain region of the Moon. Lancaster, PA: Science Press Printing. Thomson, E. (1912). The fall of a meteorite. In Proceedings of the American Academy of Arts and Sciences, 47. Tilghman, B. C. (1905). Coon Butte, Arizona. In Proceedings of the Academy of Natural Sciences of Philadelphia, pp. 887–914.

References

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Urey, H. C. (1955). Some criticisms of ‘On the origin of the lunar surface features’ by G. P. Kuiper. Proceedings of the National Academy of Sciences USA, 41(7), 423–428. Wayman, P. A., & Mullan, D. J. (1986). Obituary for Ernst Julius Öpik. Quarterly Journal of the Royal Astronomical Society, 27(3), 508–514. Wegener, A. (1921a). Die Entstehung der Mondkrater (The origin of lunar craters). Braunschweig, Germany: Friedr. Vieweg & Son (Translated by the Center for Foreign Technology, Pasadena, California, on 24 October 1969. Wegener (1921a) and Wegner (1921b) are different translations of the same original article). Wegener, A. (1921b). Die Entstehung der Mondkrater (The origin of lunar craters). Braunschweig, Germany: Friedr. Vieweg & Son (Translated by A. M. Celâl Ṣengör in 1975 and reproduced as “The origin of lunar craters.” The Moon, 14(2), 211–236. Wegener (1921a) and Wegner (1921b) are different translations of the same original article). Wilhelms, D. E. (1963). To a rocky Moon: A geologist’s history of lunar exploration. Tucson, AZ: University of Arizona Press.

Chapter 3

Pre-Apollo Theories About the Origin of the Moon

3.1

Introduction

Theories about the origin of the Moon are related to theories about its age. In this chapter, we will first discuss the pre-Apollo estimates of the age of the Moon and then consider the main theories for its formation.

3.2

Pre-Apollo Estimates for the Age of the Moon

The Moon’s surface features had been observed extensively with Earth-based telescopes. Relative ages had been assigned to some of the features, partly based on the density of craters associated with the features. The dark maria that often fill large basins on the lunar surface have a lower density of craters than the light-colored surface features, called “terrae,” “highlands,” or “uplands.” There were still arguments about the origin of some of the craters, but most lunar scientists assumed that the majority were formed by the impact of objects of various sizes in the solar system. Areas of the lunar surface that had a lower density of craters were assumed to be younger than areas with a higher density of craters. In many cases, there are lunar craters that formed in preexisting craters, which simplified the relative dating of those features. Large impacts had excavated lunar material and spread it across the lunar surface, so that different strata could be recognized. Crude relative dating of these stratigraphic units was possible, but without lunar samples for dating by radioisotope methods, it was impossible to determine absolute ages of the various lunar features. It was possible, however, to estimate the age of the Moon based on the fact that the Earth’s rate of rotation is slowing, and the Moon’s distance from the Earth is gradually increasing. Figure 3.1 illustrates the reason this is so. Friction causes the rotating Earth to carry the tidal bulges with its rotation, so that they are offset from © Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_3

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Fig. 3.1 A sketch illustrating how the Moon acts on the Earth’s tidal bulges to slow the rotation of the Earth. F1 slows the Earth’s rotation, while F2 increases it. The net effect is a slowing, because F1 is larger than F2. Adapted from Fig. 1 in Singer, S. F. (1968) “The origin of the Moon and geophysical consequences.” Geophysical Journal of the Royal Astronomical Society, volume 15, issues 1–2, page 206, and used with permission of S. F. Singer and Oxford University Press

the Earth-Moon line, as shown in the figure. (The size of the tidal bulges and the offset angle are greatly exaggerated in the figure.) The Moon exerts forces, F1 and F2, on each of the Earth’s tidal bulges. The stronger force (F1) is on the bulge that is nearest the Moon and acts against the rotational motion of the Earth. F2, which acts on the more distant tidal bulge, tends to increase the Earth’s rate of rotation. But, since F1 is stronger than F2, the result is a gradual slowing of the Earth’s spin rate. The loss of Earth’s spin angular momentum is compensated by an increase in the Moon’s orbital angular momentum, so that the total angular momentum of the Earth-Moon system remains constant. The increase in the Moon’s orbital angular momentum is accomplished by lengthening the distance between the Moon and the Earth. Thus, the ultimate result of tidal friction is to slow the Earth’s spin rate and cause the Moon to gradually recede from the Earth. It was possible to demonstrate conclusively that the Earth’s rate of rotation was decreasing, and hence the Moon must be receding from the Earth. In 1960, two geophysicists from the University of California, Walter Munk and Gordon J. F. MacDonald, published their study of the rotation rate of the Earth in which they determined from astronomical evidence that the current tidal deceleration of the Earth was such that the length of a day at the beginning of the Paleozoic Era, i.e., the beginning of the Cambrian Epoch, was 21 h, rather than the 24 h period of today (Munk & MacDonald, 1960: 250). A year later, J. Laurence Kulp, a geochemist from Columbia University’s Lamont Geological Observatory, published a paper that used radioisotope age determinations on rocks of known stratigraphic age to define an absolute time scale for Earth’s geologic history (Kulp, 1961: 1105). In 1963, a paleo-biologist from Cornell University, John West Wells (1907– 1994), published a paper in which he first combined the results of Munk, MacDonald, and Kulp to show a relation between the number of days in a year and geologic time, if the Earth was indeed slowing down in its rotation (Fig. 3.2). It had been conjectured that ridges (visible to the unaided eye) on certain fossil corals might correspond to seasonal fluctuations in the growth of their skeletal structure, perhaps driven by seasonal variations in the temperature of the

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Fig. 3.2 The relation between the number of days in a year and geological time. Reprinted with permission from Macmillan Publishers Ltd: Nature, Wells, J. W. (1963) “Coral growth and geochronometry.” volume 197, issue 4871, page 949, copyright 1963

surrounding water. These ridges were described as “annulations.” Paleontologists had also noticed very fine lines within the annulations, which were assumed to be growth increments. Wells examined some living corals and found about 360 growth lines in the space of a year’s growth. This was close enough to 365 to convince Wells that the fine ridges were daily growth lines. He then began to examine fossil corals. Most fossil corals are too degraded by wear to count the number of fine lines within the annulations, but Wells found a few that could be used for that purpose: The best of the limited fossil material I have examined so far is from the Middle Devonian of New York and Ontario, … Diurnal and annual growth rates vary in the same individual, adding to the complexity, but in every instance there are more than 365 growth lines per annum, usually about 400, ranging between extremes of 385 and 410. It is probably too much, considering the crudity of these data, to expect a narrower range of values for the number of days in a year in the Middle Devonian; many more measurements will be necessary to refine them. A few more data may be mentioned: Lophophyllidium from the Pennsylvanian (Conemaugh) of western Pennsylvania gave 390 lines per annum, and Caninia for the Pennsylvanian of Texas, 385. These results imply that the number of days a year had decreased with the passage of time since the Devonian, as postulated by astronomers, and hence that values of the isotopic dates of the geophysicists agree well with the astronomical estimates of the age of the Earth. (Wells, 1963: 950)

Since there are fewer days in a current year than there were hundreds of millions of years ago, the Earth is able to make fewer and fewer rotations on its axis in a given trip around the Sun, i.e., the Earth’s spin rate is gradually decreasing—the emphasis being on gradually. The data indicate the addition of about 2 ms per century in the period of the Earth’s rotation. The rate of recession of the Moon is determined by the rate of slowing down of the Earth’s spin. If the current rate of slowing has persisted over the life of the Moon, then the Moon would have been in contact with the Earth some 1.7 billion

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years ago (MacDonald, 1966: 181). Gordon MacDonald noted that “… It is this short lifetime, evident even in elementary calculations, that presents a major timescale problem for many theories of the origin of the Moon.” (ibid.). With the paleontological confirmation provided by Wells, the time span of about 1.7 billion years began to be considered by some scientists as the age of the Moon, even though it was based on the dubious assumption that the current rate of the slowing of Earth’s rotation has been the same for the life of the Moon. It also was at odds with the work of a prominent British geophysicist, Harold Jeffreys (1891–1989), who derived an estimated age of the Moon of 2–4 billion years (Jeffreys, 1962: 247) by solving the equation that relates the rate of change of the Earth’s rotation rate and the tidal torques of the Moon on the Earth. The best estimates of the age of the Earth were around 4.55 billion years (Patterson, 1956: 230). Alastair Graham Walter Cameron (1925–2005), about whom we will see more later, dismissed the “lunar age problem” by arguing that the Moon and the Earth were likely about the same age (4.5 billion years) and that the tidal dissipation in the past must have been considerably less that at the present time (Cameron, 1966: 266). In 1965, William K. Hartmann, about whom we will see more later, estimated the age of the lunar maria to be approximately 3.6 billion years (Hartmann, 1965: 164). Hartmann used the research of others, as well as his own work, to estimate the number of craters of diameter greater than 1 km that had been created on the Moon per square kilometer per billion years. His best estimate for this rate was 5  10−4. He then derived his estimate for the age of the maria by dividing this rate into the observed density of craters within the maria that were greater than 1 km in diameter. Hartmann apparently agreed with Cameron that the Moon’s age was about the same as the age of the Earth. Hartmann cited theoretical research of MacDonald (1961) on the thermal history of the interior of the Moon as he noted that his estimated age of the maria as 3.6 billion years was: … very consistent with the current isotopic ages of meteorites of about 4.5 to 4.7  109 years. Assuming the maria to be lava flows, it is consistent with estimates of a period of less than 2  109 years for maximum melting to have occurred near the lunar surface as a result of radioactive heating …. (ibid.)

As with other questions about the Moon, its age was in need of definite determination through the Apollo explorations.

3.3

Pre-Apollo Theories for the Formation of the Moon

As the time for the Apollo explorations drew near, theorists for the formation of the Moon presented refined versions of their ideas. The theories were largely based on dynamical considerations, because little was known about the chemical composition of the lunar soil and rocks. The main theories fell into three categories:

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1. Fission (a) Rotational: The material that now constitutes the Moon was once a part of a rapidly rotating proto-Earth and was subsequently flung away from its parent by centrifugal force. (b) Collisional: Though not widely recognized, at least one scientist of note had proposed that fission of the Earth might have occurred because of a collision with another planetary body. 2. Co-accretion: the Earth and the Moon developed near each other within the cloud of dust and gas from which the solar system would evolve. 3. Capture: The Earth and Moon developed in different regions of the solar accretion disk, the Moon’s orbit was subsequently disrupted so that it crossed the Earth’s orbit, and the Moon was eventually captured by the Earth.

3.3.1

Rotational Fission

The person primarily responsible for developing a theory for the formation of the Moon based on rotational fission is George Howard Darwin (1845– 1912) (Fig. 3.3), the second son of Charles Darwin. George Darwin began his work by studying the physical processes related to tides, and particularly how tides produced by the Moon affect the Earth. His initial motivation was to see if the Moon’s tidal effect on the Earth could explain the Earth’s obliquity, i.e., the tilt of the Earth’s spin axis from the normal to the plane of the Earth’s orbit about the Sun (Hough, 1914: ii). In the course of this investigation, Darwin developed a theory that the material that formed the Moon was thrown off a rapidly rotating Earth. Darwin reasoned that earlier in the history of the Earth-Moon system, the Earth must have been spinning faster, and the Moon must have been closer to the Earth. Following the motion of the Earth-Moon system backward in time, Darwin guessed that “… if the moon and the earth were ever molten viscous masses, then they once formed parts of a common mass” (Darwin, 1879: 535). Assuming that the common mass of the proto-Earth and Moon had the same angular momentum as the current Earth-Moon system, it would have been rotating with a period of between four and five hours. Darwin argued as follows: … Sir William Thompson has shown that a fluid spheroid of the same mean density as the earth would perform a complete gravitational oscillation in 1 h and 34 min. The speed of oscillation varies as the square root of the density, hence it follows that a less dense spheroid would oscillate more slowly, and therefore a spheroid of the same mean density as the earth, but consisting of a denser nucleus and a rarer surface, would probably oscillate in a longer time than 1 h and 34 min. It seems to be quite possible that two complete gravitational oscillations of the earth in its primitive state might occupy 4 or 5 h. But if this were the case, then the solar semi-diurnal tide would have very nearly the same period as the free oscillation of the spheroid, and accordingly the solar tides would be of enormous height.

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Fig. 3.3 Photograph of George H. Darwin, obtained from Wikimedia Commons. The image title is “George Darwin sepia tone” and is attributed to the photographic company of J. Russell & Sons

Does it not seem that, if the rotation were fast enough to bring the spheroid into anything near the unstable condition, then the large solar tides might rupture the body into two or more parts? (ibid.: 537)

Darwin was still refining and extending his calculations when in 1885 Henri Poincaré published his research on the equilibrium forms of rotating fluids (Poincaré, 1885). To the collection of equilibrium forms of rotating fluids that already included the oblate spheroid (number 1 in Fig. 3.4, which is from an article by a later adherent of the fission theory, Wise (1963)) and the cigar-shaped form (ellipsoid with three unequal axes) attributed to Carl Jacobi (number 2 in the figure), Poincaré added a pear-shaped form (number 3 in the figure). As the rotation rate changes, the form of a given series changes, and it might go from a form that is stable against small disturbances to one that is unstable. When that point is reached, the unstable form has the same shape as the unstable endpoint of another series of forms that upon further progression of the rotation rate is stable (at least for a certain range of rotation rates). Darwin saw in Poincaré’s rotating pear-shaped form the possible genesis of the Earth-Moon system. He speculated:

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Fig. 3.4 Possible evolution of shapes for a rapidly rotating Earth and separated Moon. Reproduced with permission from Wise, D. U. (1963) “An origin of the Moon by rotational fission during formation of the Earth’s core.” Journal of Geophysical Research, volume 68, issue 5, Fig. 1 on page 1548. Copyright 1963 by the American Geophysical Union

The further development of the pear shape is unknown, …. There can, however, be hardly any doubt that the pear becomes more constricted in the waist, and begins to resemble an hour-glass; that the neck of the hour-glass becomes thinner, and that ultimately the body separates into two parts. … (Darwin, 1899: 327)

He felt that his research and that of Poincaré’s had arrived at the same point, but from different directions. I have myself attacked this problem from an entirely different point of view, and my conclusions throw an interesting light on the subject, although they are very imperfect in comparison with Poincaré’s masterly work. (ibid.: 327–328)

He considered a system that: … consists of a liquid planet and liquid satellite revolving round one another, so as always to exhibit the same face to one another, and each tidally distorting the other. It is certain that if the two bodies are sufficiently far apart the system is a stable one, for if any slight disturbance be given, the whole system will not break up. … Now if the rotations and revolutions of the bodies be accelerated, the two masses must be brought nearer together in order that the greater attraction may counterbalance the centrifugal force. But as the two are brought nearer the tide-generating force increases in intensity with great rapidity, and accordingly the tidal elongation of the two bodies is much augmented.

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3 Pre-Apollo Theories About the Origin of the Moon A time will at length come when the ends of the two bodies will just touch, and we then have a form shaped like an hour-glass with a very thin neck. The form is clearly Poincaré’s figure, at an advanced stage of its evolution. (ibid.: 328–329)

In addition to the supporting evidence of Poincaré’s newly discovered pear-shaped figure, Darwin had the friendship and support of William Thompson, who took the title Lord Kelvin in 1892, upon being knighted by Queen Victoria. The young, but already distinguished British astronomer James Jeans (1877–1946), gave tacit approval of Darwin’s theory (including the Poincaré pear-shaped figure), when he adapted it to explain Algol variable stars as the observation of the birth of double stars. Harold Jeffreys was also initially supportive of Darwin’s fission theory for the origin of the Moon. Other scientists had challenged the theory on the grounds that their calculations indicated that the resonance proposed by Darwin would be insufficient for separation under his model of a homogeneous mass of the Earth (Jeffreys, 1917: 117). Using his own calculations for a two-dimensional (cylindrical) model, however, Jeffreys showed that Darwin’s resonance theory would work if the model used for the Earth were not homogeneous, e.g., if it had a central core. Thirteen years later, upon further examination of the friction of fluids at the surface of the core of the Earth, Jeffreys decided that Darwin’s theory could not be valid. It has always been recognized that the validity of this theory depends on the smallness of friction; for friction necessarily fixes an upper limit to the amplitude of an oscillation attainable by resonance. In an earlier paper I showed that the requisite coincidence of periods can be attained only if we allow for the earth’s lack of homogeneity; the central core must be considered. This introduces a possibility of friction in the tidal currents in the outer shell flowing over the central core. This friction can now be estimated; and it turns out to be sufficient to invalidate the theory. (Jeffreys, 1930: 169)

In Darwin’s theory, the material that formed the Moon came from the upper layers of the Earth; hence, the Moon should have a density that is comparable to the density of the Earth’s crust. That was known to be the case, even prior to the Apollo explorations. The Moon’s average density is 3.34 g/cm3, which is significantly lower than a comparable average density of the Earth, 4.03 g/cm3, calculated as if the material of the Earth were not compacted by its own gravitational force. The density of material in the upper layers of the Earth, however, is close to the measured average density for the Moon. Heavier elements such as iron in the Earth’s core cause the average density of the Earth to be higher than the density of material in its upper layers. Despite the negative conclusions of Jeffreys in 1930, Darwin’s theory for the formation of the Moon made a prediction that seemed accurate, and it was still “alive” at the beginning of the Apollo explorations. Perhaps another reason for the staying power of the theory had to do with a speculation of a British geologist and clergyman named Osmond Fisher (1817–1914). In 1882, Fisher suggested that the rupture of the Earth during the genesis of the Moon was the cause of the Earth’s ocean basins (Fisher, 1882: 243–244). Fisher’s idea was that the “… ocean basins are the scar, which still testifies to the place of separation.” (ibid.: 244). The cavity formed in the Pacific basin would be partially closed by the

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movement of continents. “This would make the Atlantic a great rent, and explain the rude parallelism which exists between the contours of America and the Old World.” (ibid.). In the early 1960s, proponents of the rotational fission theory developed various modifications to Darwin’s original theory. For example, to counter objections raised with regard to the inadequacies of Darwin’s tidal resonance mechanism, Donald Wise proposed that the proto-Earth-Moon was originally spinning below but near the rate necessary for fission. The critical rate for fission was reached as the Earth’s (largely iron) core settled, coalesced, compacted, or collapsed. The formation of the Earth’s core would have caused an increase in the Earth’s spin rate because of conservation of angular momentum (Wise, 1963: 1547–1554). The standard analogy is that of a spinning ice skater who increases her spin rate by bringing her extended arms closer to her body.

3.3.2

Collisional Fission

Sometime in the early to mid-1940s, the eminent Princeton astronomer Henry Norris Russell (1877–1957) (Fig. 3.5) wrote to the geologist Reginald Aldworth Daly (1871–1957) suggesting “that it might be worthwhile to study the question whether the main part of the moon’s substance represents a planetoid which, after striking the earth with a glancing, damaging blow, was captured.” (Daly, 1946: 108). Daly at the time was an emeritus professor of geology at Harvard, where he had built a distinguished career. He was a member of the National Academy of Sciences and would receive the William Bowie Medal from the American Geophysical Union in 1946. Daly followed up on Russell’s suggestion and published the results of his study in 1946. As an alternative to the fission theory of Darwin, Daly conjectured that: … a ‘planetoid,’ captured because of tangential, slicing, collision with the liquid earth, brought with it so much angular momentum as to ensure its perpetuation as a separate, revolving body—the moon we know. … initially liquid fragments were exploded out of the planet, well beyond Roche’s limit. Many of these were gravitationally aggregated by the pull of master fragment or captured ‘planetoid’ to make the substance of our moon, and the somewhat diminished earth felt a prolonged rain of other earth-fragments, large and small. (ibid.: 118)

We will continue to encounter the term “Roche limit” mentioned by Daly. The limit arises because the Earth exerts tidal forces on the Moon in the same way that the Moon exerts tidal forces on the Earth. When the Moon was in a formative state, so that its various parts were held together loosely by mutual gravitational attraction, there was a minimum distance that the Moon could approach the Earth without being torn apart by the tidal forces from the Earth. That distance is called the Roche limit, named for the French astronomer, Édouard Roche, who first derived it in 1848. The concept of a Roche limit applies to all bodies in the solar system. For the Earth, the Roche limit is a little less than a distance of 3 Earth radii from the Earth’s center.

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Fig. 3.5 Photograph of Reginald A. Daly, courtesy of the Department of Earth and Planetary Sciences, Harvard University

Daly’s paper was almost totally forgotten for forty-six years. In a belated review of the paper in 1992, Ralph Baldwin and Don E. Wilhelms noted that: Daly’s paper explicitly discussed the idea that a collision between the brand-new Earth and a planet-sized body led to the formation of the Moon. Yet his paper is not discussed in any of the modern works on the origin of the Moon … (Baldwin & Wilhelms, 1992: 3837)

3.3.3

Co-accretion

The dominant theory for the formation of the solar system in the mid-1960s featured the evolution of a slowly rotating cloud of dust, and gas that first formed the Sun and then after flattening into a disk, called the accretion disk, formed the planets. Harold Urey, among others in the U.S., and Otto Yulyevich Schmidt (1891–1956) (Fig. 3.6) of the Soviet Union worked separately on these ideas in the 1940s and 1950s. Urey published his ideas in a 1952 book titled The Planets: Their Origin and Development (Urey, 1952: 105–107). Schmidt’s theory on Earth’s origin was summarized in four lectures that he gave to the Geophysical Institute (formally the

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Fig. 3.6 Photograph of Otto Y. Schmidt, from the frontispiece of Schmidt, O. (1958) A Theory of the Earth’s Origin: Four Lectures. Moscow: Foreign Languages Publishing

Institute of Theoretical Geophysics) of the Soviet Academy of Sciences in 1948. He continued to revise his theory, and following his death in 1956, his colleagues at the Geophysical Institute published the third and final edition of his book, A Theory of Earth’s Origin: Four Lectures, Third Edition, in 1958 (Schmidt, 1958). Schmidt began his academic career at Kiev University, where as an undergraduate he published three papers on group theory, one of which contained a version of what is now called the Krull-Schmidt theorem. He received his undergraduate degree from Kiev University in 1913 and a Master’s degree in 1916. The events of World War I led Schmidt to become involved in food distribution efforts in Kiev. Following the Russian revolution of 1917, Schmidt was appointed as head of one of the divisions of the People’s Commissariat for Food, and he and his wife relocated to Moscow. He continued to pursue both political and academic roles, serving on the People’s Commissariat for Education and becoming a Professor of mathematics at Moscow State University, where he became Head of the Department of Algebra. Schmidt is perhaps best known for his exploration of the Russian Arctic, establishing polar geophysical observatories and arctic research

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Fig. 3.7 Formation of asteroid-sized intermediate bodies from the dust component of the solar nebula according to the models of Russian researchers. Reprinted from Levin, B. J. (1972) “Origin of the Earth.” In A. R. Ritsema (Ed.), The Upper Mantle Tectonophysics, volume 13, issues 1–4, Fig. 1 on page 9, Copyright 1972, with permission from Elsevier

stations, for which he was honored with the title of Hero of the Soviet Union. He was a member of the USSR Academy of Sciences, which established at his urging the Institute of Theoretical Geophysics, with Schmidt as Director (O’Connor & Robertson, 2011). Schmidt at first thought that there were planetary embryos present in the circumsolar cloud that grew into the planets. He later changed his mind, crediting his Soviet colleagues L. E. Gurevich and A. I. Lebedinsky with: … proving that even if the primordial embryos did not exist, even if the cloud had consisted exclusively of gas and dust, the condensation must have taken place. Using the methods of statistical physics they … showed that the following must occur: (a) The relative velocities of the particles are reduced by collisions; this results in the flattening of the system with a consequent increase of density leading to a still greater frequency of collisions; (b) When a certain critical density is reached the system cannot remain in its former state; under the influence of gravitation the intensive formation of condensation begins; (c) These condensations are flattened in shape and have a mass comparable to that of the asteroids; (d) The condensations in turn are bound to collide (owing to their small free path) and agglomerate into a small number of big bodies, the planets. (ibid.: 40) (Figs. 3.7 and 3.8)

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Fig. 3.8 Gradual accretion of intermediate bodies into planets, per the Russian models. Reprinted from Levin, B. J. (1972) “Origin of the Earth.” In A. R. Ritsema (Ed.), The Upper Mantle Tectonophysics, volume 13, issues 1–4, Fig. 2 on page 10, Copyright 1972, with permission from Elsevier

This is an early articulation of the idea that the planets formed from intermediate-size bodies called planetesimals, though Schmidt did not use this term. An important stage in the evolution of the cloud was the formation of a large number of intermediate bodies of asteroid size. … … It is still difficult to draw a detailed picture of the early stages of the evolution of planet embryos. Their collision led to their cohesion or to their splitting after which fragments could again be drawn into the process of accumulation. In general, the predominating process was one of the conglomeration of matter. The fragments, together with “primary” particles, constituted the dispersed matter out of which the embroyos grew, at first rapidly and then more and more slowly as they swept up the surrounding matter. When some of the embroyos had acquired the size of big asteroids, the chaotic movement of the particles again increased under their dynamic influence. As the bigger bodies grew, however, they ceased to fear collisions since the splintered material, in the majority of cases, remained within their field of gravity and fell back on them. The highest rate of growth belongs to those embryos whose effective radius is much greater than their geometric radius, especially those placed at regular distances from the Sun so that they least of all interfere with each

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3 Pre-Apollo Theories About the Origin of the Moon other in acquiring matter from the medium. From these a small number of massive bodies, the planets, is gradually formed. (ibid.: 41–42)

Schmidt was early to recognize that the cumulative effects of many collisions between planetesimals would result in “regularities” among the planets. The circular orbits result from the natural statistical averaging of the motions of the separate bodies that agglomerate to form the planets. This natural averaging provides a simple explanation for the next two basic regularities—the motion of all the planets in practically the same plane and in the same direction. Both these result from the averaging of the angular momentum of many bodies. (ibid.: 42–43)

The co-accretion theory for the formation of the Moon (and other satellites of the planets) amounted to binary planet formation from within the initial solar cloud and subsequent accretion disk. Schmidt wrote as follows: We have seen that during the evolution of the cloud its dust component has flattened and the orbits of the particles have approached the circular. Then intermediate bodies of asteroidal size are formed from the particles and these bodies disturb one another so that they begin to move in elliptical orbits. The accumulation of such bodies and particles in separate regions of the cloud (swarm) leads to the formation of planets. … … The satellites are formed in one single process together with the planets. During the process of planet formation, when particles encountered the bigger planet embryos, some of them lost their velocity to such an extent in collisions that they were captured from the swarm and began to revolve around the planets. In this way a condensation, a swarm of particles, was formed near the planet embryo and revolved about it on elliptical orbits. These particles also collided amongst themselves, thus changing their orbits. In these swarms, processes similar to the formation of planets took place on a smaller scale. The majority of the particles fell on to the planet and were absorbed by it, but some of them formed a swarm around the planet and accumulated to form independent embryos, the future satellites. The exception is the ring of Saturn which consists of small particles that have not been able to agglomerate on account of the tidal action of Saturn in whose immediate vicinity they are (an unformed satellite). As the orbits of the particles forming a satellite were averaged, the satellite acquired a symmetrical, almost circular orbit in the equatorial plane of the planet and could not fall on it. In this way satellites appeared around the planets. Thus we see that the formation of the satellites was a by-product of the formation of the planets … (ibid.: 54–59)

In 1960, another Soviet scientist, Evgenia L. Ruskol, building on the work of Schmidt and others at the Soviet Geophysical Institute, began to develop a variation of the co-accretion theory that involved the idea that: … a swarm of bodies of total mass 0.01–0.1 m♀ could have formed about the earth during its growth period. This would require the “effective” dimensions of the colliding particles to be not more than 10–100 km, for the average probability of capture in a single collision to be of the order of 0.01. The size of the swarm would be about 100 earth radii, with the density of the matter strongly concentrated toward the earth. The rotation of such swarms would have to take place in the same direction as the rotation of the protoplanetary cloud. (Ruskol, 1960: 657)

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In concert with Schmidt, Ruskol envisaged the formation of the Moon from the smaller bodies in the proto-Earth’s extended envelope. From the discussion of the scheme of formation of the satellite swarm encircling the earth, we find that the growth of the moon must have begun at a short distance from the earth, since the density of matter in the swarm was at a peak near the earth. …The swarm would have to have grown most rapidly at an earth mass of 0.3 to 0.5 of its present mass …. As a study of the earth’s rate of accumulation shows, the growth of the earth proceeded at a far more rapid rate in the beginning than later. At most, 100 to 200 million years were required to form 99% of the earth’s mass. During this same time interval, the formation of the satellite swarm must have been completed for the most part. The accumulation of the major bulk of the moon’s mass would have necessarily taken no longer a time than the accumulation of the earth. The difference in the ages of the earth and moon could not be greater than 200 million years in the light of this argument. The earth-moon distance must have always exceeded the Roche limit. … Since the bulk of the mass of the swarm was included within a range of 10 earth-radii, we may arrive at the conclusion that the moon was formed basically at a distance of 5–10 earth-radii. (Ruskol, 1963: 226)

Ruskol noted that Kuiper had come to a similar conclusion, having suggested that the Moon had pushed through a belt of earth-orbiting small bodies as it receded from the Earth because of tidal forces (ibid.). Ernst Öpik had a similar idea about a ring of material orbiting the Earth, but, in agreement with Ruskol, Öpik suggested that the Moon might have been formed from the ring, rather than moving through it. A theory (to be discussed below) that the Moon was captured by the Earth featured a closest approach of the captured Moon at distance almost equal to Roche’s limit. Öpik noted that: The closeness of the moon’s minimum distance to Roche’s limit is a curious, quite improbable coincidence in the capture process of an originally independent planet. The minimum distance could not have been less, but it could have been any value greater than this limit. If the coincidence is considered significant, it would favor the origin of the moon from matter inside the Roche’s limit. An aggregate of mass, something like the rings of Saturn, may have existed there near “zero hour”. If the primeval earth was more massive than the present one, and decreased in mass through the loss of light gases to space, the orbits of the particles in the ring would have increased (radius of orbit inversely proportional to mass of earth), until Roche’s limit was reached. At this stage the particles would coagulate under mutual gravitation, the moon being thus formed, precisely in Roche’s limit at “zero hour”. In this case the coincidence is no longer fortuitous. (Öpik, 1955: 247)

In 1960, Kuiper produced a Photographic Lunar Atlas, which featured some of the best lunar images taken at the Mount Wilson, Lick, Pic du Midi, McDonald, and Yerkes Observatories. Öpik used this atlas to examine the ellipticities of lunar craters, trying to determine if there was evidence that crater shapes had been deformed by tidal forces after they had been created. He concluded, “It is clear that the moon could not have undergone large tidal deformations after the formation of craters …” (Öpik, 1961: 66), and that: … the only mode of formation of the moon consistent with the assumptions and the observational data is its buildup in a direct orbit at a considerable distance from the earth, of the order of 6 earth radii, from earthbound material previously found at that distance. (ibid.)

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Fig. 3.9 Photograph of Alfred E. Ringwood, obtained from the AIP Emilio Segrè Visual Archives, Physics Today Collection, and used with permission

Öpik also concluded, “The last stage of accretion which gave the lunar surface its present appearance could have added only a small fraction to the lunar mass … equivalent to a layer of about 50 m. …” (ibid.: 65). Alfred Edward “Ted” Ringwood (1930–1993) (Fig. 3.9) of the Australian National University developed a variation of the co-accretion theory that he called a precipitation hypothesis. Ringwood, along with many others, was trying to find a way that the Moon could have been formed with a lower density and different chemical combination than the Earth. He argued that: During the final stages of accretion of the earth … the temperature in the atmosphere [of the early Earth] exceeded 1500 °C leading to melting at the surface. … A fractionation of silicates from metallic iron was thereby caused, with iron continuing to accrete on the earth, and silicates entering the atmosphere in the gas phase. … During or perhaps immediately after the primary accretion, the massive, primitive, reducing atmosphere escaped from the earth … As the gasses recede from the earth, expansion and cooling occur, leading to the precipitation of less volatile components in the form of smoke, condensations or

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planetesimals. According to our previous assumptions, the precipitate would consist dominantly of silicates depleted in iron. Collisions between planetesimals combined with mutual gravitation would cause this debris to collect in a “sediment ring” … surrounding the earth. If the escape of the atmosphere were partly caused by rapid rotation or controlled by magnetic fields, the plane of the ring may have lain approximately in the plane of the earth’s equator. Such a sediment-ring has been envisaged by Öpik (1955, 1961, …) as the parent of the moon. A viscous interaction between outwardly moving gasses and condensate might result in this material moving outwards from the earth and beyond Roches Limit, where aggregation into larger bodies occurred. Ultimately these bodies collected together to form the moon. (Ringwood, 1966: 81)

Harold Urey favored one or the other of each of the main theories for the formation of the Moon at different times. In 1963, Urey favored a version of the capture theory, and he raised two objections to theories based on the simultaneous and proximate aggregation of the Moon and Earth. A small earth and a small moon would attract each other with a small gravitational force. Hence the angular velocity of rotation would have to be small in the beginning in order for the centrifugal and gravitational forces to be equal. As they grew in size, the gravitational forces would become greater and the angular velocity would have to increase in a very exact way, for otherwise the two objects would either collide with each other or move an infinite distance apart. In the second place, it is necessary to assume some process in the accumulation which would account for the difference in chemical composition of the earth and moon. (Urey, 1963: 165)

Pertinent to Urey’s second point, because the Moon’s density is smaller than the Earth’s, the Moon has proportionally less heavy elements than the Earth. In particular, it probably has a proportionately smaller iron core than that of the Earth. If the Earth and Moon accumulated material from the same region of the original solar nebula, then their internal structure should be similar. As shown in a 1966 paper by Ted Ringwood and in a 1968 textbook by William Mason “Bill” Kaula (1926–2000) of UCLA, the Moon’s low iron content is a clear anomaly among the terrestrial planets. Kaula gave the estimates shown in Table 3.1 for percentages of iron-nickel in the Moon, Earth, and the other terrestrial planets. (Kaula, 1968: 415) Kaula arrived at these results through numerical integration of the force-balance equation for a rotating fluid, using a relation between density and pressure obtained from shock-compression experiments and assuming that all the iron-nickel for each planetary body has settled into a core. Table 3.1 Percentages of iron-nickel in the Moon and terrestrial planets

Mercury Venus Earth Moon Mars

65.0% 26.5% 31.5% 6.0% 19.0%

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3.3.4

Capture

Partly because of the anomaly shown in Table 3.1, the capture theories assumed that the Moon and Earth developed in different regions of the solar accretion disk or at different times. Urey, for example, proposed that the Moon accumulated primarily from the material that had already undergone heating and fractionation, i.e., separation of heavier elements from lighter ones. Urey proposed the following outline of events: First, a nebula was produced from a rotating sun as it contracted. The sun’s radiation was screened from the nebula by gas and dust in the immediate neighborhood of the sun and lying toward the plane of the equator of the sun. The nebula cooled by radiation into space until fairly low temperatures existed. … Following this process the gas escaped from the solar nebula due to radiation processes from the sun. … When the gas had escaped almost completely, a breakup of the solid objects formed in this way was produced by collisions, and silicate dust was carried into space along with the residue of the gas, thus leaving behind material containing increased amounts of high-density elements. The accumulation of the planets occurred at this time, and the planets thus have a greater amount of iron compared to the sun. The moon is one such object that by chance escaped breakup and accumulation into the bodies of the terrestrial planets and was captured in a special orbit after some capture of high-density material similar to that which makes up the earth. Its composition is thus intermediate between that of the sun and that of the earth and other terrestrial planets and meteorites. (Urey, 1963: 160)

Urey knew that the capture theories, like the co-accretion theories, depended on improbabilities, e.g., that: … it is necessary to account for differences in composition of the earth and moon by special assumptions … It should be noted that [Urey’s proposal] avoids some of these improbabilities. The moon is assumed to have accumulated primarily out of solid material of solar composition by the settling of dust particles in a gravitational field. It was one of many objects of this kind; hence it is not so improbably that one of these objects should have been captured by a terrestrial planet. In fact, if the moon was captured by the earth, it seems very probable indeed that there were at one time many moons in the solar system that were destroyed by capture into the planets … This model requires that some material of terrestrial composition has been added to a moon of solar composition. The calculated iron content of the moon is in accord with this. (ibid.: 165–166)

Thus, Urey took the view that among the planetesimals that aggregated within the solar accretion disk were many objects of the size of the Moon. Urey noted that there are six other satellites in the solar system with masses that are within a factor of two of the Moon. All other satellites have masses that are less than three percent of the mass of the Moon (ibid.: 157). Urey speculated that most of the lunar-sized planetesimals were destroyed via collisions as soon as their gaseous, cushioning envelopes were evaporated or swept away by the developing Sun’s radiation and/or solar wind (ibid.: 161). Though they might not have known each other in the mid-1960s, a Russian astronomer and planetary scientist, Victor Sergeevich Safronov (1917–1999) (Fig. 3.10), would probably have agreed with Urey about the presence of lunar-sized

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Fig. 3.10 Photograph of Victor S. Safronov, courtesy of the Meteoritical Society. Safronov was a protégé of Otto Schmidt. Following the English translation of Safronov’s 1969 book Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets, the planetary science community worldwide would become more aware of his work. In 1989, Safronov was awarded the Leonard Medal of the Meteoritical Society. In his citation for the awarding of this medal to Safronov, George Wetherill wrote, “Safronov established new standards of quantitative presentation, that impose upon all of us the obligation of following up our fantasies with real physical calculations before we or anyone else should be expected to take them seriously. …” (Wetherill, 1989: 347). In 1990, Safronov received the Gerard P. Kuiper Prize from the Division of Planetary Sciences of the American Astronomical Society

planetesimals in the early solar system. In a 1966 paper published in the journal Soviet Astronomy, Safronov calculated the size of the largest bodies falling onto the planets during their formation, based on the observed inclinations of the spin axes of the planets (Safronov, 1966: 987–981). From his calculations, Safronov concluded: … The largest bodies falling onto the earth had masses of about 10−3 of the earth’s mass, that is, they were the size of the largest asteroids. … The mass of the largest body falling onto the surface of Uranus … would have to be 0.05 of the mass of that planet. (ibid.: 987)

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Fig. 3.11 Densities of the terrestrial planets and the Moon. Reprinted from Icarus, volume 1, issues 1–6, Alfvén, H. “The early history of the Moon and the Earth,” page 357, copyright 1963, with permission from Elsevier

The mass of the planetesimal that would cause the spin axis of Uranus to rotate about 90% would be almost 60 times the mass of the Moon. There was thus a wide range of masses of planetesimals in the early solar system as envisaged by Victor Safronov. Hannes Olaf Gösta Alfvén (1908–1995) of Sweden also argued that the Moon and the Earth accumulated their materials in different regions of space. He plotted a figure that showed the densities of the Earth’s crust, Moon, and Mars in one grouping and Mercury, Venus and Earth in another (Fig. 3.11). He argued that the figure supported his view “… that the high density group Mercury-Venus-Earth originated from a cloud with a chemical composition which differed from that of the cloud which produced the low density bodies Moon and Mars.” (Alfvén, 1963: 357). Alfvén further argued that events similar to that of the Moon’s capture by the earth might have occurred elsewhere in the solar system. He pointed to Triton, the largest moon of Neptune, as a capture candidate, since it moves in a retrograde orbit around Neptune. The presence of high mountains on the Moon suggested to Urey that the lunar interior was rigid and always had been so. Urey reasoned that the mountains had been created by the impact of meteorites on the Moon, i.e., as the walls of the craters formed by the impacts, and thus would not be expected to have low density material below them to “keep them afloat.” (Urey, 1962: 484). Mountains on the Earth do have low-density “roots” that keep them “floating” above the Earth’s surface. So, Urey reasoned that: … If the moon had a “viscosity” comparable to that of the outer parts of the earth, the difference in elevation [on the Moon] could not be maintained for more than some hundreds of thousands of years. This evidence indicates that the moon is now a rigid object, a conclusion that is valid regardless of the origin of the moon or how the irregularities were established. (Urey, 1963: 126)

In other words, since the mountains had not settled, Urey concluded that the Moon has retained a high rigidity since its formation. That was part of his argument that the Moon is, in part, an aggregation of sub-lunar sized planetesimals that came

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together at low temperatures. There had been little or no melting of internal lunar material, and if it occurred it was of short duration, so that the Moon’s interior was cold and rigid (Urey, 1962: 488). But if the Moon had accumulated its mass far away from the Earth and somehow was placed in an orbit around the Sun that brought it close to the Earth, it would still have difficulty in being captured by the Earth, as the kinetic energy of its motion would be too high for capture to take place. How would the Moon have lost enough kinetic energy in a close encounter with the Earth to be captured by it? A possible answer is that kinetic energy of the pre-captured Moon could be lost (dissipated) through tidal friction. In 1955, a German school teacher named Horst Gerstenkorn (1923–1981) pursued this possibility (Gerstenkorn, 1955). In the scenario developed by Gerstenkorn, the pre-captured Moon passed around the Earth in a retrograde trajectory, i.e., moving opposite to its current prograde sense, which is in the same rotational direction as the rotation of the Earth, and with the plane of its motion inclined to the plane of the Earth’s equator by 31 degrees. In this first pass, at a closest approach of 26 Earth radii, Gerstenkorn calculated that there was enough energy lost by tidal friction to allow the Moon to be captured by the Earth. Because of its retrograde motion, the conservation of angular momentum dictated that the Moon would get closer to the Earth, instead of farther away, as the Earth’s spin was reduced owing to tidal friction, and that the inclination of the plane of the Moon’s orbit would increase. When the Moon came as close to the Earth as 4.7 Earth radii, the inclination of the Moon’s orbit relative to the Earth’s equator passed 90o, and the Moon began to orbit the Earth in a prograde sense. The captured Moon was now in a highly inclined prograde orbit, and angular momentum was conserved by lowering the inclination of the orbit while the Moon continued to get closer to the Earth. The Moon reached its closest approach at a geocentric distance of about 2.9 Earth radii, following which angular momentum was conserved by both a continued decline in the inclination of the Moon’s orbit and an increase in its distance from the Earth. Eventually, the Moon achieved its present near circular orbit (Field, 1963: 349–354). In Gerstenkorn’s theory, at the point of closest approach of the Moon, the Earth would have been spinning at such a high rate that the length of the day would have been only 4.8 h. Tidal forces were extreme, and the height of the tides would have been comparable to the mean ocean depths of today. The Earth would have been heated by the dissipation of tidal friction to such an extent that no life forms, if any existed at the time of capture, could have survived (ibid.). In a variation of Gerstenkorn’s theory, Hannes Alfvén noted that the minimum distance between the Earth and the Moon during the Moon’s closest approach would have been at the Roche limit. Alfvén suggested that if the Moon’s initial angular momentum with respect to the Earth were less than Gerstenkorn assumed, then the minimum distance of approach to the Earth would have been less than the Roche limit, and the Moon would have been partially torn apart, losing some of its material to the Earth.

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3 Pre-Apollo Theories About the Origin of the Moon From our point of view it would be possible to identify the crust as deriving from that part of the Moon which fell down on the Earth at the break-up. As the total mass of the crust is about the same as the present lunar mass, this implies that the Moon lost about half of its mass at the Roche limit. (Alfvén, 1963: 362)

There were other variations or objections to Gerstenkorn’s theory. Gordon MacDonald, for example, noted that: … The earth must have had substantial angular momentum in order to cancel out the angular momentum associated with the retrograde motion of the captured satellite … The kinetic energy of rotation [of the Earth] that must be dissipated would be more than sufficient to raise the temperature of the whole earth about 2500o and melt it. … (MacDonald, 1964: 535)

MacDonald subsequently suggested that an alternative to capture made possible by tidal energy dissipation was capture made possible by collision. “… Suppose the Earth had an initial Moon, with a mass only a fraction of the mass of the present one, in a distant orbit. The capture process would then involve a collision. …” (MacDonald, 1966: 205). MacDonald calculated that both bodies would be vaporized by the collision, however, and he assumed that was a fatal objection to such a theory (ibid.). Gerstenkorn and others estimated the age of the Moon by calculating the time it would have taken for tidal forces to result in its recession from very close to the Earth to its current distance. Most of these calculations assumed that the tidal forces have remained constant over most of the history of the Earth-Moon system. As noted above, on this basis, the best estimates for the age of the Moon were in the range 1.4–1.78 billion years (Field, 1963: 351; MacDonald, 1966: 181; and Baldwin, 1965: 41).

3.4

Concluding Remarks

In the mid-1960s, all the main theories for the formation of the Moon were at least marginally viable, but the research community was divided and unable to coalesce around a particular theory. As Urey noted, “… there is no model for the origin of the moon that is not complicated and does not appear to be very highly improbable.” (Urey, 1963: 164). Ralph Baldwin lamented: We are thus left on the multipointed horns of a dilemma. There is no existing theory of the origin of the moon which gives a satisfactory explanation of the earth-moon system as we know it. The moon is not an optical illusion or a mirage. It exists and is associated with the earth. Before 4.5 billion years ago, the earth did not exist. Somehow in this period of time, the two bodies were formed and became partners. But how? (Baldwin, 1965: 42–43)

All hoped that the upcoming exploration of the Moon would answer the question of how the partnership was formed. In the fall of 1964, following the successful

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Ranger 7 mission, the National Aeronautics and Space Administration (NASA) and the National Academy of Sciences (NAS) began to plan for a study that would help NASA prepare a program of space research, particularly in planetary exploration, astronomy, and the roles for humans in space. A summer study was held at Woods Hole, Massachusetts, during June and July of 1965. The report of the study listed 15 scientific questions in lunar exploration in three categories: (1) Structure and processes of the lunar interior, (2) Composition, structure, and processes of the lunar surface, and (3) History of the Moon (Hess, 1966: 21–22). The six questions under History of the Moon give a sense of what was unknown in 1965 about the formation of the Moon: 1. What is the age of the Moon? What is the range of age of the stratigraphic units on the lunar surface and what is the age of the oldest exposed material? Is a primordial surface exposed? 2. What is the history of dynamical interaction between the earth and the Moon? 3. What is the thermal history of the Moon? What has been the distribution of tectonic and possible volcanic activity in time? 4. What has been the flux of solid objects striking the lunar surface in the past and how has it varied with time? 5. What has been the flux of cosmic radiation and high-energy solar radiation over the history of the Moon? 6. What past magnetic fields may be recorded in the rocks at the Moon’s surface? We will see in the following chapters how the emerging lunar and planetary research community worked to answer these questions.

References Alfvén, H. (1963). The early history of the Moon and the Earth. Icarus, 1(1–6), 357–363. Baldwin, R. B. (1965). A fundamental survey of the Moon. New York, NY: McGraw-Hill. Baldwin, R. B., & Wilhelms, D. E. (1992). Historical review of a long-overlooked paper by R. A. Daly concerning the origin and early history of the Moon. Journal of Geophysical Research-Planets, 97(E3), 3837–3843. Cameron, A. G. W. (1966). Planetary atmospheres and the origin of the Moon. In B. G. Marsden & A. G. W. Cameron (Eds.), The Earth-Moon system (pp. 234–273). New York, NY: Plenum Press. Daly, R. A. (1946). Origin of the Moon and its topography. Proceedings of the American Philosophical Society, 90(2), 104–119. Darwin, G. H. (1879). On the precession of a viscous spheroid, and on the remote history of the Earth. Philosophical Transactions of the Royal Society of London, 170, 447–539. Darwin, G. H. (1899). The tides and kindred phenomena in the solar system. Boston, MA: Houghton Mifflin. Field, G. B. (1963). The origin of the Moon. American Scientist, 51(3), 349–354. Fisher, O. (1882). On the physical cause of the ocean basins. Nature, 25(637), 243–244. Gerstenkorn, H. (1955). Über Gezeitenreibung beim Zweikörperproblem. Zeitschrift für Astrophysik, 36, 245–274.

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Hartmann, W. K. (1965). Terrestrial and lunar flux of large meteorites in the last two billion years. Icarus, 4(2), 157–165. Hess, H. H. (1966). Space research: Directions for the future (National Academy of Sciences Publication 1403). Washington, DC: National Academy of Sciences—National Research Council. Hough, S. S. (1914). Sir George Howard Darwin, 1845–1912. Proceedings of the Royal Society A, 89(614), i–xiii. Jeffreys, H. (1917). The resonance theory of the origin of the Moon. Monthly Notices of the Royal Astronomical Society, 78, 116–131. Jeffreys, H. (1930). “The resonance theory of the origin of the Moon” (second paper). Monthly Notices of the Royal Astronomical Society, 91, 169–173. Jeffreys, H. (1962). The Earth: Its origin, history and physical constitution (4th ed.). New York, NY: Cambridge University Press. Kaula, W. M. (1968). An introduction to planetary physics: The terrestrial planets. New York, NY: Wiley. Kulp, J. L. (1961). Geologic time scale. Science, 133(3459), 1105–1114. Levin, B. J. (1972). Origin of the Earth. Tectonophysics, 13(1–4), 7–29. MacDonald, G. J. F. (1961). Interior of the Moon: Observations from space vehicles will provide further clues to the history of the Earth-Moon system. Science, 133(3458), 1045–1050. MacDonald, G. J. F. (1964). Tidal friction. Reviews of Geophysics, 2(3), 467–541. MacDonald, G. J. F. (1966). Origin of the Moon: Dynamical considerations. In B. G. Marsden & A. G. W. Cameron (Eds.), The Earth-Moon system (pp. 165–209). New York, NY: Plenum Press. Munk, W. H., & MacDonald, G. J. (1960). The rotation of the Earth: A geophysical discussion. New York, NY: Cambridge University Press. O’Connor, J. J. & Robertson, E. F. (2011). Otto Yulyevich Schmidt. MacTutor History of Mathematics. Obtained from http://www-history.mcs.st-andrews.ac.uk/Biographies/Schmidt_ Otto.html. Öpik, E. J. (1955). The origin of the Moon. Irish Astronomical Journal, 3(8), 245–248. Öpik, E. J. (1961). Tidal deformations and the origin of the Moon. Astronomical Journal, 66(2), 60–67. Patterson, C. (1956). Age of meteorites and the Earth. Geochimica et Cosmochimica Acta, 10(4), 230–237. Poincaré, H. (1885). Sur l’équilibre d’une masse fluide animée d’un mouvement de rotation. Acta Mathematica, 7(0), 259–380. Ringwood, A. E. (1966). Chemical evolution of the terrestrial planets. Geochimica et Cosmochimica Acta, 30(1), 41–104. Ruskol, E. L. (1960). The origin of the Moon. I. Formation of a swarm of bodies around the Earth. Soviet Astronomy, 4, 657–668. Ruskol, E. L. (1963). The origin of the Moon. II. The growth of the Moon in the circumterrrestrial swarm of satellites. Soviet Astronomy, 7(2), 221–227. Safronov, V. S. (1966). Sizes of the largest bodies falling onto the planets during their formation. Soviet Astronomy, 9(6), 987–991. Schmidt, O. (1958). A theory of the Earth’s origin: Four lectures. Moscow: Foreign Languages Publishing. Singer, S. F. (1968). The origin of the Moon and geophysical consequences. Geophysical Journal of the Royal Astronomical Society, 15(1–2), 205–226. Urey, H. C. (1952). The planets: Their origin and development. New Haven, CT: Yale University Press. Urey, H. C. (1962). Origin and history of the Moon. In Z. Kopal (Ed.), Physics and astronomy of the Moon (pp. 481–523). New York, NY: Academic Press. Urey, H. C. (1963). The origin and evolution of the solar system. In D. P. LeGalley (Ed.), Space science (pp. 123–168). New York, NY: Wiley.

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Wells, J. W. (1963). Coral growth and geochronometry. Nature, 197(4871), 948–950. Wetherill, G. W. (1989). Leonard Medal citation for Victor Sergeivitch Safronov. Meteoritics & Planetary Science, 24(4), 347. Wise, D. U. (1963). An origin of the Moon by rotational fission during formation of the Earth’s core. Journal of Geophysical Research, 68(5), 1547–1554.

Chapter 4

Exploring the Moon—The Apollo Investigations

4.1

Introduction

In 1966, as astronauts were preparing for the first mission to leave Earth orbit, NASA Administrator James Edwin Webb (1906–1992) began to look for a new mechanism to engage university researchers in the organization’s missions. Webb knew that in the past, university researchers had built their scientific instruments on their campuses; brought them to NASA for testing, launch, operation, and data capture; and then analyzed and published their data in the normal mode of academic research. Webb foresaw that future NASA missions, such as those of the Apollo program, were going to be much more operationally complex and would require a stronger means of engagement with university researchers. Webb thus turned to the National Academy of Sciences (NAS) to identify the proper mechanism/organization whereby NASA could gain the intellectual and technical support of the university researchers, and at the same time minimize the burden on these university scientists, who were often teaching and training graduate students. The president of the NAS, Frederick Seitz (1911–2008), took on the task. After lengthy deliberations, the Lunar Science Institute (LSI) was formed near the Manned Spacecraft Center (later named the Johnson Space Center) south of Houston. President Lyndon Johnson announced the creation of the LSI on 1 March 1968. In his speech, President Johnson set the international flavor for the institute. He remarked: As a further step toward joining hands with the world’s scientific community, I want to announce that we will build facilities here in Houston to help the world’s scientists work together more effectively on the problems of space. We are going to have a new Lunar Science Institute alongside this great Center. … We hope and expect that universities from all parts of the nation will join with them. This new Institute is a center of research designed specifically for the age of space. Here will come scientists—and their students—from all over the world. © Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_4

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Fig. 4.1 Photograph of William W. Rubey, courtesy of the Lunar and Planetary Institute. Rubey was a member of the National Academy of Sciences and a distinguished geologist, best known for his demonstration that the Earth’s atmosphere and hydrosphere arose from the outgassing of the Earth’s deep interior over geologic time. He received the National Medal of Science from President Johnson in 1965 (Ernst, 1978: 205–223)

We will welcome here all who are interested in the science of space. We will strengthen the cooperation between NASA and our universities. And we will set down new patterns of scientific cooperation which will have profound effects on man’s knowledge of his universe. This new Lunar Science Institute will provide new means of communication and research for the world’s scientific community. It will help unite the nations for the great challenge of space. (LBJ, 1968: 1)

Rice University was appointed the interim institutional manager for the LSI, while the academic community deliberated with the NAS over the organization that would oversee the institute. In February 1969, the Universities Space Research Association (USRA) was organized by the NAS, with the support of NASA, to manage the LSI and other facilities and programs as the need arose. USRA began as a nonprofit association of 48 major research universities and now numbers more than 111 such universities as members (Cummings, 2009). The first Director of the LSI was William Walden Rubey (1898–1974) (Fig. 4.1), and he immediately began to assemble the staff of the LSI, including the first Visiting

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Scientist,1 S. Ross Taylor of the Australian National Observatory, and the first Senior Visiting Scientist, Harold Urey of the University of California, San Diego. Time was short for the development of the LSI as an entity that would assist both NASA and the university research community in the exploration of the Moon. The first Apollo landing would take place on 20 July 1969. In the remainder of this chapter, we review the Apollo explorations and the initial discoveries.

4.2

The Apollo Explorations

Prior to the first Apollo landing, NASA sought to know as much as possible about the aspects of the Moon that would affect their mission. In addition to the Ranger missions noted in Chap. 3, five Lunar Orbiter missions were launched in 1966 through 1967 with the purpose of mapping the lunar surface. The Lunar Orbiters successfully mapped 99% of the surface of the Moon with a resolution of 60 meters or better. The precise tracking of the Lunar Orbiters also revealed unexpected lunar gravitational anomalies that could be attributed to very large mass concentrations, called mascons, beneath the center of all five nearside ringed basins (Imbrium, Serenitatis, Crisium, Nectaris, and Humorum) (Muller & Sjogren, 1968: 680–684). In almost the same period, 1966–1968, NASA sent seven Surveyor spacecraft to the Moon. These spacecraft were able to land softly on the lunar surface, rather than crashing into it, as was the case with the Rangers. Five of the seven Surveyors landed successfully and obtained close-up images of the lunar surface. The goal was to determine if the lunar terrain was safe for manned landings. All Surveyors carried television cameras, and some were able to dig trenches and test the mechanical properties of the soil. The data products from these precursor missions ultimately became part of the resource collection at the LSI, where they were made available to the broader community of scientists and interested members of the general public.

4.2.1

Apollo 11

Apollo 11 was launched on 16 July 1969. Neil Armstrong (1930–2012) and Edwin Aldrin landed on the lunar surface on 20 July, while Michael Collins orbited the Moon in the Command Module. The Lunar Lander touched down on the southwestern edge of Mare Tranquillitatis (Sea of Tranquillity) (Fig. 4.2).

1

Visiting Scientists at the LSI, later renamed the Lunar and Planetary Institute (LPI), are researchers from other organizations who make the institute their home for periods of a few weeks to several months.

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Fig. 4.2 Apollo 11 Landing Site, photograph courtesy of the Lunar and Planetary Institute

The first lunar samples were returned from the Apollo 11 mission and stored in the Lunar Receiving Laboratory at the Manned Spacecraft Center in Houston on 25 July 1969. A photograph from the LSI Newsletter of August 1969 shows the lunar samples from the first lunar rock box (Fig. 4.3). The dark rocks from Mare Tranquillitatis looked like charcoal briquettes. Ross Taylor was appointed to the Lunar Sample Preliminary Examination Team that would obtain basic data from the samples in support of their allocation and distribution to a group of 140 principal investigators around the world (Lunar Sample Preliminary Examination Team, 1969). While this preliminary examination was underway, in August 1969, the LSI held its first symposium, which had the purpose of interpreting the existing data on the geophysics of the Moon. An intense period of lunar sample analysis was about to get underway. On 4 January 1970, NASA Administrator Thomas O. Paine (1921–1992) spoke at the dedication of the Lunar Science Institute. During the following four days, lunar scientists from around the world participated in an LSI-organized, NASA-sponsored Apollo 11 Lunar Science Conference, which was reported in Science magazine by the end of the month (Lunar Sample Analysis Planning Team, 1970). Following the return of the first lunar samples, refinements of theories for the formation of the Moon and its early evolution began. For the most part, the protagonists for the main theories of lunar origin tried to retain with modifications their basic theses. Adjustments definitely had to be made, however. Perhaps one of the most consequential discoveries for the theories of the origin of the Moon was the age determination of rocks that were returned from the Apollo 11 landing site, which was in Mare Tranquillitatis. Using 40K–40Ar radioisotope

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Fig. 4.3 Samples from the first lunar rock box. Photograph courtesy of the Lunar and Planetary Institute

dating, researchers found that the igneous rocks crystallized 3.7 billion years ago, very close to the age that William Hartmann had estimated in 1965 on the basis of crater counts. Ages of rock fragments that were not part of the lavas of Mare Tranquillitatis were determined to be older by 0.5–0.9 billion years. The data indicated that the Moon was at least 4.3 billion years old, and that lava flows in the Tranquillitatis basin occurred well after the Moon was formed. The age determination for the Moon of 4.3 billion years (later to be revised to 4.5 billion years) was certainly older than the then-current estimates of Gerstenkorn, MacDonald, and others, i.e., 1.4–1.78 billion years, which they had based on the current rate of tidal dissipation. To accommodate the new data, however, one simply had to adopt Cameron’s position that the current rate of tidal dissipation was much higher than it had been in the distant past. If that were the case, it would have taken longer for the Moon to recede to its present distance from the Earth. A variable tidal dissipation rate was at least plausible. Following the development of the theory of plate tectonics in the late 1950s and early 1960s, the planetary research community was becoming aware that the configuration of Earth’s oceans had been changing, with oceans opening and closing, on time scales

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of hundreds of millions of years. Presumably, the rate of tidal dissipation would be a function of the configuration of Earth’s oceans and land masses. Chemical and mineralogical analyses of the lunar samples yielded other surprises and constraints on the theories of lunar formation. There were basically two types of rocks found at Tranquillity base: basalts and anorthosites. The basalts came from the lava flow that filled Tranquillity basin, and the anorthosites came from the lunar highlands as ejecta from craters formed there. Lunar anorthosites were not expected as the rocks of the highlands. They are light in color and weight, containing calcium, aluminum, silicon, and oxygen. The density of the lunar anorthositic rock is typically 2.9 g/cm3, compared to the density of lunar basalts, which is typically 3.3 g/cm3 as a solid but about 3.0 g/cm3 as a liquid. The density difference implied that the anorthositic highlands must be floating on a slab of anorthosite 25 km thick to account for the fact that the highlands are about 3 km above the surfaces of the maria (Wood, 1970: 6497; Wood, Dickey, Marvin, & Powell, 1970: 965–988). Free metallic iron was found to be common in lunar igneous rocks. Free metallic iron is extremely rare in earth’s basaltic magmas, where iron is found in an oxidized state. The members of the Lunar Sample Analysis Planning Team (LSAPT) found no evidence of water on the Moon, even in the mineral structure of the rocks, and they concluded that the lunar magmas crystallized in a very dry, reducing environment, with 98% of the crystallization occurring in the temperature range 1060– 1210 °C. For the igneous rocks of Mare Tranquillitatis, the LSAPT members noted that: … Some of the more volatile elements—for example, bismuth, mercury, zinc, cadmium, thallium, lead, germanium, chlorine, and bromine—are significantly depleted with respect to their presumed abundance in the primitive solar system. … (Lunar Sample Analysis Planning Team, 1970: 450)

The LSAPT members further pointed out: Many elements such as potassium, rubidium, cesium, chlorine, and thallium that occur in very low abundance in the lunar igneous rocks are strongly enriched in terrestrial crustal rocks. These terrestrial rocks are the product of igneous differentiation and are either residual liquids or the low melting fraction. On a larger scale such processes have resulted in the strong enrichment of these elements in the crust of the earth. It has been suggested that the low abundance of these elements in rocks derived from residual lunar liquids is an indication that the whole moon is depleted in a number of volatile elements. If this inference about the bulk composition is correct, one can infer that the lunar material separated from a high-temperature dispersed nebula at 1000 °C or higher. (ibid.)

The LSAPT members concluded: The results reported do not resolve the problem of the origin of the moon. However, the number of constraints that must be met by any theory have been greatly increased. For example, if the moon formed from the earth, it can now be stated with some confidence that this separation took place prior to 4.3  109 years ago. Furthermore, such a hypothesis must now take account of certain definite differences in chemical composition. (ibid.: 451)

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John Wood and his colleagues at Harvard added: Anorthosite is a specialized and unusual rock type; it is not likely to be produced at any stage as a primordial condensate from the solar nebula. A crust of anorthosite on the Moon could only have been formed by fractional crystallization, and if the crust is really 25 km thick we have to invoke fractional crystallization on a grand scale. (Wood et al., 1970: 980)

How about the thermal history of the Moon? Was Urey right in his contention that the moon was cold and had been so always? At the conclusion of the First Lunar Science Conference, Robert Jastrow, the Director of the Goddard Institute of Space Studies, made the comment that “old descriptions in terms of a hot or cold moon are an oversimplification.” He and others expressed doubts that the Moon was spun off from the Earth or that it was formed elsewhere and captured by the Earth (Compton, 1989: 215). NASA provided funding to members of the research community who wished to participate in the analyses of lunar samples. The funding was awarded through the LSI and directly from NASA via individual research grants. All awards were made on a competitive basis. The task of allocating the samples was given to a Lunar Sample Review Board, which had its first meeting on 13–14 May 1970 at the LSI. William Rubey, as Director of the LSI, chaired the review board. The LSI soon began to sponsor meetings of scientists to deliberate on issues related to the evidence contained in the lunar samples. The meetings included LSI scientists, as well as scientists from university and research organizations around the United States and the world. One of the first such meetings was on meteorite impact and volcanism, and it brought together about 80 specialists in the area of meteorite impact and volcanic phenomena. The meeting was co-chaired by Friedrich Hörz of the LSI and T. H. Foss of NASA’s Manned Spacecraft Center. In reporting on the meeting, Hörz and Foss noted that: Lunar samples from the Apollo missions display abundant evidence of the important role of meteorite impact in forming the lunar regolith. The deformations observed in these materials seem to be exclusively shock-induced. The vast majority of glasses is shock melted, although a small percentage of lunar glasses may represent primary pyroclastic material of volcanic origin. … (Hörz & Foss, 1971: 101)

Some scientists began to critique the theories of the Moon’s formation based on the evidence for the depletion of volatile elements. For example, Fred Singer, who was an early Visiting Scientist at the LSI, wrote an article for Science magazine with L. W. Bandermann that made the point: … If this depletion is explained in terms of a late accretion of volatile materials from a solar nebula with falling temperature, then the conclusion can be drawn that the moon accumulated not in earth orbit but as a separate planet, and that it was later captured by the earth. (Singer & Bandermann, 1970: 438)

Other scientists disagreed. Ted Ringwood, a Visiting Scientist at the LSI during the summer of 1970, published a paper (Ringwood, 1970a) in November, in which he briefly compared the various theories for the formation of the Moon against the Apollo 11 evidence. Ringwood argued that it was the relatively short age for the

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Moon that has been derived from tidal dissipation theories that had led to Gerstenkorn’s capture hypothesis. The great age of the lunar rocks had now removed the necessity for Gerstenkorn’s theory. He added: Capture of the moon by the earth is an event of low intrinsic probability. The two bodies must possess generally similar orbits and thus have been born in the same region of the solar system, and presumably from the same parental material. The deficiency of iron in the moon, compared to the earth, Venus, and Mars is thus inexplicable on the capture hypothesis … . (ibid.: 6473)

With regard to the co-accretion, or binary-planet, hypothesis, Ringwood noted that: … the binary-planet hypothesis implies that the earth’s mantle and the moon have been formed from the same well-mixed silicate component. Accordingly, basalts formed by partial melting in the earth’s mantle and in the moon should display a generally similar compositional range. The major chemical differences … between lunar and terrestrial basalts and between their respective source regions, particularly in major elements and volatile metals, are not readily explained on the basis of this hypothesis. (ibid.: 6474)

Ringwood noted that the same argument applies to the rotational fission hypothesis. According to this hypothesis, the material now in the moon was derived from the earth’s upper mantle after segregation of the core. Accordingly, it would be anticipated that the chemical composition of the lunar interior would be similar to that of the earth’s upper mantle and that the compositions of lunar and terrestrial basalts formed by partial melting of their respective source regions would also be generally similar. The large compositional differences between lunar and terrestrial basalts and between their respective source regions, particularly of volatile elements and major elements, are not explained by this hypothesis, at least in its earlier forms, which assumed that material from the earth’s outer mantle was ejected in a condensed state (i.e., not gaseous). (ibid.)

Ringwood argued that his precipitation hypothesis for the formation of the Moon fared better under the new Apollo 11 evidence than the other theories. As we have noted above, Ringwood’s precipitation hypothesis for the formation of the Moon was an extension of his theory for the origin of the Earth. He now connected his hypothesis with Öpik’s hypothesis of a ring of material around the Earth from which the Moon aggregated. Öpik has argued that the moon formed by coagulation of a terrestrial “sediment-ring” but did not explain the origin or composition of the latter. A possible explanation arises from an extension of the single-stage hypothesis for the origin of the earth previously advanced by the author. This maintains that during the later stages of accretion of the earth, a massive primitive atmosphere developed which was hot enough to selectively evaporate a substantial portion of the silicates which were accreting upon the earth. Subsequently, the atmosphere was driven away by particle radiation from the sun as it passed through a T-Tauri phase. The relatively non-volatile silicate components were precipitated close to the earth to form a swarm of planetesimals or moonlets, as the atmosphere was dissipated, and the moon accreted from these chemically fractionated planetesimals. The more volatile components of the terrestrial atmosphere were precipitated at lower temperatures, further

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from the earth, as fine smoke particles and were lost from the earth-moon system with the escaping gases. The hypothesis appears capable of explaining the low density of the moon, the inferred fractionation of relatively volatile elements between the earth and moon, and the different oxidation states of the terrestrial and lunar mantles. The above “precipitation” hypothesis thus implies a close genetic relationship between the earth and moon. The precipitation and fission hypotheses are also closely connected since the material now in the moon is regarded as having been derived ultimately from the earth —not the solid mantle, but from the massive primitive terrestrial atmosphere. (Ringwood, 1970b: 131)

A. G. W. Cameron quickly added his support for Ringwood’s precipitation hypothesis. Cameron’s theory of the evolution of planets from a primordial solar nebula featured high-temperature planets. It may thus be seen that the accumulation of the earth is likely to produce initial temperatures in the planet of the order of 104 K. Such a primitive planet will in no respect resemble the planetary body on which we live today; the high temperatures would result in the formation of a planet having a radius of 5–10 times the present radius of the earth, with the interior consisting of high temperature gases: gaseous iron and gaseous magnesium silicates together with their high temperature decomposition products, in the main. … … This concept of a high temperature planet with an extended gaseous silicate atmosphere is precisely the structure required by Ringwood in his suggested origin of the moon. … If the present angular momentum of the earth-moon system were concentrated in such an extended planet, then the planet would be rotationally unstable beyond about three earth radii from the center in the equatorial plane. This happens to be approximately the Roche limit for the earth, so that solid bodies can accumulate and stick together against the effects of the disrupting tidal forces of the earth at distances beyond three earth radii. Therefore it makes a great deal of sense to consider that the high temperature initial earth was in fact formed as a result of a collision between two bodies of comparable mass, which produced the very large extended object under discussion with a sufficiently rapid spin to have the angular momentum of the current earth-moon system. The outer part of the silicate envelope in the equatorial plane of the planet would then be rotationally unstable, and it would flatten down into a short disk in the equatorial plane. The moon will rather easily condense from such a disk. As the disk cools, solid condensations will occur within it, and these are capable of collecting together to form the moon. However, it is entirely possible that the dynamics of the condensation may favor the growth of the moon during the phase of strong interaction between the gases and their solid condensates, leading to a very rapid accumulation time indeed. If the moon accumulates in a time of about a century, then interior temperatures in the moon are likely to be in the vicinity of 1500 K, so that the interior would be extensively molten. In the high temperature gaseous planet under consideration, iron would condense at a higher temperature than magnesium silicates. Consequently, it is expected that liquid droplets of iron would rain out through the large gaseous atmosphere and tend to collect toward the center of gravity of the combined body, even if they had not already collected toward the centers of the fairly large bodies that collided to form the earth. In this way it is a fairly straightforward matter to understand why the moon should have a low content of iron. (Cameron, 1970: 631)

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As early as 1970, then, Cameron had considered the implications of the collision of two planetary bodies to form the Earth and the Moon. Cameron did not reference the 1946 paper of Reginald Daly, who had put forward a similar idea. Just prior to and following the return of the Apollo 11 samples, the principal advocates for the rotational fission hypothesis were John A. O’Keefe (1916–2000) of NASA’s Goddard Space Flight Center and Donald U. Wise, who at the time was on leave from Franklin and Marshall College in Lancaster, Pennsylvania, serving as Chief Scientist and Deputy Director of the Apollo Lunar Exploration Office at NASA Headquarters. O’Keefe and Wise had recently (separately) modified their theories to incorporate the idea that the hot proto-Moon would be enveloped by a large gaseous cloud, most of which would have escaped the Earth-Moon system (O’Keefe, 1969; Wise, 1969). The theories of these authors needed this escaping gas to carry off angular momentum, so as to explain the fact that the current angular momentum of the Earth-Moon system is not large enough to account for the initial rapid rotation of the Earth that would be required by the rotational fission hypothesis. They were building on Ringwood’s earlier theories for the thermal history of the Earth, which he calculated must have been initially molten owing to the heat liberated by infalling mass during the accretion process (Ringwood, 1960). Writing just at the time when the first analyses of Apollo 11 samples were being made public, Wise added the following note in proof to his 1969 article. Roasting of a newly fissioned moon adjacent to a tidally heated incandescent earth, may account for a lunar magmatic source depleted in volatile alkali elements and enriched in refractory elements as suggested by first analyses of Apollo 11 specimens. (Wise, 1969: 6044)

Following the initial investigations of the Apollo 11 samples, O’Keefe quickly wrote a paper to demonstrate that: … the Apollo 11 data support the idea that the moon was formed by the breakup of the earth, and that they suggest that after the breakup, the moon went through a heating episode that boiled away most of its mass. … … From the cosmological standpoint, the most interesting results from the Apollo 11 samples were the chemical measurements. To appreciate these, it is necessary to have some idea as to what the chemical measurements should have shown. In other words, if the moon had formed from the primeval materials of the system, what would a chemical analysis have been like? The answer to this question is called the cosmic abundance scheme. In this scheme, hydrogen forms 90% of the material, helium 10%, and the other elements, including the nonvolatile elements, constitute less than 1%. The element abundances have a fixed ratio to one another; that is, there is a definite answer to the question: What was the initial ratio of silicon to iron? A systematic listing of these ratios was first done by Henry Norris Russell in the 1930’s. The most recent and useful table is by Cameron (1968). (O’Keefe, 1970: 633)

O’Keefe then discussed the relative abundances of elements as they were found in the Apollo 11 samples. This tutorial on the characteristics of various classes of

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Fig. 4.4 Periodic table of the elements. The image of the periodic table was obtained at a public domain website titled wpclipart http://www.wpclipart.com/science/atoms_molecules/periodic_ tables/periodic_table_of_elements_BW.png.html

elements (See Fig. 4.4) is an important primer for understanding much of the subsequent analyses of lunar samples.2 In examining a lunar sample we do not really expect to find the gases still present. No object that you could hold in your hand at ordinary temperatures could consist mostly of hydrogen and helium. In any solid object that you could hold in your hand, we would have lost nearly all of the elements that would ordinarily be in the gaseous form—the so called atmophile elements. These include hydrogen, nitrogen, and the rare gases. When we study the lunar samples we find that these elements are indeed missing. Their abundances are in fact even lower than in the earth. A second important class of elements covers a large part of the righthand side of the periodic table. These elements are often associated with odors; they include things like sulfur, phosphorus, iodine, chlorine. At the present time, it is considered that the most important fact about these elements in governing their behavior is the fact that they or their oxides are volatile. The analyses of the lunar samples show that these elements are deficient in the moon to an extent that is greater than their deficiencies on the earth. The indication is that the moon has been through a period of severe heating. On the left side of the periodic table is a group of elements that are called the lithophile elements. These elements normally go into the formation of rocks; that is what their name means. They are ordinarily present in rocks in the form of their oxides, and for this reason they are sometimes called oxyphile elements. In the lunar sample most of the lithophile elements are present at a level above their cosmic abundance in somewhat the same

2 The extensive quotation of John O’Keefe is reproduced courtesy of the American Geophysical Union. The full article can be found at the Wiley Online Library http://onlinelibrary.wiley.com/ doi/10.1029/EO051i009p00633/pdf.

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4 Exploring the Moon—The Apollo Investigations proportions as on the earth. There are two interesting differences: first, the alkali elements, which are more volatile than the average, are somewhat below their terrestrial abundances although they are above their cosmic abundances; second, a group of very refractory elements including titanium, zirconium, and hafnium are present in abundances considerably higher than their cosmic abundances or their abundances in the earth. The meaning of these two peculiarities of the lithophile elements is the same as that which we have mentioned previously. The alkalies are missing because there has been strong heating; the refractories are enhanced because, apparently, the heating has been so intense as to cause the loss of a major fraction of the original mass of the moon and so to bring about a concentration of the elements that are hardest to get rid of. In the middle of the periodical table is the most interesting group of elements called the siderophile elements. Some of these, like gold and osmium, are relatively volatile; others, such as platinum or iridium, are very refractory. All have the character that their energy of oxidation, per oxygen atom, is less than that of iron. As a result oxides of nickel, for example, will not exist in the presence of molten free iron. The iron will rob the nickel of its oxygen. In many mixes of the nonvolatile constituents of the world, it is found that iron is partly oxidized and partly in the form of the free metal. … Examples are … a chondritic meteorite with silicate and iron phases [and] the earth with its mantle and core. In all such cases it is found that the siderophile elements are strongly concentrated in the metal phase. Evidently, they will always be metal because they cannot hold the oxygen; and it develops that the free metal dissolves better in metallic iron than in … silicate liquid. It is believed to be for this reason that the siderophile elements, which are not particularly rare in the universe as a whole, are quite rare in the crust of the earth. Nickel, for example, constitutes about 1% of the mass of a typical chondritic meteorite, but only a few parts per million of the abundance in the crust. Some of this is due to the crust-mantle differentiation; but a large part is also apparently owing to the concentration of the siderophiles in the core of the earth. With these considerations in mind, it is extremely interesting to see that the siderophile elements are depleted in the Apollo lunar samples by an amount that is, in many elements, comparable with the depletion in the earth. There is a strong suggestion that the material of the moon has likewise been through a process of leaching by contact with metallic iron. It is difficult to explain the pattern of siderophile deficiency which includes some quite refractory elements in any other way. (O’Keefe, 1970: 633–634)

O’Keefe then asks “Where did the moon’s nickel go?” He argues that the Apollo sample evidence implies that the nickel cannot be at the depth of the Moon from which the mare lava of the Tranquility base site originated. From other evidence, he argues: … It is essentially certain that the moon’s nickel is not concentrated, like the earth’s, in an iron-nickel core. In fact there can be at most a very small amount of free iron (whose density is about 7.8) in the moon. It has been found that the mean density of the moon is approximately 3.34 g cm−3; this is almost the density of the earth’s mantle. This density is actually slightly less that the density of the crustal rocks at the Apollo 11 site. Any significant mixture of metallic iron would give the moon a higher over-all density than is observed. The logical conclusion is then that the moon’s nickel is in the earth’s core and that the moon formed by fission of the earth after the core-mantle separation had taken place. (ibid.: 634–635)

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O’Keefe recognized the remaining drawback of the rotational fission theory, namely that the conservation of angular momentum requires that the current total angular momentum of the Earth-Moon system must be the same as the angular momentum of the proto-Earth-Moon prior to fission, and that amount of angular momentum is not enough to cause the fission. As a solution, O’Keefe suggested that there was more angular momentum in the proto-Earth-Moon than exists in the current Earth-Moon system, and specifically enough to cause fission, but that: … after the separation of the earth and moon, the strong tidal interaction between the two bodies heated both, but especially the moon, and caused the loss of large amounts of mass and angular momentum from the moon. … If we suppose that over half of the moon’s mass was boiled away at this point, then we explain simultaneously the deficiency of volatile elements in the moon, the enhancement of the refractory elements, the loss of angular momentum, and a great loss of mass which Lyttleton (1953)3 has shown is required if we are to understand the formation of the moon by fission. Theories of this kind in which the fission of the earth is followed by an episode of very strong heating in the moon were produced by Wise (1969) and O’Keefe (1969) before the Apollo 11 results became available. Thus although the results have been presented here as leading toward a theory of the origin of the moon, the historical sequence is that the facts appeared afterward as a verification for theories already in existence. It is natural to feel somewhat encouraged by this circumstance. Perhaps there is some reality in these theories after all. (O’Keefe, 1970: 634–635)

Though not necessarily focused on the problem of the formation of the Moon, other researchers were concluding from the Apollo 11 results that the early Moon had been extensively heated. John A. Wood and his colleagues at the Smithsonian Astrophysical Observatory, and Joseph V. Smith (1928–2007) and his colleagues from the University of Chicago, gave presentations at the Apollo 11 Lunar Science Conference that featured a hot early Moon. Wood argued that there must have been a substantial partial melting of the early Moon. The light anorthosite would have floated to the top of the melt to form a lunar crust of the order of 10 km thick. This crust would have solidified relatively quickly, within 0.5 billion years after the melting. The younger basalts must have been produced by subsequent melting deep below the surface of the Moon owing to internal radioactive heating. Liquid lava is about 10% less dense than solid basalt, so the basaltic lava would have risen to the surface of the Moon and filled preexisting basins that had been created by giant impacts.

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Lyttleton’s theories: In his 1953 book, the Stability of Rotating Liquid Masses, a British astronomer and mathematician, Raymond Arthur Lyttleton, first showed that rotational fission into two bodies would result in the two bodies moving infinitely distant from each other, i.e., they could not form a closed system such as the Earth-Moon system. Lyttleton argued, however, that following the loss of one part of the fissioned bodies, there might remain the main body with small satellites.

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In Wood’s model, the initial lavas that filled the basins cooled and solidified, and the crust adjusted vertically until the pressure beneath these basins was the same as the pressure under the lunar crust generally. Geologists refer to this adjustment as isostatic compensation. Later, as more basaltic lava melted at great depth below the lunar surface, owing to continued radioactive heating, an additional “topping” of lava made its way through cracks in the basin structures and overfilled the existing mare. If at least some of this lava came from beneath the anorthositic highlands, an excess mass, relative to the highlands, would have been created. Wood felt that the underlying solidified basalt would be strong enough to support the later topping of basalt in the basins, and he identified this topping of lava as the source of the mascons that had been found through analysis of the Lunar Orbiter data (Wood, 1970: 6497 ff). Joseph Smith came to a similar conclusion. In his biographical memoir of Joseph Smith, Peter Wyllie described the reaction to Smith’s paper at the Apollo 11 conference. … Apollo 11 in July 1969 returned the first lunar rock samples to Earth. The results were presented … at the first Lunar and Planetary Science Conference. Joe reported the first new mineral on the Moon, pyroxferroite, and presented a model for a hot Moon which contradicted the dominant, standard model that the Moon had remained cold during its formation. The hot Moon of Joe Smith and his Chicago team … were given a somewhat frigid reception. Joe presented a model relating the mineralogy and petrology of the rocks to the differentiation of a very large body of magma, with feldspars rising to form the light-colored highlands. He credited John Wood for a related interpretation of floating feldspars. Joe maintained that the Moon’s surface must have been extensively melted in a series of catastrophic meteorite impacts. At a press conference afterwards, Harold Urey, the eminent former Chicago faculty member, stated that he couldn’t imagine how someone from his Alma Mater dared to propose such a ridiculous scheme. The moon had accreted as a cold body, and the presence of lavas demonstrated only that some local events had temporarily caused minor heating. A decade later, magma oceans on both moon and Earth appeared to be accepted even by those who wrote in 1971: “this is not possible,” that it was “entirely lacking in supporting evidence” and “encounters a fatal difficulty.” … (Wyllie, 2010: 11–12)

A little more than a decade later, G. Jeffrey Taylor, then at the University of New Mexico, would write in a paper for a workshop at the Lunar and Planetary Institute that: The concept that the outer portions of the moon were molten very early in its history was originally developed by Wood et al. and Smith et al. to explain the apparent overabundance of plagioclase in the lunar highlands. These authors postulated that as the extensive magma system crystallized, olivine and pyroxene sank and plagioclase floated to form the anorthositic crust. The idea is a testament to scientific imagination because it was sparked by the presence of a small percentage of millimeter-sized anorthositic rock fragments in the soil returned by Apollo 11 from a mare region of the moon. … the concept has really taken hold in lunar science and has begun to be applied to other planets, including the earth … . (Taylor, 1982: 147–148)

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Apollo 12

The second exploration of the Moon began on 19 November 1969, when the Apollo 12 Lunar Lander reached the Moon. The surface explorers were Charles “Pete” Conrad (1930–1999) and Alan Bean (1932–2018), and the astronaut remaining in lunar orbit was Richard Gordon (1929–2017). The landing site was south southwest of the crater Copernicus in Oceanus Procellarum (Ocean of Storms). The site was west of the Apollo 11 landing site, in another lava-filled impact basin (Fig. 4.5). The two sites now gave the possibility to test the ideas of Wood and Smith about global magma oceans. In 1970, Hiroshi Wakita and Roman A. Schmitt of Oregon State University published a paper in Science magazine that would bolster the global magma hypothesis (Wakita & Schmitt, 1970). The first figure of their paper (Fig. 4.6) demonstrates that the soil taken from Apollo 11 and Apollo 12 samples is depleted in the rare-earth element europium (Eu), whereas lunar anorthosite samples, representative of the lunar highlands, are enriched in Eu. This phenomenon came to be known as the “complementary Eu anomaly” and was explained by the fact that Eu can substitute for calcium in plagioclase feldspar formation. The lighter lunar highland feldspars floated to the top in the global magma ocean and carried with them much of the available Eu. When the basalts later melted at depth and made their way through fissures to the lunar surface, they were correspondingly deficient in Eu. The Second Lunar Science Conference was held in Houston on 11–14 January 1971. The LSI organized the collection of papers and sponsored the publication of

Fig. 4.5 Apollo 11 and 12 Landing Sites. Photograph courtesy of the Lunar and Planetary Institute

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Fig. 4.6 Ratios of Rare-Earth Elements (REE) in Apollo 11 and Apollo 12 lunar soil (top) and in lunar anorthosites relative to REE abundances in chondrite meteorites. From Wakita, H., & Schmitt, R. A. (1970) “Lunar anorthosites: Rare-earth and other elemental abundances.” Science, volume 170, issue 3961, page 971; reprinted with permission from AAAS

the proceedings of the conference that appeared as a three-volume set as a supplement to Geochimica et Cosmochimica Acta, which is the journal of the Geochemical Society and the Meteoritical Society. Data from both the Apollo 11 and Apollo 12 missions were discussed at the conference. Many of the talks were devoted to characterizing the soil and rocks found at Tranquility Base and at the landing site for Apollo 12 in Oceanus Procellarum and comparing the data from the two sites. The igneous rocks taken from the Apollo 12 site were about a billion years younger than those found at the Apollo 11 landing site, so lava flows in the

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two basins occurred at significantly different times. The lower concentrations of volatile elements and higher concentrations of refractory elements were characteristic of both Apollo 11 and Apollo 12 sites. At least some siderophiles were depleted in the Apollo 12 samples, as well. The Lunar Sample Preliminary Examination team noted that “The chemistry of the Apollo 12 samples is not identical with any known meteorite, nickel in particular being strikingly depleted. …” (Lunar Sample Preliminary Examination Team, 1970: 1338). In a talk given at the Second Lunar Science Conference, Urey and his colleagues at the University of California at San Diego conceded that the new data “make it necessary to postulate quite an involved early lunar history.” (Urey, Marti, Hawkins, & Liu, 1971: 987). He continued to oppose the fission and “precipitation” hypotheses, however, arguing: The seismic and electrical properties and the presence of … excess masses [mascons] taken together indicate that the moon is more rigid that the earth. It has been relatively cool during the time that these excess masses have existed, and is relatively cool as compared to the earth now … . A slow accumulation of the moon from small solid objects or an accumulation in a lunar gas sphere could supply these conditions. An escape from the earth during the formation of the earth’s core or an accumulation from a vapor cloud in orbits near the earth …, however, would produce a very hot moon. (ibid.: 988)

In Urey’s somewhat revised model, the Moon still accumulated in a solid state at low temperature. Subsequently, there was surface heating from external sources, e.g., collisions, that melted a layer some 200 km deep. During the cooling and solidification of this layer, lighter material such as anorthosite rose to the top, and heaver material such as metallic iron-nickel sank to the bottom, so that there was a differentiation in the 200-km surface layer. There was subsequently a period of intense bombardment that produced the heavily cratered highlands regions of the Moon, with a few large objects in the bombardment that produced the large circular basins and, at the same time, deposited a layer of insulating dust over parts of the surface. The heat-insulating dust layer allowed the radioactive rocks to heat up and melt rock material about 4 billion years ago. This melted material rose through cracks in the anorthositic rocks to form lava layers in the large basins. Continued radiogenic heating beneath the insulating layer of dust and debris eventually caused additional heating in lower sub-layers of the 200 km surface layer, generating additional lava layers in the basins. The Urey model was similar to the Wood model, but Urey was trying to retain the idea of a cool, rigid Moon, which he thought necessary for the support of the mascons, allowing only surface and near-surface heating of the Moon, rather than the deeper heating envisaged by Wood (ibid.: 987–998). John Wood and others argued that a warmer internal Moon than postulated by Urey might be able to hold up the mascons, since “water-free rock, such as we now know the moon is composed of, is probably much more resistant to creep than are terrestrial rocks” (Wood, 1970: 6512).

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In a book chapter published in 1971, Urey and MacDonald abandoned their former hypothesis that the Earth and Moon had been close together some 1.7 billion years ago. The effects of tidal friction on the orbital elements … of the Moon’s orbit are well known, and numerous extensive calculations of these qualities have been carried out … . One feature of all these calculations is that if the present tidal lag were appropriate for all time, then the Earth and Moon would have been close together only 1.7 aeons ago. This led MacDonald (1964) to suggest that the present Earth-Moon system is of relatively recent origin and that prior to this time the Earth had several smaller moons. However, the existence of the great number of craters on the Moon’s surface and the absence of geologic evidence for abundant impact in ancient rocks makes this hypothesis unlikely. Thus, the rate of tidal dissipation may have changed greatly over geologic time, and this complication has not been included in the calculations on the dynamics of the Earth and Moon. (Urey & MacDonald, 1971: 267–268)

4.2.3

Apollo 13

The explosion on Apollo 13 prevented any lunar exploration for that mission. It was an amazing feat just to get the astronauts of Apollo 13, James Lovell, John “Jack” Swigert (1931–1982), and Fred Haise, safely back to Earth.

4.2.4

Apollo 14

Apollo 14 was launched on 31 January 1971, and its Lunar Lander touched down on the Moon’s surface on 5 February (Fig. 4.7). Alan Shepard (1923–1998) was the command pilot, and Edgar Mitchell (1930–2016) was the lunar module pilot. The third astronaut for the Apollo 14 mission was Stuart Roosa (1933–1994), who remained in the Command Module while the other two explored the Fra Mauro formation, which was part of the ejecta blanket thrown out by the impact that formed the Imbrium basin. The idea was to sample rocks that had formed prior to the later lava flows that had filled the basins. As expected, very few of the collected rocks were basaltic. As with the Apollo 11 and 12 samples, there was no evidence of water in the minerals of the rocks, and the presence of metallic iron strengthened the idea that lunar rocks were formed in an environment of low oxygen activity. The Apollo 14 samples contained a chemical composition called KREEP, which stood for potassium (chemical symbol K), rare-earth elements, and phosphorus. These are elements that are said to be “incompatible,” i.e., they would not readily be combined in a single mineral. A few KREEP samples had been found at the Apollo 12 site. The Lunar Sample Preliminary Examination Team expressed the view that this KREEP material could be widespread on the surface of the Moon, and

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Fig. 4.7 Apollo 11, 12, and 14 Landing Sites. Photograph courtesy of the Lunar and Planetary Institute

… Since this material is even further removed in composition from average solar or chondritic abundances than the mare basalts, a history of profound pre-mare lunar differentiation which produced a lunar crust is indicated. This differentiation could have occurred during the accretion of the moon with the accumulation of material rich in incompatible elements near the surface, or in the process of crust formation involving the fractionation of a considerable volume of the moon early in lunar history. The basaltic rocks from the mare regions cannot be derived by partial melting of Fra Mauro composition, and the reverse is also true. Therefore, it would appear that neither the mare nor the nonmare areas of the moon are representative of the bulk composition of the moon, but both mare and nonmare areas may represent partial melting products of the lunar interior. The extent and variability of the rocks formed in the early lunar crust cannot be determined without samples from other uplands areas; there is no compelling reason yet to assume that Fra Mauro-like material is representative of the lunar highlands. (Lunar Sample Preliminary Examination Team, 1971: 693)

4.2.5

Apollo 15

The Apollo 15 modules were launched on 26 July 1971, and the Lunar Lander touched down on 30 July (Fig. 4.8). The lunar surface explorers were David Scott and James Irwin (1930–1991), and the astronaut in the orbiting Command Module was Alfred Worden. This was the first Apollo mission to carry a lunar rover. The site was selected because it was near a mountain range (the Apennine Front) that was formed from the ejecta of the large impact that created the Imbrium basin. It was also near a valley (the Hadley Rille) that was some 300 m deep. The layers of lava flows

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Fig. 4.8 Apollo 11, 12, 14, and 15 Landing Sites. Photograph courtesy of the Lunar and Planetary Institute

observed in the wall of Hadley Rille confirmed previous evidence that the filling of mare basins was accomplished by a series of lava flows. The analysis of the lunar samples was intriguing to William Rubey, but he had agreed to commit only a few years to start up the LSI while on leave from UCLA. In the summer of 1971, Rubey resigned as director of the LSI and returned to his home university. The second director of the institute was Joseph W. Chamberlain (1928–2004), a planetary astronomer who had previously been on the staff of the Kitt Peak National Observatory in Tucson (Fig. 4.9). An important aspect of the mission was the continuation of the deployment of surface science packages, including seismometers, which would allow probing of the interior structure of the Moon, as well as thermal probe instruments that allowed measurements of surface heat flow. The Apollo 15 seismometers were now the third in a network of working units on the Moon, which was essential for the location of the source of seismic events. These measurements from the surface packages provided additional constraints on thermal models for the Moon and, as a corollary, on theories for the formation of the Moon. A team led by Gary V. Latham of the Lamont-Doherty Geological Observatory of Columbia University reported at a conference titled Lunar Geophysics, which was organized and sponsored by the LSI and held on 18–21 October 1971, that: Lunar seismic signals differ greatly from the typical terrestrial seismic signals. It now appears that this can be explained almost entirely by the presence of a thin dry, heterogeneous layer which blankets the Moon to a probable depth of a few km with a maximum possible depth of about 20 km. Seismic waves are highly scattered in this zone. Seismic wave propagation within the lunar interior, below the scattering zone, is highly efficient. …

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Fig. 4.9 Photograph of Joseph W. Chamberlain, courtesy of the Lunar and Planetary Institute

The Moon possesses a crust and a mantle, at least in the region of the Apollo 12 and 14 stations. The thickness of the crust is between 55 and 70 km and may consist of two layers. The contrast in elastic properties of the rocks which comprise these major structural units is at least as great as that which exists between the crust and mantle of the earth. Natural lunar events detected by the Apollo seismic network are moonquakes and meteoroid impacts. The average rate of release of seismic energy from moonquakes is far below that of the Earth. Although present data do not permit a completely unambiguous interpretation, the best solution obtainable places the most active moonquake focus at a depth of 800 km; slightly deeper that any known earthquake. These moonquakes occur in monthly cycles; triggered by lunar tides … … … The occurrence of moonquakes at great depths implies that (1) the lunar interior at these depths is rigid enough to support appreciable stress, and (2) that maximum stress differences occur at these depths. If the strain released as seismic energy is of thermal origin, this places strong constraints on acceptable thermal models for the deep lunar interior … The average rate of seismic energy release within the Moon is far below that of the earth. Thus, internal convection currents leading to significant lunar tectonism appear to be absent. Presently, the outer shell of the Moon appears to be relatively cold, rigid, and tectonically stable compared to the Earth. … (Latham et al., 1971: 3, 11, and 12)

Seismic evidence for the rigidity of the Moon at great depths would not have surprised Harold Urey. He presented a paper at the Lunar Geophysics Conference

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in which he continued to advance his thesis that the early solar system contained many lunar-sized objects, each of which had formed from the gravitational attraction of gas spheres. If the Moon was captured as many scientists think possible or even probable, it seems necessary to postulate the existence of many Moons of which one by some very improbable mechanism was captured by the Earth. Because of the triaxial ellipsoidal shape of the Moon as indicated by the differences in the three moments of inertia, it was suggested some twenty years ago that the Moon was very rigid, consistent with a cool origin when it was supposed to have acquired these moments of inertia, and that it had not become sufficiently hot to acquire an equilibrium shape since then. Other explanations were offered—convection currents, cold polar regions, etc. However, the presence of mass concentrations beneath the circular collisional maria definitely decided the question. The Moon must have had a cool origin and has retained a sufficiently cold rigid exterior to support these massive areas … . (Urey, 1972: 625–626)

Harold Urey did not give ground easily. A “cold” Moon turned out to be a relative concept. Urey acknowledged that the data now seemed unambiguous that the Moon’s surface had been far from cold in the past. The surface of the Moon has been melted to a considerable depth, possibly to some 200 km. This liquid had crystallized slowly producing a layer of anorthosite which crystallized and floated on the molten silicates like ice on water. … (ibid.: 626)

Urey’s model for the Moon featured various layers beneath the anorthosite layer. This included an iron-nickel layer that had sunk “… through the molten silicate and removed the siderophile elements from the lunar surface.” He also thought that the volatiles, including water, were absent from the Moon’s surface but should “… be present in solar proportions in the lunar interior.” (ibid.). He concluded: This model for the origin of the Moon can be correct only if the strong depletion of the volatile elements, as observed in the lunar surface materials, is not true for the lunar interior. It is only reasonable to assume that solids settling in a gas sphere at low temperatures should carry with them all elements except the obvious gaseous ones in solar proportions. Also the low density of the Moon can be explained, using this model, only by the presence of low density materials such as water and carbonaceous compounds. (ibid.: 628–629)

An initial heat-flow measurement at the Apollo 15 site was 33 ergs/cm2sec. (Langseth, Clark, Chute, Keihm, & Wechsler, 1972 : 299). At the Lunar Geophysics Conference, a group from MIT compared various models of the Moon with the emerging geological and physical data, including the heat-flow and seismic data, and arrived at conclusions considerably at variance with the views of Urey. In particular, they concluded that: … An initially cold Moon … would neither melt nor differentiate to account for the lunar crustal rocks and basalts with any plausible radioactive heating source abundances. Thus it is not an acceptable model. … … … Moon models based on rapid accretion satisfy the requirements for early and extended periods of magma generation. Thermal evolution for such models follows a characteristic pattern, where, with increasing time, the outer part of the Moon cools while the inner

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portion heats up. Melting progresses downward. At 4.6 b.y., most models have a partially molten core of the same composition as the initial primordial, lunar material. … Our computations indicate that a Moon model that is initially molten and differentiated to a depth of 600–800 km [i.e., the outer half of the Moon] satisfies the geologic, geophysical and geochronologic constraints for the formation of a lunar crust and mare basalts. During the first 2 b.y. of lunar history, the top of the partially molten zone remains within 300 km of the lunar surface. At the present time, these models give an average surface heat flow of about 12 to 16 erg/cm2sec … which is lower than the measured value at the Apollo 15 site … . The Apollo 15 heat flow can be matched with models having bulk radioactivity about twice the value used [in their calculations] with all other parameters the same. … (Toksöz, Solomon, Minear, & Johnston, 1972: 349)

(The heat flow measurement made at the Apollo 15 site was revised downward in 1976 to a value of 21 ergs/cm2 sec, which was closer to the models of the MIT group) (Langseth et al., 1976: 3143). The MIT group also noted that most lunar models point to a transition zone between 600–1000 km depth, and that this depth is in rough agreement with the depth of the source of the largest and most persistent moonquakes at 800 km (Toksöz et al., 1972: 350). The Third Lunar Science Conference was held on 10–13 January 1972. It was organized by the LSI and co-sponsored by the LSI and NASA. Included among the topics at this conference were reports on the analyses of lunar samples from the Apollo 14 and Apollo 15 missions and comparisons with the characteristics of samples from the earlier Apollo missions, as well as with the samples returned by Luna 16 of the Soviet Union. The range of ages of the rocks collected during the various missions extended from about 3.15 billion years to 4.1 billion years. In a report on the conference, The Lunar Sample Analysis Planning Team gave what was becoming the consensus view. It is clear by now that the moon was extensively differentiated by igneous processes early in its history …The history of magma generation in the moon appears to require that melting first occurred near the surface; then, with time and the accumulation of heat from internal radioactivity, the zone of high temperatures and the partial melting moved to progressively greater depths, affecting layers with dissimilar chemical compositions. (Lunar Sample Analysis Planning Team, 1972: 978)

Igneous layering was observed by the Apollo 15 astronauts at the Hadley-Apennine site (Fig. 4.10). A sequence of flat-lying lava flow units, typically 10 meters thick, outcrop along the wall of Hadley Rille. Prominent layering was also observed on slopes of the mountains surrounding the landing site. The layers are plane and parallel but tilted … they seem to be lava flow bedding in a pre-Imbrium crust that was shattered and heaved up in tilted blocks by the Imbrium impact. (ibid.: 980)

The history of the Moon following its creation was becoming clearer because of the Apollo explorations and the subsequent analyses of lunar samples. The Moon’s great age, comparable to the age of the Earth, was established. The various peculiar elemental abundances suggested a hot beginning, which, however, was a feature of the main theories for the formation of the Moon that were being considered, i.e., fission, precipitation, and capture.

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Fig. 4.10 Photograph of the wall of Hadley Rille, showing layering of lava flow units. The photograph is labeled AS15-89-12157 and is courtesy of NASA

Ernst Öpik, who by then was on the staff of the Armagh Observatory (now the Armagh Observatory and Planetarium) in Ireland and the editor of the Irish Astronomical Journal, began to discuss a possible “capture” history of the Moon in broader terms than had been envisaged by Gerstenkorn. Öpik had critiqued Gerstenkorn’s capture theory for the formation of the Moon soon after it appeared in 1955, because Gerstenkorn had found that the distance of closest approach to the Earth of the proto-Moon was 2.89 RE, which is almost exactly Roche’s limit. Öpik concluded in 1955 that the closeness of the moon’s minimal distance to Roche’s limit was likely not a coincidence. He continued to study the possibility and probability of an encounter of the Earth with a proto-moon that would break apart and then re-aggregate to form the Moon in orbit around the Earth. One possibility that he envisaged in 1972 was: … a near-grazing collision, the two tidally extended bulges of proto-moon and earth colliding, leaving part of the incoming mass to proceed with diminished momentum, ultimately to make the moon. In the turmoil, anything could have happened, and the ultimately separated lunar mass may have been only a fraction of that of the incoming original body. Actually, for a primitive moon independently formed somewhere near the earth’s orbit, … the ultimate fate is either a break-up and capture of about 50% of the mass or a collision. …, with the probability of a non-central grazing collision, say at more than one-half the sum of the two radii, being several times greater than of a central one. … Thus the chances are perhaps 4 to 1 for such a body to end its existence in a near-grazing collision. … (Öpik, 1972: 197)

In another of Öpik’s scenarios, the proto-moon was in an orbit around the Sun that was very similar to the Earth’s orbit. It made a close approach to the Earth, coming inside the Roche’s radius, and broke apart. The material of the inner half of

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the proto-moon would have been captured by the Earth in initially highly eccentric orbits, whereas the material that formed the outer half of the proto-moon would have been thrown out into interplanetary space but later aggregated, in the main, to the Earth. The captured material would quickly form a circular ring about the Earth, owing to collisions among the many pieces, and eventually coalesce into one body —the Moon. In a more general analysis of the possibility that the Earth captured the Moon, Öpik concluded that: … if the moon came from outside, its attachment to earth must have involved collision and/ or breakup. … … When no ad hoc collisions with other pre-existing satellites are assumed, it becomes obvious that capture into a bound orbit outside Roche’s limit is not feasible. … … From the preceding, it appears that, in the two-body encounter process (in the distant presence of the sun), capture can only be simultaneous with break-up. The other alternative, additional to break-up, is grazing or full collision. (ibid.: 224)

Öpik finally concluded that: the improbability of capture is only apparent. When the fact of inevitable breakup of the proto-moon is admitted, capture becomes a highly probably event, in 50-50 competition with the other equally probable choice—a collision with the earth itself. (ibid.: 237)

First Reginald Daly, then Alastair Cameron, and now Ernst Öpik suggested the possibility that another planetary body might have collided with the Earth to form the Earth-Moon system.

4.2.6

Apollo 16

Apollo 16 was launched on 16 April 1972, and the Lunar Lander touched down on a lunar highland region just north of the crater Dolland in the so-called Descartes formation on 21 April (Fig. 4.11). John Young (1930–2018) and Charles Duke used a lunar rover to enhance their surface exploration, while Ken Mattingly orbited the Moon in the Command Module. This was the first Apollo mission to explore the lunar highlands. A subsequent report by Noel Hinners (1935–2014), who was then the NASA director for lunar programs, gives an indication of the care that was taken in the selection of landing sites. The Apollo 16 landing site, Descartes, was selected after the Apollo 11, 12, and 14 missions, but before the Apollo 15 mission to Hadley-Apennine. The Apollo 11 and 12 flights had returned material which conclusively demonstrated that the mare fill is dominantly basalt of lava-flow origin and that the maria are actually very old, although they appear young. The isotopic age information, when used in conjunction with data on crater

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Fig. 4.11 Apollo 11, 12, 14, 15, and 16 Landing Sites. Photograph courtesy of the Lunar and Planetary Institute

densities and morphologies on many mare surfaces, suggested that mare lava generation might have been limited to the period between roughly 3 and 3.7 billion years ago. The Apollo 14 mission established that the Fra Mauro Formation is ejecta from the Imbrium Basin and that the Imbrium impact, one of the youngest basin-forming events, occurred 3.9 billion years ago. Model ages of lunar soils from all sites indicate that the Moon originated about 4.5 billion years ago. The composition of Apollo 12 putative Copernicus ray material and of Fra Mauro samples indicated extensive premare igneous differentiation that created high-alumina basalts of relatively high radioactivity. Additionally, exotic fragments at all sites indicated that large regions of the highlands might be anorthositic. The foregoing factors led to a consensus that the prime objective of both Apollo 16 and 17 should be direct sampling of highlands material that would be compositionally different from Fra Mauro and mare fill, and that would provide detail on lunar evolution before the Imbrium impact, 3.9 billion years ago. A second high-priority objective was to sample the youngest widespread lunar volcanics to determine whether the lunar heat engine really stopped 3 billion years ago. (Hinners, 1972: 1-1–1-2)

There had been recent research using photographs taken by the Lunar Orbiter that provided possible evidence for volcanism at the proposed Apollo 16 site in the lunar highlands (Trask & McCauley, 1972). Further, as Hinners noted, … in all site selection discussions for which a record exists (Group for Lunar Exploration Planning (GLEP), GLEP Site Selection Subgroups, Ad Hoc Site Selection Committee, and the Apollo Site Selection Board), both the Cayley and Descartes Formations are overwhelmingly interpreted as volcanic units. … The Descartes Formation, of higher albedo than Cayley, was thought to represent a more siliceous, higher viscosity extrusive. It was further argued by H. Masursky before the Site Selection Board that the Apollo 16 site is located on the highest topographic region of the frontside highlands, indicating that the Descartes volcanics represent remobilized highlands, and that analysis of these volcanics would shed light on the basic process of highland formation. (Hinners, 1972: 1–2)

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The Apollo 16 astronauts made use of the lunar rover to cover 27 km of the Descartes Formation. The region investigated was heavily cratered, indicative of the great age of the highlands. Most of the rocks found were anorthositic breccias. Despite the careful site selection, which intended to place the astronauts in an area of volcanoes, no evidence for volcanic rocks beneath the regolith at the Apollo 16 site was found.

4.2.7

Apollo 17

The final Apollo mission was launched on 7 December 1972, and the Lunar Lander touched down on 11 December in a highlands region of the Moon known as the Taurus-Littrow valley, which is on the rim of the Serenitatis basin (Fig. 4.12). The Taurus-Littrow valley is radial to the Serenitatis basin and is thought to be a deep graben that was formed when the lunar crust had to make an adjustment following the creation of the basin by an impacting bolide (Apollo Field Geology Investigation Team, 1973: 672). The Apollo 17 astronauts who explored the surface were Eugene Cernan (1934–2017) and Harrison Schmitt, a professional geologist. Ronald Evans (1933–1990) orbited the Moon in the Command Module. In the Introduction to the Apollo 17 Preliminary Science Report, Anthony Calio (1929–2012) described NASA’s rationale for selecting the Taurus-Littrow site. The identification and selection of the landing site resulted from Astronaut Worden’s Apollo 15 orbital observations (he noticed dark patterns that looked like cinder cones in the Littrow region of the Moon) and from detailed analysis of the Apollo 15 imagery. The rim

Fig. 4.12 All Apollo Landing Sites. Photograph courtesy of the Lunar and Planetary Institute

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of the Serenitatis basin in the Tarus-Littrow region seemed to have all the elements geologists would want to explore in the final Apollo mission. Cinder cones and steep-walled valleys with large boulders at their base presented the possibility of sampling, at the same location, both young volcanic rock from depth and older mountainous wall material. … (Calio, 1973: xiii)

The dark pattern that looked like the effluent of cinder cones to Worden could not be confirmed by the Apollo 17 astronauts. At a small crater, named Shorty Crater, near the landing site, they instead found orange and black glass spheres. As Calio noted: At Shorty Crater, orange and black glasses that were hopefully young volcanic material were observed and sampled. However, the old age of the glass and the astronaut observations and photographs suggest that this impact crater apparently excavated layers of very old pyroclastic material. Throughout this landing site, 10 to 20 percent of each soil sample consists of these “exotic” glasses, apparently brought from subsurface and distributed by the gardening effect. … (ibid.: xiii and xiv)

In the search for volcanoes on the Moon we had now traveled for more than 100 years, from the strong assertion that all lunar craters were thus formed to the failure during the final two Apollo missions to find evidence of young volcanoes from what looked like promising possibilities.

4.3

Concluding Remarks

As the Apollo program was nearing its conclusion, the LSI conducted a Summer Study on Post-Apollo Lunar Science during the week of 10 July 1972 on the campus of the University of California, San Diego. The chair of the study was Robert A. Phinney of Princeton University. There were 44 participants representing the various sub-disciplines of the lunar sciences. The purpose of the study was to make recommendations for lunar science after the Apollo program. The report is interesting in a number of respects, one of which is the summary of scientific accomplishments of lunar exploration as of 1972 that it provided. This was seven years after the National Academy study that was held at Woods Hole, Massachusetts, to formulate “Scientific Questions in Lunar Exploration.” How many of the six questions posed under the heading “History of the Moon” had been answered? Some of the answers to these questions appear in one of the figures of the report, titled Chronological interpretations of lunar history based on Apollo results (Fig. 4.13). Excerpts from the report give some elaboration. Q1. What is the age of the Moon? What is the range of age of the stratigraphic units on the lunar surface? What is the age of the oldest exposed material? Is a primordial surface exposed? Answer – “Isotopic analyses of samples from its surface show that the Moon, as a body, was formed 4.6 billion years ago. It is similar in age to the Earth and to other objects in the solar system. … (Chamberlain, 1972: 11) “Most of the recognizable major geologic events occurred during the first 1.5 billion

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Fig. 4.13 Chronological interpretation of lunar history based on Apollo results and a comparison with the chronology of Earth’s evolution. From Fig. 3 in Chamberlain, J. W. (1972) Post-Apollo Lunar Science: Report of a Study by the Lunar Science Institute, July 1972. Houston, TX: Lunar Science Institute, page 22, Copyright 1972

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Q2.

Q3.

Q4.

Q5.

years of lunar history. Except for minor lava flows and a relatively small number of large impact craters, the face of the Moon has remained largely unchanged in the last 3 billion years.” (ibid.) “The pre-mare igneous rocks so far dated were formed 4.0 to 4.1 billion years ago.” (ibid.: 24). Primordial rocks, i.e., those dating to 4.6 billion years had not been found. What is the history of dynamical interaction between the earth and the Moon? Answer – The history of dynamical interaction between the earth and the Moon was not yet clear. The report stated that: “The lunar studies of the last decade have not produced conclusive evidence against any of the theories of the Moon’s origin. However, many significant compositional differences between comparable terrestrial and lunar rocks have provided additional constraints that are difficult to explain by the hypothesis that the Moon was fissioned from the earth.” (ibid.: 27) What is the thermal history of the Moon? What has been the distribution of tectonic and possible volcanic activity in time? Answer – The thermal history of the Moon was listed as an unsolved problem because the data were in conflict. “The deep interior of the Moon must have been solid since about 3.2–3.9 billion years ago (and probably since the very beginning) in order to support the mascons and the unequal moments of inertia. However, this conclusion stands in conflict with the magnetic data which suggest that the Moon had a fluid core 3–4 billion years ago …. The outer 100 km or more of the Moon melted at least 4.3 billion years ago, forming a lunar crust consisting largely of anorthosite and norite …. It is not yet clear whether this melting took place immediately after accretion, or several hundred million years later … (ibid.: 25). The mare regions were filled with low viscosity basaltic liquids or ash flows, permitting wide areas to be covered with a single flow. These basaltic rocks are igneous products of a chemical differentiation due to internal heat sources. The times of mare fillings lie in the interval 3.2 to 3.7 billion years ago. Since these samples represent the youngest widespread lunar surface material, it must be concluded that regional lunar volcanic activity stopped 3 billion years ago.” (ibid.: 24) What has been the flux of solid objects striking the lunar surface in the past and how has it varied with time? Answer – “The large mare basins represent the last stages of impact of large bodies on the surface of the Moon. The ubiquitous and profound cratering of the highlands on all scales is in some way a record of this period of impact and cratering. The Apollo age dates indicate that the Imbrium basin, the second youngest of the mare basins, was formed 0.5 billion years after the Moon’s accretion. This raises various possibilities about the cratering history during the first half billion years. The rates of impact may have been so great at times that most of the large craters formed never survived to leave any record. A critical problem is to determine the time span for the basin-forming impacts and the nature of the impacting bodies. Such evidence is required to determine whether the basins were related to a special collisional event, the time of capture of the Moon, or the gradual sweep-up of small bodies from terrestrial space, the asteroid belt, or elsewhere. “The craters on the mare surface are a record of impacts since 3 billion years ago. The average cratering rate has been much lower than during the first 0.5 billion years. A limited number of medium-sized (20–100 km) craters such as Copernicus and Tycho have been formed since 3 billion years. The times are uncertain, although study of particles in the soil at Apollo 12 suggests that Copernicus may have been formed 850 million years ago.” (ibid.: 24–25) What has been the flux of cosmic radiation and high-energy solar radiation over the history of the Moon? Answer – Two questions about cosmic radiation were inserted into the section of the report titled Current Major Unsolved Problems. “Are there other phenomena

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preserved in the irradiation record of the lunar surface which would apply to such problems as the origin and residence time in the Galaxy of cosmic rays and the evolution of the source regions of the solar particles? Does the lunar surface material contain supernova products such as super-heavy elements and undiscovered fundamental particles?” (ibid.: 28) “…One important conclusion is that the average solar-flare activity has not changed appreciably over the past few million years. It has also been shown that solar flares were active at least 0.5 billion years ago and probably back to the original formation of lunar surface.” (ibid.: 19) Q6. What past magnetic fields may be recorded in the rocks at the Moon’s surface? Answer – “Fields of 102 or 103 gamma* are required to generate the remanent magnetism giving rise to the fields we now measure. The origin of these fields is the central and unsolved question in lunar magnetism studies. “* This is 50 to 500 times less than the Earth’s field, but 20 to 200 times greater than the field in the solar wind.” (ibid.: 17)

Many of the questions that were posed during the summer study that was held at Woods Hole, Massachusetts, during June and July of 1965 had been answered by the summer of 1972. Many of the unsolved problems, including that of the formation of the Moon, would be addressed during the next several years of the post-Apollo analyses of lunar samples and other investigations. Some questions remain unresolved today.

References Apollo Field Geology Investigation Team: Muehlberger, W. R., Baston, R. M., Cernan, E. A., Freeman, V. L., Haitt, M. H., Holt, H. E., Howard, K. A., Jackson, E. D., Larson, K. B., Reed, V. S., Rennilson, J. J., Schmitt, H. H., Scott, D. H., Sutton, R. L., Stuart-Alexander, D., Swann, G. A., Trask, N. J., Ulrich, G. E., Wilshire, H. G., & Wolfe, E. W. (1973). Geologic exploration of Taurus-Littrow: Apollo 17 landing site. Science, 182(4113), 672–680. Calio, A. J. (1973). Introduction. In R. A. Parker, R. R. Baldwin, R. Brett, J. D. Fuller, R. L. Giesecke, J. B. Hanley, D. N. Holman, R. M. Mercer, S. N. Montgomery, M. J. Murphy, & S. H. Simpkinson (Eds.), Apollo 17 preliminary science report (NASA Special Publication 330) (pp. xiii–xvii). Washington, DC: NASA. Cameron, A. G. W. (1968). A new table of abundances of the elements in the solar system. In L. H. Ahrens (Ed.), Origin and distribution of the elements (pp. 125–143). New York, NY: Pergamon Press. Cameron, A. G. W. (1970). Formation of the Earth-Moon system. Eos, Transactions, American Geophysical Union, 51(9), 628–633. Chamberlain, J. W. (1972). Post-Apollo lunar science: Report of a study by the Lunar Science Institute, July 1972. Houston, TX: Lunar Science Institute. Compton, W. D. (1989). Where no man has gone before: A history of Apollo lunar exploration missions (NASA Special Publication 4214). Washington, DC: NASA. Cummings, W. D. (2009). A documentary history of the formation of USRA. Columbia, MD: Universities Space Research Association. Ernst, W. G. (1978). William Rubey, 1898–1974. National Academy of Sciences Biographical Memoirs, 205-223. Hinners, N. W. (1972). Apollo 16 site selection. In R. Brett, A. W. England, J. E. Calkins, R. L. Giesecke, D. N. Holman, R. M. Mercer, M. J. Murphy, & S. H. Simpkinson (Eds.), Apollo

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16 preliminary science report (NASA Special Publication 315) (pp. 1-1–1-3). Washington, DC: NASA. Hörz, F., & Foss, T. H. (1971). Report of a meeting sponsored by The Universities Space Research Association October 19–23, 1970, Houston, Texas: Meteorite impact and volcanism. Eos, Transactions, American Geophysical Union, 52(3), 101–102. Langseth, Jr., M. G., Clark, Jr., S. P., Chute, Jr., J. L., Keihm, S. J., & Wechsler, A. E. (1972). The Apollo 15 lunar heat-flow measurement. In H. Alfvén, Z. Kopal, & H. C. Urey (Eds.), Proceedings of a Conference on Lunar Geophysics at the Lunar Science Institute, Houston, Texas, October 18–21, 1971 (The Moon, 4(3–4)) (pp. 390–410). Dordrecht, Holland: D. Reidel. Langseth, Jr., M. G., Keihm, S. J., & Peters, K. (1976). Revised lunar heat-flow values. In R. B. Merrill (Ed.), Proceedings of the Seventh Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 7, v. 3) (pp. 3143–3171). Elmsford, NY: Pergamon Press. Latham, G., Ewing, M., Dorman, J., Lammlein, D., Press, F., Toksöz, N., Sutton, G., Duennebier, F., & Nakamura, Y. (1971). Moonquakes and lunar tectonism. In H. Alfvén, Z. Kopal, & H. C. Urey (Eds.), Proceedings of a Conference on Lunar Geophysics at the Lunar Science Institute, Houston, Texas, October 18–21, 1971 (The Moon, 4(3–4)) (pp. 373–382). Dordrecht, Holland: D. Reidel. LBJ visits MSC, tells of Lunar Institute aims. (1968, March 2). Houston Post, p. 1. Lunar Sample Analysis Planning Team: Arnold, J., Arrhenius, G., Eglinton, G., Frondel, C., Gast, P., MacGregor, I., Pepin, R., Strangway, D., Walker, R., Wasserburg, G., & Zill, P. (1970). Summary of Apollo 11 Lunar Science Conference. Science, 167(3918), 449–451 Lunar Sample Analysis Planning Team: Burlingame, A., Burnett, D., Doe, B., Gault, D., Haskin, L., Schnoes, H., Heymann, D., Melson, W., Papike, J., Tilling, R., Toksöz, N., & Wood, J. (1972). Third Lunar Science Conference: Primal igneous activity in the outer layers of the Moon generated a feldspathic crust 40 kilometers thick. Science, 176(4038), 975-981. Lunar Sample Preliminary Examination Team: Anderson, D. H., Anderson, E. E., Biemann, K., Bell, P. R., Bogard, D. D., Brett, R., Burlingame, A. L., Carrier, W. D., Chao, E. C. T., Costes, N. C., Dahlem, D. H., Dalrymple, G. B., Doell, R., Eldridge, J. S., Favaro, M. S., Flory, D. A., Frondel, C., Fryxell, R., Funkhouser, J., Gast, P. W., Greenwood, W. R., Grolier, M., Gromme, C. S., Heiken, G. H., Hess, W. N., Johnson, P. H., Johnson, R., King, Jr., E. A., Mancuso, N., Menzies, J. D., Mitchell, J. K., Morrison, D. A., Murphy, R., O’Kelley, G. D., Schaber, G. G., Schaeffer, O. A., Schleicher, D., Schmitt, H. H., Schonfeld, E., Schopf, J. W., Scott, R. F., Shoemaker, E. M., Simoneit, B. R., Smith, D. H., Smith, R. L., Sutton, R. L., Taylor, S. R., Walls, F. C., Warner, J., Wilcox, R. E., Wilmarth, V. R., & Zähringer, J. (1969). Preliminary examination of lunar samples from Apollo 11: A physical, chemical, mineralogical, and biological analysis of 22 kilograms of lunar rocks and fines. Science, 165(3899), 1211–1227. Lunar Sample Preliminary Examination Team: Anderson, D. H., Anderson, E. E., Bean, A., Bell, P. R., Biemann, K., Bogard, D. D., Brett, R., Burlingame, A. L., Butler, Jr., P., Calio, A. J., Carrier, W. D., Chao, E. C. T., Clark, R. S., Conrad, Jr., C., Costes, N. C., Dahlem, D. H., Eldridge, J. S., Favaro, M. S., Flory, D. A., Forbes, C. D., Foss, T. H., Frondel, C., Fryxell, R., Funkhouser, J., Gibson, Jr., E. K., Goddard, E. N., Greenwood, W. R., Hait, M. H., Heiken, G. H., Harmon, R. S., Hauser, J., Hirsch, W., Johnson, P. H., Keith, J. E., Lewis, C. F., Lindsay, J. F., Lofgren, G. E., McKay, V. A., Mancuso, N., Menzies, J. D., Mitchell, J. K., Moore, C. B., Morrison, D. A., Murphy, R., O’Kelley, G. D., Reynolds, M. A., Roseberry, R. T., Schaber, G. C., Schaeffer, O. A., Schonfeld, E., Schopf, J. W., Scott, R. F., Shoemaker, E. M., Smith, D. H., Smith, R. L., Sutton, R. L., Swan, G. A., Taylor, S. R., Warner, J., Waters, A. C., Wilcox, R. E., Wones, D. R., & Zähringer, J. (1970). Preliminary examination of lunar samples from Apollo 12: A physical, chemical, mineralogical, and biological analysis of 34 kilograms of lunar rocks and fines. Science, 167(3923), 1325–1339. Lunar Sample Preliminary Examination Team: Anderson, D. H., Bass, M. N., Bennett, A. D., Bogard, D. D., Brett, R., Bromwell, L. G., Butler, Jr., P., Carrier, III, W. D., Clark, R. S., Cobleigh, T., Duke, M. B., Gast, P. W., Gibson, Jr., E. K., Hart, W. R., Heiken, G. H., Hirsch, W. C., Hörz, F., Jackson, E. D., Johnson, P. H., Keith, J. E., Lewis, C. F., Lindsay, J. F., Martin, J. R., Melson, W. C., Mitchell, E. D., Moore, C. B., Morrison, D. A., Nance, W. B.,

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Phinney, W. C., Reid, A. M., Reynolds, M. A., Richardson, K. A., Ridley, W. I., Schonfeld, E., Shepard, A. B., Sutton, R. L., Trask, N. J., Warner, J., Wilkin, R. B., Wilshire, H. G., & Wones, D. R. (1971). Preliminary examination of lunar samples from Apollo 14: A physical, chemical, mineralogical, and biological analysis of 43 kilograms of lunar rocks and fines. Science, 173(3998), 681–693. Lyttleton, R. A. (1953). The stability of rotating liquid masses. New York, NY: Cambridge University Press. Muller, P. M., & Sjogren, W. L. (1968). Mascons: Lunar mass concentrations. Science, 161(3842), 680–684. O’Keefe, J. A. (1969). Origin of the Moon. Journal of Geophysical Research, 74(10), 2758–2767. O’Keefe, J. A. (1970). Apollo 11: Implications for the early history of the solar system. Eos, Transactions, American Geophysical Union, 51(9), 633–636. Öpik, E. J. (1972). Comments on lunar origin. Irish Astronomical Journal, 10(5–6), 190–238. Ringwood, A. E. (1960). Some aspects of the thermal evolution of the Earth. Geochimica et Cosmochimica Acta, 20(3–4), 241–259. Ringwood, A. E. (1970a). Petrogenesis of Apollo 11 basalts and implications for lunar origin. Journal of Geophysical Research, 75(32), 6453–6479. Ringwood, A. E. (1970b). Origin of the Moon: The precipitation hypothesis. Earth and Planetary Science Letters, 8(2), 131–140. Singer, S. F., & Bandermann, L. W. (1970). Where was the Moon formed? Science, 170(3956), 438–439. Taylor, G. J. (1982). Pristine lunar highland rocks: Hypotheses of origin. In D. Walker & I. S. McCallum (Eds.), Workshop on magmatic processes of early planetary crusts: Magma oceans and stratiform layered intrusions (pp. 147–153). Houston, TX: Lunar and Planetary Institute. Toksöz, M. N., Solomon, S. C., Minear, J. W., & Johnston, D. H. (1972). Thermal evolution of the Moon. In H. Alfvén, Z. Kopal, & H. C. Urey (Eds.), Proceedings of a Conference on Lunar Geophysics at the Lunar Science Institute, Houston, Texas, October 18–21, 1971 (The Moon, 4 (1–2)) (pp. 190–213). Dordrecht, Holland: D. Reidel. Trask, N. J., & McCauley, J. F. (1972). Differentiation and volcanism in the lunar highlands: Photogeologic evidence and Apollo 16 implications. Earth and Planetary Science Letters, 14 (2), 201–206. Urey, H. C. (1972). Evidence for objects of lunar mass in the early solar system. In H. Alfvén, Z. Kopal, & H. C. Urey (Eds.), Proceedings of a Conference on Lunar Geophysics at the Lunar Science Institute, Houston, Texas, October 18–21, 1971 (The Moon, 4(3–4)) (pp. 383–389). Dordrecht, Holland: D. Reidel. Urey, H. C., & MacDonald, G. J. F. (1971). Origin and history of the Moon. In Z. Kopal (Ed.), Physics and astronomy of the Moon (2nd ed., pp. 213–289). New York, NY: Academic Press. Urey, H. C., Marti, K., Hawkins, J. W., & Liu, M. K. (1971). “Model history of the lunar surface.” In A. A. Levinson (Ed.), Proceedings of the Second Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 2, v. 2) (pp. 987–998). Cambridge, MA: MIT Press. Wakita, H., & Schmitt, R. A. (1970). Lunar anorthosites: Rare-earth and other elemental abundances. Science, 170(3961), 969–974. Wise, D. U. (1969). Origin of the Moon from the Earth: Some new mechanisms and comparisons. Journal of Geophysical Research, 74(25), 6034–6045. Wood, J. A. (1970). Petrology of the lunar soil and geophysical implications. Journal of Geophysical Research, 75(32), 6497–6513. Wood, J. A., Dickey, Jr., J. S., Marvin, U. B., & Powell, B. N. (1970). Lunar anorthosites and a geophysical model of the Moon. In A. A. Levinson (Ed.), Proceedings of the Apollo 11 Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 1, v. 1) (pp. 965–988). Elmsford, NY: Pergamon Press. Wyllie, P. J. (2010). Joseph V. Smith, 1928–2007. National Academy of Sciences Biographical Memoirs, 3–18.

Chapter 5

Post-Apollo Synthesis and Debate

5.1

Introduction

Following Apollo 17, the Lunar Science Institute (LSI) continued to sponsor and host conferences for the research community. On 10–12 January 1973, the Conference on the Geophysical and Geochemical Exploration of the Moon and Planets was held at the LSI. The meeting was chaired by David Strangway (1932– 2016) (Fig. 5.1), who later in the year would be appointed as Interim Director of the LSI, following the departure of Joseph Chamberlain. In the preface to the proceedings for the conference, Strangway noted, “This conference marked the start of what might be viewed as the post-Apollo program of analysis and synthesis. …” (Strangway, 1974: 2). In this chapter, we will review some important elements of this process.

5.2

O’Keefe’s Arguments for Fission

John O’Keefe gave a paper at the Conference on the Geophysical and Geochemical Exploration of the Moon in which he once again defended the fission theory for the formation of the Moon. He continued to stress his main theme: As soon as the Apollo data began to be published, most geochemists perceived the absence of volatiles, and theories were constructed to account for it …, but the siderophile deficiency was not as easy to explain. Some theorists sought to explain the whole problem of the Moon’s composition as a result of vapor fractionation in a hypothetical solar nebula. This explanation is awkward because the theorist must insist that the Moon lost the volatiles preferentially, and yet he must also explain why the Moon is deficient in platinum, rhodium, iridium, and other very refractory siderophile elements. …

© Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_5

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Fig. 5.1 Photograph of David W. Strangway, courtesy of the Lunar and Planetary Institute

It is becoming clear that the deficiency of siderophiles in the Moon requires us to suppose that the Moon initially contained a metal phase which leached out the siderophiles. From the point of view of the fission theory, this result is, of course, expected; a similar deficiency of siderophiles occurs in the Earth. … (O’Keefe, 1974: 219–220)

O’Keefe then argued that the required amount of metal to leach the siderophiles would be at least 30% of the mass of the Moon, whereas the density of the Moon would not allow more than 6% to be metal, and considerations of the moments of inertia of the Moon would not allow a metal core with more than 1% of the total mass of the Moon (ibid.: 220). He noted, “It thus seems very doubtful whether the maximum amount of metal phase allowable … would come within a factor of 20 of satisfying [the required amount to leach the siderophiles].” (ibid.: 221).

5.3 The Fourth Lunar Science Conference

5.3

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The Fourth Lunar Science Conference

On 5–8 March 1973, the LSI co-sponsored with NASA the Fourth Lunar Science Conference, which was held at the Lyndon B. Johnson Space Center, formerly the Manned Spacecraft Center, in Houston. By the time of this conference, the evidence had continued to mount that there had been extensive melting in the outermost layers of the Moon to a depth of hundreds of kilometers. The melting and consequent separation of lighter from heavier materials was thought to have occurred during or immediately after the formation of the Moon. During the first 700 million years of the Moon’s history, an enormous number of impacts shaped the lunar surface, and, in particular, large impacts excavated the great lunar basins and redistributed the crustal material over the Moon’s surface. The Lunar Sample Analysis Planning Team reported: Radiometric dating of terra [highlands] rocks samples has yielded a surprising result. Ages … almost invariably fall in the range 3.85 to 4.05  109 years. The rocks are neither as old nor as scattered in age as was expected. We are confident that the moon is as old as the rest of the solar system, 4.6  109 years, and we anticipated that terra rocks, most of which have suffered a series of shock brecciations and reheatings, would display a spectrum of apparent ages extending back to that time. The affinity of terra rocks for ages of 3.9 to 4.0  109 years is so pronounced that it appears some major cataclysm affected the moon at that time, or had affected it continuously until about 3.9  109 years ago, resetting the clocks of rocks throughout the part of the crust we have access to. The cataclysm is thought to have taken the form of a particularly intense and violent epoch of meteoroid bombardment, culminating in the giant impacts that excavated the Imbrium and Orientale basins. (Lunar Sample Analysis Planning Team, 1973: 616)

There followed at various intervals over the next 700 million years lava flows (mare basalts) that filled many of the basins on the side of the Moon facing the Earth. The lava emanated from the interior of the Moon, where it was brought to the melting point by radioactive heating. The lunar maria are broad depressions filled with dark volcanic basalts rich in iron and titanium. By now the maria have been sampled at five sites, and the range of basalt compositions and ages have been explored. Basalt ages fall in a surprisingly narrow time range 3.15 to 3.85  109 years. Apparently, the mare basalts were generated by radioactive heating and partial melting in an iron-rich, plagioclase-poor region in the interior of the moon, and were not a product of the primary differentiation that gave rise to the lunar crust. (ibid.: 617)

During the post-Apollo synthesis, there was a growing use of data from the seismic networks that had been left on the lunar surface as a part of the Apollo missions. The Lunar Sample Analysis Planning Team reported: … The deep lunar interior attenuates shear waves, which shows that it is probably hot and weak; a small fraction of the rock may be melted. The lunar lithosphere (the relatively rigid layer, which transmits shear waves) is approximately 1000 km thick. All internally generated moonquakes for which focal depths can be determined occur at depths of 700 to 1000 km, near the boundary between the lithosphere and the asthenosphere (deep attenuating zone). These moonquakes could result from stresses applied by convection in the asthenosphere, but they are clearly triggered by tidal forces; the frequency of moonquakes

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Fig. 5.2 Schematic diagram of lunar structure. The near side of the Moon is to the left of the figure. From Fig. 7 in Toksöz, M. N., Dainty, A. M., Solomon, S. C., & Anderson, K. R. (1973) “Velocity structure and evolution of the Moon.” In W. A. Gose (Ed.), Proceedings of the Fourth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 4, v. 3) page 2539, Elmsford, NY: Pergamon Press increases at lunar apogee and perigee, and also responds to a 207-day cycle of solar tides. (ibid.: 619)

The seismometers allowed the determination of the velocity of seismic waves at various depths in the interior of the Moon, i.e., the so-called “velocity structure.” The velocity data, along with other data, allowed the construction of a model lunar structure (Fig. 5.2) that, in turn, provided the basis for theories of the evolution of the Moon. At the Fourth Lunar Science Conference, the MIT group presented a paper titled “Velocity structure and evolution of the Moon,” in which they concluded: Models of the thermal evolution of the moon that fit the chronology of igneous activity on the lunar surface, the velocity structure, the stress history of the lunar lithosphere implied by the presence of mascons, and the surface concentrations of radioactive elements involve extensive differentiation early in lunar history. This differentiation may be the result of rapid accretion and large-scale melting at the time of the formation of the moon. … (Toksöz, Dainty, Solomon, & Anderson, 1973: 2529)

5.4

Harold Urey’s 80th Birthday

Harold Urey’s ideas were consistent with those of the MIT group, at least to some extent. Though differences remained, Urey had conceded that the lunar surface had experienced extensive melting. But scientific differences aside, Harold Urey was

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approaching his 80th birthday, and it was time to celebrate his contributions to lunar science. For that purpose, the LSI organized and hosted a conference that was held from 30 April through 2 May 1973. Urey had been the first Senior Visiting Scientist at the LSI, and his 1969 paper in Science magazine titled “Early temperature history of the Moon,” was Contribution No. 1 from the Lunar Science Institute. Homer Newell (1915–1983), the NASA Associate Administrator, presented a paper for the conference in which he recalled: Sometime during December of 1958 Harold Urey and Robert Jastrow came to see me at NASA Headquarters in Washington, D.C. Their mission was to press for including a serious study of the Moon in the newly developing space science program of the United States. They emphasized the importance of the Moon for achieving an understanding of the origins of the solar system and of the Earth itself. … In their conversation with me, Urey and Jastrow pointed to the fact that without an atmosphere or any visible water, the Moon’s surface had not been subjected to the erosion that over geologic time has erased much of the early history of the Earth. Nor are the sorts of mountain building processes we see on the Earth operative on the Moon to distort and confuse the early history of the lunar surface. One had to conclude that the Moon must have preserved on its surface the early record of its formation and history. Urey felt that the Moon might well be a primitive body, formed in the early days of the solar system. If this were so, the record of its early history preserved on the surface of the Moon, would provide invaluable clues to the origin and evolution of other bodies of the solar system as well. This was a powerful argument for including the Moon in the space science program. I, personally, did not need persuading, and Urey’s story provided good ammunition for moving the proposal on up the line. The persuasiveness of the argument carried the day at each stage, within NASA, in the Administration, and finally in Congress, and in due course investigation of the Moon was formally and officially a part of the NASA space science program. (Newell, 1973: 1–2)

5.5

The Lunar Petrology Conference

The growing sense of the importance of understanding the makeup of lunar uplands rocks, and in particular of identifying the earliest crustal material, was stressed in a Lunar Petrology Conference that was sponsored by the LSI on 23–25 July 1973. In a report on the conference by W. I. Ridley, A. M. Reid, and P. R. Brett, it was noted that: One of the basic requirements in understanding lunar crustal evolution is the recognition of the nature of the rock types in the lunar highlands. M. Prinz stressed that major difficulties in interpretation are presented by the complexity of the rocks that may have undergone brecciation [welding together of loose components to form a rock], cataclasis [deformation by fracturing and crushing], shock and thermal metamorphism, partial to complete melting,

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or some combination of these effects in one or in several cycles of collisional events. The textures that reflect these collisional processes are extremely complex, whereas the minerals of highland rocks and the major element compositional types are few and simple. … (Ridley, Reid, & Brett, 1974: 4)

In the final session of the Lunar Petrology Conference, participants tried to evaluate how the lunar data provided information on the earliest period of lunar evolution. Both J. Wood and H. Schmitt discussed a post-accretionary model in which the outer shell of the moon, possibly to depths of several hundred kilometers was molten at approximately 4.6 eons. Wood calculated that conservation of accretionary energy and a high rate of accretion could provide enough heat to initially melt the outer shell, which would then gradually cool with decreasing rate of impact. Schmitt noted that during formation of the crust and crystallization of the underlying silicate melt the meteorite flux, although it was decreasing, would remain extremely high. The crust would be continuously shattered and mixed with partly crystallized melt until such time as the crust became thick enough to prevent impact penetration to the underlying liquid. Rock types produced during this period would be predominantly coarse-grained cumulates possibly mixed with much finer grained shocked and metamorphosed rocks that resulted from brecciation of the initial thin cooled crust. Both Wood and Schmitt suggested that only the rare coarse-grained basic and ultra-basic clasts in Apollo 16 and 17 breccias and some cataclastic anorthosites are reasonable contenders for early crustal material. Most other highland rocks, dominated by cataclastic and metamorphic textures, would result from and reflect the long period of bombardment down to 3.9 eons in which the rigid crust underwent shattering, annealing, and possibly remelting. … Studies of siderophile, lithophile, and volatile trace elements summarized by E. Anders also indicate distinct differences between the moon and other solar system bodies. In particular the moon appears to be enriched in refractory elements (e.g., U) but grossly depleted in siderophile elements (e.g., Au) and volatile elements (e.g., Tl) compared with the earth and meteorites. These features, first noted in the Apollo 11 samples, have persisted for every mission. A moon primarily depleted in volatile elements as a consequence of either the accretionary process or early large-scale crustal melting seems to be an acceptable hypothesis to Anders. Elemental abundances and elemental ratios remain distinctly different between the moon, earth, and meteorites. … (ibid.: 7–8)

The lunar research community appreciated the role of the LSI in advancing the state of knowledge, and during the last half of 1973, James Head III (Fig. 5.3), of Brown University, agreed to serve as interim director of the LSI, while a search committee, chaired by Robert Kovach of Stanford University, was at work to find a more permanent successor to Joseph Chamberlain. Strangway and Head had important influences on the LSI, as they established a set of goals and functional responsibilities for the institute that would carry it through a critical transition period for NASA’s lunar and planetary program.

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Fig. 5.3 Photograph of James W. Head III. Photograph courtesy of the Lunar and Planetary Institute

5.6

The Fifth Lunar Science Conference

The LSI and NASA co-sponsored the Fifth Lunar Science Conference on 18–22 March 1974. As noted above, the final mission in the Apollo program, Apollo 17, had been launched on 7 December 1972, and the lunar lander touched down on 11 December of that year. There had not been enough time to adequately analyze the Apollo 17 data at the time of the Fourth Lunar Science Conference. So, the Fifth Lunar Science Conference was the first of the Lunar Science Conferences in which the lunar science research community began to synthesize the results from all of the Apollo missions. Ross Taylor and Petr Jakeš, both visiting scientists at the LSI, gave an important paper at the conference on a model for the geochemical evolution of the moon, based on data from all the Apollo missions (Taylor & Jakeš, 1974). The paper was a prelude to the second book that Taylor wrote while he was a visiting scientist at the LSI—Lunar Science: A Post-Apollo View. In this book, Taylor noted that the abstracts submitted for the conference contained a new idea for the formation of the Moon. Taylor termed the idea “Disintegrative Capture,” and, as noted above, it had

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actually been introduced as far back as 1955 by Ernst Öpik, though Öpik did not use the term. Taylor described the process as follows: … During the condensation of the solar nebula many bodies form. Selective condensation depletes the volatile elements. Melting and fractionation produce metal cores and silicate mantles in these planetesimals. A number of these approach the earth on parabolic orbits. Those approaching within the Roche limit will disintegrate. Wood estimates that most (80– 90 percent) of the bodies coming within the Roche limit would escape impacting the earth. A debris ring accumulates in earth orbit. Because of dynamical constraints, this would consist preferentially of the silicate portions of the moonlets, which accrete to form the moon. The more massive metallic core material either escapes or is accreted to the earth. (Taylor, 1975: 330–331)

The idea appeared in abstracts submitted by Joseph V. Smith of the University of Chicago (Smith, 1974) and John A. Wood and H. E. Mitler of Harvard (Wood & Mitler, 1974). Wood and Mitler in particular had extended the ideas of Öpik to propose a way of aggregating material for the Moon that would have low abundances of iron. Wood and Mitler, and in a separate paper, Mitler, argued that if the proto-Moon or planetoid (as they also termed it) that entered the Earth’s Roche zone, had already differentiated into a core and mantle, then when it disintegrated, the iron core would escape capture, whereas a fraction of the mantle of the planetoid would become part of the material that would form the Moon. In this way the Moon would have a low abundance of iron. Mitler tried several models, e.g., using a

Fig. 5.4 Photograph of Robert O. Pepin, courtesy of the Lunar and Planetary Institute

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single large planetoid and using several smaller planetoids. He found that the hypothesis of a flux of multiple small planetoids is unlikely to have produced the Moon, whereas: … there appear to be no difficulties with the hypothesis that the Moon coalesced from material captured from a protomoon formed at the same time as Earth and within the same disk. … … … We assume one large protomoon to have been the lunar source. This object would have been of Martian size and is presumed to have originated in a fairly eccentric Earth-crossing orbit with aphelion perhaps midway between Earth and Mars. … … the hypothesis of partial capture not only explains the lack of iron in the Moon, but may be consistently incorporated into a model that involves the passage of just one very large protomoon past the Earth. … (Mitler, 1975: 264–265)

The search committee for the next LSI Director recommended, and the USRA board approved, Robert Pepin (Fig. 5.4), a physicist from the University of Minnesota. Pepin’s research interests began with the study of rare gases in meteorites to understand the composition of the solar nebula and the early solar system. He studied lunar samples and, later, interplanetary dust particles, and his range of interests eventually included the origin and evolution of terrestrial atmospheres. Pepin took over the LSI Directorship on 15 June 1974.

5.7

The Conference on Satellites of the Solar System

Following the Fifth Lunar Science Conference, John Wood attended the International Astronomical Union (IAU) Colloquium No. 28—Conference on Satellites of the Solar System, which was held at Cornell University on 18–21 August 1974. At the conference, Wood gave a talk in which he reviewed the current theories for the formation of the Moon: fission, capture, and binary accretion. In the capture section, he mentioned a version of his theory that Taylor had termed disintegrative capture. It involved the close approach to Earth of many planetesimals, rather than the single large proto-Moon that Mitler had discussed. Wood discounted the multiple planetesimals scenario because it seemed to be too inefficient. Wood thought that the most promising theory was binary accretion, but he ended his review with the following reservations. … Binary accretion explains the existence of the Moon as a natural consequence of the processes operating in the early solar system. No special occurrences are invoked; this makes binary accretion the most aesthetically appealing model. But if Moon formation is inevitable, then why don’t all the planets have satellites, with dimensions comparable to Earth’s Moon? One is now forced to make special assumptions about all the other planets: Mercury’s dynamical history was too much dominated by the Sun; Venus’s moon (if it had one) was spun down to its surface by tidal interactions with a planet rotating more slowly

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than the period of the satellite (or even in a direction opposite to the satellite’s orbit) …; there were fundamental differences in the process of satellite formation for Jovian planets. Mars is probably the worst embarrassment; there is no obvious reason why it should not have accumulated a moon comparable to Earth’s. Perhaps in the end it would be more plausible to make a special assumption for only one planet, and invoke fission or capture for Earth’s Moon. (Wood, 1977: 528–529)

Wood didn’t mention another idea for the formation of the Moon that appeared at the same conference at Cornell in the summer of 1974. The talk by William K. Hartmann was published in the journal Icarus in April 1975. In that paper, Hartmann and Donald R. Davis suggested that “Collision of a large body with the Earth could eject iron-deficient crust and upper mantle material, forming a cloud of refractory, volatile-poor dust that could form the Moon.” (Hartmann & Davis, 1975: 504). This idea was similar to Öpik’s, and later Wood and Mitler’s, disintegrative capture hypothesis. Instead of a near miss, the planetesimal would actually strike the Earth off center, as had been suggested earlier by Daly, Cameron, and Öpik. Hartmann and Davis made the suggestion after their calculations on plausible size distributions of bodies near planets at the close of planet formation. Hartmann and Davis reasoned, “… If a planetary body forms in a certain zone in the solar system, there must be a second-largest body in that zone (and still smaller bodies).” (ibid.: 512). If this second largest body collided with the Earth after the Earth’s core had formed or was forming: … the ejected material would be already depleted in iron, as in the fission theory. … The material ejected into orbit forms a cloud of hot dust, rapidly depleted in volatiles. As shown by Soter (1971), the particles in such a swarm would interact and rapidly collapse into the equatorial plane, where a satellite could form. The evolution at this point resembles that postulated by Ringwood, except that an energy source is provided that does not necessarily apply to all planets. (ibid.)

In a rather striking contrast with the summary by Wood, Hartmann and Davis made a virtue of a seemingly ad hoc theory, concluding: This model has an important philosophically satisfying aspect. There has always been difficulty in accounting for all properties of all satellite systems by a single evolutionary theory. Jupiter and Saturn have “miniature” solar systems with retrograde outriders. Uranus has its spin and satellites’ angular momentum vectors radically altered. Earth is a “dual” planet with a relatively huge satellite. Mars has only two tiny moons. Venus and Mercury have none. This heterogeneity becomes more satisfyingly accountable if it is viewed as the product of events involving statistics of small numbers. Does the second-largest planetesimal in each system hit the planet after 107 years or 108 years? Is it large or small? Does it hit the planet dead center? Retrograde? A glancing blow prograde? Or is it captured? Or is it destroyed by a planetesimal-planetesimal collision so that it has no appreciable effect on the planet other than to produce many small craters? Or does it hit a preexisting satellite of the planet, perhaps converting it to several small satellites? Only one of these kinds of fates can befall the second-largest planetesimal. And this fate, the product of small-number statistical chance encounters, may determine whether the planet acquires a tilted axis, a massive circumplanetary swarm of dust, a captured satellite, or perhaps loses a larger satellite, gaining small fragmentary satellites.

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This model can thus account for the iron depletion, refractory enrichment, and volatile depletion of the Moon, and at the same time account for the Moon’s uniqueness; the Moon may have originated by a process that was likely to happen to one out of nine planets. (ibid.: 512–513)

5.8

The Sixth Lunar Science Conference

It had been assumed almost since the return of the first lunar samples that a theory for lunar formation had to account for the Moon’s iron depletion, refractory enrichment, and volatile depletion. A group from the University of Chicago, led by Robert N. Clayton, developed an additional constraint on theories for the formation of the Moon that was based on the isotopes of oxygen. Most of the oxygen in the universe is in the form 16O, with 16 atomic mass units in its nucleus. There are minute amounts of two stable isotopes of oxygen, 17O and 18O, in the universe, as well. Lunar and planetary researchers could now measure with high accuracy the ratios of 17O/16O and 18O/16O in a given sample of material. In practice, they would measure the difference between these ratios and the corresponding ratios in a standard material called the Standard Mean Ocean Water (SMOW), denoted as d17O and d18O. For example, if one measures oxygen isotope ratios in various iron meteorites and plots d17O against d18O, one finds that the data points fall along a straight line, called the mass-fractionation line. This line is found to be different for other classes of meteorites, e.g., ordinary chondrites (Fig. 5.5).

Fig. 5.5 Comparison of oxygen isotope fractionation lines from the Earth, Moon, and various classes of meteorites. The lunar data points, from sources shown in the upper left legend, fall along the “terrestrial” line. Data for differentiated meteorites are shown as fine dots. Data for undifferentiated meteorites, e.g., H chondrite group (high in metallic iron), L chondrite group (low in metallic iron), and C2 carbonaceous chondrites fall along other lines. Reprinted from Fig. 2 of Clayton, R. N., & Mayeda, T. K. (1975) “Genetic relations between the Moon and meteorites.” In R. B. Merrill (Ed.), Proceedings of the Sixth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 6, v. 2), page 1761, Elmsford, NY: Pergamon Press, copyright 1975

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At his talk at the Sixth Lunar Science Conference in Houston in the spring of 1975, Clayton argued: Due to the inhomogeneous distribution of the stable isotopes of oxygen at the time of condensation and accretion in the solar nebula, it is possible to identify bodies which formed from a common region of the nebula and distinguish them from bodies formed in other regions. On this basis, the moon is in the same group as the earth and the differentiated meteorites (achondrites, mesosiderites, pallasites, irons), and is unrelated to the ordinary chondrites or the carbonaceous chondrites. … … The fact that the moon and the earth lie on the same mass-fractionation line would not be surprising except for the observation that most of the other analyzed samples of the solar system do not. … (Clayton & Mayeda, 1975: 1761)

This was an important discovery and provided further credence to the convergence of “close encounter” ideas from Daly, Cameron, Öpik, Wood, Mitler, Hartmann, and Davis. In the modern era, Reginald Daly was perhaps the first to make this suggestion, but despite his prestige, his idea of the formation of the Moon by the collision of a planetesimal with the Earth did not gain much traction for at least three reasons: (1) it seemed ad hoc and implausible (a difficulty that Daly recognized); (2) until the work of Hartmann and Davis, models of planetary formation featured only very small objects (e.g., 10−3 Earth mass) that might impact the Earth toward the end of the accretion period; and (3) it was always assumed that material knocked off the Earth during a collision would eventually fall back on to the Earth’s surface (Wood, 1986: 42).

5.9

The Seventh Lunar Science Conference

This third objection would soon be removed. In the discussion after Hartmann’s talk at the Cornell meeting, Cameron (Fig. 5.6) remarked that he and a colleague at Harvard, W. R. Ward, were also working on a model for the formation of the Moon that involved a collision of a planetesimal with the Earth. Cameron and Ward presented their work at the Seventh Lunar Science Conference in 1976 (Cameron & Ward, 1976). They argued: A key constraint on the origin of the Earth-Moon system is the abnormally large value of the specific angular momentum of the system, compared to that of the other planets in the solar system. At an early stage, when the Moon was close to the Earth, most of the angular momentum resided in the spin of the Earth. This spin was presumably imparted by a collision with a major secondary body in the late stages of accumulation of the Earth, with the secondary body adding its mass to the remainder of the protoearth. The collisional velocity must have been close to 11 km/sec, and if the impact parameter was one earth radius, then the mass of the impacting body was comparable to that of Mars. It is probable that the largest accumulative collision should have involved a mass of this order, but the size and location of the impact parameter would have been a matter of chance. It is likely that both bodies would have been differentiated and possibly molten at the time of impact. (ibid.: 120)

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Fig. 5.6 Photograph of Alastair G. W. Cameron, courtesy of the Yeshiva University Archives and the AIP Emilio Segre Visual Archives, Physics Today Collection

In their paper, Cameron and Ward made another important point, namely that after the collision, … the mantle material of both bodies in the region of the collision would shock-unload predominantly in the forward direction relative to the collision velocity and much of the material would vaporize. The subsequent motion of this material is not just a set of ballistic trajectories; the early motion of the material is entirely governed by gas pressure gradients in the vapor which is expanding into a vacuum. (ibid.)

In other words, much of the material that would re-condense to form the Moon would be helped off Earth by being entrained in an expanding hot gas envelope. It would not fall back to Earth, as many had assumed (Fig. 5.7). In 1975, George West Wetherill (1925–2006) (Fig. 5.8) of the Carnegie Institution of Washington presented a “tentative synthesis” for the formation of the solar system and planets that prominently featured planetesimals. The principal features of such a synthesis are as follows: 1. The nebular period had a duration of 100–150 m.y., following which time condensation of solid matter took place. 2. Planetesimals up to *100 km in radius accreted rapidly (*104 year) … as a consequence of gravitational instability of solid matter in the central plane of the nebula.

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Fig. 5.7 The collisional ejection model: Expansion of ejected silicate vapor accelerates condensates into orbit around the Earth. From Fig. 10 in Wood, J. A. (1986) Moon over Mauna Loa: A review of hypotheses of formation of Earth’s Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 43, Houston, TX: Lunar and Planetary Institute. Copyright 1986

3. The interiors of at least some of these planetesimals were at temperatures of 500–1600 °C within  107 years, resulting in metamorphism and/or igneous differentiation. 4. Some of the planetesimals cooled quickly (*107 to 108 years) either because of their original small size or because of disruption during an early heavy bombardment associated with the formation of Jupiter and Saturn. 5. Most of these planetesimals and their debris accreted further to form planets and large satellites on a very uncertain time scale of 104 to 108 years. The importance of gravitational energy of accretion as a heat source is critically dependent on the length of this time scale. 6. The larger surviving planetesimals and planetary objects combined to evolve internally in the subsequent planetary era. (Wetherill, 1975: 324) The existence of planetesimals in the early solar system was assumed. The issue was the probability of a collision as envisaged by Hartmann, Davis, Cameron, and Ward. During his talk at the Seventh Lunar Science Conference, Wetherill said that what was needed was an accretional model that could give “… fairly definite predictions concerning the size distribution, orbits, and evolution of large bodies in the early solar system.” (Wetherill, 1976b: 3255). With such a model, it would be possible to make a better judgment about the probability that a Mars-sized object might have collided with the Earth to form the Moon. The existence of planetesimals in the early solar system was assumed. The issue was the probability of a collision as envisaged by Hartmann, Davis, Cameron, and Ward. During his talk at the Seventh Lunar Science Conference, Wetherill said that what was needed was an accretional model that could give “… fairly definite predictions concerning the size distribution, orbits, and evolution of large bodies in the early solar system.” (Wetherill, 1976b: 3255). With such a model, it would be

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Fig. 5.8 Photograph of George W. Wetherill from Boss, A. P. (2006) “George West Wetherill (1925–2006).” Bulletin of the American Astronomical Society, volume 38, page 1284. Copyright 2006 by the American Astronomical Society. Used with permission of Alan P. Boss. Wetherill had a very distinguished career in planetary science. He was a pioneer in the use of the decay of uranium into lead for the purpose of providing accurate dating for when rocks crystallized, and he showed that debris from meteorite strikes on Mars could find their way to Earth. Wetherill was elected to the National Academy of Sciences in 1974, and he received many other honors from professional science societies. In 1997, he received the National Medal of Science, the highest scientific award in the U. S. (Boss, 2006)

possible to make a better judgment about the probability that a Mars-sized object might have collided with the Earth to form the Moon. The interest of Wetherill and others in accretion models was broader than the “Collision Theory,” however. An open question was how the Moon had obtained enough heat to have melted its surface layer to a depth of 100 km or so, as seemed to be indicated by the analyses of lunar samples. If the accretion of lunar material to form the Moon was rapid, e.g., confined to a duration of the order of a thousand years, then heating of the Moon by bombardment during the accretion process

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would have been sufficient to melt its upper layer. On the other hand, if the lunar accretion had been stretched out for a period of the order of ten million years, as was argued by Safranov and his Soviet colleagues, then Wetherill concluded: Extensive early heating is possible on the long time scale, provided that a significant fraction of the accreting bodies are greater than or equal to 200 km in radius, permitting release and trapping of their impact energy at depths sufficiently great to preclude transfer of the heat to the surface … It is therefore of importance to estimate the fraction of large bodies in the accreting material. … (Wetherill, 1976a, p. 930)

While Wetherill was examining the heating of the Moon by impacting bodies, William M. “Bill” Kaula (1926–2000) and Charles Yoder of UCLA were taking another look at tidal heating of the Moon during lunar orbit evolution. In their talk at the Seventh Lunar Science Conference, they discussed the precession of the lunar perigee that is caused by tides raised on the Earth by the Sun. Currently, the period of this precession is about nine years, but earlier in the evolution of the lunar orbit, the period was shorter. The solar “evection” resonance took place when the perigee of the lunar elliptical orbit about the Earth had a precession period of about one year. Kaula and Yoder noted that “The solar resonance … would lead to great tidal heating and the transfer of enough angular momentum out of the earth-moon system to account for the excess required by fission hypotheses. …” (Kaula & Yoder, 1976: 442). This observation seems to have been ignored for about 20 years, but, as we will see in the Epilogue, it has recently come back into focus.

5.10

The Eighth Lunar Science Conference

The Proceedings of the Eighth Lunar Science Conference in March of 1977 featured eight papers on the formation and evolution of the Moon. A paper from a group led by Heinrich Wänke (1928–2015) of the Max-Planck-Institut für Chemie in Mainz, Germany, provided another clue about the origin of the Moon. Wänke and his Mainz colleagues submitted 12 separate research proposals to NASA for lunar samples, and seven were accepted. In the initial allocation of the Apollo 11 samples, the Mainz group received the largest amount of lunar material of any laboratory outside the United States. Wänke then became very heavily involved in NASA’s lunar analysis program. In an interview with Ursula Marvin, Wänke recollected his involvement. … I thought we should do as much as possible to analyze these highly valuable samples whatever they might be, so while we were waiting for them we developed a multi-element analysis program, in which we would do the least destructive analyses first and then more and more destructive ones in succession. … … In all, I think our Mainz group measured up to 56 elements on at least one of our lunar samples, and this was very useful. This is why our data look more coherent when compared

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to other data where major elements were measured by this group and a trace element from that group, and the other trace element from the third group. So it is no wonder that we found many of what we now call element correlations, which at the time were practically unknown in geochemistry. … .(Marvin, 2002: B85–B86)

The paper coauthored by Wänke at the Eighth Lunar Science Conference dealt with one of these element correlations, namely that between tungsten (chemical symbol W) and a soft metal called lanthanum (chemical symbol La) as found in samples taken from the Moon, the Earth and various kinds of meteorites. The bottom line in Fig. 5.9, which was included in the paper by Werner Rammensee

Fig. 5.9 The correlation between the densities of tungsten (W) and lanthanum (La) in samples from the Moon, Earth, and meteorites. From Fig. 5 in Rammensee, W., & Wänke, H. (1977) “On the partition coefficient of tungsten between metal and silicate and its bearing on the origin of the Moon.” In R. B. Merrill (Ed.), Proceedings of the Eighth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 8, volume 1), page 404, Elmsford, NY: Pergamon Press, copyright 1977. Heinrich Wänke began his career as a nuclear physicist in post-war Vienna. In 1953, he was able to join a laboratory at the University of Durham in England that was headed by Professor Friedrich Paneth, who had had to leave Germany in the 1930s because of the Nazis. Soon after Wänke arrived, however, Paneth had an opportunity to move to the Max-Planck Institut für Chemie in Mainz, Germany, and he convinced Wänke to move with him. At the institute, Wänke and his colleagues began to develop methods to determine small concentrations of gases in meteorites. Among other things, Wänke discovered that the solar wind causes the implantation of rare gases and other species into meteorites. Before the Apollo program began, he correctly predicted that the lunar soil would be greatly enriched with solar wind particles (Marvin, 2002, p. B79)

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and Wänke at the conference, shows the density of W plotted against the density of La for lunar and terrestrial samples, as well as some achondrite meteorites. The fact that the data fall along a straight line indicates that ratio of the density of tungsten to the density of lanthanum is constant for the various samples. The top line in the figure shows data taken from three carbonaceous chondrites. The ratio of W/La in these samples is also constant, but higher than the other ratio in the other samples by a factor of about 19. Tungsten is a siderophile element (high affinity for iron), whereas lanthanum is not. Rammensee and Wänke explained that their figure: … shows that for the moon the W/La ratio remains rather constant for mare basalts as well as for KREEP-rich and KREEP-poor highland rocks. A similar abundance ratio is also valid for achondrites and, with a larger scattering, which may be caused by more complex magmatic and metamorphic processes, for terrestrial samples. It can be seen that the W/La ratio is 19 times less than the chondritic (C1) W/La ratio. If we assume that the moon accreted from a material with chondritic elemental abundance ratios of, at least, the refractory elements, we have to reflect on the mechanism … which caused this remarkable depletion of W. Neither condensation or evaporation nor magmatic differentiation processes can have been the cause of this depletion. Only the experimentally verified siderophile nature of W can be responsible for this depletion … . (Rammensee & Wänke, 1977: 403)

As noted above, the siderophile nature of W means that in a melt that contains iron and tungsten, the tungsten will go where the iron goes. For the example of the Earth, most of the tungsten accompanied iron into the Earth’s core. In agreement with John O’Keefe’s observation about the dearth of nickel in the lunar samples, Rammensee and Wänke argued that there is not enough iron in the Moon to explain the observed depletion of W in lunar samples. They concluded, “… the most likely parent body of the moon and the achondrites, on which the redistribution of W and the segregation of metal could have occurred seems to be the earth …” The research of the Wänke group thus seemed to support a fission model for the formation of the Moon. Such a model incorporated a very hot environment for the Earth and Moon at the time of the Moon’s origin. The recent research of Joseph V. Smith of the University of Chicago led to a different conclusion. Smith presented a paper at the Eighth Lunar Science Conference, in which he: … suggested that the chemistry of the earth and moon can be modeled more plausibly in the context of a slow, cool accretion of the earth and either simultaneous accretion or disintegrative capture of the moon than by fission or volatilization models based on a hot earth. … (Smith, 1977: 363)

On the other hand, the research group led by Ted Ringwood of the Australian National University presented a paper at the conference in which they argued: The abundance patterns of volatile elements in the moon differ dramatically from those to be expected from the condensation of a nebula of solar composition, thereby demonstrating that the moon was not formed from components which themselves had condensed directly from the solar nebula. … It is proposed that the moon formed in earth-orbit by recondensation of material evaporated by the impact of a large planetesimal, from the earth’s hot outer mantle subsequent to core formation. (Ringwood & Kesson, 1977: 371)

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Ringwood and his Australian colleagues were thus supporting the ideas about the formation of the Moon as having been caused by the impact on the Earth of a Mars-sized object. As noted above, these ideas had been articulated recently by Hartmann and Davis and Cameron and Ward. John O’Keefe and Harold Urey had come to a conclusion similar to that of Ringwood after studying the deficiency of siderophile elements in the Moon. The deficiency of siderophiles in the crust of the Earth is customarily attributed to leaching by metallic nickel-iron, which eventually sank to form the core. A similar deficiency exists in the Moon, which has at best, a very much smaller core. Hence it is logical to consider the hypothesis that the Moon formed from the mantle of the Earth, after the siderophiles had been removed. … the non-hydrostatic figure of the Moon, and the requirement that the mascons must be supported, together with the high heat flow, imply that the metal of the Moon is collected in the core. It probably amounts to less than 1% of the Moon’s mass. Calculations show that if the core is in chemical equilibrium with the lunar silicates then the nickel has been removed from the Moon as a whole to an extent which is greater than can be explained by theories of direct formation from a nebula. … (O’Keefe & Urey, 1977: 569)

Thus, Urey had abandoned his view of 1955 that “… very probably the moon was accumulated at low temperatures from a primitive dust cloud of solar composition …” (Urey, 1955: 427). Not only that, but he now sided with the fission enthusiasts: The deficiency of siderophile elements in the Moon suggests leaching by liquid metallic iron. The small amount of metal in the Moon then suggests that the leaching took place in the protoearth, and that the Moon therefore formed from it by fission. (O’Keefe & Urey, 1977: 575)

In the face of conflicting models for the formation of the Moon, Bill Kaula (Fig. 5.10) of UCLA decided to try a methodical approach to resolve the arguments. At the Eighth Lunar Science Conference, he reviewed the evolving ideas about the theories for the origin of the Moon. Kaula listed as possible mechanisms for lunar formation: A. Impacting of a very large body into the Earth, B. Tidal disruption of sizeable differentiated planetesimals by the Earth, and C. Selective capture of differentiated planetesimal material by small moonlets. Kaula asserted: It seems almost universally agreed that the severe depletion in the lunar rocks of refractory siderophiles (osmium, rhenium, iridium, etc.), simultaneous with the … enrichment in refractory lithophiles, rules out nebula condensation processes as a means for differentiating the proto-lunar material, and requires prior differentiation in a planetary body. … (Kaula, 1977: 324)

Kaula listed four possible loci for the differentiation of proto-lunar material that had been suggested by various investigators who had proposed models for the formation of the Moon:

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Fig. 5.10 Photograph of William M. Kaula, courtesy of the American Geophysical Union and the AIP Emilio Segre Visual Archive

1. The Earth; 2. A large body, or a few large bodies, tidally disrupted by passing close to the Earth; 3. Satellites in orbit around the Earth; and 4. Planetesimals in orbit around the sun. Kaula pointed out that each of the possible loci entails four problems: (a) An energy source for the differentiation; (b) A mechanism for removal of material to geocentric orbit; (c) A means of compositional selection—refractory lithophile gain, volatile and siderophile loss; and (d) A means of attaining an initial temperature distribution satisfying the lunar thermal history constraints … : a cool interior and a hot exterior for the Moon.

Kaula noted that model A—impacting of a very large body into the Earth— would solve the above four problems better that models B and C, but “The likelihood of a Mars-sized body hitting the earth seems low …” (ibid.: 329). He concluded that “Each mechanism has its difficulties; the major unknown affecting all of them is the size distribution of planetesimals.” (ibid.: 321) and “All

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Fig. 5.11 Photograph of Thomas R. McGetchin, courtesy of the Lunar and Planetary Institute

hypotheses of lunar origin thus seem implausible and difficult to evaluate.” (ibid.: 329) (Fig. 5.11). The next director of the LSI, Thomas R. McGetchin (1936–1979) (Fig. 5.11), had wide interests in the geosciences, including the internal structure of Mars and other planets, as well as the geology of the Moon. McGetchin, became the Director of the LSI on 15 June 1977, succeeding Robert Pepin, who returned to the University of Minnesota. McGetchin was a graduate of Caltech, where his thesis adviser was Leon T. (Lee) Silver. McGetchin soon pushed to change the name of the institute from the Lunar Science Institute to the Lunar and Planetary Institute (LPI), and that change occurred on the first of January 1978. Later that year, McGetchin’s newly renamed LPI organized the Ninth Lunar and Planetary Science Conference, which took the place of the annual Lunar Science Conference.

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The Ninth Lunar and Planetary Science Conference

The kind of careful analyses and thorough discussion that was needed for an eventual consensus continued, as exemplified by papers given at the Ninth Lunar and Planetary Science Conference by research groups led by Ted Ringwood of the Australian National University and Heinrich Wänke of the Max Planck Institut für Chemie, and by Edward Anders, who led a research group at the University of Chicago. These groups had been studying the chemistry of samples from the lunar highlands. The first task—and the point of disagreement between Anders and the others—was what constituted “pristine” lunar highland rocks. In general, samples from the lunar highlands were thought to consist of meteoritic debris as well as “pristine” or “indigenous” rocks that had been part of the primary lunar crust. Some researchers, among them the Anders group, defined “pristine” highlands rocks as those that had low abundances of (iron-loving) siderophile elements, e.g., nickel. These rocks were assumed to have been produced by igneous activity internal to the Moon and thus did not contain any parts of meteorites. Chondritic meteorites would have “cosmic” abundances of the siderophile elements. Other researchers, among them the Ringwood group, argued that the so-called “pristine” lunar highlands samples had been deprived of their siderophiles during impact melts, when the siderophiles would have followed iron to lower levels of the Moon. They attempted to “subtract out” the meteoritic contamination of non“pristine” lunar highlands samples to arrive at the original lunar “siderophile signature.” Their research objective was to compare this lunar siderophile signature with that of the Earth’s upper layers. Ringwood’s group concluded that the Moon’s “siderophile signature” was comparable to that of the Earth’s upper mantle, and they argued: The Earth’s upper mantle possesses a unique “siderophile signature” that is probably a result of equilibration between mantle phases and an oxygen-bearing, Fe-rich core at very high pressures … . For the moon to exhibit a comparable “siderophile signature” when it is incapable of having experienced similar processes, implies that the moon was derived from the earth’s upper mantle after segregation of the core. … (Delano & Ringwood, 1978c: 123–124)

Anders disagreed and contended that Ringwood’s group manipulated their data. One can sense the intensity of the disagreement in the assertion by Anders that: Delano and Ringwood (1978a,b,c) contend that the siderophiles in the lunar highlands are mainly of indigenous rather than meteoritic origin. I shall endeavor to show that they reach this conclusion by Procrustean methods, stretching or chopping the evidence to fit a pre-conceived mold. … (Anders, 1978: 161)

There were counter-charges and arguments by the Ringwood group, as well as the Wänke group. The consensus view, however, seemed to be that of Ross Taylor, who spent several months at a time at the LPI as a visiting scientist from his home institution, The Australian National University. His view was namely that:

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… The complex history of the [Earth’s] mantle with respect to the distribution and abundance of siderophile elements probably makes comparison invalid for the trace elements. Thus, there is good evidence that the abundances of Os, Ir, and Pd in the Earth’s upper mantle have chondritic ratios. These are probably due to a late addition of material from meteorites. Accordingly, there is not a unique terrestrial siderophile element signature. (Taylor, 1982: 425)

In other words, as John Wood would later write, “… too many factors cloud the picture to allow mantle compositions to be used as a convincing argument either for or against formation of the Moon from the Earth. …” (Wood, 1986: 26–27). Later that year, seven years after the Apollo 11 Lunar Science Conference, Senator William Proxmire would ask in a hearing on the NASA budget, “How long does it take to study a Moon rock?” Senator Proxmire was the Chair of the Senate Subcommittee in charge of NASA’s funding, and he was also the creator of the “Golden Fleece Award.” The April 1976 Golden Fleece Award went to NASA “For requesting $2.8 million to construct an addition to the Lunar Laboratory to house 100 lb of Moon rocks.” At an August 1978 hearing of his Senate Appropriations Subcommittee, Proxmire wanted to delete all of NASA’s funding for lunar sample research. The final outcome of the debate within the subcommittee was that the NASA line item for lunar sample analysis was reduced from the requested $5.7 million to $1 million dollars. Despite this drastic cut, NASA managers were able to continue supporting the analyses of lunar samples by the university research community. It would have been imprudent to inform the Senator that it would take many more years of analyses of samples from several locations on the lunar surface, further lunar seismological studies, and extensive computer modeling to come up with a theory of the Moon’s formation with which almost all lunar scientists would agree. It would eventually happen, but not before another seven years of careful research and discussion within the lunar research community.

5.12

The Tenth Lunar and Planetary Science Conference

The LPI co-sponsored with NASA the Tenth Lunar and Planetary Science Conference, which was held at the Gilruth Center of NASA’s Johnson Space Center on 19–23 March 1979. At the conference, Ross Taylor presented a paper in which he politely disagreed with his colleagues at the Australian National University. He first reiterated his opinion that trying to compare siderophile signatures of the Earth and the Moon is a difficult, if not impossible task. Various disputes have arisen during the examination of the chemical evidence bearing on the fission hypothesis, with attention focussing mainly on proposed primitive siderophile element abundances in the lunar highlands. However, these regions do not appear as particularly suitable in which to set up tests relying on chemical similarities between the earth and the moon. … It is generally agreed that the highlands formed primarily through the flotation of plagioclase during crystallisation of the magma ocean, later invaded by the

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residual liquid (final 2%), which provides the component we know as KREEP. A primitive Mg-rich component is derived either from a primordial frozen crust on the magma ocean, possibly from liquid trapped during the crystallization of the highlands or perhaps from later infalling planetesimals during the final stages of accretion. Probably all these processes contribute. The resulting crust has been repeatedly pulverized by the catastrophic or cataclysmic bombardment, producing craters and ringed basins of all sizes. All these have added in a component of siderophile elements, and it seems to be a difficult task to disentangle all these effects to get back to the primeval unfractionated lunar composition, which we can then compare with the composition of the terrestrial mantle to test the fission hypothesis. At this stage, the comparison is complicated by the possibility that the terrestrial upper mantle may contain relics of a similar bombardment, which could contribute a signature of highly siderophile elements which would mimic that of the moon … Thus the differing evolutionary histories of the moon and of the terrestrial mantle make simple intercomparisons so complex that less equivocal tests are called for. The requirement is to compare bulk elemental abundances which can be estimated despite differing geochemical evolution in the two bodies, and which can be calculated by a variety of independent methods. (Taylor, 1979: 2018)

Taylor estimated the bulk elemental abundance of uranium, which is a refractory element, at 15–20 parts per billion (ppb) for the Earth and approximately 40 ppb for the Moon. He noted, “The enrichment of the moon in refractory elements (including the major elements Ca and Al) over the earth by a factor of two or greater is difficult to accomplish by simple fission models. …” (ibid.: 2017).

Fig. 5.12 Photograph of John R. Sevier, courtesy of the Lunar and Planetary Institute

5.12

The Tenth Lunar and Planetary Science Conference

135

In addition to the Tenth Lunar and Planetary Science Conference, during 1979, the LPI sponsored a workshop and a topical conference that furthered the discussion on the possible “genetic” relationship between the Earth and the Moon. A workshop titled Ancient Crust of the Terrestrial Planets was held on 12–14 February, and a topical conference on the Lunar Highlands Crust was held on 14–16 November. During the summer of 1979, Tom McGetchin resigned as Director of the LPI, owing to a serious medical condition. In the fall of that year, Tom died of cancer. His deputy, John R. “Jack” Sevier (1929–1999) (Fig. 5.12), took over as acting director of the institute.

5.13

Basaltic Volcanism on the Terrestrial Planets

The end of 1979 marked the conclusion of a massive three-year effort that was titled the Basaltic Volcanism Study Project. The project was funded by NASA and organized and sponsored by the LPI. Ideas for the project were developed in the fall of 1975 at an LSI-sponsored Conference on Origins of Mare Basalts and Their Implications for Lunar Evolution. The project involved a total of 105 scientists from across the U.S., as well as from non-U.S. planetary research centers from across the globe. The project led to the publication of a major research volume titled Basaltic Volcanism on the Terrestrial Planets. This book provided good summaries of what had been learned during the Apollo exploration of the Moon. For example, the book provided a brief outline of the lunar time scale: 1. A large scale lunar differentiation occurred *4.4 b.y. ago, resulting in the formation of alumina rich crust and sources of lunar basalts with characteristic chemical fractionation patterns. U and Th and other refractory elements were enriched in the outer layer of the Moon. 2. Excavation of giant ring basins by bombardment occurred prior to about 4 b.y. ago. The duration of the bombardment is not known. The youngest of the basins is the well preserved and only partially flooded basin of Mare Orientale on the western edge of the nearside of the Moon. The next youngest is that of Imbrium. With the exception of Orientale, all nearside giant basins are flooded. 3. Abrupt cessation of the intense bombardment occurred *3.9 b.y. ago. 4. Extrustion of magma to the surface and flooding of the giant ring basins occurred 3.9– 3.1 b.y. ago. Since then only relatively minor bombardment and magmatism have occurred. (Basaltic Volcanism Study Project, 1981: 948)

A summary of conclusions related to mare flooding was also provided. 1. Lunar magmatism represents a short duration (3.8–3.1 b.y.) when compared with the age of the planet, which existed as a separate body for *4.5 b.y. In view of limited sampling, however, the existence of basalts with ages outside the established range cannot be excluded …

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5 Post-Apollo Synthesis and Debate 2. The duration of flooding of individual preexisting giant basins by magmas was of the order of a few hundred million years. Eruption was associated with different degrees of mild elemental fractionation. 3. The sources (reservoirs) of mare basalts are heterogeneous. The heterogeneity was established early in time (*4.4 b.y.) and was preserved to at least the time of magma eruption. 4. The sources of lunar basalts are generally old (*4.4 b.y.) approaching the age of the planet itself. This indicates that the differentiation process could not have lasted more than *150 m.y. after accretion. 5. Despite their antiquity, these sources are not pristine primary materials but rather secondary (or higher order) differentiates. (ibid.: 959)

The book termed the problem of the evolution of the Moon as “severe,” since for the Moon, “… [the] peculiar bulk composition and early differentiation of a thick crust appear to require special circumstances.” (ibid.: xlvii). The book perhaps reflected the growing awareness of the problem related to the early differentiation of the Moon’s crust, namely that ordinary accretion of material on the Moon would not deliver enough heat to accomplish the melting of the surface to form a magma ocean that solidified into the crust. Bill Kaula, who was chair of the editorial committee for the book, had written in an article for the Journal of Geophysical Research earlier in 1979 that: It is often suggested that the terrestrial planets were quite hot at, or soon after, formation: too soon for the long-lived radioisotopes to have generated appreciable heat. … These requirements for early heat have often been misinterpreted as requirements for rapid accretion, of the order of 103–104 years’ accretion time … . This need for rapid accretion is based on the dubious assumption that the accreted material was all in very small particles, such that all the heat is deposited in a thin layer so that it can rapidly radiate away… . However, examinations of the dynamics of formation of a planet from a planetesimal swarm get 107 years or more for the accretion time … . The solution to this problem was suggested by Barrell, Schuchert, Woodruff, Lull, & Huntington, (1918) and Safronov (1972); if a significant proportion of the accreted mass was in sizable planetesimals, then some of the heat generated by their infall would have been buried deep enough not to be lost by the planet. … (Kaula, 1979: 999)

After performing a computer-based analysis of an extension of an accretion model that had been developed by Victor Safranov, Kaula concluded that his results “… favor formation of the moon as a consequence of a great impact (or impacts) into the earth …” (ibid.). The Lunar and Planetary Science Council, whose convener was John B. Adams of the University of Washington, served as a search committee for the next director of the Lunar and Planetary Institute. The Science Council recommended, and the USRA board approved, Roger Phillips (Fig. 5.13) of the Jet Propulsion Laboratory to be the next Director of the LPI. Phillips was a geophysicist whose principal interest was in understanding the state and evolution of planetary interiors. He obtained his Ph.D. at the University of California, Berkeley. Phillips took over as Director of the LPI on 15 October 1979.

5.14

The Eleventh Lunar and Planetary Science Conference

137

Fig. 5.13 Photograph of Roger J. Phillips, courtesy of the Lunar and Planetary Institute

5.14

The Eleventh Lunar and Planetary Science Conference

1980 was a busy year for the lunar and planetary community. The LPI co-sponsored with NASA the Eleventh Lunar and Planetary Science Conference during 17 through 21 March. At this conference, Ross Taylor presented a paper on his analyses of the abundances of refractory and moderately volatile elements in the Earth, Moon, and meteorites. On the basis of his own analyses and that of others, Taylor found: … evidence for widespread fractionation among refractory and moderately volatile elements, most probably during planetary accretion. This indicates that the earth-moon relationship is not unique, but represents a common situation in the inner solar system. The origin of the chemical differences between the earth and the moon must thus be placed in a wider context and would appear to favor models of origin based on some variation of the double-planet hypothesis. (Taylor, 1980: 345)

Taylor thus arrived at a different conclusion from Kaula, and the controversy over the formation of the Moon remained alive. Alive and well, actually—at the same conference, Alan Binder of the Institut für Mineralogie at the Universität

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Münster in West Germany presented a paper with the opening sentence, “There is increasing evidence supporting the model that the moon originated by binary fission from the proto-earth.” (Binder, 1980: 1931).

5.15

Concluding Remarks

At the beginning of the decade of the 1980s, several options for the formation of the Moon remained under discussion, not all of which were completely separate ideas. To the three listed by Kaula: A. Impacting of a very large body into the Earth, B. Tidal disruption of sizeable differentiated planetesimals by the Earth, and C. Selective capture of differentiated planetesimal material by small moonlets.

Taylor added: D. Some variation of the double-planet hypothesis.

and Binder revived: E. Fission from the proto-Earth.

References Anders, E. (1978). Procrustean science: Indigenous siderophiles in the lunar highlands, according to Delano and Ringwood. In R. B. Merrill (Ed.), Proceedings of the Ninth Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta, supplement 10, v. 1) (pp. 161–184). Elmsford, NY: Pergamon Press. Barrell, J., Schuchert, C., Woodruff, L. L., Lull, R. S., & Huntington, E. (1918). The evolution of the earth and its inhabitants. New Haven, CT: Yale University Press. Basaltic Volcanism Study Project. (1981). Basaltic volcanism on the terrestrial planets. Elmsford, NY: Pergamon Press. Binder, A. B. (1980). The first few hundred years of evolution of a moon of fission origin. In R. B. Merrill (Ed.), Proceedings of the Eleventh Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta, supplement 14, v. 3) (pp. 1931–1939). Elmsford, NY: Pergamon Press. Boss, A. P. (2006). George West Wetherill (1925–2006). Bulletin of the American Astronomical Society, 38, 1284. Cameron, A. G. W., & Ward, W. R. (1976). The origin of the Moon [Abstract]. Abstracts of papers submitted to the seventh lunar science conference (pp. 120–122). Lunar and Planetary Institute: Houston, TX. Clayton, R. N., & Mayeda, T. K. (1975). Genetic relations between the Moon and meteorites. In R. B. Merrill (Ed.), Proceedings of the Sixth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 6, v. 2) (pp. 1761–1769). Elmsford, NY: Pergamon Press.

References

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Delano, J. W., & Ringwood, A. E. (1978a). Indigenous siderophile element component of the lunar highlands and its significance [Abstract]. Abstracts of papers submitted to the ninth lunar and planetary science conference (pp. 238–240). Lunar and Planetary Institute: Houston, TX. Delano, J. W., & Ringwood, A. E. (1978b). Indigenous abundances of siderophile elements in lunar highlands: Implications for the origin of the Moon. The Moon and the Planets, 18(4), 385–425. Delano, J. W., & Ringwood, A. E. (1978c). Siderophile elements in the lunar highlands: Nature of the indigenous component and implications for the origin of the Moon. In R. B. Merrill (Ed.), Proceedings of the Ninth Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta, supplement 10, v. 1) (pp. 111–159). Elmsford, NY: Pergamon Press. Hartmann, W. K., & Davis, D. R. (1975). Satellite-sized planetesimals and lunar origin. Icarus, 24 (4), 504–515. Kaula, W. M. (1977). On the origin of the Moon, with emphasis on bulk composition. In R. B. Merrill (Ed.), Proceedings of the Eighth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 8, v. 1) (pp. 321–331). Elmsford, NY: Pergamon Press. Kaula, W. M. (1979). Thermal evolution of Earth and Moon growing by planetesimal impacts. Journal of Geophysical Research-Solid Earth, 84(B3), 999–1008. Kaula, W. M., & Yoder, C. F. (1976). Lunar orbit evolution and tidal heating of the Moon [Abstract]. Abstracts of papers submitted to the seventh lunar science conference (pp. 440– 442). Lunar and Planetary Institute: Houston, TX. Lunar Sample Analysis Planning Team: Bell, P., Burlingame, A., Burnett, D., Silver, L., Chang, S., Gault, D., Eberhardt, P., Haskin, L., James, O., Papike, J., Reed, G., Toksöz, N., & Wood, J. (1973). Fourth lunar science conference: Relocation of crustal materials by cataclysmic impacts dominated the first 700 million years of lunar history. Science, 181(4100), 615–622. Marvin, U. B. (2002). Oral histories in meteoritics and planetary science: IX. Heinrich Wänke. Meteoritics & Planetary Science, 37(S12), B79–B88. Mitler, H. E. (1975). Formation of an iron-poor Moon by partial capture, or yet another exotic theory of lunar origin. Icarus, 24(2), 256–268. Newell, H. E. (1973). Harold Urey and the Moon. The Moon, 7(1–2), 1–5. O’Keefe, III, J. A. (1974). The formation of the Moon. In D. W. Strangway (Ed.), Proceedings of a Conference on the Geophysical and Geochemical Exploration of the Moon and Planets at the Lunar Science Institute, Houston, Texas, January 10–12, 1973 (The Moon, 9(1–2)) (pp. 219– 225). Dordrecht, Holland: D. Reidel. O’Keefe, J. A., & Urey, H. C. (1977). Deficiency of siderophile elements in the Moon. Philosophical Transactions of the Royal Society of London A, 285(1327), 569–575. Rammensee, W., & Wänke, H. (1977). On the partition coefficient of tungsten between metal and silicate and its bearing on the origin of the Moon. In R. B. Merrill (Ed.), Proceedings of the Eighth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 8, v. 1) (pp. 399–409). Elmsford, NY: Pergamon Press. Ridley, W. I., Reid, A. M., & Brett, P. R. (1974). Lunar petrology conference. Eos, Transactions, American Geophysical Union, 55(1), 4–8. Ringwood, A. E., & Kesson, S. E. (1977). Composition and origin of the Moon. In R. B. Merrill (Ed.), Proceedings of the Eighth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 8, v. 1) (pp. 371–398). Elmsford, NY: Pergamon Press. Safronov, V. S. (1972). Evolution of the protoplanetary cloud and formation of the Earth and Planets (NASA Technical Translation F-677). Jerusalem, Israel: Israel Program for Scientific Translations, Keter Publishing. Safronov, V. S., & Zvjagina, E. V. (1969). Relative sizes of the largest bodies during the accumulation of planets. Icarus, 10(1), 109–115. Smith, J. V. (1974). Origin of the Moon by disintegrative capture with chemical differentiation followed by sequential accretion [Abstract]. Abstracts of the papers submitted to the fifth lunar science conference (pp. 718–720). Lunar and Planetary Institute: Houston, TX. Smith, J. V. (1977). Possible controls on the bulk composition of the Earth: Implications for the origin of the Earth and Moon. In R. B. Merrill (Ed.), Proceedings of the Eighth Lunar Science

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Conference (Geochimica et Cosmochimica Acta, supplement 8, v. 1) (pp. 333–369). Elmsford, NY: Pergamon Press. Soter, S. (1971). The dust belts of Mars. Strangway, D. W. (Ed.). (1974). Proceedings of a Conference on the Geophysical and Geochemical Exploration of the Moon and Planets at the Lunar Science Institute, Houston, Texas, January 10–12, 1973 (The Moon, 9(1-2)) (pp. 1-245). Dordrecht, Holland: D. Reidel. Taylor, S. R. (1975). Lunar science: A post-apollo view: Scientific results and insights from the lunar samples. Elmsford, NY: Pergamon Press. Taylor, S. R. (1979). Lunar and terrestrial potassium and uranium abundances: Implications for the fission hypothesis. In R. B. Merrill (Ed.), Proceedings of the Tenth Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta, supplement 11, v. 2) (pp. 2017– 2030). Elmsford, NY: Pergamon Press. Taylor, S. R. (1980). Refractory and moderately volatile element abundances in the Earth, Moon and meteorites. In R. B. Merrill (Ed.), Proceedings of the Eleventh Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta, supplement 14, v. 1) (pp. 333–348). Elmsford, NY: Pergamon Press. Taylor, S. R. (1982). Planetary science: A lunar perspective. Houston, TX: Lunar and Planetary Institute. Taylor, S. R., & Jakeš, P. (1974). The geochemical evolution of the Moon. In W. A. Gose (Ed.), Proceedings of the Fifth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 5, v. 2) (pp. 1287–1305). Elmsford, NY: Pergamon Press. Toksöz, M. N., Dainty, A. M., Solomon, S. C., & Anderson, K. R. (1973). Velocity structure and evolution of the Moon. In W. A. Gose (Ed.), Proceedings of the Fourth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 4, v. 3) (pp. 2529–2547). Elmsford, NY: Pergamon Press. Urey, H. C. (1955). Some criticisms of ‘on the origin of the lunar surface features’ by G. P. Kuiper. Proceedings of the National Academy of Sciences USA, 41(7), 423–428. Wetherill, G. W. (1975). Radiometric chronology of the early solar system. Annual Review of Nuclear Science, 25, 283–328. Wetherill, G. W. (1976a). The role of large impacts in the formation of the Earth and Moon [Abstract]. Abstracts of papers submitted to the seventh lunar science conference (pp. 930– 932). Lunar and Planetary Institute: Houston, TX. Wetherill, G. W. (1976b). The role of large bodies in the formation of the Earth and Moon. In R. B. Merrill (Ed.), Proceedings of the Seventh lLunar Science Conference (Geochimica et Cosmochimica Acta, supplement 7, v. 3) (pp. 3245–3257). Elmsford, NY: Pergamon Press. Wood, J. A. (1977). Origin of Earth’s Moon. In J. A. Burns (Ed.), Planetary satellites (pp. 513– 529). Tucson, AZ: University of Arizona Press. Wood, J. A. (1986). Moon over Mauna Loa: A review of hypotheses of formation of Earth’s Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 17– 55). Houston, TX: Lunar and Planetary Institute. Wood, J. A., & Mitler, H. E. (1974). Origin of the Moon by a modified capture mechanism, or half a loaf is better than a whole one [Abstract]. Abstracts of papers submitted to the fifth lunar science conference (pp. 851–853). Lunar and Planetary Institute: Houston, TX.

Chapter 6

Widening the Research Front

6.1

Introduction

The change of the name of the Lunar Science Institute to the Lunar and Planetary Institute in 1979 reflected the widening interest of the community of researchers who were originally focused on the Apollo explorations of the Moon. On the one hand, ideas about the origin of the Moon began to be discussed more in a planetary context than had been the case earlier. On the other hand, what had been learned from lunar research began to be applied to origins of the Earth and other planets in the solar system. In this chapter, we follow some of the dialog within the lunar and planetary research community through the early 1980s.

6.2

Meteorites

Comparison of various aspects of lunar samples with data from the Earth and from meteorites was an important avenue of inquiry. The LPI had recently begun to support the Meteorite Working Group, which provided guidance on the scientific use of the growing collection of meteorites recovered from Antarctica. The Meteorite Working Group held its first meeting at the LPI on 9–13 April 1980. The enthusiasm for the study of meteorites greatly accelerated following the discovery by Japanese scientists of several different types of meteorites in a small area in Antarctica. William A. “Bill” Cassidy of the University of Pittsburgh soon proposed to the National Science Foundation that he be funded to search for additional meteorites in Antarctica. His proposal was eventually funded by NSF’s Polar Research Program, and it matured into the Antarctic Search for Meteorites (ANSMET) program. Initially, the program was jointly conducted with the Japanese and soon involved NASA and the Smithsonian Institution, as well as NSF. NASA’s role was to curate © Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_6

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Fig. 6.1 Profile of an ice sheet showing an idealized model of a meteorite trap. The vertical exaggeration in the sketch is about a factor of 50. From Cassidy (2003) Meteorites, Ice, and Antarctica: A Personal Account. New York, NY: Cambridge University Press, page 309, Copyright 2003 by Cambridge University Press, and used with their permission

the meteorites that were found on the ice in Antarctica, and NASA used the Lunar Curatorial Facility at the Johnson Space Center in Houston to do that. Lunar and planetary scientists were eventually able to request samples of meteorites from Antarctica, as well as lunar samples, to study. NSF provided funds to USRA-LPI through NASA to support the Meteorite Working Group. Meteorites found on the ice in Antarctica have enormously increased the total inventory of meteorites found on Earth. Figure 6.1, taken from Cassidy’s book Meteorites, Ice, and Antarctica: A Personal Account (Cassidy, 2003), illustrates why various places in Antarctica have proven to be good “traps” for meteorites. The general idea is that ice flows from the center of the continent toward the sea, and if the ice flow encounters a barrier, such as the Transantarctic Mountains, meteorites will be left on the surface as the ice carrying them evaporates away. As Cassidy explains: Meteorites fall onto Antarctica at the same low but constant rate as they fall elsewhere. Unless it falls directly onto a stranding surface, the terrestrial history of an Antarctic meteorite includes a period of time embedded in ice. Whether a meteorite becomes part of a concentration in Antarctica apparently depends on its trajectory across the continent, carried by ice from the point where it fell to the point where it is ultimately freed of the ice surrounding it. Most are released from the ice when a floating iceberg melts, and it falls into sea floor sediments as a dropstone. A relatively few of all that have fallen on the entire continent are exposed on a stranding surface when a mass of stagnant ice experiences negative mass balance and evaporates away, leaving meteorites behind as a sort of residual deposit. (ibid.: 304)

The Antarctic meteorites are generally in pristine condition, and, relative to meteorites found elsewhere, many more stony meteorites are found, since stony meteorites found elsewhere on the Earth deteriorate because of weathering. Note from Cassidy’s discussion how greatly the ANSMET collection has increased the total number of meteorites available for study.

6.2 Meteorites

143

The two subtypes of stony meteorites are chondrites and achondrites. Among the modern falls, there are 781 chondrites and 73 achondrites that have been identified as to group. Among the ANSMET finds, the collection contains 8409 chondrites and 234 achondrites. …” (ibid.: 249)

The vast majority of meteorites are thought to be fragments from the collisions of bodies in the asteroid belt. Some meteorites found on the ice in Antarctica, however, are known to have been caused by impacts on the surfaces of the Moon and Mars.

6.3

Workshop on Early Crustal Genesis: Implications from Earth

In addition to the annual Lunar and Planetary Science Conference, the LPI sponsored a total of seven other conferences and workshops during 1981. The three that occurred in November were: November 3–5: Workshop on Early Crustal Genesis: Implications from Earth November 10–12: Conference on Multi-Ring Basins: Formation and Evolution (resulted in the publication of a book—Multi-Ring Basins) November 13–15: Workshop on Apollo 16, the first of a series of meetings that were part of NASA’s Highlands Initiative, which was aimed at an improved understanding of the early evolution of the lunar crust

The Workshop on Early Crustal Genesis: Implications from Earth, which featured key presentations by LPI scientists, illustrates the trend to widen the domain of inquiry beyond the Moon to the other planets. This workshop was a prelude to a NASA-funded Early Crustal Genesis project, which the LPI organized. An example of the linkage between the study of the Moon and research on planetary science in general is provided in part of the summary language of a document prepared for the Early Crustal Genesis project. Under the Planetary Formation section, the LPI Director, Roger Phillips, and an LPI Research Scientist, Lewis Ashwal, wrote: It appears almost certain that infalling planetesimals had a great range of sizes, with the bulk of the mass in the larger bodies. … An important consequence of the bulk of the mass being in the larger bodies is that most of the delivered heat would have been buried, rather than lost by radiation or ejecta. … … … Most of the mass in planetary formation probably came in bodies appreciably larger than that which created Mare Imbrium. Hence, probably more than half the energy of impacts was retained as heat within the forming planets, providing ample energy for crustal differentiation and raising the temperature high enough for core formation well beyond completion of growth. … (Philips & Ashwal, 1981: 24)

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As Bill Kaula noted (Chap. 5), the insight that large infalling planetesimals were needed in order to deposit heat deep enough for it to be retained in a planetary body can be traced at least as far back as 1916, when Joseph Barrell, Professor of Structural Geology at Yale University, gave the opening lecture in a series of Sigma Xi lectures at Yale. Barrell’s lecture was titled “The origin of the Earth,” and in discussing the aggregation of material to form the Earth he said in part: The energy of impact from dust-like material would be absorbed at the surface and, as heat, quickly radiated into space. The accretion of dust would favor the growth of an earth solid throughout. Larger masses would, on the other hand, carry the energy of impact into the earth. … If in addition the infall of masses was sufficiently rapid to bury the heat of previous infalls before it could be dissipated by conduction to the surface, a general heating and liquefaction of the earth would tend to take place, both from the increased compression of the deeper nucleus and the effects of impacts at higher levels. (Barrell, Schuchert, Woodruff, Lull, & Huntington, 1918: 27–28)

For these and other reasons, Barrell favored the hypothesis of an Earth that was initially molten. A molten state suggests a rapid earth-growth due to an original clustering of the matter whose convergence built up the planet. Larger nuclei hundreds of miles in diameter and smaller ones comparable to the planetoids moved in elliptic and nearly intersecting orbits. Mutual perturbations kept modifying these orbits and providing new chances for collisions, union, and growth. Such collisions led to a development of energy of impact sufficient to produce in the growing earth a molten state, at least in the outer portions. The earth kept growing at the same time sweeping up large quantities of finer material, but a molten state suggests that the greater growth was due to the infall of larger nuclei. … (ibid.: 33)

The trend of broadening the domain of inquiry beyond lunar science continued in 1981 with the Twelfth Lunar and Planetary Science Conference, where only 5 of the 9 sections were focused on the Moon. The remaining sections were focused on meteorites, Mars and Venus, Satellites of Jupiter, and other topics in solar system and planetary physics.

6.4

The Snowbird Conferences

Another example of the diversity of research interests within the planetary science community is illustrated by a conference that the LPI held later in the year. This was the first in a series of conferences on large-body impacts that came to be known as the “Snowbird Conferences,” though only the first two were held in Snowbird, Utah. The first conference was co-sponsored with the National Academy of Sciences and was held during 19–22 October 1981. It was titled Conference on Large Body Impacts and Terrestrial Evolution: Geological, Climatological, and Biological Implications. The subject of the conference was motivated in large part by the recent discovery of Luis Alvarez and his colleagues at the University of California, Berkeley, of a possible cause for the extinctions at the

6.4 The Snowbird Conferences

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Cretaceous-Tertiary boundary. The abstract of the article published in Science magazine in 1980 by Alvarez et al. read as follows: Platinum metals are depleted in the earth’s crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object’s mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record. One prediction of this hypothesis had been verified: the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other. Four different independent estimates of the diameter of the asteroid give values that lie in the range 10 ± 4 km. (Alvarez., Alvarez, Asaro, & Michel, 1980: 1095)

Many of the scientists at the first Snowbird Conference were prepared to accept the evidence of the Alvarez group that a large bolide had impacted the Earth and caused the extinction of many species, including the dinosaurs. Some were more skeptical. Among them was Thomas J. M. Schopf (1939–1984), a highly respected paleontologist from the University of Chicago, whose abstract read: The first step in any scientific program is to determine the problem to be solved. The often popular view that thousands of species of dinosaurs went extinct in the space of a year or two, world wide, is not true. The firm evidence is that during the last 2 to 3 m.y. of the latest Cretaceous … a total of about 16 species … which had been living along the margins of a large seaway (which once extended from the Gulf of Mexico to the Arctic Circle) died off as the seaway dried up. Elsewhere in the world, local populations of dinosaurs had evidently died out before the latest Cretaceous both in Mongolia and in southern Europe. Possibly a species persisted in northern Europe into the latest Cretaceous. Seen in this light, the extinction of the dinosaurs is a perfectly understandable phenomenon—indeed no different than the fate of millions and millions of previous species. The reason why the extinction of the dinosaurs has attracted so much non-scientific attention by scientists and others is that (1) it doesn’t cost anyone anything, (2) it sounds impressive, (3) it’s basically a rather unimportant scientific problem though a rather important popular problem, and (4) hard paleontological data are difficult to obtain. (Schopf, 1981: 49)

The current consensus view seems to be that the impact of the bolide that created the Chicxulub crater 65 million years ago was at least a contributing factor for the extinctions at the end of the Cretaceous. In any case, the LPI facilitated further discussion of this issue by co-sponsoring three more Snowbird Conferences during the next two decades: October 20–22, 1988: Global Catastrophes in Earth History: Conference on Impacts, Volcanism, and Mass Mortality—held in Snowbird, Utah; February 9–12, 1994: Conference on New Developments Regarding the KT Event and Other Catastrophes in Earth History—held at the University of Houston- Clear Lake; and

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July 9–12, 2000: Catastrophic Events and Mass Extinctions: Impacts and Beyond—held at the University of Vienna.

This line of research is extremely interesting and increasingly understood to be important for Earth’s future, but is only indirectly related to the problem of the formation of the Moon. Near-Earth Asteroids are linked to several extinction events on Earth and remain today a threat to our planet. Such work represents just one of many examples that could be given of the great variety of research interests that occupied the lunar and planetary research community during the early 1980s.

6.5

Discussions on the Formation of the Moon in a Planetary Context

Broader interest in planetary science in the 1980s was fueled by ever-more sophisticated spacecraft that flew by, orbited, or landed on other planets in the solar system. These spacecraft originated within the space programs of both the US and the USSR. On 1 March 1982, the Soviet spacecraft Venera 13 landed on the surface of Venus, and Venera 14 landed four days later. The Proceedings of the Thirteenth Lunar and Planetary Science Conference contained a paper by Soviet scientists on the composition of rocks on Venus, as well as a paper on the geology of Tethys based on data taken by Voyager 2 as it flew by this moon of Saturn in 1981. Furthering the understanding of the terrestrial planets via study of the Moon had been a major theme of NASA’s lunar program, and in March of 1982, the LPI published a book by Ross Taylor titled Planetary Science: A Lunar Perspective. The six workshops and topical conferences sponsored or co-sponsored by the LPI in 1982 reflected this theme and the diversity of interests in the lunar and planetary community: April 15–16: Workshop on Lunar Bases April 19–21: Workshop on Antarctic Glaciology and Meteorites April 20–22: Workshop on Venus Tectonics October 9–12: Conference on Planetary Volatiles October 15–17: Workshop on Pristine Highlands Rocks and the early History of the Moon November 15–17: Conference on Chondrules and their Origins—resulted in a book published by the LPI with the same title.

In a paper delivered at the Workshop on Pristine Highlands Rocks and the early History of the Moon, Ross Taylor brought the focus back onto the problem of the formation of the Moon, though this time in the context of broader planetary science. As Taylor continued to develop his arguments against the fission models for the formation of the Moon, he adopted the view that the inner planets, including the Moon, were formed through the accretion of planetesimals. He wrote:

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Fig. 6.2 Variations in K/U ratios in meteorites and planets. From Taylor (1983). “Lunar and planetary compositions and early fractionation in the solar nebula.” In J. Lonhghi & G. Ryder (Eds.), Workshop on Pristine Highlands Rocks and the Early History of the Moon, page 73, Houston, TX: Lunar and Planetary Institute, Copyright 1983 by the Lunar and Planetary Institute

Astrophysical observations indicate that during star formation, nebulae clear on time scales of 106 years … . Strong T Tauri Solar winds also occur on these short time scales … . Calculations for accretion of the inner planets by accretion from a swarm of planetesimals indicate time scales of 108 years for this process …, two orders of magnitude longer. Jupiter and Saturn clearly grew in a gas-rich environment and accordingly probably formed early. Earth and inner planets accrete from planetesimals in a gas-free environment (except for gas trapped in planetesimals) up to 100 million years later. Jupiter was thus able to prevent formation of a planet in the asteroid belt and stunt the growth of Mars by sweeping up planetesimals. The depletion of volatile elements in the inner Solar System, leading to low K/U ratios may have resulted from loss of volatiles during early nebula/clearing stage within a few million years of formation of the Sun. … (Taylor, 1983: 73)

Taylor based his conclusions partly on data from eucrites, which are meteorites that were created by impacts on the large asteroid Vesta. Vesta went through the processes of differentiation similar to the inner planets, but did so very early in the history of the solar system. The ratio of the concentration of potassium (K) to uranium (U) is taken to be a marker for loss (or retention) of volatile elements in a given material. Taylor argued that K/U can be markedly different for different planets but is constant for material from a given planet (Fig. 6.2), i.e., the degree of volatile depletion is characteristic for a given planet. Taylor therefore argued that: (a) Metal-silicate and volatile-refractory element fractionation occurred very early in Solar System history at a time indistinguishable from 4.57 Aeons. (b) These events occurred before the accretion of the Earth and the Moon which inherited their K/U ratios from precursors. (ibid.: 72)

Supporters of the fission theory argued that the great heat associated with the fission of the Moon from the Earth resulted in the depletion of volatile elements on the Moon. On the contrary, Taylor argued that the planetesimals that aggregated to

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form the Moon had already been depleted in volatiles. Taylor listed five implications of his research for the fission hypothesis: (a) The differences in total Fe, siderophile element content, Fe/Mg ratios and refractory/ volatile elements between the Earth and the Moon … are inherited from the accreting planetesimals. (b) No simple mechanism can produce these differences following core separation. (c) The mantle Fe/Mg ratio is fixed for the Earth before accretion, not by processes during core separation. (d) Although some material knocked off from the Earth by collisions may be incorporated into the Moon, its compositional signature is swamped by independently accreted planetesimals. (e) The W/La correlations in the Moon, Earth and eucrite parent body are due to processes occurring before accretion and have no local genetic significance … . (ibid.: 73)

As the editors of the workshop report noted, “Such a model is obviously at odds with a lunar origin by fission. …” (Longhi & Ryder, 1983: 13).

6.6

Planning for a Conference on the Origin of the Moon

In November of 1982, the Lunar and Planetary Sample Team (LAPST) held a meeting at the LPI in which they discussed the idea of a conference that would focus on the origin of the moon. William K. Hartmann recalls the course of events: A LAPST subcommittee was brainstorming about the logical subject for the next topical workshop, to follow in their series of meetings, which started in 1979: the Conference on the Lunar Highlands Crust and the Apollo 16 Workshop (November, 1980), Magmatic Processes of Early Planetary Crusts (August, 1981), Lunar Breccias and Their Meteoritic Analogs (November 1981), and Pristine Lunar Highland Rocks and the Early History of the Moon (October, 1982); the central theme of these workshops was the origin and evolution of the lunar crust. ‘Why not expand the focus to the origin of the entire Moon?’ one of the members asked. The idea was instantly appealing. It seemed to us that the decade since the last manned landing had provided time to reflect further on the origin of the Moon and indeed had provided some revisions and refinements to the earliest conclusions drawn after the lunar landings. One substantial advance, for example, was the analysis of meteoritic and lunar oxygen isotope data, showing that only the Moon and the earth shared identical oxygen isotope ratios, while material from other parts of the solar system had different ratios. This seemed an important boundary condition on the origin of the lunar material. It was thus agreed that LAPST should at some point organize a conference on lunar origin. (Hartmann, Phillips, & Taylor, 1986: viii)

Hartmann, Roger Phillips, and G. Jeffrey Taylor, who was chair of the LAPST subcommittee, were named as the organizers and conveners for a future conference on the Origin of the Moon.

6.6 Planning for a Conference on the Origin of the Moon Fig. 6.3 The principal hypotheses for the formation of the lunar crust. From Walker (1983) “Lunar and terrestrial crust formation.” In W. V. Boynton & G. Schubert (Eds.), Proceedings of the Fourteenth Lunar and Planetary Science Conference (Journal of Geophysical Research, 88, supplement), page B18, Washington, DC: American Geophysical Union, Copyright 1983 by the American Geophysical Union and reproduced with the permission of John Wiley & Sons

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6.7

The Fourteenth Lunar and Planetary Science Conference

The LPI co-sponsored with NASA and the American Geophysical Union the Fourteenth Lunar and Planetary Science Conference, which was held in Houston on 14–18 March 1983. Papers delivered at the conference continued to reflect the spreading interests in the lunar and planetary research community, as well as reexaminations of “settled opinion,” brought on by further study of the lunar samples. An example of the latter was a paper by David Walker of Columbia University, who challenged the idea that the lunar crust came about simply by the solidification of a global magma ocean. Walker summarized the evolution of concepts about the lunar crust as follows (Fig. 6.3). Our ideas concerning the origin and significance of the rocks of the lunar highlands crust have evolved. At least three major distinguishable hypotheses have been proposed. The progression from one hypothesis to another has been related to study of lunar samples. In chronological order of proposal the hypotheses are: (1) the uplands are an accumulation of cosmic sediment from the late stage of accretion; (2) the uplands are the upper flotation layers of a magmatic cumulate sequence established during the solidification of a global magma ocean that formed during the late stages, possibly as a result of lunar accretion; (3) the uplands are volcanic and plutonic complexes serially extruded and intruded throughout the first few hundred million years of lunar history. … Lunar sample studies have dominated this evolution of ideas and have further demonstrated that none of the hypotheses alone is adequate. For instance, the ‘magma ocean’ was not the whole story because there clearly was later ‘serial magmatism’. Likewise, ‘serial magmatism’ presupposes planetary formation and so ‘cosmic sedimentation’ is a precondition. … The cosmic sedimentation model was a result of the reflection of Urey (1952) on the irregular shape and unequal moments of inertia about different lunar axes. Urey argued that the moon must be relatively cold to be rigid enough to support the stresses implied by the nonuniform mass distribution. He also supposed that the moon must always have been cold and rigid, given that the moon was not expected to have cooled sufficiently by conduction to ‘gel’ from some hot, early differentiation. … In fact, the material retrieved in the Apollo and Luna sampling programs proved to be anything but undifferentiated … . The first lunar sample missions indicated that the cosmic sedimentation model for the lunar crust required replacement. It was promptly provided … in the form of the lunar magma ocean model. The lunar magma ocean provided a unifying framework for understanding many diverse observations about the chemistry, petrology, chronology, and physical properties of lunar samples. It was particularly successful in rationalizing the complementary relationships of the lunar highlands and mare basalt source regions. Stated in simple terms, the model involved the melting of at least the outer few hundred kilometers of the moon penecontemporaneous with accretion. The solidification of this ocean of magma deposited vast sequences of cumulate crystalline rocks, roughly according to their density. Buoyant plagioclase-feldspar-rich cumulates formed a flotation crust and dense complementary olivine and pyroxene-rich cumulates formed below within the moon. Essentially, all late-accreting material was processed through this immense molten bath. The lunar crust then would not be a repository of lunar raw material but would be the differentiated, buoyant scum, of the magmatic system produced in a primordial orgy of melting.

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Clearly the two hypotheses have very different implications for the constitution of the lunar crust. It was the identification of such highly feldspathic, fractionated rocks as anorthosite with sources on the highlands crust … that caused the magma ocean hypothesis to supercede the model of little-disturbed accretion. The topographic contrast between mare and highlands and gravity data was consistent with a considerable thickness of anorthosite in the highlands. To form the requisite amount of anorthosite as a globe-encircling flotation product requires an ocean of magma. The magma ocean model also provided a ready explanation for the positive Eu anomaly of the anorthosites and related feldspathic rocks compared to the negative Eu anomaly of the source regions of mare basalts. The incorporation of Eu in crustal plagioclase produced an Eu deficiency in the complementary mafic cumulates that upon remelting produced mare basalts. This and other early-established features indicating a complementary relationship of crustal and mare basalt source region chemistry have been previously summarized … . The Eu anomaly remains the most widely cited piece of evidence of a relationship produced through a global magma ocean. A number of observations … do not fit, yet the magma ocean model has remained a useful working hypothesis for more than a dozen years because it does explain so much. The magma ocean model replaced the cosmic sediment model for the lunar crust because the fractionation and complexity of lunar samples immediately made it clear that more had occurred to form the highlands crust than just accretion. These discoveries in no way disproved accretion; they merely demonstrated that it was not the whole story. So, too, a dozen additional years of discovering further fractionation and complexity in the lunar crustal samples have given cause for wondering whether the magma ocean model is sufficient. … To anticipate the conclusion, the magma ocean will not be disproved any more than accretion was disproved with the advent of the magma ocean. …The magma ocean will be argued to be an insufficient, possibly unnecessary, and nonrecurring hypothesis of planetary crust formation. … complex eruptive and plutonic volcanism made a major contribution to lunar crust formation, irrespective of any previous magma ocean contributions that were obscured in the process. (Walker, 1983: B17–B19)

Walker cited, among other evidences for his model of crust formation by serial magmatism, the lack of lateral homogeneity in the makeup of highland rocks. We will return to this topic in Chap. 7.

6.8

Conferences and Workshops on Planetary Issues— The Impact of Lunar Research

Roger Phillips took a faculty position at SMU, and he was succeeded by Kevin Burke (Fig. 6.4) as Director of the LPI on 1 September 1983. Burke had been serving as the Deputy Director of the LPI, on a part-time basis since the summer of 1982. He had come to the LPI from the State University of New York at Albany but was originally from London, England, where he had obtained his Ph.D. from the University of London. Burke’s research interests were in plate tectonics and related areas of geophysics. The Fifteenth Lunar and Planetary Science Conference was held at the Gilruth Center of the Johnson Space Center on 12–16 March 1984 and was again organized by the Lunar and Planetary Institute. Among the many points of disagreement was

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Fig. 6.4 Photograph of Kevin C. A. Burke, courtesy of the Lunar and Planetary Institute

the topic of magma oceans. One paper discussed the magma ocean on Earth, noting, “Based on the lunar results it appears plausible to postulate a global magma ocean on the earth. …” (Turcotte & Pflugrath, 1985: C541), while another paper proposed, “… to examine in detail how … anorthosites might have formed without a magma ocean [on the Moon].” (Longhi & Ashwal, 1985: C571) A little later in the year, the LPI sponsored the Workshop on the Early Earth: The Interval from Accretion to the Older Archean. The workshop organizers, Kevin Burke and Lewis Ashwal, a Research Scientist at the LPI, noted: Fifteen years of research following the first return of rocks from the Moon has enabled planetary scientists to develop a framework in which to consider the mechanisms whereby planetary bodies are constructed. Concurrently, scientists have been studying the oldest available terrestrial rocks to try to understand how the Earth operated in the most distant geologic past. The Earth is a highly dynamic planet, however, and the record of events prior to about 3.8 b.y. years ago has been almost completely obliterated. We are thus left with a gap of about 800 million years between the time of Earth’s accretion and that of the oldest preserved rocks. (Burke & Ashwal, 1985: i)

In many of the papers that were presented at the workshop, one sees the influence of lunar studies on thinking about the evolution of the early Earth. An example is the talk given by A. G. W. Cameron, who was now at the Harvard-Smithsonian Center for Astrophysics. Cameron’s paper was titled “Conditions during formation of the Earth,” and his subsection on the Moon and summary of the entire paper read as follows.

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Formation of the Moon. The angular momentum in the Earth-Moon system corresponds to that contained in a tangential collision of the protoearth with a body having the mass of Mars. If such a colliding projectile were a protoplanetary remnant, its rapid formation would have assured the prompt separation of its iron core from its mantle. In the collision itself a large amount of mantle rock will be vaporized; it is the pressure gradient established in such vapor that is responsible for accelerating some of the material into an orbit around the protoearth. This material is strongly dissipative and will spread out and tidally interact with the protoearth in such a way that much of it collects to form the Moon beyond the Roche lobe of the Earth …. The disk that is formed in orbit is self-gravitating but subject to continuing shear; the primordial atmosphere of the Earth is caused to escape by this disk and the high temperature of the environment, all except a highly mass-fractionated remnant of the xenon; the resulting fractionated pattern of isotopic abundances of the xenon characterizes the atmospheric pattern of xenon today. The collision adds about ten percent to the mass of the protoearth and deposits about 3  1038 ergs of new thermal energy in the Earth, remelting any parts of the mantle or crust that had solidified. The present terrestrial atmosphere is then the atmosphere acquired through late accretion following the formation of the Moon. All of this is expected to happen for dynamical reasons at an Earth age of roughly 108 years for a projectile that is in a reasonably eccentric Earth-crossing orbit. … Summary. The formation of the Earth starts with gravitational instabilities in the gas of the primitive solar nebula, which form giant gaseous protoplanets. The envelopes of these are thermally evaporated, but solids can gravitationally settle to the center of the protoplanets, and most of the core and mantle of the Earth were formed at that time (age about 103 years). The protoearth then survived a period of high temperatures in the surrounding gas (age about 105 years). While and after the gas was thermally stabilized and removed from the solar system, the Earth grew by bombardment of planetesimals and its early atmosphere was established. A late major collision led to formation of the Moon and loss of the primordial atmosphere (age about 108 years). The present atmosphere resulted from still later accretion. The presence of this atmosphere spreads out the mantle cooling and solidification over hundreds of millions of years. (Cameron, 1985: 13)

Another example of how lunar research was affecting research on the evolution of the Earth appears in the talk given by Richard Grieve and Marc Parmentier of Brown University, titled “Considerations of large scale impact and the early Earth.” They said: The earth’s crustal evolution during the first 800 my of its history is conjectural. The lack of a very early crust may indicate that thermal and mechanical instabilities resulting from intense mantle convection and/or bombardment inhibited crustal preservation. Whatever the case, the potential effects of large scale impact have to be considered in models of early earth evolution. The number of impacts can be estimated from the cratering record in the lunar highlands, corrected for terrestrial impact conditions … (Grieve & Parmentier, 1985: 23)

A paper by Bill Kaula and Stephen Cooperman of UCLA showed the growing acceptance of the idea of the existence of large planetesimals in the early solar system. Kaula and Cooperman wrote, in part: The formation of the Earth, which took 100 million years or less about 4.5 billion years ago, was mainly from sizeable bodies: perhaps moon-sized. The principal reason for believing this is that any mechanism ever proposed for making planetesimals depends on conditions which lead to the formation of many planetesimals. No one has ever conjectured a “magic suppressor” which allows the formation of four or five kernels for the terrestrial

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bodies but prevents formation of any other similar bodies. Models of interaction among small planetesimals which take into account only close encounters all lead to the formation of moon-sized objects …, thus leading to several 100 in the inner solar system. Longer-term interactions, such as secular resonance sweepings … are needed to get these planetesimals together to form the observed terrestrial bodies. The consequence is that the formation process is dominated by great impacts, most of whose energy is retained as heat at time of impact … . What might be called the ten-little-indians scenario seems the most plausible working hypothesis: before there were four terrestrial planets there were five; before there were five, there were six; and so forth. Hence the terminal stages of planet formation were probably dominated by great collisions, involving Mars-sized bodies. Evidences that this small-number statistics situation prevailed in the early solar system are the great differences among the planets; between Venus and the Earth, in rotation rate and presence of a satellite; among Venus, Earth, and Mars, in primordial argon retention; between Mercury and the rest, in iron content; and among all four, in size. The energy released in a collision between a mars-sized and an Earth-sized planet is more than 1038 ergs, about 106 times that creating Mare Imbrium and comparable to the total radioactive heat of the Earth throughout its history. While considerable model definition and calculation is needed, it probably is enough to put two percent of the mass involved into orbit in a highly devolatilized state. It also would have considerable implication for the Earth’s state, melting or vaporizing an appreciable fraction and stimulating core formation and outgassing. … (Kaula & Cooperman, 1985: 45)

Finally, a little more than 100 years after Osmond Fisher proposed that the Pacific Ocean basin might have been caused by the departure of the Moon from a rapidly rotating Earth, a researcher at the workshop proposed something like a mirror image, namely that the arcuate features on the Earth’s surface, e.g., the Canadian Shield and the Aleutian arc, are vestiges of the Earth’s bombardment by planetesimals. John Saul of the OREX Company in Paris wrote: Immediately following accretion, the surface of the Earth was densely patterned with circular scars which were the surface expressions of 3-D craterform structures. In the course of geologic time these structures would have become less and less visible due to the workings of the Earth’s atmosphere, surface waters, and plate tectonics regime but there is no compelling reason to assume that they have been entirely eradicated. … (Saul, 1985: 71)

Saul’s idea was given some support from another paper at the workshop that was delivered by Peter Schlutz of the LPI and titled “Lunar and Martian impact basins: exposed records of terrestrial bombardment?” By examining the crater density for the Moon, Mars, and Mercury, Schultz argued that: … the Earth should have recorded more than 500 impacts that resulted in basins larger than 300 km in diameter over its post-accretion geologic history … Each planet [Moon, Mercury, and Mars] appears to preserve one mammoth impact structure: Imbrium on the Moon (1200 km in diameter); Caloris on Mercury (1300 km); and Hellas on Mars (2000 km). … Impact basins on the Moon and Mars largely control the distribution of post-accretion volcanism and subsequent structural history. Radial grabens extend more than 2000 km from the Imbrium basin, thereby affecting more than 70% of the nearside hemisphere … . Similar radial fractures have recently been reported for the major martian basins … . Massive eruptions of basalt were localized along concentric zones of weakness that also extend well beyond the nominal raised basin rim. On Mars, concentric canyons and grabens

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encircle the Hellas basin 2400 kmfrom basin center; that is, the concentric structural zone of influence covers nearly 80% of one hemisphere. The continued re-use of these structural weaknesses underscores their importance for the early crust of these planets and perhaps provided a clue for the role of major impacts for evolution of the early Earth. (Schultz, 1985: 74)

6.9

Concluding Remarks

By the fall of 1984, it had been 15 years since the beginning of the Apollo investigations. Members of the lunar and planetary research community now was applying what they had learned about the Moon to research problems related to other planetary bodies, including the Earth. There was agreement on many facts about the Moon, e.g., its age, the cause of the great basins and the smaller craters, and the difference in the composition of the maria and the highlands. There was not yet agreement on how the Moon was formed, but that was about to change.

References Alvarez, L. W., Alvarez, W., Asaro, F., & Michel, H. V. (1980). Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, 208(4448), 1095–1108. Barrell, J., Schuchert, C., Woodruff, L. L., Lull, R. S., & Huntington, E. (1918). The evolution of the Earth and its inhabitants. New Haven, CT: Yale University Press. Burke, K., & Ashwal, L. D. (Eds.). (1985). Workshop on the early Earth: The interval from accretion to the older Archean (LPI technical report 85-01). Houston, TX: Lunar and Planetary Institute. Cameron, A. G. W. (1985). Conditions during formation of the Earth. In K. Burke & L. D. Ashwal (Eds.), Workshop on the early Earth: The interval from accretion to the older Archean (LPI Technical Report 85-01) (pp. 11–13). Houston, TX: Lunar and Planetary Institute. Cassidy, W. A. (2003). Meteorites, ice, and Antarctica: A personal account. New York, NY: Cambridge University Press. Grieve, R. A. F., & Permentier, E. M. (1985). Considerations of large scale impact and the early Earth. In K. Burke & L. D. Ashwal (Eds.), Workshop on the early Earth: The interval from accretion to the older Archean (LPI Technical Report 85-01) (pp. 23–24). Houston, TX: Lunar and Planetary Institute. Hartmann, W. K., Phillips, R. J., & Taylor, G. J. (Eds.). (1986). Origin of the Moon. Houston, TX: Lunar and Planetary Institute. Kaula, W. M., & Cooperman, S. A. (1985). Implications for the Earth of the early dynamical environment. In K. Burke & L. D. Ashwal (Eds.), Workshop on the early Earth: The interval from accretion to the older Archean (LPI Technical Report 85-01) (pp. 45-47). Houston, TX: Lunar and Planetary Institute. Longhi, J., & Ashwal, L. D. (1985). Two-stage model for lunar and terrestrial anorthosites: Petrogenesis without a magma ocean. In G. Ryder & G. Schubert (Eds.), Proceedings of the Fifteenth Lunar and Planetary Science Conference (Journal of Geophysical Research, 90, supplement) (pp. C571–C584). Washington, DC: American Geophysical Union. Longhi, J., & Ryder, G. (Eds.). (1983). Workshop on pristine highlands rocks and the early history of the Moon (LPI Technical Report 83-02). Houston, TX: Lunar and Planetary Institute.

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Phillips, R. J., & Ashwal, L. (Eds.). (1981). Concept for a research project in early crustal genesis (LPI Technical Report 81-08). Houston, TX: Lunar and Planetary Institute. Saul, J. M. (1985). Large arcuate scars: A geological legacy of the Earth’s accretionary past. In K. Burke & L. D. Ashwal (Eds.), Workshop on the early Earth: The interval from accretion to the older Archean (LPI Technical Report 85-01) (pp. 71–73). Houston, TX: Lunar and Planetary Institute. Schopf, T. J. M. (1981). Extinction of the dinosaurs [Abstract]. In Papers presented to the Conference on Large Body Impacts and Terrestrial Evolution: Geological, Climatological, and Biological Implications (p. 49). Houston, TX: Lunar and Planetary Institute. Schultz, P. H. (1985). Lunar and martian impact basins: Exposed records of terrestrial bombardment? In K. Burke & L. D. Ashwal (Eds.), Workshop on the early Earth: The interval from accretion to the older Archean (LPI Technical Report 85-01) (pp. 74–75). Houston, TX: Lunar and Planetary Institute. Taylor, S. R. (1983). Lunar and planetary compositions and early fractionation in the solar nebula. In J. Lonhghi & G. Ryder (Eds.), Workshop on pristine highlands rocks and the early history of the Moon (pp. 72–74). Houston, TX: Lunar and Planetary Institute. Turcotte, D. L., & Pflugrath, J. C. (1985). Thermal structure of the accreting Earth. In G. Ryder & G. Schubert (Eds.), Proceedings of the Fifteenth Lunar and Planetary Science Conference (Journal of Geophysical Research, 90, supplement) (pp. C541–C544). Washington, DC: American Geophysical Union. Urey, H. C. (1952). The Planets. New Haven, CT: Yale University Press. Walker, D. (1983). Lunar and terrestrial crust formation. In W. V. Boynton & G. Schubert (Eds.), Proceedings of the Fourteenth Lunar and Planetary Science Conference (Journal of Geophysical Research, 88, supplement) (pp. B17–B25). Washington, DC: American Geophysical Union.

Chapter 7

The Kona Conference—Day 1

7.1

Introduction

On 14 and 16 October 1984, the Conference on the Origin of the Moon that had been in planning at the LPI for two years was held in Kona, Hawaii. The conference was organized by the LPI and cosponsored by the LPI, the Division for Planetary Sciences of the American Astronomical Society, and NASA. The LPI published a book titled Origin of the Moon that contained some of the papers delivered at the conference plus some additional papers. The Kona Conference was a watershed event in lunar science. The papers given at the conference, others that were included in the follow-on book, and additional background papers are discussed in this chapter and in Chaps. 8 and 9. This chapter also features the presentations that were given on the first day of the conference.

7.2

Invited Reviews

The conference began with a series of seven invited reviews. John Wood of the Harvard-Smithsonian Center for Astrophysics gave an overview of the various hypotheses of lunar formation, including intact capture, coaccretion, rotational fission, collisional fission (which he called collisional ejection), and disintegrative capture. We have discussed these models in the previous chapters and will continue to discuss them below, and so will not describe Wood’s review.

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Larimer

The second review paper was given by John W. Larimer of Arizona State University. Larimer addressed the question of how the Moon’s composition compares with what one would expect from the sequence of condensations that are thought to have occurred as the solar nebula cooled. Larimer began by noting: Lunar rocks, as well as all other samples of planetary material, are the end products of their cosmic history, the long sequence of events that determine the final elemental and isotopic composition. From this broad perspective, the origin of the Moon is one step in a series of events in the chemical history of planetary matter. The history begins with nucleosynthesis, continues with a nebular, star-forming stage, and culminates in accretion. The scene is then set for planetary differentiation: the formation of a core, mantle, crust, and atmosphere. Each of these events leaves its signature, although the more ancient signatures are often obscured by the more recent events. The challenge is to decipher this history in order to understand the elemental and isotopic changes that occur at each stage. Lunar and terrestrial rocks, having passed through the full sequence of processing, are the most complex to trace. … Some theories of lunar origin reverse the historic sequence by suggesting that the Moon accreted from material that had already passed through the planetary differentiation stage. There are several currently popular ideas: fission from the Earth, a gargantuan terrestrial impact, and disintegrative capture. In these theories the Moon accretes from the debris of preexisting, differentiated bodies; the debris originates from the Earth’s mantle either by fission or impact, from a differentiated giant impactor, or from a swarm of differentiated planetesimals that breakup as they approach the Earth. An important stimulus for these theories is that they explain the Moon’s low metallic Fe content by a known process: mantles of differentiated bodies are metal-deficient because the metal has been sequestered into a core. The violent, energetic nature of these events introduces other complexities. It is possible, however, that the Moon simply accreted from planetary material in orbit around the Earth. Such an origin is generally accepted for most satellites in the solar system. Reminiscent of the Earth-Moon system, many of these satellites also have densities that differ from the planet they orbit, an indication of compositional differences. … (Larimer, 1986: 146)

Larimer then traced “the cosmic history of planetary matter from nucleosynthesis through accretion.” Along the way, he made an interesting point about the implications of oxygen isotope data in meteorites. Data collected to date indicate that each of the various groups of meteorites is derived from its own oxygen reservoir, implying a separate parent body for each group. All lunar and terrestrial samples, on the other hand, plot on a single line, to within experimental limits, suggesting an origin from the same or very similar reservoir. But the significance of this similarity is unclear. The isotopic composition of the Earth and Moon falls in the middle of the range defined by the meteorites. A few meteorite groups have virtually the same composition as the Earth and Moon, but most do not. However, no pattern is evident: relative to the Earth and Moon, some meteorite parent bodies are more enriched and others more depleted in 16O. Thus while the data appear to suggest a common origin for the Earth and Moon, they would not be inconsistent with a theory in which lunar material originates as far away as the asteroid belt. (ibid.: 148)

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Larimer concluded that: Since the Moon has only 1/80 of the Earth’s mass, it was clearly discriminated against in some way. In fact, if we consider the Moon’s composition from a different perspective, an interesting relationship emerges. Suppose the Moon is not overendowed in refractory elements, but instead is deficient in all other components. From this perspective, relative to the Earth, the Moon is depleted in Mg silicates by a factor of 2–3, in metal by a factor of 10, and in the highly volatile elements by a factor of 30. This depletion pattern parallels the condensation sequence. It also suggests competition between the Earth and Moon, perhaps implying lunar accretion in orbit around the accreting Earth … . The accretion process might then be described as modifying the old cliché to read “the rich get their share while the poor get poorer.” … (ibid.: 170)

The question that Larimer did not discuss, however, is, “How did the rich and the poor, while accreting together, wind up with so much angular momentum?” The next speaker discussed a numerical simulation that had the potential of answering that question.

7.2.2

Wetherill

For many, perhaps most, of the researchers at the conference, the giant impact theory was coming to the forefront of research. It had been around in its modern articulation for about ten years, and many felt that it was the theory that most needed a critical examination. The basic question to be answered was, “How likely was it in the early period of formation of the Earth that a Mars-sized object would have made the required glancing collision with the Earth?” George Wetherill of the Carnegie Institution of Washington began this discussion with the third review paper at the conference. For a number of years, Wetherill had been conducting computer-based research on the formation of planets, and at the conference he reported on his most recent computer simulations of terrestrial planet formation. The growing capabilities of large computers made refined modeling of planetary formation processes increasingly feasible. Wetherill began his paper with the following introduction. Because of the need for a context in which to evaluate the relative importance and probability of alternative lunar formation processes, a meaningful discussion of the way the Moon was formed requires understanding of the formation of the terrestrial planets as well. In this paper terrestrial planet formation from a heliocentric swarm of planetesimals under conditions in which nebular gas drag was of minor importance will be investigated. Particular emphasis will be placed on the question of the existence of planetesimals large enough to provide the angular momentum of the Earth-Moon system in a single event. … (Wetherill, 1986: 519)

Previous studies had shown that the angular momentum of the Earth-Moon system could not be the result of the overall average of angular momentum supplied to the forming Earth by small impacts, for the fairly obvious reason that:

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… impacts imparting angular momentum of either sign are very nearly equally probable and the average angular momentum of the resulting planet will be essentially zero. … It therefore seems very likely that the angular momentum of the geocentric swarm and the resultant Earth-Moon system was not acquired simply as a result of averaging over a large number of small bodies, but was caused by stochastic fluctuations in the angular momentum input of an accumulation history involving very large objects, only partially smoothed out by contributions from smaller bodies … For this reason, in relating lunar origin to Earth formation it is particularly important to understand the masses and velocities of the largest accumulating bodies that failed to become terrestrial planets in their own right. … (ibid.: 521)

In his computer simulations, Wetherill’s initial state consisted of 500 bodies with a range of masses determined based on the theoretical work of others, principally Victor Safronov and Yoshitsugu Nakagawa and their coworkers. The minimum mass was about three times the mass of the asteroid Ceres, and the maximum mass was one and a half times the mass of the Moon. The total mass was chosen to be about 5% more than the total mass of the current terrestrial planets. The limitation to 500 bodies was imposed by computer time and memory. It represents a 5-fold increase over earlier 100-body calculations … . The initial eccentricity [of the orbits of the 500 bodies] was assigned a random value between 0 and 0.05, the initial inclination a random value between 0 and 0.025, and the initial semi-major axis was randomly sampled between 0.7 and 1.1 AU, weighted so that the number of bodies per unit surface area was constant. It has been shown earlier that an initial distribution of semi-major axes similar to this is necessary to match the present energy and angular momentum of the terrestrial planets … . Because it is found that only a few percent of the mass, energy, and angular momentum is lost from the system during its growth (for a wide range of initial states), this matching of the initial and final values of these quantities is essential. (ibid.: 525–526)

Wetherill explained that in his computer simulations: … The evolution of the system is assumed to result from close two-body encounters between planetesimals in crossing orbits … . When the encounter distance between two planetesimals becomes less than the sum of their physical radii, the bodies are assumed to merge to form a larger body with mass equal to the sum of the masses. Otherwise, an encounter between two bodies results in gravitational perturbation to new orbits. As the calculation progresses the number of bodies becomes smaller … . Eventually, only bodies in noncrossing orbits will remain, the calculation is then terminated, and the surviving bodies are considered to be the final planets resulting from that particular accumulation calculation. (ibid.: 527–528)

In his paper, Wetherill provided in a series of snapshots: The calculated evolution of an accumulating planetesimal swarm, the final outcome of which resembles the present terrestrial planets … . Initially all of the bodies are confined to a narrow zone of semi-major axes and eccentricities (as well as inclinations), as shown in Fig. 3a (Fig. 7.1). … After only 0.5 m.y. [Figure 3b (Fig. 7.2)], nine bodies in the small “planet” mass range (2  1026 g–1027 g) have been formed, the mutual perturbations of the planetesimals have “pumped up” the mean eccentricity to about three times its initial value, and a number of bodies have begun to diffuse beyond the original boundaries of the swarm.

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Fig. 7.1 The initial state in Wetherill’s simulation. To avoid crowding in the figure, not all the 500 bodies were plotted. This figure and the following six are from Wetherill, G. W. (1986). “Accumulation of the terrestrial planets and implications concerning lunar origin.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, pages 534 (A), 535 (B and C), 536 (D), 537 (E), 538 (F), and 539 (G), Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

Fig. 7.2 Wetherill’s simulation after a computer run time corresponding to a half a million years of real time. The larger filled circles correspond to lunar-size bodies. The open circles correspond to small planets. The position of the “embryos” of the final planets are indicated

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Fig. 7.3 Wetherill’s simulation after 2.2 million years. All bodies in the simulation are now plotted

After 2.2 m.y. [Figure 3c (Fig. 7.3)] the bodies that will turn out to be the analogs of the present planets Earth and Venus are the largest bodies in the swarm, and their eccentricities are well below the mean eccentricity of the smaller bodies. The eccentricities of the smaller bodies consist more or less of a Gaussian distribution … Fourteen small “planets” now exist. On the average, their eccentricities are lower than those of the very smallest bodies. On the other hand, many of them have aphelion-perihelion distances comparable to the heliocentric spacings of the bodies that will evolve into the final planets. Even at this early time, the concept of unique feeding zones for each “planetary embryo” has little usefulness. At 10.9 m.y. [Figure 3d (Fig. 7.4)] “Earth” and “Venus” have reached 84% and 42% of their final masses. Note the 1.2  1027 g object near “Venus.” Five million years later, it will strike “Venus” with an impact velocity of 13 km/sec. The kinetic energy of the impact in the center of mass frame is *65% of the gravitational potential energy of the combined body. An impact this energetic is somewhat unusual, but not extremely so. Another object of interest is the body that will ultimately become the analog of Mercury. At this stage of accumulation, it has reached 37% of its final mass of 3.9  1026 g (* 72% of its final radius). It is the final planet with the largest semi-major axis. At 31 m.y. [Figure 3e (Fig. 7.5)] the “clean-up” epoch of accumulation has been entered. 81% of the initial mass of the system has been incorporated into the final planets. The eccentricities (and inclinations) of the remaining bodies are quite large, and some of them are evolving into “asteroidal” orbits with semi-major axes of *2.0. Other than the final planets, the largest body remaining has a mass of 3.0  1026 g (* present mass of Mercury). Seventeen million years later, it will strike the Earth, and represent the last impact of a body with mass >1027 g on the planet. “Mercury” is now beyond the present semi-major axis of Mars. During the next 3 million years a series of close encounters with “Mars,” “Earth,” and “Venus” will perturb it to a semi-major axis of 0.58 AU, inside the

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Fig. 7.4 Wetherill’s simulation after 10.9 million years orbit of “Venus.” Its survival as a final planet was simply a matter of luck. It could just as well have struck a planet during this journey through the inner solar system. By 64 m.y., the cleanup is in its final stages [Fig. 3f (Fig. 7.6)]. Mercury is near its final heliocentric distance. The last impact is that of a 5  1024 g body on “Mars” at 202 m.y. The three small remaining bodies will be ejected from the solar system following encounters with “Earth” and “Mars.” [Figure 3g (Fig. 7.7)]. (ibid. 533–539)

Wetherill reported on 11 other accumulation calculations besides the one illustrated in the accompanying figures. He noted: In these cases, the final distribution of bodies does not resemble the terrestrial planets as well as the case described by Fig. 3 … . All of these results have the common characteristic, however, that the final number of planetary bodies with masses greater than 1026 g (1.4 lunar masses) is small, almost always four or less. (In two cases a fifth body of mass 1 to 2  1026 g was found.) In half of the cases the number of “large” planets (>2  1027 g) is three instead of the observed two. … it is not known if this variable outcome is the result of the approximate nature of the modeling or represents physically realistic stochastic

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Fig. 7.5 Wetherill’s simulation after 31million years

variability in the accumulation process. If planets did form this way, however, it appears difficult to escape a highly stochastic evolution. (ibid.: 536–538)

Wetherill also noted that the existence of Jupiter was not included in his modeling, and that probably accounts for the outcome of his calculations that: … the outermost planet-size body (“Mars”) is systematically two to four times more massive than the actual planet Mars. The occasional lingering presence of subplanetary-size bodies with semi-major axes beyond “Mars” may be a related phenomenon. … If, however, as seems likely, the major planets existed during the formation of the terrestrial planets …

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Fig. 7.6 Wetherill’s simulation after 64 million years

these would reduce the stability of moderately and highly eccentric orbits beyond that of the Earth, reduce the number of planetesimals available to “Mars,” and remove the more distant residual planetesimals. (ibid.: 538–539)

As noted above, the question of interest at the conference was, “How likely was it that during the formation of the planets the Earth was impacted by a Mars-size object?” Wetherill reported that for 12 accumulation calculations: Typically, one or two impacts of bodies more massive than Mars occur for each accumulation, and about three more massive than Mercury. … These giant impacts occur most frequently after the accumulation has proceeded for 1–15 m.y. In this time interval from 15% to 70% of the mass of the Earth had already formed. … More detailed step-by-step examination of the calculated growth of the planetesimals, as well as numerical experiments with variant models, suggests that the phenomenon of giant impacts is not confined to initial states similar to those calculated but is a more general

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Fig. 7.7 Wetherill’s simulation after 239 million years

phenomenon, including planetary systems having quite different values of mass, energy, and angular momentum than our terrestrial planets. Although not supported by actual calculations, it should be valuable to try and understand better what is actually going on here, rather than simply accepting ‘what the computer tells me.’ The fundamental circumstances that lead to these impacts seems to be a primordial initial state containing many small bodies of similar (1016–1018 g) mass, together with the local nature of the accumulation process. No little planetesimal is labeled “Earth” or “Venus.” As Victor Safranov often says, “All planetesimals are created equal.” At any stage of growth, planetesimals are effective in perturbing and colliding only with bodies in crossing orbits … . For the terrestrial planets, eccentricities large enough to permit widespread exchange of material throughout the entire swarm do not occur until a number of >1027 g bodies have formed. Prior to this final stage, growth of bodies at one heliocentric distance do nothing to prevent growth of bodies of similar mass in other regions of the heliocentric swarm. In the earliest stage of accumulation studied by Greenberg et al. (1978), at first the growing planetesimals accumulate material only from a region 10−3 AU in width, and hundreds of bodies of mass  1023 g can be expected to form. As these bodies gradually perturb one another into orbits of higher eccentricity they will continue to accumulate preferentially from their neighbors, leading to a smaller number of bodies in adjacent zones that are still of comparable mass to one another. As the process of growth continues this repeated collision of bodies of comparable mass is responsible for the giant impact phenomenon. … Rather than considering giant impacts as a somewhat radical suggestion, if one is skeptical about the reality of the phenomenon, a good starting point would be to consider it a normal phenomenon that one should, at least naively, expect during planetary formation. … (ibid.: 540–543)

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Fig. 7.8 Time scale for the growth of “Earth” and “Venus,” based on six of Wetherill’s simulations. From Fig. 10 in Wetherill (1986). “Accumulation of the terrestrial planets and implications concerning lunar origin.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 546, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

Wetherill examined the time scale for accumulation of the terrestrial planets by considering six of his accumulation calculations in which the final terrestrial planets in his model resembled the actual set (Fig. 7.8). He noted: The occurrence of these giant impacts will cause large stochastic fluctuations in the growth rate of the terrestrial planets. Nevertheless, by averaging the results of several calculations, the underlying characteristic time for accumulation can be discerned … In accordance with conventional opinion, the time scale required for nearly complete growth of the terrestrial planets by gas-free accumulation is *108 years. It should be emphasized, however, that during the first ten million years the growth is much more rapid. Bodies larger than 1027 g in mass are formed within *1 m.y., and 50% growth in mass (and 79% in radius) occurs within 7 m.y. … (ibid.: 545–546)

As a result of his computer simulations, Wetherill concluded that: … for a wide range of initial conditions, terrestrial planet accumulation was characterized by giant impacts, ranging in mass up to 3 times the mass of Mars, at typical impact velocities of *9 km/sec. These large planetesimals and the impacts they produce are sufficient to explain the unexpectedly large angular momentum of the Earth-Moon system. …. (ibid.: 519)

Wetherill noted: It is particularly interesting that these large planetesimals provide in a natural way the giant impacts proposed by Hartmann and Davis (1975) and Cameron and Ward (1976) as a way of forming the Moon. In spite of earlier demonstration that impacts of this magnitude were entirely consistent with Safronov’s theory …, there has been a tendency, at least until recently, to regard these suggestions as too extreme to be taken seriously or even as

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“Velikovskian.” Although it would be presumptuous to conclude that these large planetesimals and impacts were inevitable consequences of planet formation, their probable occurrence imposes obligations of explicitly considering their consequences in any discussion of the early history of the Earth and the Moon. These include more than the question of formation of the Moon by ejecta following giant impacts discussed by Cameron and Ward (1976, 1978), Thompson and Stevenson (1983), and by these and other authors in this volume. These impacts should also have had a major effect on the thermal and chemical evolution of all the terrestrial planets, as well as that of their atmospheres … and their rotation. (ibid.: 548)

The paper by Wetherill seemed to lay to rest concerns about the implausibility of an impact on the Earth by a Mars-size body early in the life of the solar system.

7.2.3

Drake

The final 30-min review paper of the first morning of the conference was given by Michael J. Drake (1946–2011) of the University of Arizona. Drake’s task was to answer the question, “Is lunar bulk material similar to Earth’s mantle?” Drake’s answer was a qualified “yes.” Similar, but not identical. He summarized the pertinent facts: The Earth and Moon have the same ratios of 18O/17O/16O. The Moon is impoverished in metal compared to Earth. Siderophile trace element concentrations in the Moon are generally lower than in the upper mantle of the Earth, implying separation of metal from silicate following lunar assembly. Refractory elements may be enriched in the Moon compared to the upper mantle of the Earth, but identical concentrations cannot be rigorously excluded. The Moon is depleted in volatile elements compared to Earth, but detailed examination of volatile element ratios suggest that this depletion is not simply related to high temperature processes during lunar assembly. The mg# [The mg# is the ratio of abundances—magnesium/(magnesium + iron] of the lunar mantle appears to be lower than the upper mantle of the Earth, although extreme lunar estimates overlap the terrestrial value. The Fe/Mn ratio of the lunar mantel appears to be higher than the upper mantle of the Earth, although Mn concentrations in both objects are low compared to eucrites, shergottites, and CI chondrites. The upper mantle of the earth is presently more oxidized than the Moon. … (Drake, 1986: 105)

Despite his multiple caveats, Drake concluded, “On the basis of these observations the Rotational Fission hypothesis of lunar origin may be rigorously excluded.” (ibid.). Drake’s primary argument against the rotational fission hypothesis seemed to be based on his point about volatile element ratios, namely the evidence that: … Cs/Rb ratios for the Earth and Moon are not in accord with predictions based on considerations of volatility. The volatility of Cs is somewhat greater than Rb in elemental and oxide form, and in silicate melts … . With the Moon being depleted in volatile elements, one would predict that its Cs/Rb ratio should fall below that of the Earth in Fig. 2 (Fig. 7.9), rather than between the Earth and CI chondrites. … (ibid.: 113–114)

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Fig. 7.9 The concentrations of cesium versus rubidium in terrestrial, lunar, and meteoritic samples (MORB = Mid-Ocean Ridge Basalts, from Earth). From Fig. 2 in from Drake (1986) “Is lunar bulk material similar to Earth’s mantle?” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 113, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetray Institute

As for the other theories of lunar origin, Drake concluded: The Intact Capture and Disintegrative Capture hypotheses are not readily testable by geochemical criteria. Both the Collisional Ejection hypothesis and the Binary Accretion hypothesis remain viable in their present, imprecisely defined, variants. (ibid.: 105)

7.2.4

(Jeffrey) Taylor

The next three invited reviews at the conference were presented by G. Jeffrey Taylor of the University of New Mexico (“What Were the Earliest Lunar Differentiation Events?”), Lon L. Hood of the University of Arizona (“Is there a Lunar Iron Core?”), and Joseph A. Burns of Cornell University (“What Was the Moon’s Ancient Orbital History?”). Taylor’s talk on the earliest differentiation events for the Moon reviewed the evidence for and against a lunar magma ocean. At the Apollo 11 Lunar Science Conference, John Wood and his colleagues at Harvard and Joseph Smith and his colleagues at the University of Chicago had proposed that the highland rocks represented a flotation layer of an early lunar global magma ocean. The subsequent history of lunar evolution included churning of the surface by multiple impacts, which made analysis of the highland rocks difficult. Most of the highlands rocks were found to be breccias that had been shocked, and some remelting had occurred that mixed together preexisting soil and rocks. Nevertheless, following the third Lunar Science Conference, the Lunar Sample Analysis Planning Team had concluded that “The most interesting epoch of lunar history, the first half-billion years,

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is recorded in highland rocks if it is preserved anywhere. …” (Lunar Sample Analysis Planning Team, 1972: 980), and many research teams were keen to analyze the oldest lunar highlands rocks. At the time of the Conference on the Origin of the Moon, Taylor was a member of the Institute of Meteoritics at the University of New Mexico. A decade earlier, he had been at the Smithsonian Astrophysical Observatory (SAO), where he had been the lead author on an article about the composition and origin of the lunar highlands, based on the analysis of fragments returned by the Soviet Luna 20 mission. The robotic Luna 20 was the first mission to return samples from the lunar highlands. In this paper, Taylor and his colleagues at the SAO (Michael Drake, John Wood, and Ursula Marvin) began to find some order in the chaos. They found that some mineral analyses clustered into two groups, one more magnesian than the other (Taylor, Drake, Wood, & Marvin, 1973: 1987). How did this clustering come about? Did it occur in the original global magma ocean, or perhaps as John Wood thought possible, in melts that were caused by impacts? In 1975, Wood wrote of the original global magma ocean fractionation: Fractionation on such a large scale would have established certain moon-wide chemical and mineralogical trends in the rocks of the lunar crust. A primary goal of petrologic investigations must be to search for these first-order trends in the lunar highlands samples … . However, it is essential to recognize that the moon has a complex geologic and petrologic history since the time of its primary fractionation. Much crustal rock has been remelted by major cratering events, and in some cases secondary fractionation must have occurred in pools of melt rock. Impact brecciation has mixed earlier-formed lithologies, and in some cases the petrographic evidence of mixing has been obliterated by metamorphism or remelting. These secondary effects are profound, and cannot be ignored … . Chemical and petrologic trends established by the primary fractionation may be difficult or impossible to detect through this veil of secondary effects. (Wood, 1975: 1087)

The nature of the groupings discovered by Taylor and his SAO colleagues in 1973 became clearer at the seventh Lunar Science Conference, when J. L. Warner (JSC), Charles H. Simonds (LSI), and W. C. Phinney (JSC) presented a paper on the “genetic distinction between anorthosites and mg-rich plutonic rocks.” The authors reported two distinct non-overlapping trends for “lunar rocks and clasts that are petrographically interpreted as being plutonic and potentially early crustal cumulates.” (Warner, Simonds, & Phinney, 1976: 917). Figure 7.10 is from the abstract of their talk, which identified two different trend lines in a plot of data of coexisting plagioclase and mafic minerals in potentially early highland cumulates. Over the next several years, the data for this and similar plots were corrected and augmented by Jeffrey Taylor and others, particularly Paul Warren and John Wasson, as researchers searched for and analyzed pristine nonmare rocks from the Apollo collection. Pristine rocks and clasts were defined as those “Rare exceptions which have survived the heavy bombardment with compositions intact from endogeneous lunar magmatism …” (Warren & Wasson, 1978: 185). Warren and Wasson gave guidelines to determine “pristinity,” the primary ones being very low concentrations of siderophile elements and negligible KREEP components. Warren and Wasson also argued that pristine rocks could not have

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Fig. 7.10 Plot of coexisting plagioclase and mafic minerals in lunar rocks and clasts of potentially early cumulates. From Warner et al. (1976). “Genetic distinction between anorthosites and Mg-rich plutonic rocks: New data from 76255” [Abstract]. In Abstracts of Papers Submitted to the Seventh Lunar Science Conference, page 917, Houston, TX: Lunar and Planetary Institute, Copyright 1976 by the Lunar Science Institute

been formed as secondary differentiates, as proposed by Wood and others. They pointed out that “… too many pristine rocks have plutonic textures, much more so than expected within pools of impact melt” (ibid.: 211; their italics). They buttressed this point by citing samples “… with clear-cut evidence of having cooled very slowly, i.e., at depth …” (ibid.), and they noted that “… 5 lunar rocks having extremely old measured ages also appear to be pristine on the basis of texture, siderophiles, and incompatibles …” (ibid.; their italics). For the next few years, Warren and Wasson “foraged” for pristine highland lunar samples, identifying about ten per year from the Apollo collection. At the LPI-sponsored Conference on the Lunar Highlands Crust in the fall of 1979, Warren and Wasson presented an analysis of the pristine samples thus-far collected. Included in their paper was a revised version of the original plot of Warner, Simonds and Phinney, as shown in Fig. 7.11, where An = [Ca/(Ca + Na + K)]. Warren and Wasson argued that the “… wide gap between the most “Mg-rich” ferroan anorthosite and the most “ferroan” Mg-rich rock, indicates that the two groups are not cogenetic. …” (Warren & Wasson, 1980: 83). Warren and Wasson further argued that the ferroan anorthosites would be able to float in a magma ocean because of their high percentage of the light plagioclase mineral, whereas the magnesium-rich rocks would not, as illustrated in Fig. 7.12. The tentative conclusion of Warren and Wasson in 1979 was that the ferroan anorthosites formed as flotation cumulates on a lunar magma ocean but that the magnesium-rich rocks formed “… as almost all terrestrial cumulates do, along the floors of their magma chambers.” (ibid.: 91). The magma chambers in which the magnesium-rich rocks formed were thought to be later intrusions in the already-formed lunar crust.

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Fig. 7.11 Plot showing two groupings in samples of pristine lunar anorthosites. Reprinted from Fig. 1 in Warren & Wasson (1980) “Early lunar petrogenesis, oceanic and extraoceanic.” In R. B. Merrill & J. J. Papike (Eds.), Proceedings of the Conference on the Lunar Highlands Crust, page 83, Elmsford, NY: Pergamon Press, Copyright 1980

Fig. 7.12 Plagioclase contents of pristine nonmare rocks. From Fig. 2 in Warren & Wasson (1980) Early lunar petrogenesis, oceanic and extraoceanic. In R. B. Merrill & J. J. Papike (Eds.), Proceedings of the Conference on the Lunar Highlands Crust, page 84, Elmsford, NY: Pergamon Press, Copyright 1980

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Fig. 7.13 Ca/(Ca + Na) versus mg for pristine nonmare rocks. From Fig. 1 in Warren (1985) “The magma ocean concept and lunar evolution.” Annual Review of Earth and Planetary Sciences, volume 13, page 205, Copyright 1985 by Annual Reviews Inc. Reproduced with permission of Annual Reviews

Warren and Wasson’s model was able to explain some differences in the samples that were returned from the Apollo 15, 16, and 17 missions. The lunar lander for Apollo 15 touched down on the rim of the Imbrium basin, and the lander for Apollo 17 touched down on the rim of the Serenitatis basin. The Apollo 16 site, however, was distant from any major lunar basin. Warren and Wasson noted: One of the phenomena that this model can account for is the preponderance of Mg-rich rocks among pristine rocks from Apollo 15 and Apollo 17, as opposed to Apollo 16, where ferroan anorthosites are preponderant. This relationship implies that the upper part of the original crust was ferroan anorthosite, while the deeper levels plumbed only by basinforming impacts are rich in Mg-rich rocks … . Apparently the parental magmas of the intrusions were denser than the ferroan anorthosites, just as the magma ocean had been, so they tended to be emplaced below the older, ferroan anorthositic crust (but above the mantle, which in any magma ocean model consists of dense, sunken cumulates). (ibid.)

By the time of the Conference on the Origin of the Moon, the distinction between ferroan anorthosites and all other classes of lunar rocks remained apparent, as shown in Fig. 7.13 from a review by Paul Warren (Warren, 1985: 205). In the plot, Warren included data from his foragings with John Wasson and others, as well as compilations from the Lunar Curatorial Facility at the Johnson Space Center by Graham Ryder and Marc Norman. It still seemed likely, though not universally accepted, that the ferroan anorthosites were derived from a global magma ocean. The discovery of pronounced petrochemical differences associated with rare-earth elements in the non-ferroan anorthosites cast some doubt on the global magma ocean concept, however, because these differences seemed to be correlated with lunar longitude. Paul Warren, Jeffrey Taylor, Klaus Keil, Clare Marshall, and John Wasson had written about this complication for one-magma petrogenetic models in a paper presented at the 12th Lunar and Planetary Science Conference:

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It is almost conceivable that a single magma body would be so heterogeneous that essentially bimineralic, or even monomineralic, cumulates of the same mineralogy but different longitudes would be given minor- and trace-element compositions as radically different as have been observed …, but not unless minor- and trace-element compositional differences on the order of 500% existed between the different accumulation sites (in this case *1000 km, or 35o of equatorial longitude, apart). However, it is very doubtful if such conditions could have obtained in the lunar magma ocean. Certainly no existing magma ocean fractionation model predicts the observed trends. (Warren, Taylor, Keil, Marshall, & Wasson, 1981: 35–36)

Aware of these complications, and that there were some skeptics of the concept of a global magma ocean in the audience, Taylor presented a balanced review of the evidence. For so doing, he was criticized after the talk by Al Cameron, who thought that Taylor should have been more forceful in support of the global magma ocean concept, given that it was a necessary consequence of the giant impact model that was blossoming at the conference (Personal communication from Jeffrey Taylor, June, 2014).

7.2.5

Hood

The primary message from Lon Hood was that measurements to determine the characteristics of the lunar interior were not quite sufficient to unambiguously do so. … current lunar seismic velocity models and/or their interpretations are not yet adequate to strongly constrain changes in composition and density in the middle and lower mantle. Consequently, observational limits on the lunar moment of inertia do not yet strongly constrain the existence and radius of a possible metallic core. Other observations … suggest but do not prove the presence of a small metallic core with radius *330–460 km. Iron cores of this size represent *2–4% of the lunar mass and are larger than would be expected if the Moon formed entirely from terrestrial mantle material and if segregation of this amount of metal caused the depletions of lunar siderophiles. However, additions of metal to a proto-Moon composed largely of terrestrial mantle material by differentiated circumterrestrial planetesimals or by a large terrestrial impactor would circumvent this difficulty. Alternatively, models such as the binary accretion hypothesis would be capable of producing a bulk Moon in agreement with available geophysical constraints. (Hood, 1986: 361–362)

7.2.6

Burns

The paper by Joseph Burns was not included in the Origins of the Moon book, and because it was a review paper, an abstract was not submitted. Burns had been the chief organizer of a conference titled Natural Satellites that was held at Cornell University from July 5–9, 1983. Burns gave a talk on the evolution of satellite orbits at that conference, and a section of his talk was on lunar orbital history.

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The material in that talk, which was included in a book that Burns coedited and which was based on the Cornell conference, very likely gives the substance of his review talk at the Conference on the Origin of the Moon. Burns main point from his earlier work is that at this moment in the history of the Earth-Moon separation, energy dissipation is particularly high because of a near resonance between the natural period of oceanic free gravity waves and the semi-diurnal tides: Resonance is thought to play a fundamental role in the exceptional oceanic dissipation observed today. It is well known that an underdamped, linear, forced harmonic oscillator dissipates energy most effectively when driven near its natural frequency … . Tidal gauge observations of actual sea levels have confirmed that tidal forcing frequencies fall within the measured spectrum of major oceanic normal modes; for example, normal modes of 14.8 and 9.3 h have been inferred for the North Atlantic from tidal data taken at Bermuda and the Azores. … (Burns, 1986: 141)

Burns reviewed various models for energy dissipation and concluded: All of these models find that the energy dissipation, and hence the rate of lunar [orbital] evolution, was much less in the past than in the present. … While specific histories depend on the particular continent model chosen and of course on the validity of the particular assumptions in the model …, the general conclusion that the past coupling between Earth and Moon was weaker than today’s is almost surely true and is relatively insensitive to the continental configuration. (ibid.: 143)

While the mystery of the relative short age of the Moon, as calculated from tidal dissipation, was solved, Burns warned that: … past lunar orbital histories are poor guides to constrain lunar origin … . Thus, even though there is renewed interest in the question of the Moon’s origin … orbital evolution calculations are likely to assist little in refining ideas. (ibid.: 143–144)

7.3

Geophysical Constraints

During the remainder of the afternoon of the first day of the conference, eight short papers were presented that related to geophysical constraints on theories for the formation of the Moon. The following sections briefly summarize these talks.

7.3.1

Turcotte

The first paper was given by Donald L. Turcotte of Cornell University, and it was titled “Geophysical and geochemical constraints favoring the Capture Hypothesis.” Turcotte had been working for a number of years on a concept that he called “accretional capture.” The idea, which Turcotte reiterated in his talk at the conference, is that if a primary body, e.g., the Earth, were rapidly increasing its mass

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via accretion, another body in an independent orbit about the Sun might be captured in an elliptical orbit about the primary body. The problem with the idea, however, is that it requires a very rapid accretion rate for the primary body. In a paper presented at the Eighth Lunar Science Conference in 1977, Turcotte and one of his students, John Charles Nordmann, had shown that: Accretional capture is favored by a high rate of accretion. If a large fraction of the earth accreted in one to 10 yr, then the probability for accretional capture is reasonably large. However, the cross-section for accretional capture is significant for accretion times of 10– 1000 yr. … (Nordmann & Turcotte, 1977: 64)

As noted above, however, the simulations of Wetherill indicated that the accretion time of the Earth is measured in millions of years, much longer than the short accretion times that would make accretional capture feasible. Perhaps for this reason, Turcotte’s paper in the follow-on book, which he wrote with another graduate student, Louise Kellogg, focused more on the implications of the variability of the isotope data in lunar rocks than on the capture hypothesis. The main implication of the isotope data was that initially the interior of the Moon must have been cool, even though lunar accretion caused a surface layer to be molten. Turcotte and Kellogg summarized their research as follows: Measurements of isotope ratios and the associated concentration ratios have provided a wealth of data on the age of lunar rocks as well as constraints on the origin of the Moon. On the Earth the uniformity of isotope and concentration ratios for midocean ridge basalts has been taken as strong evidence for vigorous mantle convection. This convection mixes and homogenizes the upper mantle beneath the lithospheric plates. Isotope and concentration ratios for lunar mare basalts show much more variability. This has been taken as evidence for distinct source regions. Heterogeneity of the source regions implies little or no mixing and, therefore, little or no mantle convection. Currently, it is popular to associate the origin of the Moon with a massive collision between Earth and a large planetesimal. Such a hypothesis implies a hot early Moon. If the early Moon was hot, strong mantle convection would have been expected to stir and homogenize the lunar mantle, and distinct source regions would not be available for the subsequent mare volcanism. An alternate hypothesis for the origin of the Moon is accretion from relatively cool material. In this case the deep interior of the Moon would be cool but the near surface rocks would have been heated and melted by the gravitational energy associated with accretion. This would have resulted in a stable density distribution that would have inhibited mantle convection. The heating of an initially cool interior has been hypothesized to explain the mare volcanism. The distinct source regions implied by the isotope date supports a relatively cool origin for the Moon. (Turcotte & Kellogg, 1986: 311)

By the time of the Conference on the Origin of the Moon, Harold Urey had been dead for almost five years. But, as illustrated by the Cornell paper, and despite the assessment of John Wood that Urey had been wrong about an initially cold Moon, Urey’s idea was still alive. Furthermore, the hypothesis of a Moon that had accreted material on its own was still alive, as the following elaboration by Turcotte and Kellogg makes clear: The early Moon was certainly strongly depleted in volatiles. Very early in the evolution of the Moon a differentiation event led to the formation of the lunar crust. This is evidence that the outer portion of the Moon was hot but does not constrain the temperature of the deep interior.

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If the Moon accreted as an independent body and if the accreting material was cool, then it follows that the deep interior of the Moon would initially have been cool … a lunar temperature that increases with radius would suppress solid state convection. Thus heterogeneities could have been preserved. The heating of a Moon with a cool interior has been used to explain the delayed mare volcanism … . However, it is now popular to associate the formation of the Moon with a cataclysmic collision between a large planetesimal and the proto-Earth. Such a collision would have produced a circumferential cloud of vaporized material and debris. Such a scenario for the origin of the Moon would imply a hot initial state for the lunar interior. Both secular cooling of the lunar interior and the heat generation by radioactive isotopes would require vigorous convection within the lunar mantle … .(ibid.: 312–313)

It was clear from this paper that at least one reputable research lab, that of Donald Turcotte’s at Cornell, was not ready to accept the impact theory for the formation of the Moon.

7.3.2

Matsui and Abe

The next short paper at the conference was given by Takafumi Matsui and Yutaka Abe of the University of Tokyo. It was titled “Lunar magma ocean and its implications for origin of the Moon.” The corresponding paper in the follow-on book had a somewhat broader topic, “Origin of the Moon and its early thermal evolution.” The motivation of both papers, however, was to find a way to make possible a theory of binary accretion for the formation of the Moon. … the capture model seems to be inconsistent with the oxygen isotope systematics that suggest that formation of the Moon was intimately associated with formation of the Earth … . The fission model for the origin of the Moon also appears implausible because of its dynamic difficulties … . Therefore, the binary accretion model appears to be the most plausible explanation for the origin of the Moon. It has been widely accepted that a magma ocean was formed when the Moon was formed … although skepticism seems to be increasing … . One of the main reasons for skepticism is that it is not clear how sufficient heat could accumulate to form a magma ocean. One of the most important problems, therefore, in considering binary accretion is to provide a theoretical background for the formation of the lunar magma ocean. … (Matsui & Abe, 1986: 453)

Matsui and Abe argued that the formation of an insulating atmosphere around the Moon would be one of the consequences of lunar accretion by impacting planetesimals. High-velocity impact of planetesimals into the surface of a growing Moon results in crater formation. The cratering is the process of not only forming a cavity and releasing impact energy but also ejecting dust and evaporating volatile gases such as H2O and CO2 retained in the planetesimals and the surface layer. Since very frequent high-velocity collision occurs during accretion, impact-induced volatiles are expected to surround the entire surface of a growing Moon. However, escape of an impact-induced atmosphere and growth of the Moon are considered to be competitive processes. If the characteristic decay time of an impact-induced atmosphere is longer than the incremental accretion time, … an

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impact-induced atmosphere is expected to play an important role in insulating the thermal radiation from the surface layer and thus in heating the surface layer due to the release of impact energy. Recently, we showed that due to the blanketing effect of an impact-induced atmosphere, the surface of the Earth growing by planetesimal impacts was heated up to the temperature much higher than the melting temperature; thus a magma ocean covering the entire surface was formed … . For the case of the Moon, because of its small size, accumulation of such an impact-induced atmosphere may be only possible during incremental accretion of each radial shell. Therefore, in order to discuss the early thermal state of the Moon, we need to take into account both generation and escape of an impact-induced atmosphere simultaneously. Since H2O is the most abundant volatile in the Earth and in carbonaceous chondrites, for simplicity we consider an impact-induced atmosphere constituted of only H2O. (ibid.: 456)

Matsui and Abe then performed numerical calculations for 16 different models, corresponding to different adjustable parameters. They found that for values of parameters that were presumed to be appropriate for binary accretion, a magma ocean of several hundred kilometers could be formed. Anticipating questions about the contradiction between the dearth of volatiles on the Moon and the presence of an H2O atmosphere, Matsui and Abe wrote: It has been widely assumed that the Moon is depleted in volatiles. This seems to be contradictory with the models shown in this study. However, this is not the case for the lower water content (less than 1%) models. According to the petrological point of view, a water content of less than 0.1% is considered dry … . … As shown in this study, an impact-induced atmosphere can surround the entire surface of a growing Moon during accretion of each radial shell, but it will be dissipated within a few 108 years after formation of the Moon because of its low gravity. Water retained in a magma ocean will also be lost simultaneously with solidification of a magma ocean. Therefore, we believe those models with less than 1% water are not inconsistent with a dry Moon concept. … If the existence of a lunar magma ocean with a depth of several hundred kilometers is required to explain the origin of the lunar anorthositic crust, this study suggests that binary accretion is a favorable hypothesis for the origin of the Moon. (ibid.: 466)

7.3.3

Binder

The next short paper was presented by Alan Binder, who was then at the NASA Johnson Space Center. Binder had argued in the past for a fission model of lunar formation. In his talk at the conference, as well as in his extended paper for the book that was published following the conference, Binder presented the case for a Moon that was initially totally molten. This was in contrast to the conclusions of Turcotte and Kellogg, who had examined isotope data and concluded that the interior of the Moon had always been cool. Binder looked at geochemical and stress evidence and concluded that the Moon was initially totally molten.

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Binder’s geochemical argument was based on a derivation of relatively modest “bulk Moon” concentrations of incompatible, refractory elements, such as uranium, thorium, strontium, barium, and the rare-earth elements, as contrasted with high concentrations of these elements in the lunar magma source regions where they are now found. Binder concluded that this redistribution implies that: … the entire Moon differentiated, i.e., the Moon was initially totally molten, and the magma system from which the crust and mare basalt source region formed was a residual magma formed after 80% of the Moon crystallized … . (Binder, 1986: 427)

Binder’s stress argument was based on the observation of large thrust faults and shallow moonquakes in the highlands region of the Moon: He noted that: Thermal history and thermoelastic stress models indicate that if only the outer few hundred kilometers of the Moon were initially molten, and if it had a relatively cool interior, then the global, compressional stresses in the crust would be less than 1 kbar, and therefore the highlands would be free from young thrust faults and shallow, crustal, tectonic moonquakes … . If the Moon was initially totally molten, the global, compressional stresses in the crust would be much greater than 1 kbar, and therefore the highlands should have young (less than approximately 0.5 to 1  109 year old), 10-km-scale thrust faults and shallow (< 6 km deep), >1-kbar stress drop tectonic moonquakes … . Young thrust faults have been found throughout the 4.4% of the highlands that were photographed at low-to-moderate sun angles by the Apollo panoramic cameras. Extrapolation of these data indicate that some 2000 such thrust faults exist in the highlands … . (ibid.: 428–429)

Binder and his colleagues measured and analyzed the characteristics of the observed thrust faults and moonquakes and concluded that “… the agreement between the model predictions and the observation is excellent” (ibid.: 429), and that: Two independent data sets, (1) the mare basalt and highland anorthosite data on the incompatible trace element concentrations in the bulk Moon vs. those in the magma from which the crust and mare basalt source region were formed and (2) the data on the young, highland thrust faults and associated high stress, shallow moonquakes, indicate that the Moon was largely or totally molten early in lunar history. As has been shown earlier …, the chemical/petrological fractionation of an initially totally molten Moon is consistent with the known properties of the Moon, e.g., the massive feldspathic crust, the mare basalt source region, the Moon’s bulk composition, etc. Given these conclusions, any lunar origin model must be able to explain how the Moon became molten in its earliest history if the model is to be accepted. (ibid.: ” 431–432)

7.3.4

Solomon

The editors of Origin of the Moon placed behind the paper by Alan Binder one that came to an opposite conclusion. It was a paper by Sean Solomon of MIT that grew out of Solomon’s summary presentation at the end of the conference (Solomon, 1986). Solomon pointed out that, in a paper written in 1960, Gordon MacDonald had noted:

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The absence of evidence for extensive faulting on the lunar surface, and, in particular, the absence of evidence for large-scale strike-slip faults places limitations on the possible stress and thermal history of the moon. The lack of features associated with faulting implies that the volume and thus the surface dimensions of the Moon has remained more or less constant since the time of formation of the surface features of the Moon. … The change of the radius of the Moon can result from various thermal processes. If the Moon were initially at a high temperature and the rate of loss of heat was greater than the rate at which heat was being produced in the interior, then the lunar radius should have decreased with time. If, on the other hand, the rate at which heat is being produced is greater than the rate at which heat is being lost by the Moon, then its radius should be increasing with time. (MacDonald, 1960: 250)

Solomon summarized his own paper as follows: New theories for the formation of the Moon from an accretion disk thrown into circumterrestrial orbit after the collision of a planet-sized object with the Earth have led to a reexamination of the tectonic consequences of an initially molten Moon. Even the smallest estimates of radial contraction that would accompany cooling of the Moon from an initially molten state predict accumulated near-surface horizontal compressive stresses considerably in excess of the compressive strength of the upper lunar crust, estimated to be 0.5 to 1 kbar on the basis of topographic relief, the stress levels necessary to form mare ridges in mascon mare basins, and measurements of rock friction. Various mechanisms for relieving or modifying such large near-surface stresses are considered, including viscoelastic effects, widespread development of major fault systems, impact gardening, and opposing stresses arising from other global-scale processes. All of these mechanisms face substantial difficulties when tested against geological and mechanical information from the Moon and other terrestrial planets. These considerations pose a serious problem for theories of lunar origin that call for an initially molten state. (Solomon, 1986: 435)

Solomon presented a figure in his paper (Fig. 7.14) to illustrate the implications of the two possible thermal histories for the Moon. A Moon that initially had a cool interior and a hot outer layer would evolve so that it was more or less totally “warm.” The outer layer would contract as it cooled, while the interior would expand as it warmed. The result would be a present Moon that is not much different in size than it was initially. There would thus not be significant horizontal compressional stresses near the surface and hence no significant stress-induced faults. A Moon that was initially molten, on the other hand, would experience significant contraction as it cooled. As Gordon MacDonald had pointed out, one should see evidence of such contraction on the surface of the present Moon in the form of faults. It seems likely that Binder would have agreed with Solomon’s analysis. His view, however, was that one could see the faults on the film that had been returned from the Apollo panoramic camera. This camera was mounted on the Service Module that was connected to the Command Module and thus took pictures of the lunar surface as the combined Service and Command Module orbited the Moon while the surface explorations were in progress. Solomon apparently felt that MacDonald had been right, and that there was no evidence of the faulting that would be expected if the Moon had cooled from an initially molten state. Solomon added the observation, however, that one might not see the evidence of contraction if the compressional stress had been alleviated as the Moon cooled. As noted above,

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Fig. 7.14 Schematic of two alternate initial thermal states of the Moon. In (a) the outer half by volume is initially hot and the inner half cool; in (b) the Moon is initially molten. Both evolve to similar present states, but (a) maintains a nearly constant volume over the past 3.8 b.y while (b) contracts throughout lunar history. From Fig. 1 in Solomon (1986). “On the early thermal state of the Moon.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 440, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar Science Institute

Solomon considered a number of possible ways that the stress might have been mitigated, but found none that were convincing. He concluded: This result poses an obstacle to theories of lunar origin that call for a molten initial state, including at least the most straightforward scenario for the formation of the Moon from an accretion disk thrown into circumterrestrial orbit by a giant impact on the Earth … . The obstacle is not necessarily a fatal one, however, particularly inasmuch as this class of lunar formation theories is only beginning to be explored in a quantitative fashion. An effort should be made to examine the conditions under which a large impact event gives rise to several protomoons, and to the question of the evolution of multiple protomoons formed as a result of more than one large impact. Among a set of such scenarios may be at least one in which the early Moon formed by an aggregation of objects, some of which had cooled while others were still largely molten. … The Moon and the smaller terrestrial planets, with their global lithospheres and large portion of their surface stable since the time of heavy bombardment, have left us a record of their internal evolution for nearly the last 4 billion years. This record for the Moon can be read in the history of tectonic activity and the level of stress differences known to be currently supported. All new hypotheses for the formation and earliest history of the Moon should be pitted against that record as an essential test of their longevity. (ibid.: 448)

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Solomon thus raised another voice of caution against the giant-impact theory for the formation of the Moon.

7.3.5

Yoder

An important discussion point in the consideration of theories of the formation of the Moon was whether or not the Moon had a metal core and if a lunar core existed, what was its size. The size of the core would determine the amount of iron contained in the Moon and whether or not the Moon contained enough iron to account for the apparent dearth of siderophiles found in the lunar samples. The remainder of the talks for the afternoon of the first day of the conference dealt with questions related to the Moon’s core. In the first of these talks, Charles F. Yoder of the Jet Propulsion Laboratory of Caltech discussed the implications of data obtained from laser ranging. The Apollo 11, 14, and 15 astronauts had placed corner-cube reflectors on the Moon, and the Russian Lunokhod 2 unmanned lunar rover also had such a reflector. These corner-cube reflectors are called “retroreflectors” because they have the property of reflecting an incoming light beam back in the direction from which it came. In an earlier paper, Yoder described how the lunar laser ranging measurements at the McDonald Observatory in Texas worked: … A short laser pulse is fired from McDonald Observatory towards one of four cube corner retroreflectors placed on the Moon. After about 2.5 s if the aiming and atmospheric seeing are good, then maybe one of the 1018 photons in the laser pulse is electronically detected back at McDonald. Ten or so successful firings out of several hundred attempted are converted into a normal point or range. After corrections for atmospheric refraction, relativistic clock corrections and adjustments for the motion of Moon and Earth during the 2.5 s, the two-way timing measurement can be converted to a one-way range measurement with typical accuracy of about 10 cm. … (Yoder, 1981: 328)

The orbital dynamics that cause the Moon to keep one face toward the Earth also cause the Moon’s spin axis to precess about the normal to the plane of the ecliptic, with a period of precession of about 18.6 years. Actually, as shown by a figure (Fig. 7.15) in Yoder’s earlier paper (ibid.: 329), both the Moon’s spin axis and the normal to the Moon’s orbital plane precess about the normal to the plane of the ecliptic, and at the same rate. In the ideal case, the Moon’s spin axis, the normal to the plane of the ecliptic, and the normal to the Moon’s orbital plane are co-planer. The lunar laser range measurements uncovered a very small off set of the spin axis out of this plane, corresponding to 0.2 arc seconds of angle (about 0.000056 degree), in this precession. In his talk at the conference, Yoder discussed possible torques that could cause the offset, and the most likely torque was related to a viscous force between the core and the mantle of the Moon, with the radius of the core taken to be 330 km.

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Fig. 7.15 The lunar rotation axis precesses about the ecliptic normal and ca. 180o out of phase with the precessing orbit normal. Dissipation in the Moon causes a small 0.2 arc second advance in the lunar rotation axis. From Fig. 1 in Yoder (1981) “The free librations of a dissipative Moon.” Philosophical Transactions of the Royal Society of London A, volume 303, issue 1477, page 329, by permission of the Royal Society

Yoder noted that “Although the inferred core radius is certainly within the limits imposed by the Apollo seismic experiment (Rc < 500 km), it is significantly smaller than estimates of order 400–500 km from electromagnetic sounding.” (Yoder, 1984: 6).

7.3.6

Russell

One of the electromagnetic sounding measurements was the subject of the next talk, which was given by Christopher T. Russell of UCLA. During the Apollo 15 and 16 missions, a subsatellite was injected into an approximately circular orbit around the Moon at an altitude of about 100 km. The 38 kg subsatellites were spin stabilized, and they carried magnetometers to allow the sampling of the magnetic field at least every 24 s during each 2-hour orbit of the Moon (Russell, Coleman, Lichtenstein, & Schubert, 1974: 2748).

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Fig. 7.16 The Earth’s Magnetosphere. The large dark arrows outlined in white indicate the direction of the magnetic field in the two lobes of the magnetotail. Courtesy of NASA

Russell and his colleagues at UCLA had used the magnetic measurements of the Apollo subsatellites while the Moon was within the magnetic tail of the Earth to get an estimate of the size of the Moon’s metal core. The solar wind configures the Earth’s magnetic tail similar to the way that it does a comet tail, so that it is stretched out to great distances and always points away from the Sun. The magnetic field in the tail has two lobes separated by a thin current sheet (labeled “neutral sheet” in Fig. 7.16). In the northern lobe, the field lines point toward the Earth, and in the southern lobe they point away from the Earth. When the Moon is within one or the other of the lobes of the Earth’s magnetic tail, the effect is to impose an external almost-uniform magnetic field on the Moon. If the Moon has a metallic core, a magnetic moment will be induced in it by the external magnetic field of the Earth’s magnetic tail. Russell and his colleagues used a model based on the solar wind dynamic pressure to determine what the magnetic field strength at the subsatellite would have been in the absence of the induced magnetic moment in the Moon’s core. The difference between that model field and the measured field gave them the magnetic field produced by the induced magnetic moment in the Moon’s core, and with that data they could approximate the size of the Moon’s core. Using 21 data intervals over a period of nine and a half months, Russell and his colleagues estimated that the Moon’s core must have a radius a little larger than 400 km, at a minimum (Russell, Coleman, & Goldstein, 1981: 835). The lunar seismic data, the libration data, and the electromagnetic sounding data were thus all indicating a constraint for theories of lunar formation, namely that the Moon has a metal core with a radius in the range of 300–500 km.

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7.3.7

185

Cisowski and Fuller

The Moon does not currently have a magnetic field that is generated by currents in its core, but some of the lunar samples provided evidence that early in its history the Moon experienced a magnetic field that was comparable in strength to the magnetic field at the Earth’s surface. The next three papers at the conference were on the topic of lunar palaeomagnetism. The first of these was a paper titled “Lunar magnetic history” by Stanley M. Cisowski and Michael Fuller of the University of California, Santa Barbara, and these authors were asked to expand on their presentation in a paper for the follow-on book. A problem in the analysis of the magnetism of lunar samples is that the samples contain ultrafine grains (100–10,000 Å) of iron and are sometimes significantly degraded by the traditional method of determining the “paleointensity” of the magnetic field that resulted in the magnetic remanence observed in the sample, i.e., the “natural remanent magnetization” (NRM). This traditional “direct” method involves demagnetizing the sample then allowing it to cool through its Curie temperature in the presence of a magnetic field. The process has to be repeated to obtain a thermal remanent magnetization (TRM) curve, which relates the thermal magnetic remanence of the sample to the inducing magnetic field. The method is problematic for use with lunar samples because the repeated heating and cooling can result in chemical, physical, or microstructural changes in the samples (Banerjee, 1984: 9). To get around this problem, Cisowski, Fuller and their colleagues developed a “calibration curve” method in the mid-1970s to roughly determine the strength of the (inducing) field that caused the magnetization in fine grains of the lunar samples. Figure 7.17, from their paper given at the Sixth Lunar Science Conference in 1975 (Cisowski, Fuller, Wu, Rose, & Wasilewski,1975: 3133), helps to explain their approach. The bottom two “acquisition” curves (one solid and the other dashed) show the thermal remanent magnetization (TRM) that is acquired by a lunar sample as a function of applied field (H) for two different lunar samples. The thermal remanence for a sample at a given applied field was obtained by first demagnetizing the samples by applying an alternating field of 100 oersteds (107 c) and then allowing the sample to cool below its Curie temperature in the presence of the applied field. The approximately linear acquisition curves were obtained by repeating the process for different values of the applied field. One can see from the bottom two lines that a measured TRM would correspond to different values of the inducing field for different samples. This is because of the differences in the resistance to magnetization corresponding to differences in internal structures of the grains from one sample to the next. Cisowski and his colleagues discovered, however, that if you “normalize” the measured TRM by dividing it by the saturation value of the magnetization of a sample (IRMs), you get a calibration curve that can be used for all samples. This is illustrated by the near superposition of the lines of TRM/IRMs versus H for two different lunar samples, as shown in the top part of Fig. 7.17.

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Fig. 7.17 Calibration by IRMs of TRM. From Fig. 8 in Cisowski et al. (1975). “Magnetic effects of shock and their implications for magnetism of lunar samples.” In R. B. Merrill (Ed.), Proceedings of the Sixth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 6, volume 3), page 3133, Elmsford, NY: Pergamon Press, Copyright 1975

Cisowski and Fuller assumed that for most lunar samples the magnetic field measured in the lunar sample, called the natural remanent magnetization (NRM), is a thermal remanence (TRM), i.e., it was acquired as the sample cooled through its Curie temperature while it was inside the Moon or on its surface. They then used the laboratory derived relationship between TRM/IRMs and inducing field H to infer a paleointensity (inducing field) for lunar samples with a measured NRM/ IRMs. In a paper presented at the Thirteenth Lunar and Planetary Science Conference in 1983, Cisowski and his colleagues showed a correlation plot of absolute paleointensity versus normalized remanence intensity for fourteen lunar samples (Cisowski, Collinson, Runcorn, Stephenson, & Fuller, 1983: A697). Their figure is shown in Fig. 7.18. A least-squares fit to the data gives a factor of 47 for the relationship between the paleofield and the normalized intensity of the sample, with a correlation coefficient of 0.75. Cisowski and Fuller applied this rough correlation to estimate the paleomagnetic field strength derived from 67 lunar samples for which radiometric age data was available. The main point of their paper at the conference, as shown in Fig. 7.19, is that between about 3.6–3.9 billion years ago, magnetic fields on the Moon were significantly higher than in later eras. Cisowski and Fuller were cautious in interpretations of their results, and the results of others, that went beyond noting this temporal variation:

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Fig. 7.18 Correlation plot of absolute paleointensities versus normalized remanence intensity. From Fig. 3 in Cisowski et al. (1983) “A review of lunar paleointensity data and implications for the origin of lunar magnetism.” In W. V. Boynton & T. J. Ahrens (Eds.), Proceedings of the Thirteenth Lunar and Planetary Science Conference (Journal of Geophysical Research, 88, supplement), page A697, Washington, DC: American Geophysical Union, Copyright 1983 by the American Geophysical Union. Reproduced with permission of John Wiley and Sons

(Their) calibration would indicate that magnetic fields comparable to the Earth’s present field strength existed on the surface of the Moon for several hundred million years. However, paleointensity estimates involving heating of lunar samples may have overstated the intensity of the paleofield due to progressive destruction of fine-grained magnetic carriers … . … the lunar high field era may have primarily involved intensities of only thousands to a few tens of thousands of gammas, rather than the up to 1 oersted (100,000 gammas) fields indicated from other lunar paleointensity studies … . (Cisowski & Fuller, 1986: 420)

Cisowski and Fuller speculated that “Possible sources for this high field include a short-lived, core-residing dynamo, close approach to the Earth and its own internal dipole field, or some combination of the above. …” (ibid.: 423).

7.3.8

Banerjee

The next presenter at the conference was even more cautious. Subir Kumar Banerjee was an eminent geologist at the University of Minnesota. His specialty was rock

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Fig. 7.19 Absolute (left scale) and relative (right scale) paleointensity estimates on 67 lunar samples. From Fig. 8 in Cisowski & Fuller, (1986) Lunar paleointensities via the IRMs normalization method and the early magnetic history of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 421, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

magnetism, and he would later form the Institute for Rock Magnetism at the University. Still later, in 2006, Banerjee would receive the John Adam Fleming medal from the American Geophysical Union for decades of research contributions in rock magnetism. In his talk, Banerjee reminded his audience of the near impossibility of carrying out conventional paleointensity experiments, which require heating and cooling to 770 °C, without chemical, physical or microstructural changes. This difficulty caused researchers to devise non-thermal methods of determining the paleointensity of lunar magnetism. The IRM method developed by Cisowski and his colleagues was one such alternative, and Banerjee had worked on another alternative, called the anhysteretic remanent magnetization (ARM) method. Banerjee reported: I have investigated the experimental errors inherent in these alternative approaches to estimate the accuracy of limits on the calculated paleointensities. In the light of these built-in errors, a review of the up-to-date lunar paleointensity data yields the following conclusions: (1) It is most likely that lunar surface field was between 1,000 and 5,000 nT1 during its total history, (2) There is suggestive but not conclusive evidence that the field was as high as 100,000 nT at 3.9 b.y. ago, (3) It is unlikely that the 3.9 b.y. high field epoch was preceded by a weak (  1,000 nT) field lasting between 4.0 and 3.9 b.y. (Banerjee, 1984: 9).

1

A nanotesla (nT) is the same as a gamma (c).

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Banerjee, then, was not willing to concede that there was an anomalously high field within the Moon during the interval 3.9–3.6 b.y. ago. The lunar surface field that he did envisage for that time period, 1,000–5,000 nT, is, however, much larger than the current intensity of the solar wind magnetic field in the vicinity of the Moon (typically 1–10 nT).

7.3.9

Runcorn

The next speaker at the conference, Stanley Keith Runcorn (1922–1995) of the University of Newcastle upon Tyne in England, was not nearly as hesitant as Banerjee about asserting a high value for the magnetic field of the Moon early in its history. Runcorn was a distinguished geophysicist, whose research in paleomagnetism of the Earth had made him a leading figure in the development of plate tectonics. He was the first to discover evidence of the periodic reversal of the Earth’s magnetic poles. Runcorn was a Fellow of the Royal Society and a recipient of the AGU’s Fleming medal in 1983. Runcorn and his colleagues at the University of Newcastle upon Tyne had examined the magnetic properties of lunar samples from the Apollo 11 mission, and they had concluded in 1970 that one of the samples: … has a remanence showing comparable stability to similar rock types on earth; although the process by which the rock became magnetized is not yet known, the available evidence suggests that it was acquired in a lunar magnetic field considerably stronger than that existing at present and that the field was of internal origin. (Runcorn et al., 1970: 2386)

Five years later, at the Sixth Lunar Science Conference in 1975, the Newcastle upon Tyne group claimed: A survey of paleointensity determinations on lunar samples suggests that the ancient lunar field may have decreased over the period 3.9 to 3.2 AE from 1.3 to 0.05 Oe. Such a decrease would provide a strong argument against suggestions that the magnetization of the lunar rocks was acquired by local processes. Theories of an external origin for the ancient field are ruled out by a theorem due to Runcorn when used with results of Russell et al. (1974) for the global permanent lunar dipole moment. Thus it is suggested that the ancient surface field which magnetized the lunar rocks was produced by an internal source which gradually decreased in strength over the above period. (Stephenson, Runcorn, & Collinson, 1975: 3049)

Runcorn was convinced that the early Moon had a strong magnetic field of internal origin, presumably created by convection in a fluid iron core. The issue for him was what could have been the heat source that drove the lunar dynamo between 4.0 and 3.2 billion years ago. By the time of the Conference on the Origin of the Moon, Runcorn had not solved this problem, but he had not retreated from his conclusion that there must have been an early lunar dynamo. He summarized his talk with the assertion:

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Lunar paleomagnetism raises three issues relating to the origin of the Moon: the early formation of a fluid iron core, the nature of primeval heat sources in the Moon and the existence of a primeval satellite system. The remanent magnetization of the Apollo samples was interpreted as evidence for an internally generated lunar magnetic field … . The three independent methods of determining palaeointensities … are now in general agreement that the field was about 1 G 3.9 b yr ago declining exponentially to .02 G 3.2 b yr ago. Paleomagnetic directions of crustal strata have been determined from the Apollo 15 and 16 subsatellite magnetometer observations by Coleman, Russell and Hood (Hood, 1980, 1981). The question whether these are randomly directed such as would be expected from local magnetization processes or are proof of the existence of an early core dynamo field is one of the key issues of lunar science. On the core dynamo hypotheses the mean magnetic field would have been a dipole along the axis of rotation: the north magnetic pole positions so calculated from the observational data can be grouped into 3 bipolar groupings along 3 axis different from the present lunar rotation axis … . (Runcorn, 1984: 10)

To translate the units used by the Runcorn group in their presentation at the Sixth Lunar Science Conference, one could say that “… the ancient lunar field may have decreased over a period 3.9–3.2 billion years ago from 1.3 Gauss (G) to 0.05 G.” In his paper at the Conference on the Origin of the Moon, these figures were revised to a decline from 1.0 G (3 times larger than currently at Earth’s equator) to 0.02 G over the same time period. Runcorn argued that the three different axes corresponded to successive reorientations of the Moon by the objects that formed multi-ring basins when they impacted the lunar surface. He concluded: Although the presence of a lunar core was long ago suggested and there are now various different, although individually not conclusive arguments, the fit of the paleomagnetic data to the dipole hypothesis is strong evidence for the existence of a molten lunar iron core and implies a powerful heat source present in the earliest history of the Moon. (ibid.)

7.4

Concluding Remarks

During the opening day of the Kona Conference on the Origin of the Moon, the review by Wetherill seemed to have the biggest impact on the subsequent discussions. His computer simulations strongly suggested that collisions between relatively large planetary bodies almost certainly occurred as the solar system developed. Differing points of view from very good researchers continued to be expressed, however, even as the geophysical constraints on theories for the origin of the Moon were developed in a more quantitative way.

References Banerjee, S. K. (1984). Magnetic constraints on early lunar evolution revisited—Limits on accuracy imposed by methods of paleointensity measurements [Abstract]. In Papers to the Conference on the Origin of the Moon (LPI Contribution 540) (p. 9). Houston, TX: Lunar and Planetary Institute. Binder, A. B. (1986). The initial thermal state of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 425–433). Houston, TX: Lunar and Planetary Institute. Burns, J. A. (1986). The evolution of satellite orbits. In J. A. Burns & M. S. Matthews (Eds.), Satellites (pp. 117–158). Tucson, AZ: University of Arizona Press.

References

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Cameron, A. G. W., & Ward, W. R. (1976). The origin of the Moon [Abstract]. In Abstracts of Papers Submitted to the Seventh Lunar Science Conference (pp. 120–122). Houston, TX: Lunar and Planetary Institute. Cisowski, S. M., Collinson, D. W., Runcorn, S. K., Stephenson, A., & Fuller, M. (1983). A review of lunar paleointensity data and implications for the origin of lunar magnetism. In W. V. Boynton & T. J. Ahrens (Eds.), Proceedings of the Thirteenth Lunar and PlanetarySscience Conference (Journal of Geophysical Research, 88, supplement) (pp. A691–A704). Washington, DC: American Geophysical Union. Cisowski, S. M., & Fuller, M. (1986). Lunar paleointensities via the IRMs normalization method and the early magnetic history of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 411–424). Houston, TX: Lunar and Planetary Institute. Cisowski, S. M., Fuller, M. D., Wu, Y. M., Rose, M. F., & Wasilewski, P. J. (1975). Magnetic effects of shock and their implications for magnetism of lunar samples. In R. B. Merrill (Ed.), Proceedings of the Sixth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 6, v. 3) (pp. 3123–3141). Elmsford, NY: Pergamon Press. Drake, M. J. (1986). Is lunar bulk material similar to Earth’s mantle? In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 105–124). Houston, TX: Lunar and Planetary Institute. Greenberg, R., Wacker, J. F., Hartmann, W. K., & Chapman, C. R. (1978). Planetesimals to planets: Numerical simulation of collisional evolution. Icarus, 35(1), 1–26. Hartmann, W. K., & Davis, D. R. (1975). Satellite-sized planetesimals and lunar origin. Icarus, 24(4), 504–515. Hood, L. L. (1980). Bulk magnetization properties of the Fra Mauro and Reiner Gamma formations. In R. B. Merrill (Ed.), Proceedings of the Eleventh Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta), (Vol. 3, Suppl. 14, pp. 1879–1896). Elmsford, NY: Pergamon Press. Hood, L. L. (1981). Sources of lunar magnetic anomalies and their bulk directions of magnetization: Additional evidence from Apollo orbital data. In R. B. Merrill & R. Ridings (Eds.), Proceedings of the Twelfth Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta), (Vol. 1, Suppl. 16, pp. 817–830). Elmsford, NY: Pergamon Press. Hood, L. L. (1986). Geophysical constraints on the lunar interior. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 361–410). Houston, TX: Lunar and Planetary Institute. Larimer, J. W. (1986). Nebular chemistry and theories of lunar origin. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 145–171). Houston, TX: Lunar and Planetary Institute. Lunar Sample Analysis Planning Team: Burlingame, A., Burnett, D., Doe, B., Gault, D., Haskin, L., Schnoes, H., Heymann, D., Melson, W., Papike, J., Tilling, R., Toksöz, N., & Wood, J. (1972). Third lunar Science Conference: Primal igneous activity in the outer layers of the Moon generated a feldspathic crust 40 kilometers thick. Science, 176(4038), 975–981. MacDonald, G. J. F. (1960). Stress history of the Moon. Planetary and Space Science, 2(4), 249–255. Matsui, T., & Abe, Y. (1986). Origin of the Moon and its early thermal evolution. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 453–468). Houston, TX: Lunar and Planetary Institute. Nordmann, J. C., & Turcotte, D. L. (1977). Numerical calculations of the cross-section for the accretional capture of the Moon by the Earth. In R. B. Merrill (Ed.), Proceedings of the Eighth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 1, v. 1) (pp. 57–65). Elmsford, NY: Pergamon Press. Runcorn, S. K. (1984). Implications of lunar paleomagnetism for the origin of the Moon [Abstract]. In Papers presented to the Conference on the Origin of the Moon (LPI Contribution 540) (p. 10). Houston, TX: Lunar and Planetary Institute. Runcorn, S. K., Collinson, D. W., O’Reilly, W., Battey, M. H., Stephenson, A., Jones, J. M., Manson, A. J., & Readman, P. W. (1970). Magnetic properties of Apollo 11 lunar samples. In A. A. Levinson (Ed.), Proceedings of the Apollo 11 Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 1, v. 3) (pp. 2369–2387). Elmsford, NY: Pergamon Press. Russell, C. T., Coleman, Jr., P. J., & Goldstein, B. E. (1981). Measurements of the lunar induced magnetic moment in the geomagnetic tail: Evidence for a lunar core? In R. B. Merrill & R.

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Ridings (Eds.), Proceedings of the Twelfth Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta, supplement 16, v. 1) (pp. 831–836). Elmsford, NY: Pergamon Press. Russell, C. T., Coleman, Jr., P. J., Lichtenstein, B. R., & Schubert, G. (1974). The permanent and induced magnetic dipole moment of the Moon. In W. A. Gose (Ed.), Proceedings of the Fifth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 5, v. 3) (pp. 2747– 2760). Elmsford, NY: Pergamon Press. Solomon, S. C. (1986). On the early thermal state of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 435–452). Houston, TX: Lunar and Planetary Institute. Stephenson, A., Runcorn, S. K., & Collinson, D. W. (1975). On changes in the intensity of the ancient lunar magnetic field. In R. B. Merrill (Ed.), Proceedings of the Sixth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 6, v. 3) (pp. 3049–3062). Elmsford, NY: Pergamon Press. Taylor, G. J., Drake, M. J., Wood, J. A., & Marvin, U. B. (1973). The Luna 20 lithic fragments, and the composition and origin of the lunar highlands. Geochimica et Cosmochimica Acta, 37(4), 1087–1106. Thompson, A. C., & Stevenson, D. J. (1983). Two-phase gravitational instabilities in thin disks with application to the origin of the Moon [Abstract]. In Abstracts of Papers Submitted to the Fourteenth Lunar and Planetary Science Conference (pp. 787–788). Houston, TX: Lunar and Planetary Institute. Turcotte, D. L., & Kellogg, L. H. (1986). Implications of isotope data for the origin of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 311–329). Houston, TX: Lunar and Planetary Institute. Ward, W. R., & Cameron, A. G. W. (1978). Disk evolution within the Roche limit [Abstract]. In Abstracts of Papers Submitted to the Ninth Lunar and Planetary Science Conference (pp. 1205–1207). Houston, TX: Lunar and Planetary Institute. Warner, J. L., Simonds, C. H., & Phinney, W. C. (1976). Genetic distinction between anorthosites and Mg-rich plutonic rocks: New data from 76255 [Abstract]. Abstracts of papers submitted to the Seventh Lunar Science Conference (pp. 915–917). Houston, TX: Lunar and Planetary Institute. Warren, P. H. (1985). The magma ocean concept and lunar evolution. Annual Review of Earth and Planetary Sciences, 13, 201–240. Warren, P. H., Taylor, G. J., Keil, K., Marshall, C., & Wasson, J. T. (1981). Foraging westward for pristine nonmare rocks: Complication for petrogenetic models. In R. B. Merrill & R. Ridings (Eds.), Proceedings of the Ttwelfth Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta, supplement 16, v. 1) (pp. 21–40). Elmsford, NY: Pergamon Press. Warren, P. H., & Wasson, J. T. (1978). Compositional-petrographic investigations of pristine nonmare rocks. In R. B. Merrill (Ed.), Proceedings of the Ninth Lunar and Planetary Science Conference (Geochimica et Cosmochimica Acta, supplement 10, v. 1) (pp. 185–217). Elmsford, NY: Pergamon Press. Warren, P. H., & Wasson, J. T. (1980). Early lunar petrogenesis, oceanic and extraoceanic. In R. B. Merrill & J. J. Papike (Eds.), Proceedings of the Conference on the Lunar Highlands Crust (pp. 81–99). Elmsford, NY: Pergamon Press. Wetherill, G. W. (1986). Accumulation of the terrestrial planets and implications concerning lunar origin. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 519–550). Houston, TX: Lunar and Planetary Institute. Wood, J. A. (1975). Lunar petrogenesis in a well-stirred magma ocean. In R. B. Merrill (Ed.), Proceedings of the Sixth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 6, v. 1) (pp. 1087–1102). Elmsford, NY: Pergamon Press. Yoder, C. F. (1981). The free librations of a dissipative Moon. Philosophical Transactions of the Royal Society of London A, 303(1477), 327–338. Yoder, C. F. (1984). The size of the lunar core [Abstract]. In Papers presented to the Conference on the Origin of the Moon (LPI Contribution 540) (p. 6). Houston, TX: Lunar and Planetary Institute.

Chapter 8

The Kona Conference—Day 2

8.1

Introduction

The Conference on the Origin of the Moon had been organized so that geophysical constraints on the various theories would be discussed on the afternoon of the first day, and constraints imposed by chemical and petrological evidence on the morning of the second day. Discussions of dynamical constraints would be taken up on the afternoon of the second day, along with the first of two sessions titled My Model of Lunar Origin. In this chapter, each of the short talks of these sessions will be reviewed, along with expanded versions of these talks that appeared in the follow-on book, as well as additional background papers in some cases.

8.2

Chemical and Petrological Constraints

The Sunday afternoon talks at the conference had been focused on geophysical constraints for theories of the formation of the moon. A common thread among them was that the geophysical evidence indicates that the Moon has a metal core. The next morning, the focus would be on chemical and petrological constraints, but the discussion about the Moon’s core continued.

8.2.1

Newsom

The first talk of the Monday morning session was given by Horton Newsom of the University of New Mexico and was about the constraints imposed on theories of the formation of the Moon because of the low lunar abundances of molybdenum and other siderophile elements.. © Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_8

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As far back as the Second Lunar Science Conference in 1971, Ross Taylor and his Australian colleagues had reported a significant depletion of molybdenum (and other siderophile elements) in lunar samples from the Apollo 12 mission. The depletion is relative to the abundances of the element(s) found in carbonaceous chondrites, i.e., presumably relative to abundances of the elements in the original solar nebula. Newsom’s recent analysis confirmed the results of Taylor and his colleagues and also demonstrated that molybdenum is depleted relative to abundances in the Earth’s mantle, as well. Figure 8.1 from his talk plots the concentration of molybdenum against the concentration of neodymium. Neodymium, like molybdenum, is a refractory element, but, unlike molybdenum, it is not a siderophile. The ratio of the concentration of molybdenum to neodymium for rocks on Earth is found to be relatively constant. For the impact theory of the formation of the Moon, the depletion of lunar molybdenum below the abundances found in the Earth’s mantle suggested that the Moon must have a small metal core, the aggregation of which would have entrained molybdenum and other siderophiles. If the Moon did not come from the outer layers of an already segregated Earth, i.e., if the Moon had an “independent origin,” then a larger metal core would be required to account for the depletion of molybdenum and other siderophiles. Newsom and Mike Drake had been recently championing the “independent origin” hypothesis for the formation of the Moon, and Newsom observed:

Fig. 8.1 Molybdenum and neodymium concentrations in terrestrial samples (dot symbols) and in lunar samples (open symbols). From Fig. 2 in Newsom, H. E. (1986) “Constraints on the origin of the Moon from the abundance of molybdenum and other siderophile elements.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 208, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

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The question of a core in the Moon is, unfortunately still in some doubt from the perspective of siderophile elements. The significant depletion of siderophiles in the lunar silicates, even relative to the Earth’s mantle, strongly suggests that at least some metal segregation has occurred in the Moon. The existing models for the origin of the Moon, however, leave open the possibility that some or all of the metal required to explain the siderophile depletions was not incorporated into the Moon. However, some geophysical data, including paleomagnetic data …, strongly favor the existence of a relatively large Fe, Ni metal core. Because an independent origin model requires a large core, and this model actually fits the siderophile data and the geophysical data the best, an independent origin may be the closest to being true. (Newsom, 1986: 223)

With regard to future work needed to tighten the constraints on the various theories of lunar formation, Newsom offered the suggestion that “The largest advance will come from a definitive geophysical measurement of the size of the lunar core. …” (ibid.).

8.2.2

Ringwood and Seifert

The next paper in the morning session was coauthored by Ted Ringwood and Stefan Seifert, both of the Australian National University. Since at least 1977, Ted Ringwood had been supportive of the idea of a large impact on the Earth as the source of material that would condense into the Moon. He did not favor the “independent origin,” aka “independent planet,” hypothesis. At the conference, Ringwood’s paper with Seifert presented data on the abundances of nickel and cobalt in the Moon. They argued that the data showed: The mean abundances of cobalt and nickel in the bulk Moon are similar to those in the Earth’s mantle. … The cobalt content of the lunar mantle is similar to that of the Earth’s mantle, whereas the nickel content of the lunar mantle is depleted by a factor of about three. The above conclusion can be reconciled by the segregation of a small core (0.4% by mass) within the Moon. Equilibrium considerations dictate that the core contains about 40% Ni and 0.7% Co. … (Ringwood & Seifert, 1986: 274)

By contrast, the Earth’s core is thought to contain about 6% nickel and 0.3% cobalt (ibid.: 273). Ringwood and Seifert made two points (1) that: The similarity in Ni and Co contents of the bulk Moon and the Earth’s mantle strongly suggests that the material now in the Moon was derived from the Earth’s mantle subsequent to separation of the Earth’s core. (ibid.: 275)

and (2) that: These drastic differences between the compositions of the metallic phases present in the interiors of the Earth and Moon are not explained by any of the current versions of the independent planet hypotheses. (ibid.: 273)

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Delano

The next talk at the conference was given by one of Ringwood’s former Australian colleagues, John Delano, who was then at the State University of New York in Albany. Ringwood and Seifert had used data for nickel and cobalt abundances from a variety of lunar sample types, whereas Delano used lunar glasses of volcanic origin to estimate the abundances of nickel and cobalt in the “silicate portion” of the Moon. Twenty-five glass samples were used. These glasses had been erupted onto the lunar surface in fire fountains. They had come rapidly to the surface of the Moon from a depth of a few hundred kilometers without undergoing any crystallization, and presumably little or no chemical fractionation, during their journey. Thus, they are “pristine” samples of the hot, chemically uniform, lunar magmas from the Moon’s mantle. The glasses that have the lowest concentrations of titanium oxide (TiO2) were found to have the highest abundances of nickel and cobalt among the glass samples. Even so, as shown in Fig. 8.2, the concentration of nickel in the low-Ti glasses Fig. 8.2 Comparison of nickel and magnesium concentrations in terrestrial samples and in lunar glasses. From Fig. 4 in Delano, J. W. (1986) “Abundances of cobalt, nickel, and volatiles in the silicate portion of the Moon.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon,page 236, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

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(TiO2
1.0 wt%) was a factor of four (indicated by the dashed curve) below terrestrial concentrations, when plotted against the concentration of magnesium oxide (MgO). The nickel depletion reported by Delano suggested to him the requirement for a small metallic core within the Moon. Delano also surveyed and summarized what was known about volatile gases that had condensed on the surface of the glass pieces or had been trapped within vesicles of the glasses. The analysis of these volatile elements suggested to Delano and others that they likely occur in approximately CI-chondrite abundances. The Earth is depleted in volatile elements, though not as much as was characterized by the samples found on the lunar surface. Delano suggested that there might be volatile-rich reservoirs at depth in the Moon, and that: This component may occur as discrete chunks of volatile-rich debris from the outer solar system that became entrained in the circumterrestrial accretion disk. If this view is correct, then this component would be expected to have had a different history/origin from the bulk of the matter comprising the Moon (e.g., Earth-fissioned material). (Delano, 1986: 241)

8.2.4

Dickinson and Newsom

Tamara Dickinson and Horton Newsom, both of the University of New Mexico, gave the next talk, which was on the analysis of germanium abundances in the lunar mantle. From their literature survey, Dickinson and Newsom found that the germanium concentration varied from one Apollo site to another, and they estimated the average absolute lunar abundance to be 3.52 parts per billion. They reported, “The moon is depleted [in germanium], relative to chondritic abundances, by a factor of 38,000 normalized to Si. …” (Dickinson & Newsom, 1984: 16). The abundance of germanium in the Earth’s crust was known to be about 1.7 parts per million (Burton, Culkin, & Riley, 1959). So, germanium, which is a siderophile, was found to be depleted in lunar samples relative to the Earth’s crust as well as to chondritic abundances. This was more evidence for metal segregation in the Moon.

8.2.5

Goodrich and Barnes

In a paper presented at the Fifteenth Lunar and Planetary Science Conference in 1983 (Newsom, 1984), Horton Newsom had reported that his model for the independent origin of the Moon could account for the depletions of the siderophile elements found in lunar rocks, except for phosphorus, where his calculated depletion is not as great as that observed. Newsom suggested that phosphorus might have been depleted in proto-lunar material because of its volatility.

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The next paper at the Conference on the Origin of the Moon was essentially a follow-up on Newsom’s 1983 paper. It was coauthored by a colleague of Newsom’s at the University of New Mexico, Cyrena Anne Goodrich, and by Stephen Barnes, who was an NSF Fellow at NASA’s Johnson Space Center. The paper by Goodrich and Barnes reported on experiments to find another explanation for the depletion of phosphorus, namely the presence in the Moon’s interior of phosphorus-rich silicates. On Earth, phosphorus-rich olivine is sometimes, but rarely, found. Goodrich and Barnes thus experimented with the behavior of phosphorus and olivine under simulated lunar conditions. They found that it was unlikely that phosphorus would be compatible with olivine under any widespread lunar igneous conditions (Goodrich & Barnes, 1984: 17). They concluded that phosphorus is depleted in the Moon for the reason that Newsom had suggested, i.e., because of its volatility.

8.2.6

Warren and Rasmussen

The next paper that was presented at the conference was coauthored by Paul H. Warren of UCLA and Kaare L. Rasmussen, who was a visiting scientist at UCLA but whose home institution was the University of Copenhagen. Their talk concerned the revised estimate for heat flow given by Marcus Langseth and his colleagues at the Seventh Lunar Science Conference in 1976, and the associated estimate for the bulk concentration of uranium in the Moon. The revised estimates of Langseth et al. for an average global heat flow was 18 ergs per square centimeter per second, and the corresponding estimate for the concentration of uranium, assuming that the radioactivity of uranium is responsible for the heat being generated in the Moon, was 46 parts per billion, which is about twice the estimated concentration of uranium in the Earth’s mantle (Langseth, Keihm, & Peters, 1976: 3143). Warren and Rasmussen took account of the fact that the Apollo 15 and Apollo17 sites, where heat flow measurements were made, were atop unusually thin layers of the “megaregolith.” They also corrected for the fact that the Apollo 15 site was found to be in an exceptionally uranium-rich region of the Moon, noting that measurements of heat flow by Langseth et al. at the Apollo 15 site was 50% higher than at the Apollo 17 site. The revised estimates of Warren and Rasmussen for the average lunar heat flow was 11 erg s−1 cm−2. Their estimate for the corresponding lunar bulk concentration of uranium was 20 parts per billion. They concluded that “The Moon’s bulk composition is less exotic than generally assumed.” (Warren & Rasmussen, 1984: 18). In subsequent presentations and discussions, Warren was to reiterate this point. He was in the camp that favored an origin of the Moon from a circumterrestrial swarm of planetesimals, as is evident from a paper that he had presented with his UCLA colleague, John Wasson, in 1979 at the Tenth Lunar and Planetary Science Conference. There, they presented their model for the formation of the Moon from differentiated planetesimals of chondritic composition (Wasson & Warren, 1979).

8.2 Chemical and Petrological Constraints

8.2.7

199

Warren

Paul Warren was the next speaker at the conference, and he gave a companion paper to the one he had coauthored with Karre Rasmussen. His topic was the so-called “mg ratio,” which is the ratio of the concentration of magnesium oxide to the sum of the concentrations of magnesium oxide and ferrous oxide, i.e., [MgO]/ [MgO + FeO]. In this calculation the concentration is given by the number of moles of the substance per unit volume. In the summary of his talk at the Conference on the Origin of the Moon, Warren noted: The bulk-Moon mg ratio is essentially the same, within a few percent uncertainty, as that of the Earth (about 0.895). The recently popular Earth-impact model, wherein the Moon forms out of vaporized ejecta from a collision between the Earth and one of its largest protoplanets, may have difficulty accounting for the similarity in mg ratio between the Moon and the Earth. Supporters of the Earth-impact model claim that it would help account for the Moon’s volatile-trace element depletions, and even putative refractory-major element enrichments, due to fractional condensation of the vaporized ejecta. However, fractional condensation could easily lead to a different (most likely higher) mg ratio for the Moon compared to the Earth. Closely similar mg ratios are consistent with models that form the Moon by accretion from a circumterrestrial swarm of fragments of previously differentiated asteroid-sized bodies, provided the mechanism(s) for depleting the Moon of FeNi did not simultaneously enrich it in materials from the shallowest parts of the previously differentiated bodies. (Warren, 1986: 279)

The model of Wasson and Warren featured a means of depleting the Moon of FeNi based on “compositional sorting.” They: … emphasized collisions as a means of break-up [of the circumterrestrial swarm of planetesimals], but suggested that the Moon’s low FeNi content resulted from a tendency for the collisions to yield larger FeNi fragments than silicate fragments, due to the greater mechanical strength of FeNi … . Because larger fragments have greater momentum, and hence greater probability of passing through the circumterrestrial swarm of planetesimals and striking the Earth, the swarm was depleted in FeNi and enriched in silicates, compared to the Earth. (ibid.: 302–303)

The idea of compositional sorting would return to the discussions later in the conference. As we shall see, it had its origin in the earlier research of Egon Orowan of MIT and Evgania Ruskol of the Soviet Geophysical Institute.

8.2.8

Goettel

The next paper at the conference was given by Kenneth Goettel of Brown University. Goettel took the view that the leading contenders for a theory for the formation of the moon were a fission hypothesis, which might include the Earth-impact idea, and an independent origin of the Moon from materials in orbit about the Earth. Goettel professed to be unbiased, writing that “The focus of the present paper is not to argue for either of these models, but rather to address an

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implication that is common to both.” (Goettel, 1984: 20). In his talk, Goettel discussed the implications for the two models of the difference in concentration of FeO between the Moon and the Earth’s mantle. The FeO content of the moon, about 13%, is substantially higher than the present FeO content of the Earth’s mantle, about 8%. If the moon formed by fission from the Earth’s mantle, then the conclusion that the Earth’s mantle must have been much richer in FeO at the time of the fission appears firm; there do not appear to be viable mechanisms to fractionate FeO so markedly during fission or to add FeO to the moon after fission and not add FeO to the Earth as well. If the moon formed independently in geocentric orbit, then the FeO contents of the two bodies should be similar, because both would be accreting from the same source of silicate material and it is very difficult to envision a process that would so markedly fractionate FeO between Earth and Moon. Therefore, Earth’s mantle at the time of lunar formation probably had an FeO content quite similar to the present FeO content of the moon. … (ibid.)

Goettel’s conclusion thus seemed to favor the fission hypothesis over the independent origin model of the Moon. He seemed to be more interested in the broader implications for planetary compositions, however. If the Earth had more FeO than previously thought, then the composition differences between Earth and Mars are less than previously believed. This suggests that condensation temperature and heliocentric distance may have been less important in governing planetary compositions and other mechanisms, including iron/silicate fractionation may have been more important. … (ibid.)

8.2.9

Shervais and (Lawrence) Taylor

The next paper at the conference tried to relate the way the Moon came together with the different rock types that had been reported for the different Apollo landing sites. The coauthors of the paper were John W. Shervais of the University of Tennessee and the University of South Carolina and Lawrence A. Taylor (1938– 2017) of the University of South Carolina. They noted that in Ross Taylor’s 1982 book, Planetary Science: A Lunar Perspective, Taylor had emphasized the remarkably constant volatile/refractory element ratios for lunar rocks found at all sites. Ross Taylor had written: The close correlations between volatile and refractory elements (K/U. Ba/Rb. K/Zr, K/La) both in mare and highlands samples indicates that both regions were originally homogeneous with respect to volatile and involatile elements. This is a primary piece of evidence for homogeneous accretion of the Moon, or homogenization following accretion. (Taylor, 1982: 229)

Many lunar scientists subscribed to “homogenization following accretion” via a global magma ocean. But, Shervais and Taylor asked in their paper, how do you then explain the observed geochemical differences found in highland rock samples from the Apolo 14, 15, 16, and 17 sites that implied “lateral heterogeneity” in the lunar crust (Shervais & Taylor, 1986)? Their suggested answer was that the lack of

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lateral homogeneity in the lunar crust was a result of the impact of giant, basinforming projectiles in the later stages of lunar accretion (ibid.: 197). One consequence of these impacts would be to gouge out regional sections of rocks that had been formed from the solidification of the magma ocean and relocate them to other parts of the Moon. By moving differentiated lunar material to the surface from great depths, the impacts would have created the observed lateral heterogeneity in rock types at the surface. Shervais and Taylor concluded that, regardless of whether or not their hypothesis was correct: … the observed large-scale, lateral variations in the petrology and geochemistry of the lunar crust and mantle reflect the processes by which the Moon formed and evolved. These processes are not independent; the general trends in lunar evolution were to a large degree preordained by the circumstances of its origin. By studying one, we may elucidate the other. (Shervais & Taylor, 1986: 173)

8.2.10 Kreutzberger, Drake, and Jones The next paper in the morning session on chemical and petrological constraints for theories of the formation of the Moon was from researchers at the University of Arizona who were affiliated with a group known as the Tucson Lunar Origin Consortium. The paper was coauthored by Melanie E. Kreutzberger, Michael Drake, and John H. Jones, and it cast doubt on the fission hypothesis for the formation of the Moon. They wrote: Fission of the Moon from the Earth following core formation is a hypothesis for the origin of the Moon … . Supporting arguments include the low density of the Moon corresponding to the density of the Earth’s mantle and the low volatile content of the lunar rocks vs. those of terrestrial origin. Vapor pressures of the alkali elements and their oxides increase in the following order: Na, K, Rb, and Cs. The Moon should, therefore, be more depleted in Cs relative to Rb, Rb relative to K, and K relative to Na than the Earth if the fission model is correct. Analysis of lunar mare basalts and terrestrial mid-ocean ridge and other young basalts indicate that this behavior is not observed; for example, the Moon shows a higher Cs/Rb ratio than the Earth – i.e., it lies between chondrites and the Earth. (Kreutzberger, Drake, & Jones, 1984: 22)

This was the same point that Drake had made in his opening review address. The authors considered the possibility: … that monovalent alkali elements might be lost from silicate materials in a different order than that inferred from elemental and oxide vapor pressures, as a result of differences in the way they are bound in silicate materials. … (ibid.)

To test this possibility, they constructed a synthetic basalt that contained 1% by weight each of sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs). They heated this material and then measured the loss of the volatile elements as a function of time.

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Analyses indicate that the behavior of volatiles dissolved in a silicate melt is similar to that inferred from elemental and oxide vapor pressures. At 1400 °C …, Cs is most readily volatilized, followed by Rb, K, and Na … . … We conclude that alkali element ratios in the Earth and Moon are not readily interpreted in terms of the Fission hypothesis. (ibid.)

8.2.11 Koeberl The next paper at the conference was also on the topic of volatile elements. The author was Christian Koeberl of the University of Vienna. Koeberl had recently graduated from the University of Graz in Austria and would later receive several appointments as a Visiting Scientist at the Lunar and Planetary Institute as he deepened his expertise in the geochemistry of planetary impacts. Koeberl’s view of the production of lunar glasses is shown in Fig. 8.3 from a figure in a 1984 publication that he coauthored with his colleagues at the University of Vienna. The volcanic glasses were produced through:

Fig. 8.3 Schematic of lunar lava fountaining through a vapor layer. From Fig. 2 in Koeberl, C., Kiesl, W., Kluger, F., & Weinke, H. H. (1984). “A comparison between terrestrial impact glasses and lunar volcanic glasses: The case of fluorine.” Journal of Non-Crystalline Solids, volume 67, issues 1–3, page 643, Copyright (1984), with permission from Elsevier

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… fire fountaining with quenching of the liquid droplets during a short ballistic flight through the simultaneously released vapor cloud consisting of volatiles, leading to the accumulation of volatile elements in and on the solidifying glasses. (Koeberl, Kiesl, Kluger, & Weinke, 1984: 642)

The issue for Koeberl as well as the Arizona group and others, was the implication of finding volatiles on the surfaces of the lunar glasses. In his talk at the conference, Koeberl noted: Models of the origin of the moon have been developed based on the assumption that the moon is depleted in volatiles. … Recently the surface correlated elements have been interpreted to indicate that the moon has some reservoirs that are enriched in volatiles, and that the deep lunar interior is not depleted in volatile elements. … Of course, a lunar interior which is not depleted in volatile elements has important implications on the origin of the moon. So it would be of interest to know something about the amount of volatiles present in the lunar interior. Since most of these elements are abundant only in the form of very thin surface layers on the glasses, and since the glasses themselves do not show a similar enrichment, the source should be of rather limited extent … . (Koeberl, 1984: 23)

Whereas the paper by Kreutzberger et al. cast doubt on the fission hypothesis, the conclusion of Koeberl’s paper was more favorable, in that it argued against the existence of large reservoirs of volatile elements in the Moon’s interior.

8.2.12 Swindle, Caffee, and Hohenberg The next paper at the conference was coauthored by Timothy D. Swindle, Marc W. Caffee, and Charles M. Hohenberg of Washington University in St. Louis. For the corresponding article in the follow-on book, Ross Taylor of the Australian National University was added as a coauthor. The focus of the talk and the article was the use of a particular method of radio-isotope dating to determine the relative ages of the Earth and the Moon. One part of the method makes use of the fact that an isotope of iodine, 129I, decays with a short half-life (17 million years) into a stable isotope of the rare gas xenon (129Xe). John H. Reynolds (1923–2000) of the University of California, Berkeley, was the first to measure excess amounts of 129Xe in a stone meteorite in 1960. Charles Hohenberg, one of the coauthors of the paper with Swindle, Caffee, and Taylor in the follow-on book on the Origin of the Moon, was a colleague of Reynolds at Berkeley in the 1960s. With Frank A. Podosek, Hohenberg and Reynolds published an important paper in 1967 that showed that the minerals in chondritic meteorites in our solar system cooled simultaneously within 1 or 2 million years (Hohenberg, Podosek, & Reynolds, 1967). Figure 8.4 from Fig. 1 in the 1960 paper by Reynolds shows the results of a mass spectrogram analysis for the isotopes of xenon in a sample of the Richardton meteorite. The short horizontal lines for each isotope indicate the comparable levels of the xenon isotope in a sample of Earth’s atmosphere that has the same 132Xe content. The large excess of 129Xe is obvious from the figure (Reynolds, 1960: 8).

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Fig. 8.4 Mass spectrum of xenon from the Richardton meteorite. Horizontal lines show the comparison spectrum of terrestrial xenon Reprinted Fig. 1 with permission from Reynolds, J. H. (1960). “Determination of the age of the elements.” Physical Review Letters, volume 4, issue 1, page 8, 1960, Copyright 1960 by the American Physical Society. http://journals.aps.org/prl/ abstract/10.1103/ PhysRevLett.4.8

The stable isotope of iodine is 127I; it does not decay into something else over time. In a given sample of material, e.g., a lunar sample or a meteorite, one can measure the ratio of the amount of “excess” 129Xe to the amount of 127I. Then, by means of a chain of assumptions and estimates, one can “determine” the time interval between the end of the formation of 129I in the part of the galaxy that became the solar system and the onset of entrapment of 129I in the material that became the lunar sample or meteorite. Let Ro ¼

129  127 I =½ Iinitially

and Rsample ¼

129

 Xe =½127 Imeasured

where 129 Xe* is the “excess” amount of 129Xe in the sample. The amount of 129Xe* is assumed to be equal to the amount of 129I that had not yet decayed into 129Xe when the decaying iodine was first unable to escape from the sample. Suppose, for example, that the ratio

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Rsample =Ro ¼ 1=128 The ratio 1/128 (= 1/27) would represent seven half-lives of exponential decay of I. The time interval between the formation of 129I (and presumably the formation of the other heavy elements in our solar system) and the point when 129I was first trapped in the sample would then be determined to have been about 119 million years (119 = 7  17). In the measurement for the Richardton meteorite, Reynolds determined this time interval to be 350 million years. Reynolds could then estimate the time interval from the present to when the elements were created: “There is a large body of evidence that the chondrites were formed at a time close to 4.6  109 years ago. Thus the age of the elements is close to 4.95  109 years.” (ibid.). The second part of the method described by Swindle, Caffee, Hohenberg, and Taylor in the follow-on book was based on the fact that an isotope of plutonium (244Pu) fissions with a half-life of 82 million years into daughter products that include stable isotopes of xenon (136Xe,134Xe,132Xe, and 131Xe). In the I-Pu-Xe process to determine the age of a sample, the quantity to be measured is the ratio of the amount of excess 129Xe to the amount of 136Xe, where the former is taken to be evidence for now decayed 129I, and the latter is taken to be evidence for now extinct (removed by fission) 244Pu. The current ratio of the amount of excess 129Xe to the amount of 136Xe is then proportional to the ratio of the initial amount of 129I to the initial amount of 244Pu, i.e., [129I/244Pu]o, times an exponential function of time that decays with a known half-life. As with iodine/xenon dating, the I-Pu-Xe process for arriving at the age of the Moon (and the Earth) involves a number of assumptions and estimates. The authors reviewed the best estimates from several researchers for the age of the Moon and Earth, using the I-Pu-Xe process, and found that the calculated ages for the Earth’s atmosphere and mantle were between 80 million years after and 30 million years before the oldest lunar ages. The best estimate would then be that the Earth’s atmosphere and mantle formed some 25 million after the origin of the Moon. They argued: 129

If the Moon were derived from the Earth by fission or by spalling, this age [the age of the Earth] should predate the age of any lunar samples. Under the current best estimates, it does not. While the present uncertainties in the ages of both bodies are still too large to say definitively that the Earth is younger than the Moon, the data tend to be inconsistent with fission or impact models. (Swindle, Caffee, Hohenberg, & Taylor, 1986: 352–353)

8.2.13 (Ross) Taylor The Washington University group had found an ally in Ross Taylor of the Australian National University. Taylor had by now authored or coauthored three books that covered topics in lunar geology, geochemistry, mineralogy, etc., and he had long been skeptical of the fission hypothesis for the formation of the Moon.

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Taylor gave the next talk at the conference, and he opened with one of his mantras: “The progress of science depends on the construction of testable hypotheses. …” (Taylor, 1984: 25). In his ten-minute talk, Taylor focused on tests of the fission hypothesis, and by “fission,” he seems to have meant the rotational fission hypothesis, in which material for the Moon was thrown off a rapidly rotating Earth. Some viewed the impact origin of the Moon as a variation of the fission hypothesis, but Taylor treated them separately. After reviewing the various comparisons of terrestrial and lunar composition, including the depletion of volatiles, Taylor concluded: If the moon fissioned from the Earth following accretion over 108 years, then the volitization required to remove K, Rb, Pb, Bi, Tl, H2O, etc., will remove Xe, and so Xe derived from I or Pb should be rare in the moon. If the moon accreted from a separate group of fractionated planetesimals, which underwent metal and volatile element fractionation at To, then 129I and 244Pb can be present in the precursor planetesimals. The combination of chemical and isotopic problems suggests that the fission hypothesis is no longer viable, and separate terrestrial and lunar accretion from a population of fractionated precursor planetesimals provides a more reasonable explanation. (ibid.)

In other words, whereas the previous paper used the concentrations of various isotopes of xenon in lunar samples to construct an argument against the fission hypothesis, Taylor was suggesting that the mere presence of these isotopes of xenon in the lunar samples was an argument against the fission hypothesis and supported a model of separate lunar accretion. In the follow-on book, Taylor was given the opportunity to expand on his talk. In his contribution, he described a scenario of events that included very early solar activity, such as a strong T Tauri type solar wind that swept out gases and volatile elements from the inner part of the developing solar system (Taylor, 1986: 125– 143). This was followed by the formation of volatile-depleted planetesimals where the terrestrial planets (Mercury, Venus, Earth, and Mars) later formed. If the Moon were formed by a giant impact of a Mars-sized object with the Earth, there might have been a second round of depletion of volatiles caused by the extreme heat of that event. If the Moon were formed as a double planet with the earth, there would be only the initial depletion of volatiles in the planetesimals that formed the Moon. A test between the two possibilities might then be the determination of the time history of volatile depletion of material that formed the Moon. Regarding the remaining viable models for the formation of the Moon, Taylor arrived at similar conclusions to those of Drake. His summary read in part: Capture models for the origin of the Moon do not meet chemical, isotopic, or dynamical constraints and have effectively been abandoned. The composition of the bulk Moon differs in several important ways from that of the terrestrial mantle, ruling out fission hypotheses unless substantial element fractionation occurs during or following fission. Large Mars-sized impactor models cut several Gordian knots, since the lunar material is derived mostly from the impactor, whose chemical and isotopic composition become free parameters. Both volatile and siderophile element depletions, observed in the Moon, are common in meteorites (e.g., eucrites) formed at 4.5 aeons, so that the lunar composition is nonunique, and a population of precursor planetesimals of appropriate composition existed. Of the five hypotheses for lunar origin, either the large impactor models or the double-planet

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models remain as viable candidates. … Whole Moon melting follows accretion. Core formation in the Moon depletes the mantle in siderophile elements. … Crystallisation results in an anorthositic crust and a differentiated lunar mantle while the final residual liquid (KREEP) invades the crust at 4.3–4.4 aeons. (ibid.: 125)

As an interesting aside, Taylor noted the following further consequences and predictions related to his ideas: The satellites of the outer planets have low densities, although Ganymede, Callisto, Titan, and Triton are about the size of Mercury. Accordingly, it appears that free metal was not available in large amounts beyond the asteroid belt. Probably volatile-refractory element fractionation, which may be linked to early solar flare-ups in the inner solar system, was not effective at and beyond 5 AU (as is shown, inter alia, by the large amounts of condensed water/ice in the Jovian and Saturnian satellites). Accordingly, it may be predicted that the volatile-refractory element ratios (e.g., K/U ratios) in these satellites are about the same as those in CI meteorites. This would have the consequence that radioactive heat generation is generally higher in these bodies than for the inner planets. … (ibid.: 140)

8.3

Dynamical Constraints

Ross Taylor’s talk concluded the Monday morning session on chemical and petrological constraints for theories for the formation of the Moon. The afternoon session featured talks on dynamical constraints, and these will be reviewed next.

8.3.1

Vanyo

The opening talk was given by James P. Vanyo of the University of California, Santa Barbara. Vanyo and his colleague at UC Santa Barbara, Stanley Awramik, had been studying the growth patterns of columnar stromatolites, using an approach somewhat similar to that of John Wells, who had studied the fine ridges in fossil corals to determine the number of days in the Devonian year, i.e., within the Cambrian period, which began some 0.57 billion years ago. Fossilized stomatolites extend much further into the Precambrian, and their study offered the potential to place constraints on the history of the Earth and Moon further back in time, as discussed by Vanyo. … in the Precambrian, life forms were restricted to subaqueous unicellular algae and bacteria. Fossils of such organisms are abundant in the geological record. Their existence is also evidenced by abundant fossilized structures (stromatolites) consisting of many thin layers of usually darker algae-bacterial growth alternating with layers of usually lighter sediment-precipitate. The earliest of these have been dated to 3.5 billion years ago, … . (Vanyo, 1984: 29)

Vanyo and Awramik had discovered fossil stromatolites in which the layered columns, instead of being in a straight line, had a sine-wave shape. They interpreted

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the sinusoidal growth pattern as having resulted from stromatolites forming in shallow water and with the tendency for the top layers to “follow” the sun through the course of a year. The length of the sine wave would then correspond to a year’s growth of the stromatolite, and by counting the layers, one could determine the number of days in the year when the stromatolite was growing. Using this method for one of their samples, they determined that 850 million years ago there were approximately 435 days in a year (ibid. See also Vanyo and Awramik, 1982: 1125–1128). This result is consistent with the tidal-friction research of Walter Munk of the University of California, San Diego. In 1966, Munk had estimated that 850 million years ago there were between 420 and 440 days in a year (Munk, 1966). Recall that, using coral fossils, John Wells had determined that there were approximately 400 days per year in the Middle Devonian period, which was from 385 to 398 million years ago. The new Precambrian data point would seem to continue the upward trend of Cambrian data points for the number of days in a year versus the time span from the present to the time the fossils were created. Vanyo thus contributed to a constraint on theories for the formation of the Moon, namely that there seems to have been a decline in the spin rate of the Earth, and a corresponding increase in the Earth-Moon distance, extending back to at least 0.85 billion years ago.

8.3.2

VanArsdale

The next talk at the conference was given by William VanArsdale of the University of Houston. His challenge was to develop a constraint on theories for the formation of the Moon based on lunar orbital history. First, some background on the conventional wisdom that lunar-induced tidal dissipation on Earth occurs primarily in the oceans, rather than in the more solid parts of the Earth, i.e., bodily tides. In his 1968 lecture in honor of Harold Jeffreys, Walter Munk had remarked: Those who have been heavily involved in calculating past orbits [of the Moon] have a vested interest in dissipation by bodily tides rather than ocean tides, for the solid Earth is less ephemeral than the ocean basins. I would conclude that a straightforward interpretation of the seismic evidence … leads to the opposite conclusion. This view has also the advantage that one does not melt the Earth. Moreover, the oceanographic evidence indicates that the oceans definitely are an important factor. … (Munk, 1968: 356)

And Kurt Lambeck, an eminent Australian scientist, expressed the conventional wisdom when he wrote in 1975: Because the dissipation occurs principally in the oceans, extrapolation of the lunar orbit into the past becomes an impossible task, since we know that a very major reordering of ocean-continent distribution occurred during at least the last 200 m.y. (Lambeck, 1975: 2924)

It was well known that if one calculates lunar orbits backward in time, assuming a constant rate of tidal dissipation, as was done by Gordon MacDonald in 1966, the Moon arrives near the Earth only 1.7 billion years ago. It was clear from the

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analyses of lunar samples that this time span was not long enough, hence, the time-scale problem. Lambeck concluded that “… only a variable energy sink can solve the time-scale problem and the only energy sink that can vary significantly with time is the ocean.” (Lambeck, 1980: 288). In his talk at the conference, VanArsdale took a view contrary to the conventional wisdom: … while the oceans undoubtedly contribute to energy dissipation, it seems implausible that such a small amount (0.02%) of the Earth’s mass could play such a dominant role in this process. An alternate hypothesis assumes that most of the dissipation is associated with solid-body tides. This mechanism is also capable of varying significantly over time for viscoelastic bodies. (VanArsdale, 1984: 30)

VanArsdale used a mathematical model in which the Moon and the Earth were assumed to have both viscous and elastic properties that could vary over time. A body that is totally and perfectly elastic can dissipate no energy. Likewise, a body that is totally and perfectly inviscid (having no viscosity) cannot dissipate energy. The Earth is neither perfectly elastic nor perfectly inviscid. Whether the Earth was initially cold and then was heated by radioactivity in its interior, or was initially hot and then gradually cooled, the viscosity and ability to dissipate energy in its various parts has likely changed over time. This seems to be the main point of VanArsdale’s talk, namely to suggest that the time-scale problem might be solved if solid-body tidal dissipation can be shown to occur within an Earth in which the dissipation of energy varies significantly over time. In a paper written at the time of the publication of the follow-on book on the Origin of the Moon, VanArsdale showed how in his model the mean tidal moment on the Earth varies with the Earth’s viscosity, η. Figure 8.5 from Fig. 1 in VanArsdale’s paper shows how the tidal moment (in VanArsdale’s model) exerted

Fig. 8.5 The variation of the mean tidal moment exerted by the Moon on the Earth as a function of η in VanArdale’s model. From Fig. 1 in VanArsdale, W. E. (1985) “Orbital dynamics of a viscoelastic body.” Journal of Geophysical Research-Solid Earth, volume 90, issue B8, page 6890, Copyright 1985 by the American Geophysical Union, reproduced with the permission of the American Geophysical Union and John Wiley and Sons

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by the Moon on the Earth varies as a function of η for two values of its shear modulus, µ. The top curve, with µ = 0, corresponds to a viscous fluid. The lower curve uses the value of µ = 1.45*1012 dyne/cm2, which corresponds to the approximate current rigidity of the Earth. The current magnitude of the lunar torque is shown with an arrow on the left ordinate of the figure. VanArsdale’s point was that the current lunar torque is near the maximum as a function of η, which suggested that the orbital parameters of the Earth-Moon system were currently changing at a comparatively large rate, and that the rate would have been smaller in the past if η had been significantly different, either larger or smaller, from its current effective value for the Earth.

8.3.3

Yoder

The next talk given at the conference was coauthored by a group from the Jet Propulsion Laboratory, with Charles Yoder as the lead author. They presented more results from the Lunar Laser Ranging experiment described above, using data obtained from August 1969 to May 1982. They agreed with VanArsdale that the lunar orbital parameters must be changing at a faster rate now than in the distant past, but they ascribed the change to the changing configuration of the oceans, i.e., they assumed that the required dissipation was occurring via ocean tidal friction. They found that the mean distance from the Earth to the Moon was increasing at a rate of 3.7 ± 0.2 cm/yr, and they argued that “If the moon has orbited the earth since its formation, this must be an anomalously high value presumably due to changes in dissipation in the oceans due to continental drift. …” (Yoder, Williams, Dickey, & Newhall, 1984: 31).

8.3.4

Hartung

The point of departure for the next speaker was the observation that the Moon’s center of mass is displaced from its center of figure about 2 km in an Earthward direction, and most maria are on the side of the Moon that faces the Earth. The speaker, Jack B. Hartung (1937–2015), had been working on the implications of this observation for at least eight years, since he had given a talk on the subject at the Seventh Lunar Science Conference in 1976. His home institution was then the State University of New York at Stony Brook. In his talk at the conference, Hartung argued, as follows: The Moon’s center of mass is displaced from its center of figure about 2 km in a roughly earthward direction … . Most maria are on the side of the Moon which faces the Earth … . It has been argued that because the center of mass is so displaced and the crust of the Moon has a lower density than underlying material, the lunar crust must be thinner on the Moon’s

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earth-facing side … . The observed distribution of lunar maria is said to support this conclusion because it may be expected that mare basalt magma would reach the surface through a thinner crust more easily. A problem for this model is to explain how the lunar crust thickness asymmetry developed in the first place. We suggest an alternative view and assume the Moon was initially spherically symmetric, or nearly so. The emplacement of mare basalts represents a transfer of mass which produces most of the observed center of mass displacement toward the Earth. As a worst case, if all of the center-of-mass displacement is due to basalt emplacement, then a mass equal to all of the mare basalts on the Moon … would have had to have been transferred a distance equal to the diameter of the Moon. Such a requirement might be satisfied, given the uncertainties involved, if mare basalt magmas were in “communication” over distances of thousands of km. The remaining question in this case is what causes the asymmetric distribution of lunar maria. Because the Moon is in a spin-orbit-coupled relationship with the Earth, the effect of the Earth’s gravity on the Moon is asymmetric … .(Hartung, 1984: 32)

Using a figure (Fig. 8.6) in his 1976 talk, Hartung explained: … An asymmetry results because on the front side the acceleration due to the earth’s gravity exceeds that due to rotation by an amount which is greater than the amount acceleration due to rotation exceeds that due to the earth’s gravity on the back side. … (Hartung, 1976: caption for Fig. 2, p. 3101)

This asymmetry causes a lunar equipotential surface to be offset in the Earthward direction, as illustrated in Fig. 8.7. Hartung noted:

Fig. 8.6 A schematic drawing of the Moon showing directions and relative magnitudes of forces and accelerations acting on material at the surfaces of the front and back sides of the Moon. From Fig. 2 in Hartung, J. B. (1976) “The asymmetric distribution of lunar maria and the Earth’s gravity.” In R. B. Merrill (Ed.), Proceedings of the Seventh Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 7, volume 3, page 3101, Elmsford, NY: Pergamon Press, Copyright Elsevier (1976), used with permission

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Fig. 8.7 A schematic drawing, not to scale, showing an equipotential surface that coincides with the lunar surface only at a point on the back side of the Moon. The equipotential surface is above the lunar surface on the front side of the Moon, so that basalt magma preferentially forms there. From Fig. 5 in Hartung, J. B. (1976). “The asymmetric distribution of lunar maria and the Earth’s gravity.” In R. B. Merrill (Ed.), Proceedings of the Seventh Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 7, volume 3, page 3106, Elmsford, NY: Pergamon Press. Copyright Elsevier (1976), used with permission.)

Only on the front side of the moon does the equipotential surface rise above the actual surface of the moon. Less work is required to move magma formed at a given depth to the surface on the front side than on the back side. … (ibid.: 3106)

At the Conference on the Origin of the Moon, Hartung made the point that, … The earth-facing side of the Moon is a gravitationally favored location for the extrusion of mare basalt magma in the same way that the topographically lower floor of a large impact basin is a gravitationally favored location. This asymmetric effect increases inversely with the fourth power of the Earth-moon distance. If the Moon were one-tenth its present distance from the Earth, the front-back asymmetric effect of the Earth’s gravity would be equivalent to an elevation difference on the Moon of about 3 km … . (Hartung, 1984: 32)

In his 1976 talk, Hartung had noted: … Arguments against the hypothesis are that the moon’s orbit evolves extremely rapidly when the earth and moon are close together; therefore, the moon was not likely to have been near the earth when extrusion of mare basalt magmas began, about 4 b.y. ago, and could not have remained sufficiently near the earth throughout the period of mare basalt magmatism, about 0.5 b.y. … (Hartung, 1976: 3097)

In his talk at the Conference on the Origin of the Moon, Hartung offered a solution to this objection: A scenario for history of the Earth-Moon system consistent with the above discussion includes: formation of the Moon by accretion processes in a heliocentric orbit near that of

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the Earth; a gravitational encounter with the Earth about 4 billion years ago resulting in capture of the Moon into a geocentric orbit and heating of the Moon through dissipation of energy related to tides raised during close approaches to the Earth … to produce basalt magma; and evolution of the Moon’s orbit to its present position, slowly at first to accommodate more than 500 million years during which magmas were extruded. (Hartung, 1984: 32)

Jack Hartung was thus led along a somewhat circuitous path to a capture hypothesis for the formation of the Moon.

8.3.5

Singer

A more direct path to the capture hypothesis was taken by S. Fred Singer, who did not attend the conference but was invited to include a paper in the follow-on book. Singer, then at George Mason University, had been developing a theory for lunar capture for at least 16 years. His paper was titled “Origin of the Moon by Capture.” In the introduction, Singer wrote that he would: (1) review ‘frequency-dependent (push-pull) tidal theory’ and explain how it affects the evolution of the Moon’s early orbit, so as to make capture acceptable; (2) turn to the probability of capture and discuss several plausible mechanisms to accomplish capture involving three-body theory; (3) explain why the Moon’s chemical differences from the Earth (including low iron content, depletion of volatiles, similar oxygen isotope ratios) are entirely consistent with a lunar formation away from the Earth (but in an Earth-like orbit) followed by later capture; (4) point to inevitable physical consequences to the Earth of a capture process and show that it will lead to rapid heating, melting, and core formation; the despinning of the Moon at the beginning of capture will create a magma ocean. (5) I speculate that the early formation of Earth’s water oceans, atmosphere, and even life may be due to the capture of the Moon. (Singer, 1986: 471–472)

Singer knew that the theory developed by Gerstenkorn for the capture of the Moon required the proto-Moon to approach the Earth in such a way that its initial trapped orbit would be retrograde. The initial spin angular momentum of the Earth would have to be very high so that the total angular momentum of the Earth and the proto-Moon would be equal to the total angular momentum of the Earth-Moon system today. Much of the initial kinetic energy of rotation of the Earth would have to be dissipated as heat, and that was a problem for Gerstenkorn’s theory, as had been pointed out by MacDonald in 1964. Singer argued that in his critique of Gerstenkorn’s paper, MacDonald had not done the calculations for orbital evolution accurately enough. Specifically, he had allowed the phase angle “d” between the Earth-Moon line and the Earth’s tidal bulge line (Fig. 8.8) to be constant for a given revolution of the Moon about the Earth. In Singer’s extended tidal theory, the tidal dissipation was “frequency

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Fig. 8.8 Schematic diagram showing the Earth’s tidal bulge displace by the angle d. This displacement leads to a torque that decelerates the Earth’s rotation and accelerates the Moon’s orbital motion. From Fig. 1 in Singer, S. F. (1968) “The origin of the Moon and geophysical consequences.” Geophysical Journal of the Royal Astronomical Society, volume 15, issues 1–2, page 206, used with permission of S. F. Singer and Oxford University Press

dependent,” where he defined the frequency “f” as the relative angular rate of the Moon and the Earth. Singer briefly explained the crux of his theory as follows: … The phase angle is zero, and therefore the perturbing torque is zero, when the planet is perfectly elastic and exhibits no dissipation and therefore no time delay in raising a tidal bulge. However, an imperfectly elastic planet may also experience no dissipation provided the frequency of the applied forcing function is zero. This occurs when the satellite is in a synchronous orbit in which it revolves with the same angular velocity as the planet so that df/dt = n = X. [In his paper, Singer used X as the symbol for the spin angular velocity of the Earth and “n” as the mean orbital angular velocity of the Moon.] If the satellite’s orbit is elliptic and has a perigee that is within the synchronous orbit, then we obtain the situation shown in (Fig. 8.9). Near apogee the satellite will be moving slower than the planet; therefore, the tidal bulge will lead and produce a perturbation force, S, as shown. Near

Fig. 8.9 Schematic drawing illustrating the tangential perturbing forces, S, acting on an elliptical orbit whose perigee is within the Earth’s synchronous orbit. From Fig. 1 in Singer, S. F. (1986) “Origin of the Moon by capture.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 473, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

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perigee, however, the satellite will be moving more rapidly than the planet, the tidal bulge will lag, and the perturbation force will be opposite and, of course, much larger because of the strong dependence of tidal perturbation on distance. At perigee, the tidal bulge “pulls back” on the satellite and therefore brings the apogee in; at apogee, the tidal bulge “pulls forward” on the satellite and therefore raises the perigee slightly. The net effect of this push-pull mechanism will be to decrease the semi-major axis and eccentricity. … The major result of the push-pull tidal theory is the fact that under certain conditions an orbit can be “reflected,” i.e., approach to a minimum distance and then expand again, even if the inclination of the orbit is zero. This result follows since d may reverse its sign over a sufficient portion of the orbit to yield an effect due to force S which reverses sign. … (ibid.: 473–474)

Singer’s model allowed for capture of the Moon into a highly elliptical, low-inclination, prograde orbit around the earth. He pointed out that in the frequency-independent calculations of MacDonald, high orbital inclinations of the proto-Moon had to be invoked and a much larger amount of kinetic energy had to be dissipated than in his frequency-dependent theory. “… The use of push-pull theory thus removes a major objection against the capture origin of the Moon.” (ibid.: 474). Singer relied on calculations that assumed the Moon is a small body moving under the influence of both the Sun and the Earth to provide trajectories that would result in the “temporary capture” of the proto-Moon in the vicinity of the Earth. Thomas A. Heppenheimer (1947–2015) and Carolyn Porco had given examples of such trajectories in a 1977 paper, and an example is shown in Fig. 8.10

Fig. 8.10 Example of a trajectory during a temporary capture of a proto-Moon by the Earth. From Fig. 3 in Heppenheimer, T. A., & Porco, C. (1977) “New contributions to the problem of capture.” Icarus, volume 30, issue 2, page 387, Copyright 1977 by Academic Press, Inc., with permission of Elsevier

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(Heppenheimer & Porco, 1977: 387). In this example, the proto-Moon enters the “bottleneck” at left and makes five prograde orbits around the Earth before exiting. Heppenheimer and Porco gave examples that featured hundreds of orbits in the temporary capture region. The idea is that if energy can be dissipated while the proto-Moon is in the temporary capture region it might be permanently captured in Earth orbit. Singer gave four plausible mechanisms for energy dissipation, including: 1. Energy dissipated by tidal interaction (or aerodynamic drag) as the moon passes close to the Earth’s surface one or more times. The tidal energy losses of each encounter are cumulative and can be added, a fact that greatly increases the probability of capture … 2. On a close pass the tidally distended moon is disrupted, with part escaping but the remainder captured … (Singer, 1986: 477–478)

The latter idea had been proposed earlier by Öpik (1972: 195). Singer knew that his capture theory faced a major challenge related to the chemical composition of the Moon: The gross features of the Moon are its low density, indicating a low iron content, and its low content of volatiles relative to the Earth. These features have been used to argue against capture, but such reasoning may be erroneous. We grant that the Moon must have had an Earth-like orbit in order to have the small geocentric velocity that gives capture an appreciable probability. But to explain the different composition requires an extension of the conventional theory of planet formation from a primordial solar nebula. (Singer, 1986, p. 478)

Singer began with the point earlier made by Schmidt, Safronov and their colleagues and successors, namely that there are two important processes at work as the primordial solar nebula cools (1) the condensation of small particles and (2) the accumulation or agglomeration of particles to form planetesimals. “The final accumulation of the planets then proceeds by the gravitational interactions and actual collisions of … planetesimals …” (ibid.: 479). Singer emphasized, however, the importance of the time scales for condensation and agglomeration and, in particular, the importance of the differences among the elements in the solar nebula. If iron condenses before the silicates, then: … there is formed at the Earth’s orbit, 1 AU, a single iron body, which is the core of the proto-Earth. … …while the accumulation of this iron core is progressing, the gas has cooled sufficiently to allow the condensation of the silicates into grains, which then … [results in] the formation of some 10–50 lunar-sized objects containing mainly silicates. The final accumulation of the Earth takes place by gravitational interactions among 10 to 50 bodies, with the iron core being the largest, and therefore gravitationally most important. With these bodies in very similar orbits, they have a chance to approach each other frequently. In most cases, corresponding to distant “impacts,” nothing will happen, except for a slight perturbation of the orbit. Occasionally there would be a real impact in which the proto-Earth gradually acquires a silicate mantle. The explanation of the chemical difference between Earth and Moon described here makes use of the fractionation we know to have occurred in the formation of the solar system.

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Other theories of lunar origin (such as fission, precipitation, etc.) require a special fractionation of Earth material subsequent to the formation of the Earth. The depletion of volatiles on the lunar surface can be explained directly by noting that an object of lower mass will accrete less of the condensed volatiles than would the Earth. … Finally, we have the evidence of the similarity of oxygen isotope ratios for Earth and Moon. This similarity supports a nearby origin of lunar material rather than in some remote part of the solar nebula … (ibid.: 480)

Singer’s vision for the origin of the Moon featured the formation of an iron core planetesimal that accumulated other silicate planetesimals to form the Earth, and the accumulation of other silicate planetesimals in orbits similar to the Earth to form the proto-moon. The proto-moon would be in an orbit about the Sun similar to that of the Earth, and, under the gravitational influence of the Sun and the Earth, it would be temporarily captured by the Earth. While temporarily captured, enough energy would be dissipated so that the Moon would be permanently captured by the Earth. Tidal interactions would then circularize the Moon’s orbit around the Earth.

8.3.6

Conway

Time and again, lunar researchers had ruled out capture as a viable hypothesis for the formation of the Moon. Fred Singer took the contrary view, as did the next speaker at the conference, Bruce A. Conway of the University of Illinois, Urbana. Conway had recently graduated from Stanford University, where his PhD thesis topic was titled “On the History of the Lunar Orbit.” Consistent with Singer’s approach, Conway used a frequency-dependent model of tidal friction to determine the evolutionary history of the Earth-Moon system. Conway concluded: The resulting history is consistent with a capture origin for the Moon … . It appears to rule out, in agreement with a previous result …, origin of the Moon by fission. … Tidal dissipation within the Moon, during what would be the immediate-post-capture period, is shown to be capable of significantly heating the Moon. This result is consistent with large-scale melting of the lunar surface dated at about 4.45  109 yr … . (Conway, 1984: 33)

In Conway’s analysis, immediately after capture, the Moon’s orbit carried it within the Earth’s Roche limit. Conway’s pointed out that Roche’s limit of 2.89 RE actually applies to a small liquid satellite and should not be applied to solid bodies.

8.3.7

McKinnon and Mueller

The talks of Hartung and Conway at the conference, and the invited paper by Singer for the follow-on book, explored and defended the capture hypothesis for the formation of the Moon. The next three papers at the conference were focused on the

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fission hypothesis. The first of these was coauthored by William B. McKinnon and Steven W. Mueller of Washington University in St. Louis. McKinnon was a recent graduate of Caltech, and Mueller was a graduate student at Washington University at the time of the conference. Both went on to study the outer planets and satellites, among other things, and their interest in the subject of their talk, “A reappraisal of Darwin’s fission hypothesis and a possible limit to the primordial angular momentum of the Earth,” seems to have been short lived. McKinnon and Mueller re-examined George Darwin’s idea that fission of the Moon from the Earth might have occurred because of the resonance between the tides induced by the Sun and the free oscillation of a fluid Earth. They confirmed the results of Jeffreys that resonant fission seems unlikely. They noted however, that based on recent (1969) calculations of Subrahmanyan Chandrasekhar of the University of Chicago, one of the modes for the free oscillation of a fluid Earth had a period that was almost identical with that of the primordial solar tides on the Earth. They wrote: If resonant fission is not possible, what is the significance of the commensurate periods …? Numerical coincidence? We suggest not; perhaps solar resonant tides acted as a brake on the spin of the primordial partially molten earth. (McKinnon & Mueller, 1984: 34)

Some fission-based theories for the formation of the Moon from the Earth speculated that the primitive Earth had more mass and angular momentum than the current Earth-Moon system and that mass and angular momentum were lost during the fission process. McKinnon and Mueller were suggesting, however, that because of the regulating action of the solar tides on the Earth’s spin rate, “The primordial earth-moon system may have had nearly the same angular momentum as it has today.” (ibid.).

8.3.8

Durisen, Gingold, and Scott

The next paper at the conference was coauthored by Richard H. Durisen of the University of Indiana, Robert A. Gingold of the Australian National Observatory, and Eugene H. Scott of the National Space Science Data Center. They were astronomers, and Durisen, in particular, had been working on numerical methods to explore the outcomes of dynamic instabilities in rotating stars, e.g., the formation of binary stars. Durisen and his colleagues had concluded that “… for fluids with the compressibility of stars, dynamic fission instabilities lead to spiral-arm ejection of mass and angular momentum in the form of a ring or disk of debris, not as a single body.” (Durisen, Gingold, & Scott, 1984: 35). Durisen’s article with Gingold in the follow-on book showed the numerical simulation of dynamic fission with parameters for compressibility more appropriate to terrestrial material. Figure 8.11 shows particle positions projected onto the equatorial plane for an evolution that goes to 10 revolutions of the central figure. They concluded:

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Fig. 8.11 Results of numerical modeling of a counterclockwise rotating fluid. From Fig. 1 in Durisen, R. H., & Gingold, R. A. (1986) “Numerical simulations of fission.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page, 492, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

Regardless of compressibility, the dynamic bar instability leads to spiral arm ejection of substantial mass and angular momentum fractions; it does not lead to classic binary fission. A ring or disk of debris around a central, usually barlike remnant remains. This resembles the starting point of many other lunar origin theories and represents a natural way to produce a “geocentric swarm” of planetesimal-like bodies. … There are important differences between a geocentric swarm produced by fission and those produced in other lunar origin theories. The ejected matter is the initially outermost part of the unstable body and so would be predominantly mantle material if the Earth’s core had already segregated. Because the remnant Earth after fission is nonaxisymmetric, the swarm will not collapse rapidly back onto the Earth, as it does in other lunar theories. … Thus, fission produces a swarm that is much more massive than the Moon but that must be mostly lost from the system, not reaccreted by the Earth. The swarm, once in place, can be contaminated by heliocentric material, and so a Moon formed as a product of fission need not resemble Earth mantle material exactly. In addition, the collisional aggregation process would tend to deplete volatile elements. The inclination of the Moon’s orbit to the Earth’s equator plane in the past poses no particular difficulty for our modified fission theory, because the Moon can form at many Earth radii and will contain only a fraction of the original ejecta. A last few collisions or gravitational scatterings with large geocentric (or heliocentric) planetesimals could have disturbed its inclination. … (Durisen & Gingold, 1986: 493–494)

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Durisen and Gingold recognized that an unresolved problem for the fission theory was how the proto-Earth attained enough angular momentum for fission to come into play. They estimated that the proto-Earth would have to have had three to five times more angular momentum than the angular momentum of the current Earth-Moon system. They noted: … our simplistic estimate suggests that one or several grazing collisions by Mars-sized or larger bodies are necessary to trigger fission. If this reasoning holds up, the fission theory becomes embarrassingly similar to the giant planetesimal impact theory. In fact, it may ultimately prove necessary to incorporate elements of both theories in a successful and complete explanation of lunar origin … . Fission instabilities triggered by a last large impact would guarantee ejection of a geocentric swarm that would not rapidly reaccrete onto the Earth, because the remnant Earth would be nonaxisymmetric and would truncate the inner edge of the swarm through gravitational torques. Fission would also guarantee that a substantial fraction of the swarm’s mass would come from the Earth as well as the planetesimal. … (ibid.: 495)

8.3.9

Boss and Mizuno

Durisen and Gingold recognized that their conclusions were based on the existence of a fluid proto-Earth. Dynamical instabilities would likely be suppressed if the proto-Earth were mostly solid. This was in fact the topic of the next talk at the conference; the first of a pair of talks by Alan P. Boss and Hiroshi Mizuno of the Carnegie Institution of Washington. Boss was the lead author of the first paper, and he began by referring to the computer calculations of Durisen and his colleagues that examined the growth of dynamic instabilities in a non-viscous model for the rapidly rotating proto-Earth: … The dynamic instability was found to degenerate into the ejection of a ring of matter with a substantial fraction of the mass, leaving behind a central body with most of the mass, which is still an attractive means for forming the Moon. … We have used both the linearized analytical approach and the more recent numerical approach to show that dynamic fission probably does not occur in rocky protoplanets. … … Any rocky body, even with considerable partial melt or a molten core, should be stable to dynamic fission; any rotational instability that occurs can only result in equatorial mass loss. (Boss & Mizuno, 1984: 36)

8.3.10 Mizuno and Boss Hiroshi Mizuno was the lead author for the second of the papers by Boss and Mizuno, and his topic was “Tidal Disruption and the Origin of the Moon.” He began with:

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The dynamic problem of the tidal disruption of a rocky planetesimal has been solved by a direct integration of the fully three dimensional, nonlinear equations of motion. Tidal disruption has long been thought to be a crucial process in the accumulation of the terrestrial planets. Previous work has been limited to static, equilibrium models in circular orbit about a primary, and the result that stable equilibria do not exist inside a critical radius (= Roche limit) has been hypothesized to mean that any object that passes within the Roche limit is disrupted. We have disproven this hypothesis. … The calculation show that a rocky body … which passes by the Earth on a parabolic orbit with a perigee within the Roche limit (= 3 REarth) is not tidally disrupted, even for a grazing incidence (= 1.2 REarth). At most, a few percent of the mass is lost from the surface of the planetesimal. … … … any rocky body, even with considerable partial melt, or with a molten core, should be able to resist disruption. In the accumulation theory of the formation of the terrestrial planets (including the Moon), one of the important factors for determining the relative velocity in the swarm of planetesimals (and hence the growth time) is the mass spectrum. We have shown that tidal disruption can be ruled out as a mechanism for reducing planetesimal masses. Furthermore, mechanisms for forming the Moon which rely upon tidal disruption (e.g., tidal disruption by Earth in order to capture a circumplanetary ring of matter which can later accumulate into the Moon), are unlikely to be correct. (Mizuno & Boss, 1984: 37)

Mizuno’s talk was allotted seven minutes in the conference, but it had quite an important effect of the development of a consensus within the planetary science community about the formation of the Moon, because it allowed numerical simulations, such as those by George Wetherill, to ignore breakup of planetesimals during close approaches.

8.3.11 Cox One such exponent of numerical simulation was the next speaker, Larry P. Cox, who had obtained his PhD from MIT in 1978 with a thesis titled “Numerical Simulation of the Final Stages of Terrestrial Planet Formation.” Cox had subsequently worked with his thesis advisor, John S. Lewis, and with George Wetherill and others to refine his approach to numerical simulation of planetary formation by the multiple collisions of a set of planetesimals with initial orbital parameters. In particular, he and others had worked on understanding the limits of applicability of the numerical models, and this would be of growing importance as the capabilities of large computers continued to expand to meet the needs of ever more sophisticated models. In numerical simulations by Cox, Wetherill, and others, an encounter between two bodies was calculated in a reference frame in which the center of mass of the two bodies moved with a velocity it would have in a circular orbit about the Sun at the heliocentric distance of the center of mass. This two-body approximation breaks

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down when the relative velocity of the two bodies is small (Wetherill & Cox, 1984). Numerical simulations using integration without the two-body approximation, i.e., considering more accurately the gravitational influence of the Sun during the encounter between a pleanetsimal and a target body, were impractical with the available computing power in the 1980s. The introduction of yet another body, e.g., the Earth, in an accurate simulation of the accretion of the Moon by impacting planetesimals was even more daunting. That, however, was the computational task if you desired to investigate via numerical simulations the scenario of lunar origin in which the Moon is assumed to have accreted most of its mass while in orbit about the Earth. That was Cox’s desire, and he described the initial steps of his approach as follows: A planetary impact trajectory is calculated by assuming that the planetesimal has hit the surface of the moon at an assumed location, traveling in an assumed direction, and with an assumed impact speed. Next, the equations of motion are numerically integrated backward in time in order to determine where the planetesimal has come from … . In this way those volumes in heliocentric orbital element space which contribute trajectories that directly impact the moon … can be mapped out. (Cox, 1984: 38)

From his earlier work with Wetherill, Cox knew that a body such as the Earth could attract to close approach planetesimals that were far beyond the separation distance for “normal” gravitational focusing as determined by the two-body approximation. Cox wanted to know to what extent this “anomalous gravitational focusing” by the Earth of low-velocity heliocentric planetesimals might benefit the Moon in the late stages of its accretion. Because it involved the late stages of lunar accretion, Cox’s project was applicable to all the origin theories under consideration at the conference. In his 10-minute talk, however, he was only able to give a status report on his efforts.

8.3.12 Hartmann William Hartmann of the Planetary Science Institute in Tucson gave the last talk in the session on Dynamical Constraints at the conference. His title was Stochastic 6¼ ad hoc. Hartmann’s point was: Many lunar origin theorists have felt constrained by Occam’s razor to avoid postulating a role for stochastic events, such as large impacts. This attitude is a misreading of Occam’s razor, which asks that we not posit ad hoc events. Some classes of influential events in solar system history are class-predictable but not event-predictable: we believe the class of events occurred, but we cannot predict times and magnitudes of individual events. The events are thus stochastic, but not ad hoc. (Hartmann, 1984a: 39; his underlining)

Hartmann included in his paper for the follow-on book a section on this topic. An example of the problem of class-predictable events in planetary science is the probable Cretaceous-ending asteroid impact. Since the 1960s, asteroid statistics have implied such

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events every few 107–108 years, but we could not convincingly tie specific geologic effects to specific impacts. In the absence of such evidence, impacts of this size tended to be ignored; as scientists, we should have pursued the geologic and climatic consequences of these class-predictable events instead of waiting for iridium-rich layers to take us by surprise. … Impacts and close encounters with large objects during planet formation are thus class-predictable. A stochastic, class-predictable event, such as a large impact capable of triggering ejection of Earth-mantle material into a circumterrestrial cloud, should not be rejected as ad hoc or too catastrophic, nor should it be set aside as a “theory of last resort.” Rather, a way to deal with such an event scientifically is to investigate its consequences; if they can be shown to fit the known constraints on lunar origin well, the proposed event becomes a viable concept among hypotheses of lunar origin. (Hartmann, 1986: 587–588)

8.4

My Model of Lunar Origin I

The next session of the conference was titled My Model of Lunar Origin I. As the title suggests, authors were asked to make a case for their favorite theory for the origin of the Moon.

8.4.1

Malcuit

The first two talks were given by Robert J. Malcuit and his colleagues from Denison University. For the two talks, they were given a total of six minutes to describe the group’s capture model for the formation of the Moon. Malcuit had been working on the model since at least 1975, and it was based in part on the observation that the large circular maria seem to lie along a great circle of the Moon. Figure 8.12 shows the locations of the centers of Mare Orientale (O), Oceanus Procellarum (OP), Mare Imbrium (I), Mare Serenitatis (S), Mare Crisium (C), and Mare Smythii (SI). The mare-filled crater Tsiolkovsky on the Moon’s far side also lies along the same great circle. Malcuit argued that the great-circle pattern, along with some of the features of the maria, e.g., their ellipticity/ asymmetry, could be interpreted as basin formation by the impact of basaltic spheroids, which had been pulled off the pre-capture Moon during close encounter(s) with the Earth. … such basaltic spheroids may be necked off a lava column during the tidal disruption phase of a very close gravitational encounter between Earth and Moon early in Solar System history. In this tidal disruption model, each major basaltic spheroid impact zone should be characterized by a basaltic lava “lake” surrounded by anorthositic crust “flooded” by the overflow of basalt resulting from the collapse of the basaltic spheroid onto the lunar surface. A raised rim could then result from the rebound of the area in response to excess loading during impact. (Malcuit, Winters, & Mickelson 1984b: 44)

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Fig. 8.12 A lunar great circle superimposed on a photograph of the Moon, showing the alignment of the major impact basins. From Fig. 1 in Malcuit, R. J., Byerly, G. R., Vogel, T. A., & Stoeckley, T. R. (1975). “The great-circle pattern of large circular maria: Product of an Earth-Moon encounter.” The Moon, volume 12, issue 1, page 56, Copyright 1975 by D. Reidel Publishing Company, reproduced with kind permission from Springer Science and Business Media

Malcuit and his colleagues divided their gravitational-capture scenario into five eras: ORBITAL ERA I. Jupiter-Perturbed Heliocentric Orbital Era (*4.6 to *4.3 b.y.). The model begins with formation of the planets from nebular condensates about 4.6 b.y. ago. The Earth accumulates in the vicinity of its present orbit but Luna (= pre-capture Moon) forms as a small planetary unit on the inner edge of the Asteroid Zone at about 1.7–2.0 A.U. Gravitational perturbations by Jupiter cause an increase in eccentricity of Luna’s orbit so that it becomes Mars-crossing and eventually Earth-crossing. During this orbital era, the lunar magma ocean would lose heat and eventually become mainly crystalline by 4.3 b.y. ORBITAL ERA II. Earth-Crossing Heliocentric Orbital Era (*4.3 to *3.9 b.y.). This orbital era commences when Luna’s orbit becomes Earth-crossing. At this point, occasional energy-dissipating gravitational encounters of the non-capture type can occur between Luna and Earth. A large quantity of orbital energy (*4  1038 ergs) must be dissipated within the bodies of Luna and Earth during this orbital era. Although much of this energy can be absorbed by the Earth’s upper mantle over this time interval, periodic tidal activity would be effective in reheating Luna’s magma ocean zone. ORBITAL ERA III. Luna’s Capture (*3.9 b.y.). As Luna undergoes periodic gravitational encounters with Earth while in a near Earth-like orbit, the lunar body grows warmer, more deformable, and thus more capturable. However, gravitational capture of a lunar-sized body during a close encounter would entail dissipation of about 2  1035 ergs if Luna had a v∞ of about 0.2 km/sec. … Since energy dissipation is related to r-6 (r = distance of separation between the two bodies), only encounters within 1.4 Re could be expected to result in

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Luna’s capture. … such a close encounter of a sufficiently “warm” Luna could result in tidal dissipation during the time of the encounter. ORBITAL ERA IV. Early Post-Capture Geocentric Orbital Era (*3.9 to *3.6 b.y.). After the lunar body is inserted into a geocentric orbit within the stability limit of the Earth-Moon system, a geocentric orbital evolution suggests that circularization of the orbit occurs rapidly, i.e., within a few 105 years after capture. About 5  1035 ergs more energy must be absorbed by the Earth and Moon during this orbital era. ORBITAL ERA V. Subsequent Geocentric Orbital Evolution (*3.6 b.y. to Present). Following orbital circularization at about 35 to 40 Re (prograde), the lunar orbit gradually expands to its present dimensions by way of “normal” tidal friction processes. (Malcuit, Winters, & Mickelson, 1984a: 43)

The conference attendees had now heard from a number of researchers who advocated a capture process to explain the formation of the Moon. At least for Fred Singer, Jack Hartung, Bruce Conway, and Robert Malcuit, the capture hypothesis for the formation of the Moon was still viable.

8.4.2

Cassidy

The next presentation in the My Model of Lunar Origin I session of the conference was given by William A. Cassidy of the University of Pittsburgh. Cassidy’s model was in the class of double planet hypotheses for the formation of the Moon, but it had some similarities with Singer’s capture model. Cassidy summarized his research as follows: Given certain conditions, there should be no problem in forming the moon as a sister planet of the earth, from the same reservoir of material, within the same time frame. The model described here requires fractional vapor/liquid condensation, planet accumulation during condensation, a late start for accumulation of the moon, and volatile accretion to the surfaces of each planet only near the end of the accumulation process. … In the model, initial accumulation of small objects is helped if the agglomerating particles are somewhat sticky, as liquids might be, but still there would be a critical size range within which rotating bodies are unstable. Assuming that growth proceeds through this range, agglomeration continues. If the reservoir of vapor is being preferentially depleted in iron by fractional condensation, an iron-rich planetary core forms. As the temperature decreases, condensing material becomes progressively richer in silicates and poorer in iron, forming the silicate-rich mantle of an already differentiated earth … . Continuing to develop the model, a second center of agglomeration successfully forms near the growing earth after most of the iron in the reservoir around 1 A. U. has been used up. The bulk composition of the moon then is similar to the outer mantle of the accumulating earth, since from some instant in time they drew on the same iron-depleted reservoir. As each body increased in cross-sectional area and mass it became a more efficient collector of additional material, but the earth had started sooner and became more efficient more rapidly. Volatiles accumulated late and were incorporated into surface materials of both planets, but disproportionately more were gathered by the earth because by that time it was competing much more successfully for the remaining available material. (Cassidy, 1984: 45)

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Ringwood

Cassidy’s ideas about different rates of condensation for different materials were similar to the ideas of Ted Ringwood, who was the next speaker to describe his favorite model for the origin of the Moon. Ringwood was invited to expand his ten-minute talk into a paper that was included in the follow-on volume to the conference. As we have seen, and will continue to see below, various researchers were conducting numerical simulations of the impact of a Mars-size object with the Earth. Some of the numerical simulations were suggesting that the ejecta from a giant impact came mostly from the outer layer of the impactor, while other simulations were suggesting that both the Earth’s mantle and the outer layers of the impactor would provide material to form the Moon following the impact. Ringwood had adjusted his sediment-ring theory to accommodate the giant impact model, and he took the view that most of the material that formed the Moon following a giant impact must have come from the earth’s mantle. The arguments that he presented in his paper at the conference and in the follow-on book were not based on numerical modeling, but rather on lunar element geochemistry, particularly siderophile geochemistry. Evidently, Ringwood had not been persuaded by Ross Taylor, John Wood, and others that close comparison of the siderophile compositions of the Moon and the Earth was a hopeless task. As was the case with some of the other protagonists in the lunar origin debate, Ringwood’s paper was in large part a recapitulation of his research over the prior twenty years. He first reviewed the current understanding of lunar abundances for major elements, starting with oxygen. About half of the Moon and the Earth’s mantle (by weight) is composed of oxygen. Clayton, Onuma, & Mayeda, (1976) showed that the oxygen isotopic composition of lunar and terrestrial basalts were identical. In contrast, oxygen in all classes of meteorites except enstatite chondrites and achondrites differ significantly from oxygen in the Earth and Moon. It is particularly notable that oxygen from shergottites, which are widely believed to be derived from Mars, is displaced from the terrestrial fractionation line. These differences in 16 O, 17O, and 18O abundances are believed to be caused by both an inhomogeneous distribution of 16O within the solar nebula and by chemical fractionations, probably as a result of different temperatures of equilibration with nebula gas … . The identity of oxygen isotopic composition in Earth and Moon is strongly suggestive of a genetic relationship between the two bodies … . (Ringwood, 1986: 676)

Ringwood then discussed the abundances of uranium and thorium in the Moon’s interior. These radioactive elements are responsible for most of the heat that is now being radiated from the lunar surface. When the lunar heat flow measurements were corrected, Ringwood was among the first to demonstrate that “… the revised [heat flow] values were consistent with terrestrial abundances of uranium and thorium in the lunar interior.” (ibid.: 677). So, Ringwood noted, the isotopic composition of oxygen is identical with that of the Earth’s mantle, and lunar abundances of uranium and thorium seem to be the same as terrestrial abundances. Furthermore, Ringwood’s earlier research:

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… concluded on the basis of experimental investigations into the petrogenesis of mare basalts and the lunar crust that, with the exception of FeO and volatile elements, the bulk chemical composition of the Moon was similar to that of the Earth’s mantle. This conclusion has been supported by other workers, using entirely different data and arguments … (ibid.)

Ringwood then began to discuss the exception, i.e., the abundances of FeO and the volatile elements. There is, nevertheless, one important difference in chemical composition between the Moon and the Earth’s mantle. Seismic P- and S-wave velocities together with petrologic data strongly suggest that the lunar mantle contains about 12–16% of FeO … . This is substantially higher than the terrestrial mantle, which contains around 8% FeO … and has been cited by Drake (1983) as a difficulty for hypotheses that seek to form the Moon from material derived from the Earth’s mantle. Recent investigations of the composition of the Earth’s core have shown that it probably contains about 40 wt% of dissolved FeO (Ringwood, 1977, 1979, 1984; McCammon, Ringwood, & Jackson, 1983). These authors concluded that the FeO now in the core was originally accreted as a component of the silicate phase, which was correspondingly richer in FeO than the present mantle. During core formation, FeO in the mantle dissolved in the core near its upper boundary, accompanied by strong convection and effective mixing throughout the mantle. The FeO/(FeO + MgO) molar ratio of mantle silicates fell from an initial value of about 0.3 to about 0.12 (the present value) during the core formation process, which may have exceeded 108 years. Jagoutz and Wänke (1982) and McCammon et al. (1983) pointed out that if the Moon had formed from material removed from the Earth’s mantle before the core formation process was complete, it would contain more FeO than is now present in the mantle. Thus, the evidence that the FeO content of the Moon is higher than the present FeO content of the Earth’s mantle is not inconsistent with fission-type hypotheses for the origin of the Moon … (ibid.)

Ringwood was arguing that when material separated from the Earth’s mantle to form the Moon, the abundance of FeO in the Earth’s mantle was higher than it is now. After the separation of the lunar material, the abundance of FeO in the Earth’s mantle was subsequently reduced as mantle convection continuously brought FeO to the surface of Earth’s core, where it was dissolved into the core. Ringwood was aware that “timing is everything.” One needs for the Earth’s core to have mostly formed before the lunar material is separated from the Earth, so that the Moon will have less iron than the Earth, but the separation needs to occur before the “leaching” of FeO from the Earth’s mantle is complete, so that one can explain the larger abundance of FeO in the Moon compared to the Earth’s mantle. Along these lines, Ringwood wrote: Most probably, the major element composition of the Moon was very similar to that of the Earth’s mantle during the later stages of core formation but before the transport of FeO from mantle to core had been completed. … (ibid.: 680)

For volatile elements, Ringwood noted, “It is firmly established that the Moon is strongly depleted in volatile elements as compared with the abundances of these elements both in chondrites and in the Earth’s mantle.” (ibid.: 677). As we have seen above, Ringwood had been developing his version of formation of the Moon since the mid-1960s. He referred to it as the “precipitation hypothesis,”

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as its key feature was the precipitation of silicates in the early massive and hot atmosphere of the Earth to form a “sediment ring” of planetesimals in Earth orbit. The planetesimals subsequently coagulated to form the Moon. Ringwood invoked the presumed T-Tauri phase of the early Sun, with its massive solar wind, as the mechanism to sweep away volatile elements from the material that would form the Moon. His model, therefore, could account for the absence of volatile elements in the lunar samples, at least in a general way. No theory, including Ringwood’s, seemed to be able to account for the details of the depletion pattern of the Moon relative to the chondrites and/or Earth’s mantle, however. As Ringwood noted: Protagonists of the principal hypotheses of lunar origin have weighed the previous evidence on lunar bulk composition and volatile abundances differently and with a conspicuous lack of consensus. One is therefore led to enquire whether there is any other source of compositional evidence that can be satisfied unambiguously by only one of the existing hypotheses of origin. The author believes that the answer is provided in the siderophile geochemistry of Earth and Moon. (ibid.: 681)

As we have discussed above, there had certainly not been a unanimous opinion within the lunar research community as to the implications of the data on siderophile geochemistry. On the one hand, Ross Taylor and others thought that there were too many unknowns associated with the terrestrial siderophile data to make sense of a comparison with the lunar siderophile data. On the other hand, Mike Drake of the University of Arizona published a paper in 1983 in which he used siderophile lunar/Earth comparisons to argue that the dissimilarity of the abundances of certain elements, such as rhenium (chemical symbol Re), in terrestrial and lunar basalts posed a major problem for Ringwood’s theory (though not for Drake’s double-planet hypothesis) (Drake, 1983: 1759). Drake had pointed out that “Re abundance inferred for the primitive mantle of the Earth is higher than that inferred for the Moon by a factor of approximately 250. …” (ibid.: 1762). Ringwood’s estimate for the ratio of abundances of rhenium in the terrestrial and lunar mantles was 80, rather than 250 (Ringwood, 1986: 688), but he knew that even that overabundance of rhenium in the Earth’s mantle relative to the lunar samples posed a problem for fission theories, such as his precipitation hypothesis, because rhenium is by no means a volatile element. The abundances in the lunar samples and the Earth’s mantle for other siderophile elements, e.g., cobalt, tungsten, phosphorus, iridium, osmium, sulfur, and selenium were approximately equal (within a factor of two). Both the Earth’s mantle and the lunar samples were depleted in these elements relative to carbonaceous chondrites. The explanation for the depletion of these elements in the Earth’s mantle was that, as siderophiles, they tended to follow iron into the Earth’s core. But if the Moon began as a body similar to the Earth, it was difficult to use the same explanation for the depletion of these elements in the lunar samples, since the Moon’s core was thought to be so small. Ringwood had argued that this circumstance was a strong argument in favor of the idea that the Moon was derived from the Earth’s mantle.

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If that theory worked for the above-mentioned siderophiles, why didn’t it work for rhenium (or for other siderophiles that posed a similar, if lesser, problem, e.g., gold, molybdenum, and nickel)? Ringwood’s answer was that some siderophiles are more siderophilic than others, and that there was a correlation between degrees of depletion in the lunar samples and the siderophilic character of the problem elements, i.e., the more siderophilic, the greater the depletion. Molybdenum, rhenium, and gold, in particular, are so strongly siderophilic that they could be leached from the lunar mantle into a lunar core, even if the lunar core was as small as 0.4% of the lunar mass. The nickel budget of the Moon [nickel is depleted in the lunar mantle by a factor of three relative to the Earth’s mantle, but the overall abundance of nickel, including the amount in the Moon’s core, might be the same as in the Earth] can be readily satisfied if a small, nickel-rich (*40% Ni) core amounting to 0.4% of the lunar mass is present. Newsom (1984b) and others pointed out that the observed depletions of Mo, Re, and Au in the Moon could be explained by segregation of a core of this size. Thus the correlation between degree of depletion and siderophile nature is simply explained. This model appears capable of providing a quantitative explanation of the abundances of the less volatile siderophile elements within the Moon. (ibid.)

Ringwood was therefore arguing that for moderately siderophilic elements, the small lunar core would not affect their abundances in the lunar mantle, and these abundances would be similar to the abundances of the same elements in the Earth’s mantle, assuming that some variation of a fission theory is correct. Only the lunar abundances of highly siderophilic elements would be different (and smaller) than their corresponding abundances in the Earth’s mantle. Ringwood had been focusing his discussion of siderophile comparisons using abundances from lunar basalts that were classed as “low-titanium” basalts, i.e., basalts that contained relatively low concentrations of titanium. These basalts were thought to be younger and derived from greater depths in the Moon than those with higher concentrations of titanium and were therefore more likely to represent indigenous lunar material, i.e., not “contaminated” by impacting bolides. It was thought by Anders and others that the lunar highlands were hopelessly contaminated by impacting material, but Ringwood and his colleague, John W. Delano, had: … estimated the indigenous abundances of siderophiles in Apollo 16 highland breccias using a very simple procedure … . They assumed that the projectiles striking the Moon possessed compositions similar to ordinary chondrites. Meteoritic contamination was subtracted from each sample, making the conservative assumption that all of the iridium present had been derived from the meteorite. Residuals so obtained for each element were averaged and corrected to a common Al2O3 content (to allow for plagioclase dilution). It was found that the residual abundances of W, Ni, Co, P, Cu, Ga, S, and Se were similar (within a factor of about two) to the abundances of these elements in low-Ti mare basalts, which are undoubtedly indigenous. It is striking that such a simple model has proven capable of extracting an ordered and meaningful result from the apparently chaotic data base represented by available chemical analyses of Apollo 16 highland breccias. The agreement between the highland siderophile residuals and the mare basalt abundances strongly suggests that the former represent genuinely indigenous siderophiles. … (ibid.: 687)

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Ringwood’s idea was based on the facts that: (1) The concentration of iridium in ordinary (undifferentiated) chondritic meteorites is much higher than it is in the mantles of the Earth or the Moon, as iridium is highly siderophilic, and presumably mostly resides in the iron cores of these bodies. He subtracted out the abundances of all the meteoritic elements from the lunar highlands samples in proportion to the amount of iridium in the sample, based on the assumption that the meteoritic contamination of such a sample would have chondritic abundances. (2) Ringwood and another Australian colleague, Susan E. Kesson, had estimated that Al2O3 would have made up about 18% of the overall “parental magma” of the lunar highlands. Plagioclase-rich lunar material in the magma ocean that had “floated to the top” of the magma ocean would contain higher concentrations of Al2O3, but Ringwood wanted to assess the siderophile abundances of the total “parental magma” of the highlands, not just the material that formed the top layer. He therefore used the concentration of Al2O3 in a particular sample and the 18% figure to correct for “plagioclase dilution.”

With this “data processing,” Ringwood was able to show that the siderophile abundances of the lunar highlands, as well as the mare basalts, matched the corresponding abundances of the Earth’s mantle reasonably well. Ringwood’s favored model for the formation of the Moon, his “precipitation” or “sediment ring” model, was consistent with this finding, but he knew that the relatively newer “giant impact” model could be consistent, as well. He noted: The attractive feature of these impact models as a means of placing material from the Earth into geocentric orbit is that they rely on a process that must inevitably occur during the accretion of planets and can supply the enormous amounts of energy required. Nevertheless, although the new models are very promising, they will require further development in order to explain the Moon’s chemical composition as distinct from the dynamics of its formation.

In the author’s opinion, a successful model for the terrestrial origin of the Moon should preferably satisfy the following conditions: 1. The proportion of vapourized material that was permanently removed from the Earth-Moon system during formation of the Moon should represent a limited fraction (e.g., 5% by weight) lunar core, while the terrestrial core contains 31% of the Earth’s mass, has been one of the major facts behind the fission model. This difference in the core sizes would be a direct result if fission occurred near or at the end of the formation of the terrestrial core. … (Binder, 1986: 509)

and The observed depletion of volatile elements was a predictable consequence of the fission of the Moon from a hot proto-Earth … (ibid.: 510)

and The second major compositional consequence of fission of the Moon from the Earth would be that the bulk composition (excluding volatiles, Fe, and siderophiles) would be very similar to that of the parental terrestrial mantle. … … it has become relatively clear that the bulk major oxide composition of the Moon is very close to that of the terrestrial mantle, a finding that is certainly compatible with the fission origin of the Moon. Similarly, the bulk nonvolatile, nonsiderophile trace element content of the Moon is essentially equal to that of the Earth’s mantle. … Finally, as has been known since the Apollo mission, the stable isotopic ratios for O, C, Si, and S are equal to or within the terrestrial range of values … . This clearly indicates that there is a close genetic relationship between the Earth and the Moon. In summary, the bulk composition of the Moon is very nearly equal to that of devolatilized, terrestrial mantle material. … (ibid.: 509–513)

For Alan Binder, the fission model for the formation of the Moon was alive and well.

8.4.5

Wänke and Dreibus

The fission model had been alive and well for Heinrich Wänke of the Max-Planck-Institut für Chemie in 1977, when he and a colleague made a presentation at the Eighth Lunar Science Conference in March of that year, giving geochemical evidence that supported the fission hypothesis. At the Conference on the Origin of the Moon seven years later, Wänke and Gerlind Dreibus, another colleague at the Max Planck-Institut für Chemie, gave the next paper, which was titled “Geochemical Evidence for the Formation of the Moon by Impact-Induced Fission of the Proto-Earth.” For Wänke, then, the giant-impact hypothesis was a variant of the fission hypothesis. Wänke and Dreibus were asked to elaborate on their conference paper for the follow-on book. In their paper for the book, Wänke and Dreibus “… put together and added new geochemical arguments for the formation of the Moon from material of the Earth’s mantle and, in particular, evidences for an impact-induced fission.” (Wänke & Dreibus, 1986: 650).

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Their geochemical arguments were in some cases brief recapitulations, e.g.: Oxygen Isotopes—From the work of Clayton and Mayeda (1975), it is well known that lunar samples lie, within the analytical uncertainties, on the mass-fractionation line defined by terrestrial material. This fact becomes even more significant since it is known that eucrites as well as the SNC meteorites (shergottites, nakhlites, and chassignites) fall distinctly off the terrestrial oxygen isotope fractionation line. … (Wänke & Dreibus, 1986: 650)

As noted above, the parent body for the eucrite meteorites was thought to be Vesta, now defined as a minor planet in the asteroid belt. The parent body for SNC meteorites is thought to be Mars. The point being that the oxygen isotope ratios for the Earth’s mantle and the Moon are virtually identical but very different for material in other parts of the solar system. Another, corrective, recapitulation was: Refractory Elements—Contrary to ideas generated immediately after the first lunar samples were received, it is now generally accepted that the bulk of the Moon is not significantly enriched in refractory elements compared to the Earth’s mantle … . (ibid.: 650–651)

In agreement with Ringwood, Wänke and Dreibus suggested that the lower FeO content of the Earth’s mantle relative to the Moon “… may be due to a gradual transfer of some FeO into the [Earth’s] core during the first few hundred million years of the Earth’s history …” (ibid.: 651). For other elements, Wänke and Dreibus used data from prior work in their group at the Max-Planck-Institute für Chemie in Mainz, as well as data from other researchers, to show the similarity of lunar and upper Earth-mantle geochemistry. For example (Fig. 8.13), they showed the large depletion of manganese oxide (MnO) in lunar and terrestrial basalts relative to the parent bodies for eucrite meteorites (EPB) and shergottite metorites (SPB). Fig. 8.13 MnO in basalts of the Earth, Moon, and in the parent bodies for eucrite and shergottite meteorites. From Fig. 1 in Wänke, H., & Dreibus, G. (1986) “Geochemical evidence for the formation of the Moon by impact-induced fission of the proto-Earth.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 652, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

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Fig. 8.14 In addition to Mn, Cr and V are also depleted in the Earth’s mantle relative to C1 abundances. Vanadium is a refractory element and its abundance on the Earth and Moon has to be compared with the abundance of refractory elements such as Ca. From Fig. 2 in Wänke, H., & Dreibus, G. (1986) “Geochemical evidence for the formation of the Moon by impact-induced fission of the proto-Earth.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 653, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

In another figure (Fig. 8.14), they showed the relative abundances compared to chondritic meteorites (C1) of calcium (Ca), which is a refractory element; chromium (Cr), vanadium (V), and manganese (Mn), which are strongly depleted in the lunar samples and the Earth’s mantle relative to C1 meteorites; and sodium (Na) and potassium (K), which are moderately volatile elements. Both the Earth’s mantle and lunar samples are similarly enriched in calcium and strongly depleted in sodium and potassium relative to C1 abundances. Wänke and Dreibus argued: The most likely explanation for the depletion of Mn and that of Cr and V in the Earth’s mantle is their removal into the Earth’s core, … Independent of whatever the cause of the depletion of Mn and that of Cr and V in the Earth’s mantle was, this depletion is a very characteristic feature of the Earth’s mantle and is strongly coupled to the accretion mode of the Earth. It is striking that the Moon shows very similar depletions of all three elements. Because of the small mass of the Moon and core mass, which is at least ten times smaller, almost none of the depletion mechanisms discussed for the Earth’s mantle could operate on the Moon. … (ibid.: 653)

Again, in agreement with Ringwood, the similarity of terrestrial mantle and lunar sample abundances of manganese, chromium, and vanadium help make the case for a fission theory that allows for the abundances of these elements on the Moon to be

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the same as their abundances in the Earth’s mantle. A co-accretion theory would have difficulty explaining this similarity because the different proportional sizes of the metal cores of the Earth and the Moon would result in different abilities of those cores to leach the elements from the upper layers of the two bodies. In a paper written in 1982, Wänke and Dreibus wrote: We believe that the similar depletion of Mn, Cr and V on Earth and Moon to be the strongest argument for a close genetic relationship of Earth and Moon. The Moon must have formed somehow from the material of the Earth’s mantle after core formation. (Wänke & Dreibus, 1982: 340)

In their article for the book that followed the Conference on the Origin of the Moon, Wänke and Dreibus next discussed the lunar abundances of various siderophile elements, e.g., phosphorus (P), cobalt (Co), nickel (Ni), tungsten (W), and molybdenum (Mo). They concluded: … In general the siderophile elements in pristine lunar highland rocks can be clearly explained by assuming concentrations of siderophile elements in a lunar magma ocean identical to those in the Earth’s mantle. (Wänke & Dreibus, 1986: 661)

At the time of the formation of the Moon, the Earth’s mantle and its core were presumably in near equilibrium as far as the partition of the siderophile elements and the Earth’s metal core are concerned. After the Moon was formed, there was some leaching of siderophile elements into the Moon’s core, despite its small size. Using data from their Mainz lab, as well as data from other labs, Wänke and Dreibus showed that the abundance of siderophile elements in the lunar mantle relative to the Earth’s mantle is a reflection of the effective metal/silicate partition coefficients of the respective elements. They demonstrated this correspondence in Fig. 8.15, which shows that the higher the metal/silicate partition coefficient of the element, the greater was the amount of the element that was leached into the Moon’s core, and the lower was the ratio of lunar mantle to Earth’s mantle concentration. Wänke and Dreibus also discussed the presence of unfractionated primary matter in the lunar highlands. When the first papers on the observation of unfractionated primary matter in lunar highland breccias were published, it seemed difficult to reconcile this observation with the general models of the formation and evolution of the Moon. How could it be that one finds on top of a thick, highly fractionated feldspathic crust material that obviously has escaped any significant fractionation? Impact-induced fission provided an excellent scenario to account also for the primary component in highland breccias. Collision of the proto-Earth at its late stage of accretion with a large object will vaporize considerable portions of the Earth’s mantle as well as of the projectile itself. The highly turbulent cloud surrounding the remaining central body will be loosely coupled to its surface by gas friction. … On cooling, condensation will occur, allowing some fractionation of elements according to their volatility. In this way the lower concentration of all elements more volatile than Na can be explained. …

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It is conceivable that only a fraction of the total mass originally ejected finally ended up in the Moon. The major part probably fell back on the Earth, and some fraction may have left the gravity field of the Earth while it was coming into heliocentric orbits. It is also conceivable that in addition to one large moon, one or several smaller moons were formed. Objects in orbits outside the orbit of the proto-Moon would be swallowed by the proto-Moon on its tidal retreat from the Earth. Such a scenario would be in accordance with the model advocated by Runcorn (1983), in which the large lunar basins were formed by impacts of lunar or terrestrial satellites. … Formation of the lunar basins by objects made out of material identical to that of which the proto-Moon was formed would clearly add unfractionated material on top of the solidified crust. Primary matter was mainly observed in Apollo 16 breccias. This landing site is dominated by material ejected by the Imbrium event … . It seems that highland breccias from areas far away from the big maria do not contain the primary component found in Apollo 16 breccias. … (Wänke & Dreibus, 1986: 661–664)

Wänke and Dreibus concluded their paper with the following summary: The close similarity of the composition of the lunar mantle with the very characteristic composition of the Earth’s mantle makes a close genetic relationship of the Moon with the Earth’s mantle an almost unavoidable consequence. Impact-induced splash off of the Earth’s mantle material from which the Moon was formed is the model that best fits the geochemical observations. The presence of unfractionated primary matter compositionally identical to the Earth’s mantle in the lunar highland seems to indicate that besides the proto-Moon at least one smaller satellite formed that was swallowed by the proto-Moon on its tidal retreat from the Earth on a comparatively small time scale. (ibid.: 666–667)

Fig. 8.15 Relative to the present (upper) Earth’s mantle, the lunar mantle shows an overabundance of Fe, W, and Mn and a depletion of Co, P, Ni, Mo, Au, and Re. The sequence in the abundance ratios of these elements is parallel to their effective metal/silicate partition coefficients. From Fig. 10 in Wänke, H., & Dreibus, G. (1986) “Geochemical evidence for the formation of the Moon by impact-induced fission of the proto-Earth.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 666, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

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Concluding Remarks

The paper by Wänke and Dreibus concluded the presentations for the second day of the Kona Conference on the Origin of the Moon and ended the first session on My Model of Lunar Origin. Various models were still being argued, but the impact/ fission model seemed to be gaining adherents. This trend would continue as the final day of the conference began the next morning.

References Binder, A. B. (1986). The binary fission origin of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 499–516). Houston, TX: Lunar and Planetary Institute. Boss, A. P., & Mizuno, H. (1984). The dynamic fission instability and the origin of the Moon [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 36). Houston, TX: Lunar and Planetary Institute. Burton, J. D., Culkin, F., & Riley, J. P. (1959). The abundances of gallium and germanium in terrestrial materials. Geochimica et Cosmochimica Acta, 16(1–3), 151–180. Cassidy, W. A. (1984). The ‘problem’ of iron partition between Earth and Moon during simultaneous formation as a double planet system [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 45). Houston, TX: Lunar and Planetary Institute. Clayton, R. N., & Mayeda, T. K. (1975). Genetic relations between the Moon and meteorites. In R. B. Merrill (Ed.), Proceedings of the Sixth Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 6, v. 2) (pp. 1761–1769). Elmsford, NY: Pergamon Press. Clayton, R. N., Onuma, N., & Mayeda, T. K. (1976). A classification of meteorites based on oxygen isotopes. Earth and Planetary Science Letters, 30(1), 10–18. Conway, B. A. (1984). The Moon’s orbit history and inferences on its origin [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 33). Houston, TX: Lunar and Planetary Institute. Cox, L. P. (1984). A numerical investigation of planetesimal collision trajectories with a moon accumulating in Earth orbit. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 38). Houston, TX: Lunar and Planetary Institute. Delano, J. W. (1986). Abundances of cobalt, nickel, and volatiles in the silicate portion of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 231– 247). Houston, TX: Lunar and Planetary Institute. Dickinson, T., & Newsom, H. (1984). Ge abundances in the lunar mantle and implications for the origin of the Moon [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 16). Houston, TX: Lunar and Planetary Institute. Drake, M. J. (1983). Geochemical constraints on the origin of the Moon. Geochimica et Cosmochimica Acta, 47(10), 1759–1767. Durisen, R. H., Gingold, R. A., & Scott, E. H. (1984). Numerical simulations of fission [Abstract]. In papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 35). Houston, TX: Lunar and Planetary Institute. Durisen, R. H., & Gingold, R. A. (1986). Numerical simulations of fission. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 487–498). Houston, TX: Lunar and Planetary Institute. Goldreich, P. (1966). History of the lunar orbit. Reviews of Geophysics, 4(4), 411–439.

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Goodrich, C. A., & Barnes S. (1984). Is phosphorus predictably incompatible in igneous processes? [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 17). Houston, TX: Lunar and Planetary Institute. Goettel, K. A. (1984). Bulk composition of the Moon in the context of models for condensation in the solar nebula [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 20). Houston, TX: Lunar and Planetary Institute. Hartmann, W. K. (1984). Stochastic 6¼ ad hoc [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 39). Houston, TX: Lunar and Planetary Institute. Hartmann, W. K. (1986). Moon origin: The impact-trigger hypothesis. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 579–608). Houston, TX: Lunar and Planetary Institute. Hartung, J. B. (1976). The asymmetric distribution of lunar maria and the Earth’s gravity. In R. B. Merrill (Ed.), Proceedings of the Seventh Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 7, v. 3) (pp. 3097–3112). Elmsford, NY: Pergamon Press. Hartung, J. B. (1984). Two lunar global asymmetries [Abstract]. In Papers Presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 32). Houston, TX: Lunar and Planetary Institute. Heppenheimer, T. A., & Porco, C. (1977). New contributions to the problem of capture. Icarus, 30 (2), 385–401. Hohenberg, C. M., Podosek, F. A., & Reynolds, J. H. (1967). Xenon-iodine dating: Sharp isochronism in chondrites. Science, 156(3772), 233–236. Jagoutz, E., & Wänke, H. (1982). Has the Earth’s core grown over geologic times? [Abstract]. Abstracts of papers submitted to the Thirteenth Lunar and Planetary Science Conference (pp. 358–359). Lunar and Planetary Institute: Houston, TX. Kaula, W. M., & Bigeleisen, P. E. (1975). Early scattering by Jupiter and its collision effects in the terrestrial zone. Icarus, 25(1), 18–33. Koeberl, C. (1984). Volatile elements in and on lunar volcanic glasses: What do they tell us about lunar genesis? [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 23). Houston, TX: Lunar and Planetary Institute. Koeberl, C., Kiesl, W., Kluger, F., & Weinke, H. H. (1984). A comparison between terrestrial impact glasses and lunar volcanic glasses: The case of fluorine. Journal of Non-Crystalline Solids, 67(1–3), 637–648. Kreutzberger, M. E., Drake, M. J., & Jones, J. H. (1984). Origin of the Moon: Constraints from volatile elements. [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 22). Houston, TX: Lunar and Planetary Institute. Lambeck, K. (1975). Effects of tidal dissipation in the oceans on the Moon’s orbit and the Earth’s rotation. Journal of Geophysical Research, 80(20), 2917–2925. Lambeck, K. (1980). The Earth’s variable rotation: Geophysical causes and consequences. New York, NY: Cambridge University Press. Langseth, M. G., Keihm, S. J., & Peters, K. (1976). Revised lunar heat-flow values.” In R. B. Merrill (Ed.), Proceedings of the Seventh Lunar Science Conference (Geochimica et Cosmochimica Acta, supplement 7, v. 3) (pp. 3143–3171). Elmsford, NY: Pergamon Press. Lin, D. N. C. (1981). On the origin of the Pluto-Charon system. Monthly Notices of the Royal Astronomical Society, 197, 1081–1085. Malcuit, R. J., Byerly, G. R., Vogel, T. A., & Stoeckley, T. R. (1975). The great-circle pattern of large circular maria: Product of an Earth-Moon encounter. The Moon, 12(1), 55–62. Malcuit, R. J., Winters, R. R., & Mickelson, M. E. (1984a). A testable gravitational capture model for the origin of the Earth’s Moon [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 43). Houston, TX: Lunar and Planetary Institute. Malcuit, R. J., Winters, R. R., & Mickelson, M. E. (1984b). Directional properties of ‘circular’ maria: Interpretation in the context of a testable gravitational capture model for lunar origin [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 44). Houston, TX: Lunar and Planetary Institute.

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McCammon, C. A., Ringwood, A. E., & Jackson, I. (1983). A model for the formation of the Earth’s core. In W. V. Boynton & T. J. Ahrens (Eds.), Proceedings of the Thirteenth Lunar and Planetary Science Conference (Journal of Geophysical Research, v. 87, supplement) (pp. A501–A506). Washington, DC: American Geophysical Union. McKinnon, W. B., & Mueller, S. W. (1984). A reappraisal of Darwin’s fission hypothesis and a possible limit to the primordial angular momentum of the Earth [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 34). Houston, TX: Lunar and Planetary Institute. Mizuno, H., & Boss, A. P. (1984). Tidal disruption and the origin of the Moon [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 37). Houston, TX: Lunar and Planetary Institute. Munk, W. H. (1966). Variation of the Earth’s rotation in historical time. In B. G. Marsden & A. G. W. Cameron (Eds.), The Earth-Moon system (pp. 52–69). New York, NY: Plenum Press. Munk, W. (1968). Once again—tidal friction. Quarterly Journal of the Royal Astronomical Society, 9, 352–375. Newsom, H. E. (1984). The abundance of molybdenum in lunar samples, new evidence for a lunar metal core [Abstract]. Abstracts of papers submitted to the Fifteenth Lunar and Planetary Science Conference (pp. 605–606). Lunar and Planetary Institute: Houston, TX. Newsom, H. E. (1986). Constraints on the origin of the Moon from the abundance of molybdenum and other siderophile elements. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 203–229). Houston, TX: Lunar and Planetary Institute. Öpik, E. J. (1972). Comments on lunar origin. Irish Astronomical Journal, 10(5–6), 190–238. Reynolds, J. H. (1960). Determination of the age of the elements. Physical Review Letters, 4(1), 8–10. Ringwood, A. E. (1977). Composition of the core and implications for origin of the earth. Geochemical Journal, 11(3), 111–135. Ringwood, A. E. (1979). Origin of the Earth and Moon. New York, NY: Springer-Verlag. Ringwood, A. E. (1984). The Earth’s core: Its composition, formation and bearing upon the origin of the Earth. Proceedings of the Royal Society of London A, 395(1808), 1–46. Ringwood, A. E. (1986). Composition and origin of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 673–698). Houston, TX: Lunar and Planetary Institute. Ringwood, A. E., & Seifert, S. (1986). Nickel-cobalt abundance systematics and their bearing on lunar origin. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 249–278). Houston, TX: Lunar and Planetary Institute. Rubincam, D. P. (1975). Tidal friction and the early history of the Moon’s orbit. Journal of Geophysical Research, 80(11), 1537–1548. Runcorn, S. K. (1983). Lunar magnetism, polar displacements and primeval satellites in the Earth-Moon system. Nature, 304(5927), 589–596. Shervais, J. W., & Taylor, L. A. (1986). Petrologic constraints on the origin of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), origin of the Moon (pp. 173–201). Houston, TX: Lunar and Planetary Institute. Singer, S. F. (1968). The origin of the Moon and geophysical consequences. Geophysical Journal of the Royal Astronomical Society, 15(1–2), 205–226. Singer, S. F. (1986). Origin of the Moon by capture. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 471–485). Houston, TX: Lunar and Planetary Institute. Swindle, T. D., Caffee, M. W., Hohenberg, C. M., & Taylor, S. R. (1986). I-Pu-Xe dating and the relative ages of the Earth and Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 331–357). Houston, TX: Lunar and Planetary Institute. Taylor, S. R. (1982). Planetary science: A lunar perspective. Houston, TX: Lunar and Planetary Institute.

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Taylor, S. R. (1984). Tests of the lunar fission hypothesis [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 25). Houston, TX: Lunar and Planetary Institute. Taylor, S. R. (1986). The origin of the Moon: Geochemical considerations. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 125–143). Houston, TX: Lunar and Planetary Institute. VanArsdale, W. E. (1984). Constraints on the origin of viscoelastic bodies [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 30). Houston, TX: Lunar and Planetary Institute. VanArsdale, W. E. (1985). Orbital dynamics of a viscoelastic body. Journal of Geophysical Research-Solid Earth, 90(B8), 6887–6892. Vanyo, J. P. (1984). Constraints on lunar origin: Evidence preserved in Precambrian stromatolites [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 29). Houston, TX: Lunar and Planetary Institute. Vanyo, J. P., & Awramik, S. M. (1982). Length of day and obliquity of the ecliptic 850 MA ago: Preliminary results of a stromatolite growth model. Geophysical Research Letters, 9(10), 1125–1128. Wänke, H., & Dreibus, G. (1982). Chemical and isotopic evidence for the early history of the Earth-Moon system. In P. Brosche & J. Sündermann (Eds.), Tidal friction and the Earth’s rotation II (pp. 322–344). Berlin, Germany: Springer. Wänke, H., & Dreibus, G. (1986). Geochemical evidence for the formation of the Moon by impact-induced fission of the proto-Earth. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 649–672). Houston, TX: Lunar and Planetary Institute. Warren, P. H. (1986). The bulk-Moon MgO/FeO ratio: A highlands perspective. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 279–310). Houston, TX: Lunar and Planetary Institute. Warren, P. H., & Rasmussen, K. L. (1984). Megaregolith thickness, heat flow, and the bulk composition of the Moon [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 18). Houston, TX: Lunar and Planetary Institute. Wasson, J. T., & Warren, P. H. (1979). Formation of the Moon from differentiated planetesimals of chondritic composition [Abstract]. Abstracts of papers submitted to the Tenth Lunar and Planetary Science Conference (pp. 1310–1312). Lunar and Planetary Institute: Houston, TX. Wetherill, G. W., & Cox, L. P. (1984). The range of validity of the two-body approximation in models of terrestrial planet accumulation: I. Gravitational perturbations. Icarus, 60(1), 40–55. Yoder, C. F., Williams, J. G., Dickey, J. O., & Newhall, X. X. (1984). Tidal dissipation in the Earth and the Moon from lunar laser ranging [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 31). Houston, TX: Lunar and Planetary Institute.

Chapter 9

The Kona Conference—Day 3

9.1

Introduction

The final day of the Kona conference on the Origin of the Moon began with the second session titled My Model of Lunar Origin. During the afternoon, summaries were given and time allowed for open discussion. In this chapter, we continue to review the presentations that would lead to a consensus view within the lunar research community as to how the Moon was formed.

9.2 9.2.1

My Model of Lunar Origin—II Greenberg

The second session on My Model of Lunar Origin began with a paper from the Tucson Lunar Origin Consortium, which consisted of eight researchers (Clark R. Chapman, Donald Davis, Michael Drake, Richard Greenberg, William Hartmann, Floyd Herbert (1942–2010), John H. Jones, and Stuart J. Weidenschilling) who were associated either with the Lunar and Planetary Laboratory of the University of Arizona or the independent Planetary Science Institute, both situated in Tucson. At the conference, the ideas of the group were described in a series of short talks by consortium members. The first was an overview that was given by Richard Greenberg. None of the three major categories of models of lunar origin readily explains the Moon’s properties: The fission model suffers from dynamical uncertainties and from compositional inconsistencies with the mantle of the Earth; the model of growth in circum-terrestrial orbit suffers from the gross bulk compositional differences between the Earth and Moon, e.g., the latter’s lack of metallic iron; the capture hypothesis requires some unknown capture mechanism to slow a full-sized Moon into a bound orbit, and also fails to address the problem of low iron content. © Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_9

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Our consortium considers each of those models to represent an end member of a more general scenario: As the Earth grew by planetesimal bombardment, a circum-terrestrial cloud of particles was created from a combination of impact-ejected mantle material and planetesimals captured directly into orbit around the Earth. Such a swarm continued to capture planetesimals and to receive ejecta until the bombarding population thinned, the Earth stopped growing, and the Moon accreted in orbit. If Earth-mantle material dominates the swarm, the model resembles the fission hypothesis; if small planetesimals dominate, the model represents the “growth in Earth orbit” end-member; if the swarm were dominated by a single large planetesimal, we would essentially have a capture model. A model intermediate between these extremes appears most promising. In this context, we see two ways to explain compositional properties. First is the stochastic route: A few big late planetesimals of diverse composition are captured in orbit and/or hit Earth. Such rare events control the final composition of the Earth and Moon. This approach may be reasonable because some late planetesimals were probably big, and they had large orbital eccentricities and thus may sample a range of compositional zones. However, a stochastic explanation is difficult to analyze or test. Some might call it ad hoc, yet such events are quite plausible. We consider a second route which may provide a systematic explanation of composition: The circum-terrestrial swarm acts as a filter, preferentially capturing small weak silicate bodies, while passing large iron planetesimals (cores of broken parents). … There are a number of issues related to this hypothesis … : What other geochemical properties must be explained? How was the swarm produced in the first place? And finally, how was the swarm maintained in orbit? Or more specifically, can captured material contribute enough angular momentum? The answer to the latter seems to be no, a seeming stumbling block to any model involving a circum-terrestrial swarm. Unless some systematic source of angular momentum can be identified, we may be forced to rely on stochastic dynamical process for an explanation. (Greenberg et al., 1984: 51; his underlining)

9.2.2

Hartmann

The next five talks at the conference were from members of the Tucson Lunar Origin Consortium. Each talk addressed one or more of the issues raised in the overview talk by Greenberg. The first of these was given by William Hartmann (Fig. 9.1), who spoke on the role of giant impacts in lunar formation. Hartmann’s scenario for the accretion of the Earth within a time duration consistent with theoretical models (between 30,000 years and 150 million years) required an impact flux between 107 and 1012 times the present flux. The high primordial flux implies 1 impact/week of bodies ranging about 2–50 km across and about 1013 to 1017 kg. A small fraction of the resulting ejecta reaches near-Earth space, with sub-orbital stay-times of the order of a week. … Therefore, the “weekly impactors” maintain a time-varying circum-Earth swarm or disk; … . The swarm may have interacted with incoming material, or accreted onto a small proto-moonlet already (captured?) in orbit. Less frequent 100+ km impactors ejected transient surges of mass into the circum-Earth swarm. The largest impactors probably approached or exceeded lunar size. One or more giant impacts may have added enough heated, volatile-depleted upper-mantle material

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Fig. 9.1 Photograph of William K. Hartmann, reproduced with his permission

(from a magma ocean?) to contribute much of the moon’s mass. Lunar formation in such a swarm is supported by Thompson and Stevenson, possibly within 100 yr (Thompson & Stevenson, 1983: 787). If Earth spun rapidly, as in fission models, the largest impact may have introduced enough angular momentum and energy to trigger the ejection/fission event. Impact-induced fission thus overcomes certain problems of classical fission models. The suggestion by Hartmann and Davis (1975) and later by Cameron and Ward (1976) that lunar origin involved giant impacts remains attractive. Large planetesimals are consistent with current accretion models, and may have been widely scattered in the early solar system: their existence is a reasonable, if not necessary, assumption in moon-origin models. Furthermore, isotopic data require the moon’s formation primarily from local material resembling Earth’s upper mantle, not material from elsewhere in the solar system. Giant impacts are stochastic, class-predictable events that would provide the required type of ejected Earth-mantle material without requiring large moons to form near other planets (a problem with less stochastic processes). Such material may have mixed with incoming meteorites during lunar formation, affecting lunar chemistry. … (Hartmann, 1984: 52)

Hartmann expanded his ten-minute talk into a paper for the follow-on book. It was titled “Moon origin: The impact-trigger hypothesis.” In both the talk at the conference and the follow-on paper, Hartmann used the phrase “impact-trigger hypothesis” for what others were calling the “giant-impact hypothesis.” Early in his paper, Hartmann gave an historical note about the origin of the impact-trigger model. The impact-trigger model was first suggested by Hartmann and Davis (1975) and independently by Cameron and Ward (1976). (Historical note: This model was presented by Hartmann at the Conference on Satellites of the Solar System that was held at Cornell University in the summer of 1974; in a response from the floor, Cameron noted that he and Ward were pursuing a similar model, with similar positive results.) … (Hartmann, 1986: 580)

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Hartmann then reviewed the “first-order properties and constraints on lunar origin.” • Iron deficiency and gross similarity to Earth’s upper mantle: After Apollo, lunar rock geochemistry led to the consensus that the lunar material crudely resembles Earth’s mantle … • Volatile depletion: The volatile depletion pattern of the Moon has always been difficult to explain in detail. However, at a first-order level, it appears consistent with a strong heating of most lunar material, probably in pulverized form to allow volatile escape, perhaps to temperatures of 1400–1800 K, and possibly additional chemical processing. … The hypothesis of an impact ejecting hot, finely disseminated material thus appears to be a step forward in understanding lunar volatiles …, but the chemistry of impact processing clearly requires further study. • Angular momentum considerations: As Cameron and Ward (1976) emphasize, a giant impact provides a plausible mechanism to explain the unusually high value of angular momentum in the Earth/Moon system, relative to other planets. … Indeed, a large impact is the ideal mechanism to produce Earth’s final spinup to the effective period of 4.1 h, matching the angular momentum of the present system … • Oxygen isotope ratios: … lunar samples … fall on the chemical mass fractionation line characteristic of Earth materials and are indistinguishable from Earth … In summary, the O-isotope data require that the Moon formed from material that originated in the same terrestrial “feeding zone” that contributed material to the Earth, and not as far away as the “feeding zone” of Mars. [his italics] • Bulk iron content: The estimated bulk elemental iron content of the Earth’s mantle and the Moon are: Earth mantle: Moon:

7% iron by weight … 7–9% iron by weight …

The similarity is predictable if the Moon formed from ejected upper mantle material (especially if some projectile iron were added), but is an odd coincidence in other theories. • Density: The mean densities of the Moon (3.344 ± 0.002 g/cm3) and upper mantle (3.3 to 3.4 g/ cm3) are virtually identical … . This is directly explained if the Moon formed from ejected upper mantle material, but is an odd coincidence in other theories. (ibid.: 582– 586)

Hartmann recognized that an unresolved issue for the impact-trigger model was whether the ejected material that formed the Moon largely originated in the Earth’s mantle or, on the other hand, was mainly composed of material from the impactor. Hartmann wrote: If it could be shown that such an impact would eject primarily only Earth-mantle material into orbit (for example, if the impactor were vaporized and it ejected both vaporized and solid Earth-mantle material, and if most of the vapor phase or its resultant condensates escaped from the Earth/Moon system), then the origin of the Moon would be largely solved. Heated, pulverized Earth-mantle dust would accrete in orbit. This possibility was an initial attraction of the impact-trigger hypothesis; … such material would already make a first-order match to lunar composition in terms of bulk density, iron content, volatile loss, etc.

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However, many models of giant impacts … suggest that most of the ejecta is vaporized planetesimal material. On the other hand, some studies … suggest that the fraction of mass vaporized depends primarily on impactors’ velocity, not size, implying that the percentage of vaporized ejects in a giant impact should not necessarily be 100%. … … I regard as still open the question of how much heated material, pulverized, solid Earth-mantle dust could be carried into a circumterrestrial cloud during a giant impact. Gas expansion will accelerate dust by “second burn” drag effects. Models that eject primarily gas and virtually no high-speed dust have yet to demonstrate that they are successful when scaled down to smaller events: they need to account for the thick ejecta blankets stretching *103 km from lunar basins. Just as significantly, these models need to show that they are consistent with the compelling evidence that lunar and Martian rocks have been ejected into space with negligible thermal alteration. (ibid.: 589–590)

Hartmann discussed the effect of the impact parameter on the outcome of the giant impact, as shown in his “trilogy” diagram (Fig. 9.2). Each corner represents the dominance of a particular physical effect: in (a) it is shearing (mechanical or tidal) of the planetesimal and forward jetting of ejecta associated with a nearly tangential impact; in (b) it is spinup and consequent fission due to angular momentum input from a nearly tangential impact; in (c) it is the more symmetric ejection of a massive swarm of debris associated with cratering, without invoking a near-tangential impact. The diagram is a continuum; the real process of lunar origin could be represented as a dot on any edge (combining two effects) or in the interior (combining all three effects). … (ibid.: 590–591)

Hartmann made a suggestion similar to one made by Ringwood, namely that the impactor might have come from the outer solar system, rather than from the local (to Earth) region. For such bodies: Fig. 9.2 Hartmann’s “trilogy diagram,” showing various possible lunar origin models based on the impact-trigger hypothesis. From Fig. 2 in Hartmann, W. K. (1986). “Moon origin: The impact-trigger hypothesis.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 591, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

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… the energy/kg would be much greater, allowing dramatic possibilities for ejecting large amounts of mass. Their compositions would be an opposite extreme from that of local bodies. The silicate materials would be carbonaceous, … and they would be at least 50% ice by mass … . They might resemble Callisto, Rhea, Phoebe, Chiron, or Pluto. Collisions with such bodies should be modeled along with those of terrestrial-type impactors. They may offer a better chance of driving dust into orbit because of the “second burn” effects of expansion of the enormous masses of volatiles vaporized. The cloud of (dissociated) water vapor (or other hydrogen-rich compounds) would offer an interesting environment for chemical alteration of the ejected or recondensed dust, especially oxidation/reduction reactions. O-isotope ratios would be a problem, but perhaps it can be shown that much of the vaporized planetesimal ice would escape the system, and that most of the silicates forming the resultant Moon came from terrestrial mantle materials carried aloft by the expanding gas. (ibid.: 592–593)

In his analysis of the probable size of the impactor, Hartmann concluded that “… an impactor of at least several lunar masses appears necessary to generate and orbit the minimum amount of material necessary to make the Moon.” (ibid.: 594). Hartmann had conducted lab studies of the effect of a high-velocity impact on various materials. Extrapolating from these studies and from the analyses of the amount of material ejected from impact-formed craters on the Moon, he developed sets of probable values of various impact-related parameters and concluded that, at least for impact velocities in the range 21–24 km/sec: … an impactor of 0.8 MMars or somewhat larger could loft more than a lunar mass into a circumterrestrial swarm. The debris would be mantle material from Earth and impactor, largely pulverized, partially recondensed, and low in volatiles and iron. (ibid.: 601)

The high impact velocities suggest a non-local origin of the impactor. For lower impact velocities, in the range 10–17 km/sec, higher masses for the impactor are required to put a corresponding amount of material into the circumterrestrial swarm. Hartmann concluded: Because of the uncertainties in modeling giant impacts, I consider all these models as demonstrating the plausibility more than the necessity of the impact-trigger hypothesis. The spectrum of models, from smaller, high-speed impactors (more distant origin?) to large, low-speed, local impactors, remains a valid regime for further work. … (ibid.: 603)

9.2.3

Hartmann and Vail

In the follow-on book, Hartmann teamed with S. M. Vail of the Planetary Science Institute on a paper that used computer simulations to examine plausible sizes and populations of planetesimals that might impact the early Earth. Their computer model: … was developed to simulate a gravitational encounter in a two-body system, planet and impactor. The planet rotates in a specified initial period. A run of 500 impacts was used to determine the most likely value and range of resulting angular momentum of the planet, following collision with a planetesimal of m1/m, considerably less than unity. … where:

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m1 =m ¼ masslargestimpactor =masstargetplanet … The model allows planetesimals to approach from random directions at specified speeds, and computes the final angular momentum after collision, giving the obliquity and period. In each run of 500 impacts, the obliquities were divided into 180 1o-wide bins, the periods into 200 bins, covering whatever range of periods was generated in the run. “Most frequent value” is defined as the value of obliquity or period in the most populated bin. … (Hartmann & Vail, 1986: 552)

Victor Safronov had worked on this problem in the mid to late 1960s, publishing a book in Russian in 1969 that was translated into English in 1972 (Safronov, 1972). Hartmann and Vail acknowledged the work of Safronov and others. Safronov … considered each planet, one at a time, and calculated the most likely size of impactor to have created the observed obliquity, assuming a specified approach velocity at large distance. We take the inverse of the same problem: We consider a fixed population of planetesimals and consider the range of results on the ensemble of planets. We use this approach to focus on the overall planetesimal population, looking for dynamically plausible populations that could produce observed planetary properties. (Hartmann & Vail, 1986: 553)

Safronov concluded that to give the Earth its observed obliquity, the ratio of the mass of the largest impacting planetesimal to the mass of the Earth was between 0.001 and 0.01. Safronov realized that numerical approaches would be required to get constraints on the mass ratio based on considerations of the Earth’s spin rate, but in 1969 he did not make that attempt (Safronov, 1972: 124). Hartmann and Vail considered the constraints of spin rate as well as obliquity, so that there analysis was different from Safronov’s in that respect. The physical basis of the computer modeling of Hartmann and Vail was also at least slightly different from what had emerged from the most recent work of Wetherill. Wetherill’s computer modeling seemed to indicate that the terrestrial planets arrived at their current configurations as accumulations in the course of a series of collisions of planetesimals of significant size. Hartmann and Vail assumed that the: Orbital and rotational characteristics of the solar system may be thought of as regularized to first order by cumulative statistical effects of the accretion of innumerable small planetesimals, but with irregularities produced by stochastic impacts of a few larger bodies. … (Hartmann & Vail, 1986: 551)

In their computer modeling, therefore, Hartmann and Vail assumed that the initial axis of their target planets was perpendicular to the solar ecliptic, and the initial spin period was 10 h. They wanted to see the effect of impacts of larger planetesimals on an initial condition that was consistent with the predictions of theory for accretion of the planets by much smaller bodies. For a given run of 500 impacts, Hartmann and Vail would set the parameters for the target planet, e.g., the Earth, and choose a value for m1/m, e.g., 0.025, and the approach velocity of the impactor, e.g., 20 km/sec. They would then record the most frequent value of obliquity and spin period of the target planet. As shown in Figs. 9.3 and 9.4, they found that the obliquity of the target increased, and its period of rotation decreased, as the ratio of m1/m increased.

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Fig. 9.3 Dependence of impact-induced obliquity on mass of impactor. From Fig. 2-A in Hartmann, W. K., & Vail, S. M. (1986) “Giant impactors: Plausible sizes and populations.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 555, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

Fig. 9.4 Dependence of impact-induced rotation period on mass of impactor. From Fig. 2-B in Hartmann, W. K., & Vail, S. M. (1986) “Giant impactors: Plausible sizes and populations.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 556, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

The goal of Hartmann and Vail was to discover “… what impactors would be most likely to change the assumed preimpact obliquity and period, about 0o and 10 h, to the postimpact values characteristic of the primordial Earth-Moon system.” (ibid.: 555). The current tilt of the Earth’s spin axis is 23.5o, but Peter Goldreich had determined in 1966 that when the Moon and Earth were about 10 RE apart, the Earth’s obliquity was between 10o and 15o. The spin period at that distance must

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have been about 4.1 h, given the current angular momentum of the Earth-Moon system. With these post-impact values for obliquity and spin period, and the data from their simulations shown in Figs. 9.3 and 9.4, Hartmann and Vail found that: … in the velocity range of 5–30 km/s, impactors of around 0.01–0.04 ME would be most likely to produce the obliquity. Somewhat larger impactors, around 0.03–0.12 ME, are the size that would be most likely to produce a period of about 4.1 h, depending on approach speed and initial period. This important result, that planetesimals of about 0.01–0.12 ME can well explain Earth’s spin properties, is consistent with the size of impactor (*0.1 ME  MMars) suggested in several versions of the impact-trigger theory of lunar origin. (ibid.: 556)

Hartmann and Vail concluded that: … our study finds it highly plausible that Earth was struck by a body of between several MMoon and MMars (*0.03–0.12 ME) with enough energy and angular momentum to dislodge mantle material and form the present Earth-Moon system, consistent with the impact-trigger model of lunar origin. (ibid.: 565)

9.2.4

Herbert and Davis

Floyd Herbert and Donald Davis from the Tucson Lunar Origin Consortium gave the next talk at the conference. Their topic, and challenge for some lunar formation models, was whether and how angular momentum could be added to a circumterrestrial swarm. Models of lunar origin in which the Moon accretes in orbit about the Earth from material approaching the Earth from heliocentric orbits must overcome a fundamental problem: the approach orbits of such material would be, in the simplest approximation, equally likely to be prograde or retrograde about the Earth, with the result that accretion of such material adds mass but not angular momentum to circumterrestrial satellites. Satellite orbits would then decay due to the resulting drag, ultimately impacting onto the Earth. (Herbert & Davis, 1984: 53)

Herbert and Davis conducted some numerical calculations/“experiments,” investigating the delivery of angular momentum to a circumterrestrial swarm from planetesimals with a wide variety of heliocentric orbits. These preliminary experiments show that heliocentric planetesimals passing through the Earth environment possess significant angular momentum. However it also appears that these same planetesimals impacting a circularized circumterrestrial planetesimal swarm would likely remove angular momentum (though possibly increasing mean kinetic energy), presumably promoting both swarm infall upon the Earth and escape to heliocentric space. … (ibid.)

The removal of angular momentum from the circumterrestrial planetesimal swarm is the opposite of what is needed to bring the angular momentum of the eventual Earth-Moon system to its current value. The result of the research by Herbert and Davis was negative for the hypothesis that the Moon formed by co-accretion with the Earth.

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Weidenschilling

The next two talks at the conference were given by Stuart Weidenschilling, one of the members of the Tucson Lunar Origin Consortium. In the first of these talks, Weidenschilling continued the negative assessment of Herbert and Davis. The lunar origin model considered by the Tucson Consortium involves processing of proto-lunar material through a circumterrestrail swarm of particles. Once such a swarm has formed, it can gain mass by capturing infalling planetesimals and ejecta from giant impacts on the Earth, although the angular momentum supply from these sources remains a problem. Here we examine the first stage of formation of a geocentric swarm by capture of planetesimals from initially heliocentric orbits. The only plausible capture mechanism that is not dependent on very low approach velocities is the mutual collision of planetesimals passing within Earth’s sphere of influence. … We have tested this capture scenario directly by many-body numerical integration of planetesimal orbits in near-Earth space. … … These results agree with those of Davis and Herbert (Herbert & Davis, 1984) that the systematic contribution of angular momentum is insufficient to maintain an orbiting swarm under heavy bombardment. Thus, a circumterrestrial swarm can be formed rather easily, but is hard to sustain because the mean net angular momentum of a many-body swarm is small. The requisite angular momentum can be supplied by a single collision (or a few, at most) of large bodies within Earth’s sphere of influence. A swarm formed in this way could be subsequently accreted up to several times its original mass without collapsing onto the planet, possibly allowing enough processing of proto-lunar material to produce iron-silicate fractionation. (Weidenschilling, 1984a: 54)

The second talk by Weidenschilling also raised problems for the co-accretion model, as well as the rotational fission hypothesis. Formation of the Moon by classical Darwin-type fission of a rapidly spinning proto-Earth requires approximately 3½ times the present angular momentum of the Earth-Moon system. Proponents of fission have proposed mechanisms for the escape of the excess after fission, but have generally assumed that the proto-Earth could acquire the requisite angular momentum during its accretion. Both numerical and analytical studies yield the result that the systematic angular momentum delivered by impacting planetesimals is small, and should produce a slowly rotating planet. … These results appear to rule out acquisition of enough angular momentum for fission by the accretion of small bodies directly impacting the planet. … There is a lesser but similar problem with co-accretion models. To avoid orbital collapse by accretion drag, the Moon must gain angular momentum from the rapidly rotating Earth via tides. Accumulation of the planet by direct impacts of planetesimals does not supply enough angular momentum. It would require processing approximately 0.2 ME through an accretion disk, or else a giant impact, to spin up the proto-Earth before the lunar embryo formed. Thus, co-accretion requires an earlier event that might in itself have sufficed to form the Moon. (Weidenschilling, 1984b: 55)

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Chapman and Greenberg

The next talk at the conference expanded on the circumterrestrial compositional filter model (Fig. 9.5) for the origin of the Moon. The coauthors were Clark Chapman and Richard Greenberg, of the Planetary Science Institute. They were also members of the Tucson Lunar Origin Consortium. A major question about the moon is its under-abundance of iron. We seek (as others have) to understand whether a metal-silicate fractionation of heliocentric bodies can be achieved through collisional interactions with a circumterrestrial swarm. Large, dense metallic cores of disrupted, differentiated planetesimals could pass through such a swarm relatively unimpeded, while silicate fragments would be filtered out and captured by the cloud, which might later accrete into a metal-depleted moon. We envision planetesimals left over during the very late stages of accretion of the Earth, with heliocentric orbits extending a few tenths of an AU beyond the Earth’s orbit. They may have been heated and geochemically differentiated into silicate bodies with iron cores, just as has apparently happened to a major fraction of the asteroid population. Such bodies would diffuse toward Earth’s orbit. We consider the rates of such diffusion and of the mutual collisional destruction within the population. We then consider the interaction of the differentiated planetesimals and their collisional products (both silicate mantle-fragments and iron cores) with a swarm of Earth-orbiting lunesimals (perhaps ejecta from the Earth) of km-scale, totaling 0.1 lunar mass, extending out 10 or 20 Earth radii. We find that such a small near-Earth population of lunesimals can filter out silicate-rich portions of the planetesimals. This silicate-separation process and accretion of lunesimals into the moon must go to completion in a time short compared to 107 yr, which is the timescale for the planetesimals to be swept up by, or scattered away from, the Earth.

Fig. 9.5 Pictorial illustration (not to scale) of the circumterrestrial filter model. The figure shows a massive iron core passing through the circumterrestrial disk, while smaller silicate particles are trapped. From Fig. 1a in Herbert, F., Davis, D. R., & Weidenschilling, S. J. (1986) “Formation and evolution of a circumterrestrial disk: Constraints on the origin of the Moon in geocentric orbit.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 703, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

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… The whole scenario works only if there is a way to maintain the hypothesized circumterrestrial swarm, which otherwise would collisionally diffuse on a timescale of 103 yr (much of it collapsing on the Earth); the source of angular momentum to maintain the swarm remains a mystery. (Chapman & Greenberg, 1984: 56)

9.2.7

Herbert, Davis, and Weidenschilling

In a paper for the follow-on book, Herbert, Davis and Weidenschilling proposed a solution to the mystery of how to maintain the angular momentum of the circumterrestrial swarm. Their extended research for the paper in the book was more positive for the co-accretion theory, as the authors noted: This paper explores possible mechanisms for material to be captured into geocentric orbit with enough specific angular momentum to remain in orbit. The results of our present work are in contrast with the conditionally negative results given at the Kona meeting … . The more positive current result comes principally by extending our earlier calculations to larger geocentric distances. While there remain many unresolved issues …, we no longer feel that an angular momentum deficiency is necessarily a fatal flaw to this scenario. (Herbert, Davis, & Weidenschilling, 1986: 705)

In her 1960 paper, Ruskol estimated that the circumterrestrial disk would extend to about 100 Earth radii. Herbert, Davis and Weidenshilling found that the disk could be maintained, and subsequently form the Moon, if most of the mass is captured at distances greater than 80 RE (ibid.: 703). The capture mechanism would be by collisions of planetesimals that were in heliocentric orbits.

9.2.8

Weidenschilling et al.

In the follow-on book, all of the members of the Tucson Lunar Origin Consortium were coauthors of a paper on the formation of the Moon from a circumterrestrial disk. Stuart Weidenschilling was the lead author of the paper. The authors viewed the circumterrestrial disk (CTD) model as an alternative to the giant impact (or impact-trigger) model. Their model was motivated in part by the fact that the Moon is depleted in metal. They argued that the model also meets other geochemical constraints, e.g., that of oxygen isotopes. The Earth and the Moon fall on an identical oxygen isotopic mass fractionation line. All other sampled solar system objects (except the enstatite chondrites and aubrites) fall off this line … . The association of many undifferentiated meteorites with the asteroid belt … suggest that the Earth and Moon are made predominantly of material originating in the vicinity of 1 AU or closer to the sun. This strong constraint must be satisfied by any model of lunar origin. In particular, the trigger-impact model must either derive most of the lunar material from Earth’s mantle (“fission” rather than “capture”), or else the impacting body must have the same oxygen isotopic composition as the Earth. The plausibility of the latter

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condition depends on the isotopic homogeneity of the terrestrial planet region, which is not known. (Weidenschilling et al., 1986: 737)

To explain the lack of volatiles on the Moon, the CTD model depends on a population distribution within the disk that is biased towards small planetesimals, because “Differentiation within kilometer-scale bodies may result in greater loss of volatiles than from larger bodies.” (ibid.: 759). They also noted: We have argued that it is plausible that much of the mass of the planetesimal population is in small bodies, although this has not been demonstrated directly. The low relative velocities that we have assumed for the planetesimals are consistent with this assumed size distribution. The nature of the Earth-zone planetesimal population during the late stages of the Earth’s formulation remains a critical issue; our model will succeed best if there is a balance between a massive population of kilometer-scale planetesimals plus a significant number of larger ones. The latter may be required in order to agitate the heliocentric population of planetesimals so that silicates are smashed and cores exposed, or possibly to produce the initial CTD, if it is due to an Earth impact or chance collision within the sphere of influence. The existence of a moderate number of larger (  100 km) heliocentric planetesimals does not have any direct effect on the CTD, since they are neither captured nor disrupted by passage through the CTD. Like smaller iron cores, they are eventually accreted by Earth or removed by being perturbed into Jupiter-crossing orbits. (ibid.: 758)

At the conclusion of their paper, the authors drew a distinction between the CTD and giant-impact models for the formation of the Moon. Our CTD model involves a relatively quiescent heliocentric swarm with low orbital eccentricities. This condition would tend to limit the degree of mixing between planetary zones, and to preserve isotopic and compositional differences between the various terrestrial planets. In contrast, the accretion model of Wetherill (1986) involves the scattering of large bodies throughout the inner solar system; in that case, the differences in composition between, for example, Mercury and Mars must be ascribed to stochastic effects of large-body accretion, rather than to smooth variations that are a function of the planets’ present heliocentric distances. … We have mentioned … the constraints on the giant impact model implied by the identical oxygen isotope compositions of the Earth and Moon. That condition can be met if the inner solar system was isotopically homogeneous before planetary accretion, but the SNC meteorites, if they are derived from Mars, suggest otherwise. … (Weidenschilling et al., 1986: 759)

9.2.9

Wasson and Warren

The next talk in the My Model of Lunar Origin session of the conference was given by John Wasson and Paul Warren of UCLA. As noted above, these researchers also favored a CTD model for the formation of the Moon, and they credited Evgenia Ruskol for its development. In her 1977 study, Ruskol pointed out that during the accretion of planetary bodies the size distribution of planetisemals is such that: A large part of the mass of the bodies is concentrated in the largest bodies, … and a large part of the total surface is concentrated in the smaller fraction. This means that in process where the main role is played by the frequency of interactions, the small particles have the

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greatest activity. We have shown that in capture in the swarm the size distribution of the particles changes in the direction of enrichment with the smaller fraction. … (Ruskol, 1977: 817)

Ruskol asked: Could this feature of the formation of the swarm lead to its enrichment in silicates or somehow to the depletion of iron? … Recent experiments on collision of metallic particles have demonstrated that in a large velocity range, from 0.5 km/s to 10 km/s metallic particles weld themselves together, while at velocities less than 0.5 cm/s [sic—should be 0.5 km/s] they undergo semielastic repulsion … . Experiments with silicate particles confirmed … that behavior of silicate particles is completely opposite to the behavior of metallic ones; i.e., in the impact velocity range from 1.5 km/s to 9.5 km/s destruction of the particles predominates over their agglomeration. … Only at velocities of less than 0.4 km/s is agglomeration possible … . Investigations of the changes in materials “treated” by impacts with velocities of several km/s have shown that brittle material—silicates, hydrides —are broken up into very fine particles of micron and submicron sizes which cannot be obtained by other mechanical methods … . But metals display only plastic deformation. … . (ibid.: 817–818)

Ruskol gave credit to Egon Orowan, a distinguished mechanical engineer at MIT, who first pointed out implications of the difference in the collisional behavior of metals and silicates in a 1969 paper in Nature, titled “Density of the Moon and nucleation of the planets” (Orowan, 1969). Ruskol acknowledged that the presence of oxides of iron in the protoplanetary cloud complicates the picture, as “The oxides are rather more brittle than plastic materials.” (Ruskol, 1977: 818). But, the data available on the pulverization of oxides of silicon and iron seems to provide “… a basis for enrichment of the circumterrestrial swarm in silicate materials by means of the predominant capture of the smaller fraction.” (ibid.). In reviewing Ruskol’s work, however, Wasson and Warren questioned “… whether the mean particle size of the metallic particles would have been great enough to prevent their capture into the swarm.” (Wasson & Warren, 1984: 57). To make sure the metal objects were large enough to pass through the circumterrestrial swarm and either escape or be absorbed by the Earth, Wasson and Warren suggested: … that near 1 AU asteroidal differentiation occurred before the bulk of the protolunar material had been captured into a circumterrestrial swarm. These differentiated bodies broke up as a result of mutual impacts. Fragmentation tended to reduce the more brittle silicate crusts and mantles to relatively small (perhaps less than approximately 1 m) size, whereas because of their much greater strength many of the metallic cores may have survived intact. Collisions of small silicate fragments with debris already in Earth orbit led to the orbital capture, but the debris cloud was essentially transparent to the metallic cores, and these objects continued in heliocentric orbit until removed by a close planetary encounter of the first or second kind. (ibid.)

The favorite model of Wasson and Warren, therefore, was a modified version of Ruskol’s circumterrestrial swarm. Neither model featured a giant impact.

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9.2.10 Cameron The favorite model of Alastair G. W. Cameron, who was the next speaker at the conference, did feature a giant impact, as he and William Ward were credited, along with Hartmann and Davis, with developing the idea in 1974–1975. In the book that followed the conference, the paper by Cameron was essentially a reiteration of the ideas contained in his earlier paper with Ward. Cameron did, however, submit his historical note as to his role in the development of the impact theory for the formation of the Moon: In 1975 Hartmann and Davis … noted that the spectrum of bodies bombarding the proto-Earth could have contained a number of quite large objects, having masses extending up to 10−1 Earth masses. They also noted that the collision of a large object with the proto-Earth would eject material into space, forming a “cloud of hot dust, rapidly depleted in volatiles.” They then stated their expectation that the particles would interact and collapse into the equatorial plane where a satellite could form. Their paper suggested that an impactor of two lunar masses might be adequate …. When Hartmann first presented these ideas at a 1974 meeting, I objected that such a theory did not solve the angular momentum problem, and that this could only be done with a single collision involving a body having at least 10−1 Earth masses (i.e., about the mass of Mars). That idea was then under development by W. R. Ward and me and was presented the next year. … . The two theories are often thought to be essentially the same, but in fact there is a profound difference in their approaches to the problem. The question Ward and I started with was how big an object would have to be such that, striking the proto-Earth a glancing blow, it would impart to the proto-Earth and to any material that might be placed in orbit a total angular momentum equal to the angular momentum of the present Earth-Moon system. The answer was that the projectile should be about 0.1 Earth masses, or about the mass of Mars. Since the geometry of the collision was chosen in this question in an optimum way, the real answer to this question is that the minimum mass of the projectile should be about the mass of Mars, and the actual mass could be substantially larger if the collisional impact was more centrally directed. … This single collision with a major body became the central theme of the ideas developed by Ward and me. It satisfied the angular momentum problem by definition, and the major question that perhaps one should ask is whether it is plausible that such a major body should have been present in the early solar system. We were also very much concerned with the question of how any of the material that is ejected from the proto-Earth could have gotten into orbit, since all material ejected from the Earth on a trajectory leaving the surface must return to below the surface unless escape velocity has been achieved. We realized that at the collisional velocities involved, large quantities of rock vapor would be produced, and hence that, in addition to the usual gravitational forces in the problem, acceleration of material could take place by means of gas pressure gradients. … The Hartmann and Davis picture is consistent with ours only in the case where their collision involves a body of the same large mass. They also did not address the need for nongravitational forces in the problem of placing material into orbit. These two issues were the starting points of our own approach. (Cameron, 1986: 609–610)

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Cameron gave an example to illustrate the process of getting material into orbit around the Earth following a collision, i.e., the formation of an accretion disk around the Earth: … The gas in the vapor cloud rising above the surface of the proto-Earth is subject to acceleration by pressure gradients as well as by gravity. This is the crucial difference that makes the formation of an accretion disk around the proto-Earth possible. For example, if the projectile hits the proto-Earth tangentially at 12 km/sec, and if the gas cloud were to receive an equal contribution of mass from the proto-Earth and from the projectile, then the gas would have a mean transverse velocity of 6 km/sec, which is slightly suborbital. The gas on the forward edge of the cloud then need only receive nearly 2 km/sec additional velocity in the direction of the collision due to pressure gradient acceleration to reach orbital velocity. It should be clear that the expected yield of material in orbit is much less than half of the vaporized mass. It should also be clear that if the gas components derived from the projectile and the proto-Earth do not mix efficiently, then a considerable amount of the gas derived from the projectile may be lost from the system because it exceeds escape velocity, while most of that derived from the proto-Earth will fall back on the surface. (ibid.: 612)

Cameron carried out some numerical simulations of the giant impact to see if successful disk formation occurred and, if so, under what conditions. He reported: The results showed that [successful disk formation] can be achieved under a wide variety of conditions. For the best yields the gas should be neither too “hot” nor too “cold,” the collisional velocity of the projectile should be a few km/sec greater that the escape velocity minimum, and most of the volume of the projectile should be part of the source region for the vapor that is produced. In all cases, the majority of the mass that goes into orbit following the collision is derived from the projectile. These initial conditions are optimally met for a projectile in orbit about the sun in a proto-Earth-crossing trajectory of significant eccentricity. This is consistent with a time scale for the collision on the order of 108 years after the formation of the solar nebula The collision should lead to the loss of the original atmosphere of the proto-Earth … . Judging from the fact that Venus has an atmospheric content of rare gases that is very large compared to Earth, one can conclude that the loss of the atmosphere occurred after the bulk accumulation of volatile-containing planetesimals had occurred. This is another argument in favor of the collision happening about 108 years after formation of the solar nebula. (ibid.: 614)

In the final section of his paper, Cameron discussed some outcomes of the giant impact model that seem to be consistent with observations: The iron core of the projectile is probably not significantly vaporized. Assuming … that projectile is a differentiated planetary body, then the iron core should have concentrated and extracted the siderophile elements from throughout the body of the projectile. Hence the accretion disk and the Moon should be depleted in these elements, as is observed to be the case. The deposition of mass into the proto-Earth by the collision should release a great deal of energy, which I have estimated to raise its surface temperature to about 6000 K … . Dissipation within the accretion disk probably maintains the temperature near 2000 K in the vicinity of the Roche lobe. Hence when the Moon forms it should do so in a temperature field that will have prevented most of the elements of medium volatility from condensing with any significant abundances. Again, this is observed to be the case.

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It should be noted that the accretion disk need not form precisely in the equatorial plane of the proto-Earth initially, nor should one expect the orbit of the Moon to lie in that plane today. The accretion-disk-forming collision adds a very large angular momentum vector to the proto-Earth, which will nevertheless add to the previous angular momentum vector. Thus the initial plane of the disk will differ from that of the equator … . (ibid.: 615)

Cameron mentioned: It has generally been assumed that the equality of the oxygen isotope ratios in the Earth and the Moon indicates that the Moon was formed out of material that condensed at the Earth’s orbital distance from the sun. Until the origin of the variations in oxygen isotope ratios is properly understood, this principle will remain empirical, but it seems reasonable to accept it provisionally. … (ibid.)

9.2.11 Benz, Slattery, and Cameron In the follow-on book, Cameron was a coauthor on a paper that illustrated a three-dimensional modeling of a giant impact. The lead author of the paper was Willy Benz, a Swiss scientist and expert at numerical simulations on leave at the Los Alamos National Laboratory. Cameron and W. L. Slattery of the Los Alamos National Laboratory were the two other authors. In the simulation shown in Fig. 9.6, and included in their paper, an impactor approaches the proto-Earth at a velocity of 3 km/sec. Plotted in the “snapshots” are velocity vectors at particle locations projected onto the plane defined by the center line between the two planets and the trajectory of the impactor. Benz et al. noted that in their simulations: Starting with molten or solid planets does not make any difference. After the impact, part of the impactor forms a clump orbiting around the proto-Earth. This clump, however, is on a very eccentric orbit, bringing it back well inside the proto-Earth’s Roche limit; it is therefore destroyed and spread out into a disk. It is worth noting that the material ending in the clump originates completely from the side of the impactor opposite to the proto-Earth at the time of the impact. (Benz, Slattery, & Cameron, 1986: 617)

The simulation of Benz et al. was three dimensional, but it did not feature iron cores in either the proto-Earth or the impactor. The material in both was assumed to be granite.

9.2.12 Kaula and Beachey The next talk at the conference was coauthored by Bill Kaula and A. E. Beachey of UCLA. They were invited to submit an extension of their talk as a paper in the follow-on book. The growing availability of large-capacity computers for numerical simulations was evident in many of the talks that were presented at the conference

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Fig. 9.6 A numerical model showing snapshots of events taking place between the impact and the formation of a clump in orbit. From Fig. 2 in Benz, W., Slattery, W. L., & Cameron, A. G. W. (1986) “Short note: Snapshots from a three-dimensional modeling of a giant impact.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 619, Houston, TX: Lunar and Planetary Institute, Copyright 1986 the Lunar and Planetary Institute

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Table 9.1 Collision cases tested and outcomes Protoplanet masses 1-Smaller 2-Larger M⊕ M⊕

Approach velocity km/ sec

Approach offset R⊕

Pericenter overlap R⊕

Protoplanet 1 Merge Orbit

Escape

0.1 0.9 2.95 4.79 0.00 0 0 91 0.1 0.9 2.95 3.95 0.43 91 0 0 0.1 0.9 6.6 2.22 0.23 62 29 0 0.5 0.5 2.52 5.85 0.00 0 0 88 0.5 0.5 2.52 4.94 0.43 88 0 0 M⊕ = 5.97 1024 kg; R⊕ = 6371 km The outcomes are the eventual locations of the spherical body elements From Table 1 in Kaula, W. M., & Beachey, A. E. (1986) “Mechanical models of close approaches and collisions of large protoplanets.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 569, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

and papers in the follow-on book, including the paper by Kaula and Beachey titled “Mechanical models of close approaches and collisions of large protoplanets.” Kaula and Beachey modeled protoplanets as sets of small spherical bodies, closely packed, with typically 90 spherical bodies per protoplanet. They then followed the trajectories of the small spherical bodies under conditions where two protoplanets collided with each other or nearly did so. As shown in the accompanying table, which was included in their paper, Kaula and Beachey found nothing unexpected from the simulations of near collisions, nor from relatively deep collisions when the approach velocity of the protoplanets was small, e.g., about 3 km/ sec, nor from collisions when the two protoplanets were of equal mass. For the third case shown in Table 9.1, the smaller protoplanet was 10% of the mass of the Earth, and the larger protoplanet was 90% of the mass of the Earth. The relative velocity between the two protoplanets was 6.6 km/sec, and the collisional overlap was 23% of an Earth radius. In that case 62 of the 91 spherical body elements of the smaller protoplanet “merged” with the larger one, while 29 of the spherical body elements went into orbit around the larger protoplanet. Figure 9.7 shows the positions of the spherical element for the protoplanets in the immediate aftermath of the collision. The spherical elements for the larger protoplanet are shown as Xs and the numbers represent the original radial loci rounded off to the nearest tenth of the smaller protoplanet radius. (The numerical calculations of Kaula and Beachey were two dimensional, so the “collision” is really between two cylindrical protoplanets, rather than between two spherical protoplanets.) As shown in the figure, and as noted by Kaula and Beachey: A result of interest in the cases that do place material into elliptic orbits, 0.1 ME, 0.9 ME and 6.6 km/sec, is that this material comes from the outer parts of the smaller protoplanet … This result suggests that if a collision occurs between protoplanets that are already differentiated, the material placed in orbit is deficient in iron. Also, this material is appreciably heated … so that it would be expected to be depleted in volatiles. (Kaula & Beachey, 1986: 574)

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Fig. 9.7 Snapshot of a numerical simulation of the collision of two protoplanets of masses 0.1 ME and 0.9 ME at approach velocity of 6.6 km/sec. The distribution of 89 spherical elements of the smaller protoplanet is shown by the numbers, which are the original radial loci. From Fig. 1 in Kaula, W. M., & Beachey, A. E. (1986) “Mechanical models of close approaches and collisions of large protoplanets.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 574, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

Kaula and Beachey summarized the main results of their simulations as follows: Placing material in orbit around the main mass appears to require a combination of the unequal mass ratio with the approach velocities above 6.0 km/sec, corresponding to perturbations from the asteroid belt or the vicinity of Mercury. (ibid.: 567)

9.2.13 Stevenson The final talk in the My Model of Lunar Origin session at the conference was given by David J. Stevenson of Caltech. Stevenson, originally from New Zealand, came to the United States as a Fulbright Scholar and obtained his Ph.D. in theoretical physics from Cornell University. In the year of the conference on the Origin of the Moon, Stevenson was awarded the first Harold C. Urey Prize, which was established by the Division of Planetary Science of the American Astronomical Society to recognize and encourage outstanding achievements in planetary science by a young scientist. The title of Stevenson’s talk was “Lunar Origin from Impact on the Earth: Is it Possible?” He billed his effort as “… the first attempt to replace handwaving with physics.” (Stevenson, 1984: 60).

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In the giant-impact model of Cameron and Ward, a disk of material formed around the proto-Earth following the collision. The material was initially hot because of the heat energy associated with the collision, and it was thought to have cooled via the radiation of heat into space. The result was a “cool” particulate disk around the Earth that eventually formed the Moon. Stevenson and a colleague at Caltech, Christopher Thompson, had given a talk at the Fourteenth Lunar and Planetary Science Conference in 1983 in which they showed that gravitational instability, i.e., clumping together of material, can occur in a hot disk if the material of the disk is a two-phase system, e.g., a froth of liquid and vapor silicates (Thompson & Stevenson, 1983: 787–788). The two-phase sound speed can be orders of magnitude smaller than the sound speed in either single phase. As a result, instabilities can occur in situations previously not thought possible; specifically, in hot disks. Following a suggestion of Cameron and Ward (Cameron & Ward, 1976; Ward & Cameron, 1978) we show that a massive, silicate-rich disk orbiting the Earth would undergo gravitational instabilities while still hot. Such a disk could lead to the formation of the Moon. The formation timescale could have been as short as *100 years and the Moon could have formed completely molten. (Thompson & Stevenson, 1983: 787)

At the conference on the Origin of the Moon, Stevenson focused on an analysis of possible mechanisms to give ejected material a “second burn” to get it into earth orbit following a giant impact. He considered pressure gradient acceleration of material, i.e., the idea of Cameron and Ward that the vaporized part of the ejecta will be expanding into a vacuum and can entrain some of the solid material, giving an opportunity for both vapor and solid material to enter Earth orbit; the effect of non-central gravity because of the distribution of material near the place and time of the impact; and viscous spreading, whereby a jet of material emanating from the impact site and arcing over the Earth might bring into orbit material that was adjacent to it on its outer edge. Stevenson concluded: Although “second burn” is possible, the amount of material emplaced in orbit tends to be small if the impact velocity is approximately the escape velocity. This difficulty is reduced if very large impacts occur since they cause an earth-encircling superrotating atmosphere (T * 104 K) of MgO-SiO-O plus liquid which can bleed out through the new equatorial plane to form a disk. … The earth is then enveloped in a highly oblate and opaque silicate atmosphere which has a photospheric temperature of approximately 2000 K and a cooling time of approximately 102 yr. In this time, the disk can evolve and emplace at the Roche limit sufficient material to form one or more proto-Moons. It is concluded that orbital emplacement of material is possible and that the process is much more efficient for very large impacts than small (radius less than approximately 102 km) impacts. …” (Stevenson, 1984: 60; his underlining)

Stevenson later wrote: The postimpact Earth may be like a brown dwarf star for about 100 yr. An immense amount of energy may be dumped into the Earth, causing a transient global magma ocean and a transient atmosphere of silicate vapor. The Earth may radiate from an extended photosphere at T approximately 2000 K; such radiation would be detectable by infrared astronomers orbiting nearby stars! … (Stevenson, 1987: 274)

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9.2.14 Melosh and Sonett Both the simulations of Benz, Slattery, and Cameron and those of Kaula and Beachey placed material into orbit around the proto-Earth following the giant impact, but the material going into orbit seemed to come from the impactor. The simulation of Kaula and Beachy did not involve a “core” for either protoplanet. Another paper in the follow-on book did, in a way. The paper was coauthored by H. Jay Melosh and Charles P. “Chuck” Sonett (1924–2011) of the University of Arizona. Their paper dealt with the role of jetted vapor plumes in the giant impact model for the formation of the Moon (Fig. 9.8). Melosh and Sonett noted: The major difficulty with models that attempt to derive the Moon from the Earth by impact processes is that ejected debris that follows Keplerian orbits from the instant of ejection must, according to standard ballistics, either escape the Earth entirely (hyperbolic orbits) or reimpact the Earth after completing only part of an orbit. Elliptical orbits return to their starting points, which by hypothesis, lie on the Earth’s surface. Stevenson (1984) epitomized the situation by calling on a “second burn,” in analogy with the burn-coast-burn strategy of injecting spacecraft into high orbits. Cameron and Ward (1976), however, pointed out that if vapor (as opposed to solid material) was ejected, the vapor would expand under the influence of both pressure gradients and gravity. Condensates from a hydrodynamically expanding vapor cloud would not decouple from the gas and enter Keplerian orbits until some time after the impact, when they are already high above the Earth. There is thus a possibility that some of the ejected material will achieve stable, closed orbits around the Earth. Stevenson (1984) also suggested that the complex and changing gravitational field induced by a massive impactor could aid in inserting the ejected debris into orbit. Neither Cameron and Ward (1976) nor Stevenson (1984) examined the mechanical conditions of a very large impact. Their work is mainly concerned with the evolution of the vapor after ejection. There is a difference of opinion about the origin of this vapor: Cameron and Ward (1976) and Cameron (1985) suppose that it is predominantly projectile material,

Fig. 9.8 Schematic illustration of the collision between the proto-Earth and a Mars-size protoplanet. During the early contact phase of the collision, a plume of vapor jets from the interface of the two impacting bodies. From Fig. 1 in Melosh, H. J., & Sonett, C. P. (1986) “When worlds collide: Jetted vapor plumes and the Moon’s origin.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 623, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

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while Ringwood (1984), arguing from geochemical premises, supposes that it is predominantly Earth mantle material. This paper examines the impact event itself. An approximate calculation provides details on the amount of the highest speed, most highly shocked ejecta and its ejection pattern, speed, angles of launch, etc. as a function of the impact parameter and the ratio of the sizes of the colliding protoplanets. We find that in a certain range of parameters, a great deal of vapor (up to circa 40% of the projectile mass) may be ejected at speeds approaching the impact speed, consisting of an approximately equal mixture of projectile and target mantle material. The results of these computations are in accord with the mega-impact hypothesis. (Melosh & Sonett, 1986: 622–623)

In an appendix to their paper, Melosh and Sonett discussed the various classes of ejecta events that can occur, including vapor expansion, spallation, an ejecta curtain, ricochets, and: Jetted ejecta. The highest speed ejecta is thrown out during the earliest stage of impact cratering, when the projectile first contacts the target …. The oblique convergence of portions of the projectile’s surface with the target’s surface generates a jet of highly shocked, melted (at 6 km/s impact velocity), or vaporized ejecta that travels faster that the projectile’s initial velocity. … (ibid.: 633)

Melosh and Sonett found that: The jetted vapor is derived from both the projectile and target in roughly equal proportions. … Since the width of the jet can be at most r2/2, no jetted material is derived from deeper than one-half the projectile’s radius. Thus if the projectile has a core that is smaller than this it will not enter the jet, although core material may, of course, be ejected later as part of the lower velocity ejecta curtain. … (ibid.: 629)

Figure 9.9 shows the regions that are ejected in the vapor plume. Melosh and Sonett concluded: Of all the types of ejecta thrown out by an impact, only the portion that arises from jetting can make a significant contribution to a cloud of debris orbiting about the proto-Earth. Contrary to expectations based on laboratory experiments with planar targets, the calculations reported here indicate that a large quantity of material, up to one-half the mass of the projectile, may be ejected as an early, high-velocity vapor plume from a moderately oblique collision of two comparable size spheres. The detailed characteristics of such a collision seem to eminently qualify it as the event that created the Earth’s Moon. The narrow range of impact parameter over which a great deal of material is jetted makes the formation of a large satellite relatively improbable, perhaps explaining why only the Earth, of the four terrestrial planets, has a large satellite. … … an impact on Earth would have produced abundant vapor whose subsequent condensation, first into small silicate pebbles in space, then into a large orbiting body, naturally accounts for the “high temperature condensate” geochemical character of the Moon. The incorporation of large amounts of Earth mantle material in the jet accounts for the otherwise great geochemical similarity between the Earth and the Moon, while some differences may be attributed to the admixture of projectile material in the jet. Since jetting is a near-surface phenomenon, the vapor jet will not include core material, at least not if the projectile’s core is smaller than one-half its radius. This may explain the Moon’s relative deficiency in iron and low mean density compared to the other terrestrial planets. (ibid.: 632)

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Fig. 9.9 A cross section of the projectile and target planets showing the regions that are ejected in the vapor plume. From Fig. 5 in Melosh, H. J., & Sonett, C. P. (1986) “When worlds collide: Jetted vapor plumes and the Moon’s origin.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 629, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

Because it showed the incorporation of Earth’s mantle into the jetted ejecta, the simulation of Melosh and Sonnett began to suggest a model that could account for the near equality of oxygen isotope ratios of the Earth and the Moon.

9.2.15 Kipp and Melosh The other simulation incorporated into the follow-on book was by Marlin E. Kipp of Sandia National Laboratories and Jay Melosh of the University of Arizona. Their simulation was two-dimensional, so their computations actually represented the collision of two cylinders, not spheres. Each body was assumed to have an iron core with a radius equal to one half the body’s radius. Their modeling did not include gravity, so they could only follow the initial stage of the orbital evolution of the ejected plume. Kipp and Melosh described their sequence of four snapshots as follows: … The first frame, Fig. 1a (Fig. 9.10), shows the two planets just before contact. The projectile planet is exactly half the size of the target proto-Earth. Figure 1b (Fig. 9.11) documents the beginning of a fast forward-moving jet of hot, highly shocked vapor and a slower, cooler backward-moving jet. These jets evolve further in Fig. 1c (Fig. 9.12). The tip of the fast jet is at a temperature of about 10,000 K, has an average density near 60 kg/ m3, and is traveling in excess of 20 km/sec. This material escapes the Earth entirely and is not of interest for the origin of the Moon. This vapor cloud continues to translate and expand in Fig. 1d (Fig. 9.13). It is composed predominantly of projectile mantle material. The “neck” of the hot plume in Fig. 1d, (Fig. 9.13) however, is more likely to become trapped in Earth orbit. Its velocity of circa 10 km/sec is less than Earth escape velocity

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Fig. 9.10 In a numerical simulation a projectile planet, moving horizontally, strikes the target proto-Earth. Both bodies have cores. This figure and the subsequent three are from Figs. 1a, 1b, 1c, and 1d in Kipp, M. E., & Melosh, H. J. (1986) “Short note: A preliminary numerical study of colliding planets.” In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon, page 644, Houston, TX: Lunar and Planetary Institute, Copyright 1986 by the Lunar and Planetary Institute

Fig. 9.11 6 min later in the simulated collision. From Fig. 1b in Kipp & Melosh (1986)

(11 km/sec), but higher than low Earth orbital velocity (7.7 km/sec). Its density ranges between 300 and 1000 kg/m3 and its temperature is in the vicinity of 6000 K. Although it is difficult to extrapolate this to three dimensions, it seems probable that at least one lunar mass is ejected in this “neck.” It contains roughly equal amounts of projectile and target mantle material. Neither planet’s core is ejected in the jet, although the projectile’s core remains at more than 6000 K after the release wave decompresses it. It will thus eventually

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Fig. 9.12 6 min later in the simulation. From Fig. 1c in Kipp & Melosh (1986)

Fig. 9.13 6 min later in the simulation. From Fig. 1d in Kipp & Melosh (1986)

vaporize and mix with vaporized mantle material from both the projectile and target. This vapor, however, moves too slowly to attain orbit directly. These preliminary numerical computations are in substantial agreement with the conclusions of Melosh and Sonett (1986). They thus support the possibility that the Moon originated from material ejected in a collision between the Earth and a Mars-size protoplanet early in solar system history (Kipp & Melosh, 1986: 643–647).

9.3 Summary and Open Discussion

9.3

269

Summary and Open Discussion

The last afternoon of the conference on the Origin of the Moon was devoted to summaries and open discussion. The review by Stanton Peale (1937–2015) of UCLA gave the dynamical perspective, but also the general consensus of the meeting. Peale and Alan Boss of the Carnegie Institution of Washington enlarged upon the talk for the follow-on book. They concluded: Rotational fission and disintegrative capture appear to be dynamically impossible for viscous protoplanets, while precipitation fission (precipitation of Moon-forming material from a hot, extended primordial atmosphere of volatilized silicates), intact capture, and binary accretion appear to be dynamically implausible. Precipitation fission and binary accretion suffer chiefly from having insufficient angular momentum to form the Moon, while intact capture requires forming the Moon very close to the Earth without encountering any perturbations prior to capture. The only mechanism proposed so far that is apparently not ruled out by dynamical constraints and that also seems the most plausible involves formation of the Moon following a giant impact that ejects portions of the differentiated Earth’s mantle and parts of the impacting body into circumterrestrial orbit. The Moon must have accreted subsequently from this circumterrestrial disk. The giant impact model contains elements of several of the other models and appears to be dynamically consistent with the absence of major satellites for the other terrestrial planets. While the giant impact mechanism for forming the Moon thus emerges as the theory with the least number of obvious flaws, it should be emphasized that the model is relatively new and has not been extensively developed nor thoroughly criticized. … (Boss & Peale, 1986: 59)

9.4

Concluding Remarks

The conclusion of Peale and Boss apparently mirrored the majority opinion of the participants of the conference. In his report on the conference, Richard Kerr of Science magazine wrote: Things were moving so slowly toward understanding the origin of Earth’s moon—a major scientific objective of the Apollo landings 15 years ago—that researchers convened the Conference on the Origin of the Moon in October to see what might be done. The outcome could not have pleased them more. The classic theories of lunar formation—long seen as moribund—were largely dispensed with, and a decade-old suggestion was raised to bandwagon status: what if the moon is the remnant of material blown off the newly formed Earth by the impact of a huge object, a planet in its own right? … In the large-impact hypothesis, which was first formally proposed by William Hartmann and Donald Davis of the Planetary Science Institute in Tucson in 1975, the moon originated in the most catastrophic event in Earth history. There have been many impacts since. Sixty-five million years ago a 10-kilometer-wide asteroid hit Earth and possibly drove microscopic plankton and the dinosaurs to extinction. But that impactor would be miniscule, a mere mote, beside the moon’s suggested progenitor, which might have been as large as Mars—half the size of Earth and one-tenth its mass. George Wetherill of the Carnegie Institution of Washington reassured conference participants that, toward the end of the agglomeration of the solar nebula that formed the inner planets, there would have been at least a few bodies of such size not yet swept up by the present planets, according to his recent calculation. An early, large impact seems quite likely.

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In mathematical simulations of the catastrophe that were prepared for the conference by A. G. W. Cameron of the Harvard-Smithsonian Center for Astrophysics, a particular kind of large impact appears quite capable of supplying the energy needed to separate the lunar material from the Earth. In the most successful simulations, the object falls toward Earth at more than 40,000 km per hour, slides into it at an oblique angle (grazing it actually), vaporizes its own rocky outer mantle and an equal amount of Earth’s, and blows that material into orbit. Lifting enough material into orbit had seemed a problem. If it was only the instantaneous force of the impact that shot the material outward, like a hammerblow splintering a rock, all of the debris would either fall back to Earth or shoot off into space, never to return. None would be put into orbit where the moon could form from it. Cameron and William Ward of the Jet Propulsion Laboratory solved that problem in 1976 when they pointed out that the prolonged acceleration of material by expanding gasses produced by the impact might provide the required “second burn,” the way the burning of a rocket is able to orbit a satellite. In many of Cameron’s simulations, the impact’s second burn effect lofted more than twice the moon’s mass into orbit, … … Of course, the large impact hypothesis would not be so popular if, at least at first glance, it did not help explain some of the major mysteries of the moon’s origin. Such a heavy glancing blow to the proto-Earth could account for the unusually large amount of angular momentum in the Earth-moon system without producing the embarrassing excess of the fission model. The contributions of rocky material from the mantles of Earth and the impactor—their mantles already having lost much of their iron during the formation of metallic cores—would explain the scarcity of iron in the moon. The heating of the disk could have driven off much of its volatile elements, which are depleted in the moon. And what little iron there was in the disk could have scavenged elements particularly compatible with it and sequestered them in the small lunar core, explaining the depletion of such elements in lunar samples. A large impact would also meet a more philosophical need. As pointed out by Hartmann and Davis, a single evolutionary process—one that would presumably be part of the formation of every planet—would seem hard-pressed to explain the variety in satellite systems. Neptune’s major satellite orbits in “reverse,” Uranus’s system revolves in a plane perpendicular to all others, the satellites of Earth and Pluto are large enough to be considered sister planets, Mars has two large boulders as its only moons, and Venus and Mercury have no moons at all. The sensitivity of the outcomes of a planet’s largest impact to everything from the impactor’s size to its direction of approach might account for some of this variety. (Kerr, 1984: 1060–1061)

References Benz, W., Slattery, W. L., & Cameron, A. G. W. (1986). Short note: Snapshots from a three-dimensional modeling of a giant impact. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 617–620). Houston, TX: Lunar and Planetary Institute. Boss, A. P., & Peale, S. J. (1986). Dynamical constraints on the origin of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 59–101). Houston, TX: Lunar and Planetary Institute. Cameron, A. G. W. (1986). The impact theory for origin of the Moon. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 609–616). Houston, TX: Lunar and Planetary Institute.

References

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Cameron, A. G. W. (1985). Formation of the prelunar accretion disk. Icarus 62(2), 319–327. Cameron, A. G. W., & Ward, W. R. (1976). The origin of the Moon [Abstract]. Abstracts of papers submitted to the Seventh Lunar Science Conference (pp. 120–122). Lunar and Planetary Institute: Houston, TX. Chapman, C. R., & Greenberg, R. (1984). A circumterrestrial compositional filter [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 56). Houston, TX: Lunar and Planetary Institute. Greenberg, R., Chapman, C. R., Davis, D. R., Drake, M. J., Hartmann, W. K., Herbert, F. L., Jones, J., & Weidenschilling, S. J. (1984). An integrated dynamical and geochemical approach to lunar origin modelling [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 51). Houston, TX: Lunar and Planetary Institute. Hartmann, W. K. (1984). Lunar origin: Role of giant impacts [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 52). Houston, TX: Lunar and Planetary Institute. Hartmann, W. K. (1986). Moon origin: “The impact-trigger hypothesis”. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 579–608). Houston, TX: Lunar and Planetary Institute. Hartmann, W. K., & Davis, D. R. (1975). Satellite-sized planetesimals and lunar origin. Icarus, 24(4), 504–515. Hartmann, W. K., & Vail, S. M. (1986). Giant impactors: Plausible sizes and populations. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 551–566). Houston, TX: Lunar and Planetary Institute. Herbert, F., & Davis, D. R. (1984). Models of angular momentum input to a circumterrestrial swarm from encounters with heliocentric planetesimals [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 53). Houston, TX: Lunar and Planetary Institute. Herbert, F., Davis, D. R., & Weidenschilling, S. J. (1986). Formation and evolution of a circumterrestrial disk: Constraints on the origin of the Moon in geocentric orbit. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 701–730). Houston, TX: Lunar and Planetary Institute. Kaula, W. M., & Beachey, A. E. (1986). Mechanical models of close approaches and collisions of large protoplanets. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 567–576). Houston, TX: Lunar and Planetary Institute. Kerr, R. A. (1984). Making the Moon from a big splash. Science, 226(4678), 1060–1061. Kipp, M. E., & Melosh, H. J. (1986). Short note: A preliminary numerical study of colliding planets. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 643–647). Houston, TX: Lunar and Planetary Institute. Melosh, H. J., & Sonett, C. P. (1986). When worlds collide: Jetted vapor plumes and the Moon’s origin. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 621–642). Houston, TX: Lunar and Planetary Institute. Orowan, E. (1969). Density of the Moon and nucleation of planets. Nature, 222(5196), 867. Ringwood, A. E. (1984). The origin of the Moon [Abstract]. In Papers Presented to the Conference on the Origin of the Moon (LPI Contribution 540) (p. 46). Houston, TX: Lunar and Planetary Institute. Ruskol, Ye. L. (1977). The origin of the Moon. Soviet-American Conference on Cosmochemistry of the Moon and Planets, Pt., 2, 815–822. Safronov, V. S. (1972). Evolution of the protoplanetary cloud and formation of the Earth and planets (NASA Technical Translation F-677). Jerusalem, Israel: Israel Program for Scientific Translations, Keter Publishing. Stevenson, D. J. (1984). Lunar origin from impact on the Earth: Is it possible? [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 60). Houston, TX: Lunar and Planetary Institute. Stevenson, D. J. (1987). Origin of the Moon—The collision hypothesis. Annual Review of Earth and Planetary Sciences, 15, 271–315.

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Thompson, A. C., & Stevenson, D. J. (1983). Two-phase gravitational instabilities in thin disks with application to the origin of the Moon [Abstract]. Abstracts of papers submitted to the Fourteenth Lunar and Planetary Science Conference (pp. 787–788). Lunar and Planetary Institute: Houston, TX. Ward, W. R., & Cameron, A. G. W. (1978). Disc evolution within the Roche limit. In Lunar and Planetary Science Conference, vol. 9, (pp. 1205–1207). Lunar and Planetary Institute: Houston, TX. Wasson, J. T., & Warren, P. H. (1984). The origin of the Moon [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 57). Houston, TX: Lunar and Planetary Institute. Weidenschilling S. J. (1984a). Capture of planetesimals into a circumterrestrial swarm [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 54). Houston, TX: Lunar and Planetary Institute. Weidenschilling, S. J. (1984b). The lunar angular momentum problem [Abstract]. In Papers presented to the conference on the origin of the Moon (LPI Contribution 540) (p. 55). Houston, TX: Lunar and Planetary Institute. Weidenschilling, S. J., Greenberg, R., Chapman, C. R., Herbert, F., Davis, D. R., Drake, M. J., et al. (1986). Origin of the Moon from a circumterrestrial disk. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 731–762). Houston, TX: Lunar and Planetary Institute. Wetherill, G. W. (1986). Accumulation of the terrestrial planets and implications concerning lunar origin. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 519–550). Houston, TX: Lunar and Planetary Institute.

Chapter 10

Assessments

10.1

Introduction

Questions about the origin of the Moon had been the subject of speculation and research for centuries. The Kona Conference on the Origin of the Moon represents a major turning point in our ideas about how the Moon was formed. In this chapter, we try to assess the factors that contributed to this major milestone in science.

10.2

Looking Back

In sporting events, one often hears, “It’s not over until it’s over.” With scientific research, the corresponding phrase would be “It’s not over.” There is always the chance that further research will change a perception, or at least aspects of a perception, that had previously enjoyed widespread acceptance in a research community. Still, there are major turning points in the advancement of science, and the Kona Conference on the Origin of the Moon caused a revolution in thinking, as Jeffrey Taylor (Fig. 10.1) remembered fourteen years after the meeting: The conference was revolutionary. The traditional ideas for lunar origin were tossed aside by almost all attendees in favor of the giant impact hypothesis. Beyond the giant impact hypothesis being a good idea, several factors came into play to raise it to its pedestal. The three old ideas (fission from the Earth, capture, and binary accretion) had their adherents, but most of us were dissatisfied with all of old hypotheses. Each had serious flaws. Computer methods had improved significantly, so simulations of the giant impact could be done. Our understanding of impact processes was stronger than ever because of experiments and studies of large terrestrial craters. Finally, and perhaps most important, our ideas of how planets accumulated had achieved a new paradigm that depicted planets accumulating from objects that were themselves still accumulating, leading to several large bodies

© Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_10

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Fig. 10.1 Photograph of G. Jeffrey Taylor, courtesy of Jeffrey Taylor

near each other. In this view, a giant impact was almost certain to happen. At the end of the three-day conference, the traditional hypotheses were discarded by most of us - a revolution in our thinking! … … the giant impact idea … provides the context in which we think about planet formation, much the way plate tectonics provides the context in which we try to understand the geology of the Earth. (Taylor, 1998)

Taylor’s assessment that the Kona Conference resulted in a paradigm shift in thinking about the formation of the Moon has been confirmed by talks given at the meeting on the Origin of the Moon that was sponsored by the Royal Society and held in London in 2013. The Royal Society published a focus issue after the meeting, and some of authors in that issue were protagonists in the research debates leading up to the Kona Conference in 1984, including William Hartmann, Jay Melosh, David Stevenson, Jeffrey Taylor, William Ward, and Paul Warren. The giant impact model was the dominant hypothesis for lunar origin under discussion at the meeting, with much of the discussion focused on possible variants of it to address remaining problems. The Epilogue provides a brief synopsis of recent research on the formation of the Moon, based on papers presented at the meeting.

10.3

10.3

Contributing Factors

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Contributing Factors

While Richard Kerr’s report captured the major outcome of the conference on the Origin of the Moon, it did not dwell on an assessment of the contributing factors that produced the outcome. Jeffrey Taylor mentioned some of these factors above, and in his overview article for the follow-on book, John Wood touched on some as well: There was widespread expectation that the Apollo exploration of the Moon would settle the question of its origin; this had been cited frequently as one of the scientific goals of the Apollo program. However, the Moon turned out to be a highly differentiated body, one that preserved a priceless record of the earliest igneous activity and geochemical fractionations in a small planet, but not how the body formed. The only clear reading on lunar origin to come from Apollo data was that Urey’s concept of a cold, primitive Moon had been wrong. As Apollo scientists set to work studying the geological evolution of the Moon, the question of origin receded into the background. Recently the question has been reopened by another conference on “The Origin of the Moon,” this one held at Kona, Hawaii (October 14–16, 1984) and cosponsored by the Division of Planetary Science of the AAS, the Lunar and Planetary Institute (Houston), and NASA. It was the first conference on lunar origin since Apollo; the first, in fact, since the Institute for Space Studies meeting 20 years earlier. The conference revealed that more progress had been made in understanding lunar origin than most of us suspected. Apollo science contributed to the advance, but in relatively indirect ways. Most of the gain in understanding (if it is not illusory) has come from more sophisticated dynamical studies and from a more mature appreciation of the context of the problem, the larger question of the origin of the planetary system. A major share of credit probably goes to digital computers, which have come into widespread use in the years since the Goddard Institute for Space Studies meeting. (Wood, 1986: 18–19)

Without challenging the conclusions of Taylor or Wood, it is possible to identify other contributing factors by taking a longer and broader view.

10.3.1 Early Research As we have seen, there was initially a lengthy battle over the cause of the Moon’s surface features, e.g., whether the craters were of volcanic origin or had been caused by the impact of smaller bodies with the Moon. The outcome of that debate helped to set the context for an early solar system with many planetesimals available to form the planets. Grove Karl Gilbert in the late 19th century built on the work of still earlier researchers in his advocacy of the impact hypothesis. Ralph Baldwin in the mid-20th century was also an important proponent of the impact hypothesis as the origin of the circular features on the surface of the Moon. There had been igneous activity on the Moon, as the existence of the mare illustrated, and dating rocks from the outflow of lunar, Vesuvius-type, volcanoes would be important for establishing the thermal history of the Moon. The search for such volcanoes continued during the planning of missions for the Apollo

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explorations. In 1972, Noel Hinners, the NASA director for Lunar Programs, wrote a lessons-learned report on why the Apollo 16 landing site was chosen, in part because of the incorrect prediction from lunar photo-geology that igneous rocks from young volcanoes would be sampled. Instead of igneous rocks, impact breccias predominated among the returned samples from the Apollo 16 mission (Hinners, 1972).

10.3.2 NASA and the University Research Community There are many reasons why NASA should receive enormous credit for the advancement of knowledge that resulted in a paradigm shift in our thinking about the formation of the Moon. The technical and management achievement of the agency that resulted in the Moon landings and return of the lunar samples made possible the shift in our outlook. From our perspective, of equal importance was the willingness and skill with which NASA engaged the research community in the analysis of the lunar samples and in the support of lunar and planetary science in general. John Wood alluded to the research of George Wetherill, for example, in developing “… a more mature appreciation of the context of the problem, the larger question of the origin of the planetary system.” Wetherill’s research that resulted in his landmark paper titled, “Accumulation of the Terrestrial Planets and Implications Concerning Lunar Origin,” was supported by NASA grants. And the same can be said for almost all of the U.S. lunar and planetary researchers. Through its competitive grant program and the establishment of the Lunar Science Institute, NASA helped to create the lunar and planetary research community. NASA scientists/managers, most notably William L. “Bill” Quaide (1927– 2004) (Fig. 10.2), worked with leaders of the LSI/LPI and the university research community to plan and justify the ongoing lunar and planetary research program. As Bill Quaide’s NASA colleague, Bevan French, remembers: With the end of the Apollo program, Bill moved to NASA Headquarters in 1976 to be program scientist for what was then the Lunar Data Analysis and Synthesis Program. This was a critical time for planetary science in NASA: the Apollo program had ended in 1975, and there were abundant arguments around NASA that the original lunar science programs were no longer necessary and should be terminated. Working under several associate administrators and division directors—far-sighted, active scientist-administrators like Noel Hinners, Ted Flinn, and Geoff Briggs—Bill played a crucial role in preserving, combining, and reorganizing the formerly separate lunar and planetary science programs into a few combined programs that would continue to provide support, excitement, and scientific rewards for planetary exploration during the coming years. This process involved a major reorientation in the way NASA thought about and carried out ground-based planetary science. The goals of the research efforts were broadened. Poor projects were phased out. A uniform and rigorous peer review process was introduced, based on the National Science Foundation’s science panel system. The roles and influence of science advisory committees were strengthened. A special effort was made to ensure that, despite the end of the Apollo program, good lunar science projects continued to be funded in the post-Apollo age. … (French, 2005: 140)

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Fig. 10.2 Photograph of William L. “Bill” Quaide, courtesy of NASA

The point is that scientists-administrators at NASA such as Bill Quaide were important contributing factors for the establishment of the new view of the formation of the Moon, because, in addition to their own research contributions, they created and sustained the NASA grant programs that made possible the necessary research of others. The support of research groups at universities allowed some of them to develop tremendous technical capabilities, for example in mass spectroscopy. Almost all of the U.S. research universities that participated in NASA’s grant program were able to use the funds to provide graduate education to a large number of young people, who represented and continue to represent a benefit to the nation.

10.3.3 Non-U.S. Researchers In constructing this history, I have been impressed with the importance of the foreign participation in the overall process of developing the new paradigm. Prior to the Apollo explorations, George Darwin (United Kingdom), Horst Gerstenkorn

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(Germany), Ernst Öpik (Ireland), Hannes Alfvén (Sweden), and Otto Schmidt (Soviet Union) made significant contributions. When NASA opened the allocation of the Apollo lunar samples to the worldwide community of researchers, on a competitive basis, outstanding laboratories led by researchers such as Heinrich Wänke (Germany), Ted Ringwood (Australia), and Keith Runkorn (United Kingdom) took advantage of the opportunity and make significant contributions to the lunar program. As we have seen, other non-U.S. researchers such as Victor Safronov and Evgenia Ruskol from the Soviet Union made important contributions to lunar and planetary theory, as well.

10.3.4 The Lunar and Planetary Institute The Lunar Science Institute, which was renamed the Lunar and Planetary Institute (LPI) in 1978, was an important contributing factor in the process of developing a new paradigm about the formation of the Moon. President Lyndon Johnson’s call for “new patterns of scientific cooperation” in his 1968 speech implied a strong collaboration between NASA and the international university research community that was made possible by the LPI. The management and staff of the LPI worked closely with NASA managers to achieve the research goals of the lunar and planetary program. For example, the LPI developed and managed the peer review panel referenced above by Bevin French. It was called the Lunar Science Review Panel, later renamed the Lunar and Planetary Review Panel (LPRP). The LPI was asked by NASA to organize, manage and operate the LPRP: … to provide review and evaluation of proposals submitted to the appropriate NASA Program Offices; to submit scientific and fiscal recommendations concerning these proposals to NASA; and to produce and submit to NASA an Annual Report of the Panel which evaluates the present status and future directions of the Programs, both individually and collectively, in terms of programmatic balance and the overall state of trends of lunar and planetary science … (USRA, 1979)

The LPI helped advance lunar science by orchestrating a perhaps unique collaboration between NASA science managers and the university research community. We have noted many times in the foregoing chapters the importance of the annual Lunar and Planetary Science Conferences, which were always co-sponsored by the LPI and NASA, and with the co-sponsorship from time to time of other organizations. These conferences were, and still are, the primary venue for dialogue within the lunar and planetary research community; the site of often vigorous debate, as well as important, informal hallway discussions. In addition to the annual conferences, the LPI worked with NASA managers and key researchers from the university research community to develop, plan, and manage topical conferences and workshops. Many of these conferences and workshops are cited above. These meetings were important venues for the

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development of ideas that led to the new concept about the formation of the Moon. The topical conferences and workshops also were, and still are, an extremely important way that the research community is able to work with NASA to examine the feasibility of possible areas of future research in the lunar and planetary sciences. NASA managers such as Bill Quaide would use the products (reports) of these topical conferences or workshops when he engaged in deliberations with the leadership at NASA Headquarters to determine future NASA-funded grant programs for the university research community. There were, and remain, more good ideas for future research programs within the various disciplines supported by NASA than could (can) be funded. A successful new initiative required a strong case from the research community. In the lunar and planetary sciences, NASA managers worked with the research community, through the conferences and workshops that were developed and managed by the LPI, to construct these cases. In line with the original ideas of James Webb and President Johnson, the LPI was asked by NASA to develop facilities to support visiting scientists selected predominantly from universities in the United States and abroad. Many of these scientists and their students came to the institute to conduct their research, using the collections of research-quality lunar photography, other imagery, and maps maintained at the Institute, as well as a library of lunar sample information, a geophysical data facility, and a superb lunar and planetary science publications library. The LPI developed the ability to compile and edit major documents for publication, starting with the proceedings of the annual Lunar and Planetary Science Conferences, but expanding to include many other volumes, as well. These include the products of study projects, e.g., Basaltic Volcanism on the Terrestrial Planets; handbooks, e.g., Lunar Sourcebook: A User’s Guide to the Moon; and many of the books that grew out of topical conferences, e.g., Multi-Ring Basins, which grew out of the Conference on Multi-Ring Basins: Formation and Evolution; Chondrules and their Origins, which grew out of a topical conference of the same name; and, of course, Origin of the Moon, which grew out of the conference of the same name that was held in Kona, Hawaii, in the fall of 1984. A major part of the success of the LPI can be attributed to the support it received from the lunar and planetary research community. Members of the research community recognized the important role of the institute in exercising a partnership between the community and NASA. At critical points in the history of the institute, leaders from the university research community, notably David Strangway (U. of Toronto), James Head III, (Brown U.), and Robert Pepin (U. of Minnesota), served as directors for the institute. Many leaders in the community agreed to serve on the planning committees for workshops and topical conferences sponsored and organized by the institute. Leaders in the lunar and planetary research community also served on an advisory committee for the institute known as the Lunar and Planetary Science Council. This council was appointed by the Board of Trustees of the Universities Space Research Association (USRA) and normally had nine members who served three-year terms. At least annually, a convener of the Science Council called the group together to review the various aspects of the operations of the institute.

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Table 10.1 Science Council members for the LSI/LPI (1974–1984) John B. Adams (U. of Washington) Arden Albee (Caltech) Albert Bally (Rice U.) Alfred Duba (U. Cal. Livermore) Ronald Greely (Arizona State U.) James Head, III (Brown U.) William Kaula (UCLA) Gary Latham (UT Galveston) Lynn Margulis (Boston U.) Harry McSween (U. of Tennessee) Richard O’Connell (Harvard U.) Stanton Peale (UCSB) Leon T. Silver (Caltech) Sean Solomon (MIT) Joseph Veverka (Cornell U.) John Wood (Harvard)

Donald Anderson (Caltech) Raymond Arvidson (Washington U.) Clark Chapman (Planetary Sci. Inst.) Fraser Fanale (Caltech/JPL) Richard Grieve (Brown U.) Donald Hunten (U. of Arizona) Klaus Keil (U. of New Mexico) Gunter Lugmair (UCSD) Thomas McCord (U. of Hawaii) F. Curtis Michel (Rice U.) James Papike (SUNY Stony Brook) Robert Pepin (U. of Minnesota) Laurence Soderblom (USGS) David Strangway (U. of Toronto) Robert M. Walker (Washington U.)

Members of the Lunar and Planetary Science Council who served in the interval 1974–1984 are shown in the accompanying table, with conveners shown in bold type (Table 10.1).

10.4

Concluding Remarks

There were a number of factors responsible for the turning point in lunar science that culminated at the conference on the Origin of the Moon in Kona, Hawaii, in the fall of 1984: Firstly, there was an international research community of innovative and hard-working scientists who engaged in a vigorous dialogue. Then, there were important technological advances in such areas as mass spectrometry and computer simulations. There were also far-sighted science managers at NASA who made the research possible, and a research institute (LPI) under the management of a university association (USRA) that worked closely with the research community and NASA to advance the science.

References French, B. M. (2005). Memorial: William L. Quaide, 1927–2004. Meteoritics & Planetary Science, 40(1), 139–141. Hinners, N. W. (1972). Apollo 16 site selection. In R. Brett, A. W. England, J. E. Calkins, R. L. Giesecke, D. N. Holman, R. M. Mercer, M. J. Murphy, & S. H. Simpkinson (Eds.), Apollo

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16 preliminary science report (NASA Special Publication 315) (pp. 1-1–1-3). Washington, DC: NASA. Taylor, G. J. (1998, December 31). Origin of the Earth and Moon [Online comment]. Planetary Science Research Discoveries. Retrieved from http://www.psrd.hawaii.edu/Dec98/reminisces. html. Universities Space Research Association. (1979). LPI statement of work: Proposal for the continued operation of the Lunar and Planetary Institute, USRA Board minutes for the meeting held on 2-3 November 1979, Appendix F (pp. 1–6). Columbia, MD: Universities Space Research Association. Wood, J. A. (1986). Moon over Mauna Loa: “A review of hypotheses of formation of Earth’s Moon”. In W. K. Hartmann, R. J. Phillips, & G. J. Taylor (Eds.), Origin of the Moon (pp. 17– 55). Houston, TX: Lunar and Planetary Institute.

Chapter 11

Epilogue

11.1

Introduction

Research on the formation of the Moon has continued since the Kona Conference in 1984. There have not been additional Apollo-like surface explorations of the Moon, but a number of spacecraft missions since the conference have made more data available for analysis. In addition, improved technology has allowed better computer modeling and more precise measurements of the isotopic composition of lunar samples and meteorites. In this chapter we briefly review recent developments, particularly the Discussion Meeting on the Origin of the Moon that was sponsored by the Royal Society in 2013.

11.2

Continuing Research Issues

Ross Taylor (Fig. 11.1) recently observed that: The origin of the Moon, thought to have been settled for a generation by the Giant Impact hypothesis …, is once again in a state of flux with new geochemical data and geophysical models drawing a more direct link to the Earth as the primary source of proto-lunar material, and allowing a wider range of dynamical conditions that includes both larger … and smaller impactors than the canonical Mars-sized impactor … . (Taylor, 2014: 673)

Perhaps the severest challenge to the giant impact hypothesis is the similarity of the isotopic composition of the Moon and the Earth. As noted by David Stevenson and Alex Halliday in the focus issue of the Discussion Meeting on the Origin of the Moon that was sponsored by the Royal Society in 2013: Every non-volatile element [found on the Moon] analysed so far has an almost identical isotopic composition to that in the Earth, even for elements for which meteorites from elsewhere in the Solar System are different. (Stevenson & Halliday, 2014: 2)

© Springer Nature Switzerland AG 2019 W. D. Cummings, Evolving Theories on the Origin of the Moon, Historical & Cultural Astronomy, https://doi.org/10.1007/978-3-030-29119-8_11

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Fig. 11.1 Photograph of S. Ross Taylor, courtesy of the Lunar and Planetary Institute

This is potentially a problem for the giant impact hypothesis because in the computer simulations for the impact that is the current working hypothesis, most of the material that formed the Moon came from the impactor planet, sometimes called Theia. Stevenson and Halliday offered three possible explanations for the similar isotopic compositions of the silicate Earth and the Moon: (1) The innermost Solar System from which these two objects formed was not so heterogeneous after all or Theia accreted at a similar heliocentric distance to the Earth … (2) The atoms of the Moon were derived from the Earth after core formation and the ‘traditional’ simulations are incorrect … . New dynamic models have been proposed in which the angular momentum constraint is violated (i.e., the Earth-Moon system began with over twice its current angular momentum) and the excess is extracted by a resonance involving the Sun … . (3) There was isotopic equilibration between the atoms in the lunar accretion disc and those in the Earth’s magma ocean … (ibid.) In his paper for the focus issue, William Hartmann noted: … at the 2013 Royal Society Discussion Meeting, the canonical view was that, if an impact happened, the impactor had ‘alien’ isotopes and its material would make up a significant fraction of the Moon, so the Moon would necessarily have measurably different isotopic composition from the Earth. It seems fair to comment that the isotopic evidence for lunar

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material coming from relatively Earth-like material is directly empirical, while the idea that the impactor must have had ‘alien’ isotope ratios is based on theoretical modeling. (Hartmann, 2014: 6)

Enstatite chondrite (EC) meteorites have isotopic compositions very similar to that of the Earth, and Hartmann suggested, “… the Moon may have been formed by giant impact between proto-Earth and an object similar to the EC parent body …” (ibid.: 9). Hartmann noted, however, that the similarity of tungsten (W) and silicon (Si) isotopic ratios of the Moon and Earth is “serious” for the giant impact hypothesis, because the isotope ratios of these elements “reflect the history of coremantle evolution, and it seems unlikely for Earth and an Earth-impactor of different size to have similar histories …” (ibid.: 10). In his paper for the focus issue, Jay Melosh described the problem of isotope similarity as more than “serious.” He used the phrase “isotopic crisis.” With the following statements and questions, Melosh suggested five possible solutions to the “crisis:” (a) The Earth and the impactor had identical isotopic compositions … All scenarios of this type have difficulty, however, in explaining the similarity of the W isotopes, that imply a history of accretion and differentiation that is very similar to that of the Earth. (b) Are the existing numerical simulations adequate? (c) Material equilibration between the orbiting disc and the proto-Earth … but the fact that the Earth and disc must exchange a mass comparable to, or greater than, the mass of the disc itself while the disc remains in orbit seems incredible … (d) Fast, ice-rich impactors (e) Is the angular momentum constraint valid? … if there is some mechanism that removes the angular momentum constraint then there may be new ways in which a giant impact can loft a larger fraction of the Earth’s mantle into orbit.” (Melosh, 2014: 5–8)

The angular “momentum constraint” is the assumption that the angular momentum of the Earth-Moon system at the time of the giant impact is the same as the current angular momentum of the system. In her paper for the focus issue, Robin Canup of the Southwest Research Institute reviewed scenarios that involve high angular momentum impacts. In these scenarios, the excess angular momentum, i.e., the original, post-impact, amount of angular momentum in excess of the current angular momentum of the Earth-Moon system, is eventually removed by the solar evection resonance that was suggested by Bill Kaula and Charles Yoder in their talk at the Seventh Lunar Science Conference in 1976 (Kaula & Yoder, 1976). Canup discussed two classes of high-angular momentum impacts. One was her own, which she calls the “half-Earth impact” scenario, by which she means that the impactor and the proto-Earth have comparable masses. … if the impactor’s mass is large compared with that of the target, then a disc and planet with equal compositions can occur even if the disc contains substantial material from the impactor, because, in this case, the impactor substantially affects the composition of both the disc and the final planet. (Canup, 2014: 10)

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Canup also discussed a “fast-spinning Earth impact” scenario, which had been developed in 2012 by Matija Ćuk and Sarah Stewart (Ćuk & Stewart, 2012), both then at Harvard University. In this scenario: … a high-velocity, relatively head-on impact by a sub-Mars-sized impactor into a protoearth that was rapidly spinning before the Moon-forming impact can produce a massive, iron-poor disc derived overwhelmingly from the target’s mantle. In this scenario, the Moon-forming impact itself adds little or no angular momentum to the Earth, but causes the planet, which is already rotating at near its rotational stability limit, to eject a portion of its mantle into a disc. (ibid.: 8)

This sounds like the “impact-trigger” terminology that Hartmann used at the Kona conference in 1984. In his contribution to the Royal Society’s focus issue, Hartmann wrote: At this point, it is interesting to review briefly the three Apollo-era hypotheses. The co-accretion hypothesis did not work because it could not explain a body growing next to Earth without an iron core. The capture hypothesis seems ruled out today by the inconsistencies of Earth-like isotopes arriving in an ‘alien’ object. That leaves the fission process as the only one of the three that seems to have gained from the ‘isotope crisis’. In my paper for the Kona conference volume …, I was concerned that the term ‘giant impact model’ might seem too simplistic, or too restrictive in terms of implied process, and I proposed the name ‘impact-trigger hypothesis’ to allow for a wider range of processes than a one-step giant impact. The phrase is still occasionally encountered …, but never caught on; it may be more attractive now. I visualized a triangular phase-space of impact models, with three extremes being (i) vertical impact, in which crater-like mass ejection is the main effect; (ii) near-tangential prograde impact, perhaps scattering more debris into orbit because of forward momentum and Earth’s rotation; and (iii) a near tangential prograde impact involving ‘enough angular momentum to make Earth rotationally unstable’. Points within that triangular ‘impact phase-space’ represent various combinations of these physical effects. … Such approaches may still be useful in fine-tuning the impact models to fit newly refined isotopic ratios …. (Hartmann, 2014: 10)

Donald Wise, Professor Emeritus of Geology at the University of Massachusetts Amherst, championed the rotational fission model for the formation of the Moon in the Apollo era, as we have noted above. In a recent letter to Physics Today, Wise did not exactly use the phrase “fine tuning the impact models.” He wrote: The new models have the same initial conditions as fission models of the 1960s—namely, a very rapidly spinning, partially segregated, early Earth. Elimination of excess angular momentum by the new mechanism [the solar evection resonance] removes once fatal objections to fission hypotheses. … … … Authors of the new model note that it “blends aspects of the original impact hypothesis … and the fission hypothesis.” (Ćuk & Stewart, 2012: 1051) In reality giant impact is an unnecessary complication. Reinstated fission models could include possibilities for separation into two bodies, multiple moonlets, or equatorial fragmentation and reassembly. All avoid celestial dynamic baggage and impactor contamination while invoking the same rapid rotation and momentum-reduction mechanism as the recent giant impact models. …” (Wise, 2014: 8–9)

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Concluding Remarks

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Concluding Remarks

Few lunar and planetary researchers would go as far as Wise; most seem to regard the “canonical” impact hypothesis, or some variation of it, as likely to be shown to reflect how the Moon was formed. But most lunar and planetary researchers would also agree with Ross Taylor that the research topic of the formation of the Moon is once again in a state of flux—and they are excited about that.

References Canup, R. M. (2014). Lunar-forming impacts: Processes and alternatives. Philosophical Transactions fo the Royal Society A, 372(20130175), 1–14. Ćuk, M., & Stewart, S. T. (2012). Making the Moon from a fast-spinning Earth: A giant impact followed by resonant despinning. Science, 338(6110), 1047–1052. Hartmann, W. K. (2014). The giant impact hypothesis: Past, present (and future?). Philosophical Transactions of the Royal Society A, 372(20130249), 1–11. Kaula, W. M., & Yoder, C. F. (1976). Lunar orbit evolution and tidal heating of the Moon [Abstract]. Abstracts of papers submitted to the Seventh Lunar Science Conference (pp. 440– 442). Lunar and Planetary Institute: Houston, TX. Melosh, H. J. (2014). New approaches to the Moon’s isotopic crisis. Philosophical Transactions of the Royal Society A, 372(20130168), 1–12. Stevenson, D. J., & Halliday, A. N. (2014). The origin of the Moon. Philosophical Transactions of the Royal Society A, 372(20140289), 1–3. Taylor, S. R. (2014). The Moon re-examined. Geochimica et Cosmochimica Acta, 141, 670–676. Wise, D. U. (2014). Alternative models of the Moon’s origin. Physics Today, 67(1), 8–9.

Glossary

Accretion The growth of planets from smaller objects, one impact at a time. Achondrite meteorites Stony meteorites lacking chondrules; most show evidence of having been melted. Aeon An aeon (AE) is a unit of time used in geological research equal to 1 billion years. Aerolites, aerolitic masses Old terms for stony metorites. Albedo The percentage of the incoming radiation that is reflected by a natural surface. Alkali elements Highly reactive metallic elements such as lithium, sodium, and potassium. Alps (lunar) A mountain range on the Moon that forms part of the northeastern rim of Mare Imbrium (Sea of Rains). Anaxagoras (lunar) A lunar crater with a diameter of about 51 km, located near the north pole of the near side of the Moon. Angular momentum (for the Earth-Moon system) The angular momentum of the Earth-Moon system is (approximately) the sum of the spin angular momentum of the Earth and the orbital angular momentum of the Moon. Anorthosite (lunar) A rock found in the lunar highlands that is light in color and in weight. It contains calcium, aluminum, silicon, and oxygen. Anorthositic feldspar A feldspar contains a high percentage of calcium. Aphelion In an orbit of a planetary body about the Sun, the aphelion is the point in the orbit at which the body is most distant from the Sun.

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Aperture The opening, or the diameter of the opening, that allows light into an optical device. Apogee In an orbit of a body about the Earth, the apogee is the point in the orbit when the body is most distant from the Earth. Appenines (lunar) A mountain range on the Moon that forms part of the southeastern border of Mare Imbrium (Sea of Rains). The Apollo 15 landing site was chosen, in part, because it was near the Appenine mountains, which were thought to be composed of the ejecta from Mare Imbrium. Aristarchus (lunar) A crater on the Moon that is about 40 km in diameter and located to the west of Mare Imbrium. Asteroids Bodies that are orbiting the Sun without showing any of the accompanying gas and dust that are characteristic of comets. Many asteroids are found to be orbiting the Sun in a belt between Mars and Jupiter. Atmophile elements Chemical elements that are ordinarily in their gaseous form, e.g., hydrogen, oxygen, and nitrogen. Aubrite An achondrite that is composed primarily of enstatite and which probably originated from a near-Earth asteroid designated as Asteroid 3103 (McSween, 2000: 167). Bar The international unit of pressure (one bar = 106 dynes/cm2). Basalt (lunar) The dark igneous rock that makes up the lava beds in the lunar mare. These fine-grained rocks are composed primarily of plagioclase feldspar and pyroxene minerals. Bolide A general term for a body in the solar system, such as a comet, asteroid, or large meteoroid that strikes a planetary body. Breccia A rock that is composed of rock fragments that are held together in a fine-grained matrix. On the Moon, these rocks are common and were formed primarily as the result of impacts of bolides with the lunar surface. Brecciation Welding together of loose components to form a rock. On the Moon, brecciation was accomplished by the heat generated by an impacting body onto the lunar surface. Calcareous Containing calcium, either the element, calcium, or in the form of calcium carbonate, calcium oxide, or calcium hydroxide. Carpathian mountains (lunar) A mountain range that forms part of the southern border of Mare Imbrium. Cataclasis Deformation of a rock by fracturing and crushing.

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Carbonaceous chondrites The most primitive stony meteorites, in which the abundances of all but the most volatile elements (such as hydrogen and helium) are thought to approximate those of the primordial solar nebula. Caucasus mountains (lunar) A mountain range that forms part of the eastern border of Mare Imbrium. Chassignite meteorites See SNC meteorites. Chondrites, chondritic meteorites Stony meteorites that have not undergone melting. Thought to represent the relative abundances of the non-volatile material of the very early solar system. From George Wetherill: Meteorites may be classified into two broad types: undifferentiated and differentiated. The undifferentiated meteorites are also called chondrites, because most of them contain *1 mm diameter spherical objects, called chondrules, which consist of silicate minerals. They are described as undifferentiated because their relative concentrations of rock-forming nonvolatile elements (such as Si, Mg, Fe, Ca, Al, Na, and trace elements with similar geochemical properties) are very similar to the relative abundance of these elements in the sun and in average solar system material. They are greatly depleted, relative to the sun, in extremely volatile elements, such as hydrogen, helium, and other inert gases, and nitrogen. They vary in their depletion in more moderately volatile elements such as carbon, oxygen, and sulfur. The carbonaceous chondrites, particularly those designated as type I, are the least depleted in these volatile elements. … (Wetherill, 1975: 294) CI chondrites A sub class of carbonaceous chondrite meteorites with an elemental composition that closely matches the abundance of elements in the Sun. Chondrules Small, rounded bodies in meteorites (generally less than 1 mm in diameter); see Chondrites, chondritic meteorites. Clasts A discrete particle or fragment of rock or mineral; commonly included in a larger rock. Condensation sequence The sequence by which solids form from the solar nebula in response to decreasing temperature. Copernicus (lunar) A crater on the Moon with a diameter of about 93 km that is located just south of Mare Imbrium. Core Dense, metal- or sulfide-rich central region of a planet. Earth’s core is composed principally of metallic nickel-iron. Crust Outer, highly differentiated region of a planet. Cumulates Igneous rocks composed chiefly of crystals accumulated by sinking or floating from a magma.

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Curie temperature The temperature in a ferromagnetic material above which the material becomes substantially nonmagnetic. C1 meteorites Carbonaceous meteorites of petrologic type 1 (C1 meteorites) have experienced aqueous alteration of their mineral structure, as opposed to having changed their mineral structure because of high temperature and/or pressure (metamorphism). These meteorites probably came from parent bodies that have reacted with liquid water, or they have reacted with water vapor in space after breaking away from their parent bodies. CI meteorites are of petrologic type 1 (McSween, 2000: 61). Descartes formation A highland region on the Moon, southwest of Mare Tranquillitatis. Apollo 16 landed in the Descartes formation. Differentiation The process by which planetary bodies develop concentric zones that differ in chemical and mineralogical composition. The zones are often separated by density differences. Eccentricity of an orbit A parameter that expresses the extent to which an orbit differs from circular. A circular orbit has an eccentricity of 0. The orbit of the Earth about the Sun has an eccentricity of 0.0167. The average eccentricity of the orbit of the Moon about the Earth is 0.0549. The orbit of Pluto about the Sun has an average eccentricity of 0.2488. Ecliptic plane The plane defined by the Earth’s orbit. Ejecta Material ejected from a crater because of the impact of a bolide or because of a volcanic explosion. Enstatite chondrites A stoney meteorite that is compositionally in the calcium-iron-magnesium pyroxene group of minerals, but chemically consists of only magnesium, silicon, and oxygen, i.e., without calcium or iron. Eolation The erosive action of wind on land surfaces. Eucrites Meteorites consisting mostly of feldspar and pyroxene; thought to have been created by impacts on the large asteroid Vesta, which went through the process of differentiation similar to the terrestrial planets, but very early in the history of the solar system. The planetary scientist, Klaus Keil, has written of Vesta: Because much of the geological history of Vesta (i.e., heating, melting, fractionation, extrusion, and solidification of the basaltic crust) took place in the first 10 m.y. of solar system history …, it is of great interest for understanding the earliest differentiation of solar system bodies at the dawn of the solar system. (Keil, 2002: 573) Feldspar A rock-forming mineral that contains aluminum (Al), silicon (Si), and oxygen (O) plus varying amounts of sodium, calcium, or potassium, i.e., NaAlSi3O8, CaAl2Si2O8, or KAlSi3O8. Feldspar minerals are common in the Earth’s crust and in the lunar highlands.

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Feldspathic An adjective meaning rich in feldspar. Ferroan Iron rich. Fines Lunar material arbitrarily defined as less than 1 cm in diameter; synonymous with “soils”. Focal length The distance from a lens or concave mirror to the point where it focuses an image. Fractional crystallization Formation and separation of mineral phases of varying composition during crystallization of a silicate melt or magma, resulting in a continuous change in composition of the magma. Fractionation The separation of chemical elements from an initially homogeneous state into different phases or systems. Fra Mauro formation Located just below the lunar equator, due south of Mare Imbrium, the material of the formation is thought to be part of the ejecta of the Imbrium basin. The landing site of Apollo 14 was on the Fra Mauro formation. Fra Mauro was a 15th century Venetian monk who was a famous mapmaker. Gamma (c) See Magnetic units. Gardening The process of turning over the lunar soil or regolith by meteorite bombardment; it is accompanied by fragmentation and melting of soil constituents. Gauss (G) See Magnetic units. Gaussian distribution A commonly occurring probability distribution in the physical sciences, describing the probability of occurrence about a mean value that declines exponentially with the square of the “distance” from the mean. The distribution function is symmetrical about the mean and has a “bell-shaped” appearance. Graben A graben, from the German word for trench, is a trench-like depression in the surface of a planet caused by stress-induced faults. Grabens are often created by the stresses created by the impact of bolides on the surface of a planet. They can be linear, radiating from the crater, or curved, encircling the crater. Granite An igneous rock composed chiefly of quartz and alkali feldspar; though common on Earth, granite is rare on the Moon. Half life The time interval during which a number of atoms of a radioactive nuclide decay to one half that number. Heat flow The rate of heat energy leaving a planet’s surface per unit area. Impact parameter A term referring to the geometry of a collision between two spheres unequal in size; it is the offset between the spheres’ centers projected perpendicular to the line of approach. Thus, for a head-on impact this parameter

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is zero; for a grazing impact it is equal to the sum of the radii of the impactor and target. Inclination The angle between the orbital plane of a planetary body and the plane of the ecliptic. Incompatible elements Those chemical elements “… possessing ionic radii and/or ionic charges that do not permit their ions to substitute readily into the principal crystalline phases of the mantle. As a result, they are strongly partitioned into magmas during partial melting processes. Examples are Cs, Rb, K, Ba, Pb, La, Ce, U, Th, Ta, Nb, and P” (Ringwood, 1979: 5; footnote). Inviscid A term used to describe an ideal fluid with zero viscosity. Iron meteorites Meteorites that consist almost entirely of nickel-iron metal alloys. Isostasy A condition of a planet’s crust when there is a balance of vertical forces achieved by a rising or sinking of blocks of mass. Isotopes Atoms of a specific element which differ in number of neutrons in the nucleus; this results in different atomic weight, and very slightly differing chemical properties (for example, 235U and 238U). Jetting The process that occurs during the earliest stages of an impact in which highly shocked ejects is thrown out at the highest speeds. Kbar 103 bars; see Bar. Kepler (lunar) A crater on the Moon of about 32 km in diameter, near the lunar equator and west of Copernicus. KREEP An acronym composed of the letters K (the atomic symbol for potassium), REE (Rare Earth Elements) and P (phosphorus). KREEP elements are “incompatible” with many rock-forming mineral structures, and in a melted rock fluid they tend to form a remnant. On the Moon, KREEP rocks that had been excavated by large impacts were included in the samples returned by the Apollo astronauts. Lithosphere The upper layers of a terrestrial (rocky) planet that do not accommodate easily to stress, in contrast to lower layers that are hotter and give under stress, i.e., undergo plastic deformation. Lithophile elements Rock-forming chemical elements with a geochemical affinity for silicate or oxide phases. Because they preferentially bond with oxygen, forming oxides that are typically lighter than other materials, lithophiles “float to the top” in a melt of rock materials. Refractory lithophiles are those lithophiles that maintain their structure at high temperatures. Examples are calcium, aluminum, titanium and the rare earth elements. These elements were found to be in relatively high abundance on the Moon’s surface.

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295

Lunation A length of time corresponding to a complete cycle of lunar phases, e.g., from new moon to full moon and back to new moon. Mafic minerals and rocks Minerals and rocks rich in magnesium and iron. Olivine and pyroxene are examples of mafic minerals. Basalt is an example of a mafic rock. Magma Molten rock material. Magma ocean A hypothetical layer of magma thought to have surrounded the Moon when it formed and from which the original, feldspar-rich lunar crust and at least some mare basalt source regions formed. Magmatic differentiation The production of rocks of differing chemical composition during cooling and crystallization of magma by processes such as removal of early formed mineral phases. Magnesium number The molar ratio Mg/(Mg + Fe); abbreviated as “mg#”. Magnesium ratio The molar ratio MgO/(MgO + FeO); abbreviated “mg”. Magnetic induction (B) See Magnetic units. Magnetic units A researcher typically thinks (visually) of a unit of magnetic flux as a line. The denser the lines, i.e., the more lines crossing perpendicularly through a square centimeter, the stronger the “magnetic induction” (B). A gauss (G) is a unit of B that corresponds to one line per square centimeter. A gamma (c) is 10−5 gauss. A tesla is a unit of B that equals 104 gauss. A nanotesla (nT) is 10−9 tesla and thus 10−5 gauss, or the same as a gamma (c). The value of B for the Earth at its equator is about 31,000 nT or, equivalently, about 0.31 G.In a laboratory setting, one can “induce” a value of B inside a long cylinder by wrapping the cylinder with wires and running an electric current through the wires. The magnetizing field (H) is measured in ampere-turns per unit of length along the cylinder. The more amps of current in the wires and/or the greater the linear density of the wires, the stronger is H and the stronger is the induced field B inside the cylinder. An oersted (Oe) is a unit of H. In a vacuum, 1 Oe of H produces 1 G of B. Magnetizing field (H) See Magnetic units. Mantle A layer between the core and the crust of a planetary body. Mare Large dark circular regions on the Moon that are now understood to be lavafilled impact basins, hundreds of kilometers across. Astronomers before the modern era thought that these regions were seas (Latin singular—mare; plural— maria). Mare Crisium The lava in a basin on the eastern section of the Moon with a diameter of about 555 km. The name means “Sea of Crises”.

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Mare Humorum The lava that fills a basin in the southwest section of the Moon’s near side, above and to the west of the crater Tycho. The basin is about 389 km across. The name means “Sea of Moisture”. Mare Imbrium The lava that fills a basin with a diameter of 1,146 km near the north-central part of the near side of the Moon. The name means “Sea of Rains” or “Sea of Showers”. Mare Nectaris The lava that fills a basin in the southern hemisphere of the Moon, below and to the east of Mare Tranquilitatis. It is approximately 333 km across. The name means “Sea of Nectar”. Mare Orientale The lava that partially fills a multi-ringed basin on the far western edge of the Moon as visible from the Earth. The basin is about 900 km across. The name means “Eastern Sea”. Mare Serenitatis The lava that fills a basin with a diameter of about 674 km. It is located to the east of Mare Imbrium. The name means “Sea of Serenity”. Mascons Regions on the Moon of excess mass concentrations per unit area identified by positive gravitational anomalies and associated with mare-filled multi-ring basins. Mesosiderites A class of stony-iron meteorites consisting of roughly equal proportions of metal and silicates. Per McSween, “The silicate fraction of mesosiderites are … basically igneous rocks from the crust of an achondrite parent body.” (McSween, 2000: 210–211). Mesosiderites are thought to have been created “… by a collision between two already differentiated asteroids, allowing the still-liquid core of one body to mix with the solidified crust of the other.” (ibid.: 212). Metamorphism Recrystallization in a rock resulting from high temperature and/or pressure. Meteorites From George Wetherill: Meteorites are extraterrestrial objects moving in heliocentric orbits that intersect the orbit of the earth and possess sufficient mechanical strength to survive passage through the atmosphere. Atmospheric alteration is confined to a thin surface layer and the interior of the meteorite is in the same physical and chemical state as it was in interplanetary space. The source of meteorites are not clearly understood, but meteorites are certainly derived from minor bodies in our solar system, probably asteroids, comets, or both. The unknown source is referred to as “the parent body” without intention to limit the number or variety of these parent bodies … … … the formation of most meteorite parent bodies took place near the beginning of the solar era … (Wetherill, 1975: 294–295) Meteoroids Bodies of up to 100 meters across that are orbiting the Sun in the vicinity of the Earth.

Glossary

297

Model age The age of a rock sample determined from radioactive decay by assuming an initial isotopic composition of the daughter product. Moment of inertia A quantity that gives a measure of the density distribution within a planet, specifically, the tendency for an increase of density with depth. It is derived from gravity and dynamical considerations. Momentum The mass times the velocity of a body. Nakhlite meteorites (See SNC meteorites). Nanotesla (nT) See Magnetic units. Natural remanent magnetization (NRM) The portion of the magnetization of a rock that is permanent and usually acquired by the cooling of ferromagnetic minerals through the Curie temperature. Norite A class of rock that occurs in igneous intrusions, i.e., underground flows of lava. On the Moon, some norite rocks were found to contain KREEP. Obliquity The tilt of a planet’s spin axis from the perpendicular to the plane of its orbit around the Sun. Oceanous Procellarum “Ocean of Storms,” the largest of the lunar mare, lies to the southwest of Mare Imbrium. Its north-south axis measures about 2,500 km. The landing site for Apollo 12 was in Oceanous Procellarum. Oersted (Oe) See Magnetic units. Olivine A rock-forming mineral in which silicon and oxygen are paired with either magnesium (Mg) or iron (Fe). Magnesium and iron can substitute for each other in the olivine mineral, which is a mixture of Mg2SiO4 and Fe2SiO4. Since olivine contains magnesium and/or iron, rather than aluminum, it is generally heavier than feldspar. Ordinary chondrites The most common class of stony meteorites. Pallasites A class of stony-iron meteorites consisting of metal and isolated crystals of olivine. From McSween: “Pallasites are thought to have formed at the outer fringes of cores [of asteroids] where molten metal was in contact with the overlying mantle silicates.” (McSween, 2000: 219). Partial melting The process in which rocks melt over a range of temperatures, producing liquids with compositions different from the original unmelted rock and different from the residual unmelted crystals. Partition coefficient The ratio of the concentration of a trace element in one phase to its concentration (by weight) in a second phase with which it is in equilibrium. Perihelion In an orbit of a planetary body about the Sun, the perihelion is the point in the orbit when the body is closest to the Sun.

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Perigee In an orbit of a body about the Earth, the perigee is the point in the orbit when the body is closest to the Earth. Period The time for a complete cycle of a repeating phenomenon. In planetary science, it is the time elapsed during one complete orbit of a planetary body or its complete revolution on its axis. Petrology The study of the formation, distribution, characteristics, etc., of rocks. Plagioclase feldspar Feldspars are rock-forming minerals that contain aluminum (Al), silicon (Si), and oxygen (O) paired with either sodium, calcium, or potassium. Sodium and calcium can interchange with each other in a feldspar crystal structure, and a mineral with a mixture of sodium aluminosilicate (NaAlSi3O8) and calcium aluminosilicate (CaAl2Si2O8) is called plagioclase. Planetesimals Bodies in the solar system up to hundreds of kilometers in diameter that formed during the planet-formation process, with some accreting to form the planets. Plato (lunar) A lunar crater with a diameter of about 100 km and located on the northeastern edge of Mare Imbrium. Plutonic rocks Igneous rocks formed at great depth in a planetary body, as opposed to volcanic rocks that form when magmas erupt on the surface. Presession of a spinning planetary body The change in the orientation of the spin axis of the planetary body caused by a torque acting upon it. In the case of the Earth-Moon system, the tidal bulges on the Earth exert a net torque on the Moon. Precession of planetary orbits The slow change in the orientation of the elliptical orbit of a planetary body caused by the gravitational forces of the planets in the solar system on the body. P-wave velocity The velocity if seismic body waves associated with particle motion (alternating compression and expansion) in the direction of wave propagation. Seismic p-waves are not attenuated as they propagate through liquids, whereas shear waves (s-waves) are. Pyroclastic material Material that is part of the eruption of a volcano. The hot, fluidized rocks of the pyroclastic flow can quickly cool during the eruption and form glassy material. Pyroxene A rock-forming mineral in which the silicate elements can be paired with a number of different elements, including sodium (Na), calcium (Ca), iron (Fe) and magnesium (Mg), which substitute for one another in the mineral structure. Pyroxene minerals are generally heavier than feldspars and are found in the Earth’s upper mantle, below the crust. Radiogenic A term referring to an isotope having been formed from a radioactive parent; for example, 206Pb is formed from the decay of 238U, whereas 204Pb is nonradiogenic.

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Radioisotope dating To illustrate the principles of radioisotope dating, consider the following example. 40K is an isotope of potassium that decays into the gas argon (40Ar) with a half life of 1.26 billion years. Naturally occurring argon gas would have escaped from the molten igneous material (lava). After crystallization, the argon derived from the decay of 40K would have been trapped in the rock. The total amount of 40K and 40Ar in a rock sample must therefore be equal to the original amount of 40K. The ratio of 40Ar to the total is the fraction of 40K that has decayed. If that ratio is 0.5, for example, then the age of the rock must be 1.26 billion years, since that is the half life of 40K. In general, the age of the rock in billions of years would be 2.52 times the measured ratio. Rare earth elements Per McSween, rare earth elements refers to elements with atomic numbers 57 (lanthanum) to 71 (lutetium). Because of their size and ionic charge, almost none of these elements fit easily in most minerals during crystallization. An exception is europium (atomic number 63), which can substitute for calcium in plagioclase (McSween, 2000: 124). Reducing environment/atmosphere An environment or atmosphere that has little or no free oxygen. It is the opposite of an oxidizing environment or atmosphere. Refracting telescope A telescope that uses a converging lens, as opposed to a reflecting mirror, to focus the image of a distant object. Refractory elements Chemical elements that remain in the solid state (rather than a liquid or vapor state) at relatively high temperatures. Examples are titanium, tungsten, and molybdenum. Regolith (lunar) The top surface of the Moon that has been broken into small pieces by the bombardment of bolides. The depth of the lunar regolith depends on how long the bolides have been churning the upper layers of a given region of the Moon. For the lunar highlands, the depth is up to 15 m. Residual liquid In planetary science, the material remaining after most of a magma has crystallized. Roche limit The minimum distance from the center of one planet to another object that is held together by self-gravity alone at which the second body will remain intact. Closer than the Roche limit, the second body will be torn apart owing to the different gravitational forces on different parts of the smaller body. Schickard Crater A lava-flooded crater, 227 km in diameter, in the southwest part of the Moon, as viewed from the Earth. Secular resonance A condition such that the orientation of the elliptical orbit of a (small) planetary body precesses at the same rate as the precession of one of the giant planets. Selenography, selenology The study of the Moon’s surface features (selenography) and, more broadly, the study of the Moon (selenology), In Greek mythology, Selene was the goddess of the Moon.

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Semimajor axis In planetary science, half of the long axis of the elliptical orbit of a planetary body. Shergottite meteorites (See SNC meteorites). Shock lithification The formation of rocks from loose aggregations of soil or small rocks by the heating and wielding effects of the passage of shock waves. Shock waves caused by the impact of bodies on the Moon can result in shock lithification. Siderophile elements Chemical elements with a geochemical affinity for metallic iron. Examples are iron itself and nickel, cobalt and tungsten. Silicate A phase (mineral or liquid) whose crystal structure is controlled by siliconoxygen bonds. Sinus Iridum “Bay of Rainbows”—A lava-filled basin on the northwest edge of Mare Imbrium. Sinus Iridum has a diameter of about 236 km. SNC meteorites SNC stands for shergottites, nakhlites, and chassignites, which are three classes of meteorites that have young ages. From a paper by D. Bogard and P. Johnson: … The igneous formation ages of all SNC meteorites are about 1300 million years, which is far younger than the ages of any other known extraterrestrial material (including lunar rocks), and implies geologically recent igneous activity … . These properties have led several investigators to conclude that these meteorites must have formed on a geologically complex and relatively large parent body, most probably the planet Mars … . (Bogard & Johnson, 1983: 651) Solar nebula The primitive disk-shaped cloud of dust and gas from which all bodies in the solar system originated. Solar wind The stream of charged particles (mainly ionized hydrogen) moving outward from the Sun with velocities typically in the range 300–500 km/sec. Spallation The separation of fragments from a planetary surface as the result of an impact. Stochastic processes Processes that involve random events. Stratigraphy The study of rock strata to determine, for example, relative ages and different compositions of successive layers. S-wave velocity Seismic body waves with a shearing motion perpendicular to the direction of wave propagation. Seismic s-waves are attenuated as they propagate through liquids, whereas p-waves are not. Taurus-Littrow Valley A valley on the near side of the Moon that was the landing site for Apollo 17. Tectonic activity Mountain building, earthquakes, and volcanoes, now known to be related, primarily, to the movements of large plates of land mass that make up

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the crust of a planet. Before “plate tectonics” was discovered in the middle part of the 20th century, however, “tectonic activity” usually referred to volcanism. Thermal remanent magnetization (TRM) Magnetization acquired by a rock sample when a magnetic field is imposed upon it as it cools through its Curie temperature. Thrust faults A fault in a planetary surface caused by compressive stresses. Topography The configuration of the surface of a planetary body, particularly the study of the structural relationships of its features. Trace elements Chemical elements found in very low (trace) amounts; generally less than 1000 parts per million. T-Tauri phase of stellar evolution During the early life of a star that has less than about 3 solar masses, while it is contracting under the influence of its own gravity, it goes through a so-called T Tauri phase. The name comes from a star that is currently going through this phase. Many of these stars still have accretion disks, and many have large outward flows of particles, called stellar winds. These winds are typically many times more powerful than the solar wind that currently flows from our sun. Tycho (lunar) A crater on the Moon with a diameter of about 86 km in the southern lunar highlands. It is notable for the prominent rays that emanate from the crater. Viscosity A measure of a fluid’s resistance to motion. Volatile elements Chemical elements that vaporize at relatively low temperatures; examples are potassium, sodium, and lead. Wrinkle ridges (lunar) Raised curvilinear features on lunar maria, caused by compressional stresses within the basalt-filled basin.

References Many of the terms in this glossary are taken from publications of the Lunar and Planetary Institute, specifically, Origin of the Moon and Planetary Science: A Lunar Perspective. Where other sources were used, they are cited with the glossary term. Bogard, D. D., & Johnson, P. (1983). Martian gases in an Antarctic meteorite? Science, 221(4611), 651–654. Keil, K., (2002). Geological history of asteroid 4 Vesta: The ‘smallest terrestrial planet.’ Asteroids, III, 573

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McSween, H. Y. (2000). Meteorites and their parent planets (2nd ed.). Cambridge: Cambridge University Press. Ringwood, A. E. (1979). Origin of the Earth and Moon. New York, NY: Springer. Wetherill, G. W. (1975). Radiometric chronology of the early solar system. Annual Review of Nuclear Science, 25(1), 283–328.

Index

A Abe, Yutaka, 177, 178 Accretion, 60, 63, 64, 66, 68, 83, 84, 86, 95, 98, 106, 114, 116, 119, 122, 124–126, 134, 136, 137, 144, 146–148, 150–154, 158, 159, 174, 176–178, 180, 181, 197, 199–201, 206, 207, 212, 222, 225, 230, 231, 233, 236, 237, 244, 245, 249, 251–253, 255, 258, 259, 269, 273, 284, 301 Adams, John B., 136, 280 Age of the Moon, 51, 54, 72, 104, 175, 205 Albee, Arden, 280 Alfvén, Hannes Olaf Gösta, 70, 71, 278 Althans, Carl Ludwig, 7–9, 19, 23, 45 Althans, Ernst Friedrich, 7, 23, 27 Alvarez, Luis W., 26, 144, 145 Anders, Edward, 116, 132 Anderson, Donald, 280 Angular momentum, 52, 55, 59, 64, 71, 72, 85, 86, 89, 120, 122, 126, 153, 159, 160, 166, 167, 218, 220, 231–233, 244, 246–248, 251, 252, 254, 257, 259, 269, 270, 284–286, 289 Anorthosites, 82, 83, 89, 91–93, 98, 106, 116, 151, 152, 170–173, 179, 289 Apollo astronauts Aldrin, Edwin, 79 Armstrong, Neil, 79 Bean, Alan, 91 Cernan, Eugene, 103 Collins Michael, 79 Conrad, Charles “Pete”, 91 Duke, Charles, 101

Evans, Roland, 103 Gordon, Richard, 91 Haise, Fred, 94 Irwin, James, 95 Lovell, James, 94 Mattingly, Ken, 101 Mitchell, Edgar, 94 Roosa, Stuart, 94 Schmitt, Harrison, 103, 116 Scott, David, 95 Shepard, Alan, 94 Swigert, John “Jack”, 94 Worden, Alfred”, 95, 104 Young, John, 101 Apollo missions Apollo 11, 79, 80, 83, 84, 86, 88–92, 94–96, 101, 102, 116, 126, 133, 169, 182, 189 Apollo 12, 91–97, 101, 102, 106, 194, 297 Apollo 13, 94 Apollo 14, 94–96, 99, 101, 102, 293 Apollo 15, 95, 96, 98, 99, 101–103, 173, 182, 183, 198 Apollo 16, 101–103, 116, 143, 148, 173, 229, 238, 276 Apollo 17, 102–104, 111, 117, 173, 198 Arvidson, Raymond, 280 Ashwal, Lewis, 143, 152 Asterios, 13, 19 Asteroids, 23, 26, 36, 43, 44, 46, 62, 63, 69, 106, 143, 145, 147, 158, 160, 199, 207, 222, 224, 235, 253, 254, 262, 269, 290, 292, 296 Awramik, Stanley, 207

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304 B Baldwin, Ralph Belknap, 19, 39–43, 46, 47, 60, 72, 275 Bally, Albert, 280 Bandermann, L. W., 83 Banerjee, Subir Kumar, 187–189 Barnes, Stephen, 198 Barrell, Joseph, 136, 144 Barringer, Daniel Moreau, 24, 25 Basalts, 82, 84, 89–91, 95, 99, 101, 113, 128, 135, 136, 151, 176, 179, 201, 211–213, 226, 228–230, 235, 295 Beachey, A. E., 259, 261, 262, 264 Becquerel, Henri, 16 Beer, Wilhelm, 7, 13, 19 Benz, Willy, 259, 260, 264 Biesbroek, George Van, 39 Bigeleisen, Paul E., 230 Binder, Alan, 137, 138, 178–180, 231–234 Bolide, 4–6, 19, 26, 29–32, 45, 103, 145, 229, 290, 292, 299 Boss, Alan P., 125, 220, 269 Boynton, W. V., 149, 187 Brecciation, 113, 115, 116, 170 Brett, P. R., 115 Briggs, Geoff, 276 Burke, Kevin C. A., 152 Burns, Joseph A., 169, 174, 175 Burton, J. D., 197 Byerly, G. R., 224 C Caffee, Marc W., 203, 205 Calio, Anthony, 103 Caloris crater on Mercury, 154 Cambrian Epoch, 52 Cameron, Alastair Graham Walter, 54, 81, 85, 86, 101, 120, 122–125, 129, 152, 167, 168, 174, 231, 243, 245, 246, 257–260, 263, 264, 270 Campbell, William Wallace, 38 Canup, Robin, 285 Carpathian Mountains of the Moon, 16 Carpenter, James, 11–13, 15, 44, 45 Cassidy, William A. “Bill”, 141, 142, 225, 226 Cataclasis, 115 Ceres, 160 Chamberlain, Joseph W., 96, 97, 105, 111, 116 Chandrasekhar, Subrahmanyan, 218 Chapman, Clark R., 243, 253, 254, 280 Chicxulub crater, 26, 145 Chondrules, 146, 279, 291 Cisowski, Stanley M., 185–187

Index Clasts, 116, 170, 171 Clayton, Robert N., 121, 122, 226, 235 Coleman, Paul J. Jr., 190 Compositional sorting, 199 Conway, Bruce A., 217, 225 Cooperman, Stephen, 153 Copernicus lunar crater, 10, 29, 91 Cox, Larry P., 221, 222 Craterlets, 38 Cretacious, 26 Ćuk, Matija, 286 D Daly, Reginald Aldworth, 59, 60, 86, 101, 120, 122 Dana, James Dwight, 9, 19 Darney, Maurice, 38 Darwin, George Howard, 55, 56, 58, 59, 218, 233, 277 Davis, Donald R., 120, 124, 129, 167, 243, 245, 251–254, 257, 269, 270 Davis, William Morris, 22, 24 Delano, John W., 132, 196, 197, 229 Delmotte, Gabriel, 37, 38 Dickinson, Tamara, 197 Dietz, Robert Sinclair, 41 Differentiation, 82, 88, 90, 93, 95, 102, 106, 113, 124, 128–130, 135, 136, 143, 147, 150, 158, 169, 176, 255, 256, 285, 292 Drake, Michael J., 168–170, 194, 201, 206, 227, 243 Dreibus, Gerlind, 234–239 Duba, Alfred, 280 Durisen, Richard H., 218–220 E Eu anomaly, 91, 151 F Fanale, Fraser, 280 Fauth, Philipp Johann Heinrich, 28 Fisher, Osmond, 58, 154 Flinn, Ted, 276 Foss, T. H., 83 Fractional crystallization, 83 Fractionation, 66, 68, 85, 95, 111, 118, 121, 135–137, 147, 151, 170, 174, 179, 196, 200, 206, 207, 216, 226, 230, 237, 246, 252–254, 275, 292 French, Bevan, 276 Frequency-dependent tidal theory, 213 Fuller, Michael, 185–188

Index G Gardening, 104, 180 Gerstenkorn, Horst, 71, 72, 81, 84, 100, 213, 277 Gilbert, Grove Karl, 1, 2, 6, 9, 19–27, 29, 33, 36–39, 41, 43, 44, 275 Gingold, Robert A., 218–220 Global magma ocean, 91, 150–152, 169, 170, 173, 174, 200, 263 Goddard Space Flight Center, 86, 233 Goettel, Kenneth, 199 Goldreich, Peter, 233, 250 Goodrich, Cyrena Anne, 198 Graves, William Hoyte, 25 Greely, Ronald, 280 Greenberg, Richard, 166, 243, 244, 253 Greenstein, Jesse, 39 Grieve, Richard, 153, 280 Gruithuisen, Franz von Paula, 4–6, 16, 19, 29, 33, 36, 41 Günther, Adam Wilhelm Siegmund, 29, 30, 32 Gurevich, L. E., 62 H Hadley Rille, 95, 96, 99, 100 Halliday, Alex, 283, 284 Hartmann, William K., 54, 81, 120, 122, 124, 148, 169, 181, 188, 194, 196, 214, 219, 222, 235, 238, 243–245, 247, 250, 253, 260–262, 264, 266, 267, 269, 270, 274 Hartung, Jack B., 210–213, 217, 225 Head, James W., III, 117, 279, 280 Heat flow, 96, 99, 129, 198, 226 Hellas crater on Mars, 154, 155 Heppenheimer, Thomas A., 215, 216 Herbert, Floyd, 243, 251–254 Herschel, William, 2, 3, 5 Hinners, Noel, 101, 102, 276 Hohenberg, Charles M., 203, 205 Hood, Lon L., 169, 174, 190 Hooke, Robert, 2, 10 Hörz, Friedrich, 83 Hoyaux, Max, 37, 38 Hunten, Donald, 280 I Isotopes, 121, 122, 177, 203, 205, 206, 254, 284–286 Isotopes of Oxygen, 121, 122, 148, 158, 177, 213, 217, 235, 246, 254, 255, 259, 266

305 J Jacobi, Carl, 56 Jagoutz, E., 227 Jakeš, Petr, 117 Jastrow, Robert, 83, 115 Jeans, James, 58 Jeffreys, Harold, 54, 58, 208, 218 Johnson, President Lyndon, 77, 78, 113 Johnson Space Center, 77, 133, 142, 151, 173, 178, 198, 231 Jones, John H., 201, 243 Joule, James Prescott, 23 K Kaula, William Mason “Bill”, 67, 126, 129, 130, 136, 137, 144, 153, 259, 261, 262, 264, 285 Keenan, Philip, 39 Keil, Klaus, 173, 280, 292 Kellogg, Louise, 176, 178 Kepler lunar crater, 29 Kerr, Richard, 269, 270, 275 Kesson, Susan E., 128, 230 Kiesl, W., 202 Kilauea-type volcanoes, 9, 26 Kipp, Marlin E., 266–268 Kluger, F., 202 Koeberl, Christian, 202, 203 Kovach, Robert, 116 KREEP, 94, 128, 134, 170, 207, 294 Kreutzberger, Melanie E., 201, 203 Kring, David, 26 KT event, 145 Kuiper, Gerard Peter, 39, 43–47, 65, 69 Kulp, J. Laurence, 52 L Lambeck, Kurt, 208, 209 Langseth, Marcus, 98, 99, 198 Larimer, John W., 158, 159 Latham, Gary V., 96, 97, 280 Lava, 9, 13, 15, 16, 20, 34, 41, 43, 45, 46, 54, 77, 81, 82, 88–96, 99–102, 106, 113, 202, 223, 290, 295–297, 299, 300 Lebedinsky, A. I., 62 Lee, Oliver, 39 Lewis, John S., 221 Lin, Douglas, 232 Lithophile elements, 87, 88, 231, 294 Lonhghi, J., 147

306 Lugmair, Gunter, 280 Luna 16, 99 Luna 20, 170 Lunar and Planetary Institute (LPI), 26, 78–81, 90, 91, 95–97, 102, 103, 111, 112, 117, 118, 124, 131–137, 141–148, 150–152, 154, 157, 161, 167, 169, 171, 181, 188, 194, 196, 202, 214, 219, 235, 236, 238, 247, 250, 253, 260–262, 264, 266, 267, 273, 275, 278–280, 284 Lunar chemistry aluminum (Al), 77, 82 aluminum oxide (Al2O3), 229 argon (Ar), 154 barium (Ba), 179 bismuth (Bi), 82 bromine (Br), 82 cadmium (Cd), 82 calcium (Ca), 77, 82, 236, 290 carbon (C), 98, 121, 122, 128, 178, 194, 228, 248, 291, 292 cesium (Cs), 82, 169, 201 chlorine (Cl), 82, 87 chromium (Cr), 236 cobalt (Co), 195, 196, 228, 237 copper (Cu), 229 europium (Eu), 91 ferrous oxide (FeO), 199, 200 gallium (Ga), 229 germanium (Ge), 82, 197 gold (Au), 229 hafnium (Hf), 88 iodine (I), 87 iridium (Ir), 24, 111, 228, 229 iron (Fe), 24, 46, 58, 67, 68, 82, 84, 85, 88, 113, 119, 121, 128, 129, 169, 174, 182, 185, 190, 216, 227, 243, 246, 265, 270 lanthanum (La), 127, 128, 148, 200, 294, 299 lead (Pb), 82, 125, 206, 294 magnesium (Mg), 85, 134, 148, 159, 168, 171, 173, 196, 291, 292, 295, 297, 298 magnesium oxide (MgO), 197, 199, 227, 263, 295 manganese (Mn), 168, 236–238 manganese oxide (MnO), 235 mercury (Hg), 82 molybdenum (Mo), 193, 194, 229, 237, 238 neodymium (Nd), 194 nickel (Ni), 24, 67, 88, 93, 98, 128, 129, 132, 195–197, 229, 237, 238, 291, 294, 300 osmium (Os), 88, 129, 133, 228

Index oxygen (O), 77, 82, 88, 94, 111, 121, 122, 132, 148, 158, 177, 213, 217, 226, 235, 246, 254, 255, 259, 266, 290–292, 294, 297–300 phosphorus (P), 87, 94, 197, 198, 228, 237, 294 platinum (Pt), 24, 88, 111, 145 plutonium (Pu), 205 potassium (K), 82, 94, 147, 201, 236, 292, 294, 298, 299, 301 rhenium (Re), 129, 228, 229, 238 rhodium (Rh), 111 rubidium (Rb), 82, 168, 169, 200–202, 206, 294 selenium (Se), 228, 229 silicon (Si), 77, 82, 86, 197, 234, 256, 285, 289, 291, 292, 297, 298, 300 silicon oxide (SiO), 256, 263 sodium (Na), 171, 173, 201, 202, 236, 237, 289, 291, 292, 298, 301 strontium (Sr), 179 sulfur (S), 13, 15, 87, 228, 291 thallium (Tl), 82, 116, 206 thorium (Th), 135, 179, 226, 294 titanium (Ti), 88, 113, 196, 229, 294, 299 titanium dioxide (TiO2), 196 tungsten (W), 127, 128, 228, 237, 285, 299 uranium (U), 44, 125, 134, 135, 147, 179, 198, 226, 294 vanadium (V), 236, 237 water (H2O), 3, 8, 10, 16, 29, 34, 36, 53, 77, 82, 93, 94, 98, 115, 121, 154, 177, 178, 206–208, 213, 248, 292 xenon (Xe), 153, 203–206 zinc (Zn), 82 zirconium (Zr), 88, 200 Lunar minerals feldspar, 90, 91, 150, 289, 290, 292, 295, 298 mafic, 151, 170, 171, 295 olivine, 90, 150, 198, 295, 297 plagioclase, 90, 91, 113, 133, 150, 151, 170–172, 229, 230, 290, 298, 299 plagioclase feldspar, 91, 290, 298 pyroxene, 90, 150, 290, 292, 295, 298 Lunar Orbiter, 77, 79, 90, 102 Lunar origin models accretional capture, 175, 176 binary accretion, 169, 174, 177, 178, 269, 273 binary-planet, 64, 84 capture, 177, 206, 223, 225, 244 circumterrestrial disk (CTD), 243, 253–255, 269

Index co-accretion, 193, 252 collisional ejection, 124 collisional fission, 157 disintegrative capture, 157, 158, 169, 269 double-planet hypothesis, 137, 138, 228 giant impact, 174, 226, 231, 232, 255, 258, 264, 269, 274, 283, 286 impact-induced fission, 234–238, 245 impact-trigger, 245–248, 251, 254, 286 intact capture, 157, 169, 269 precipitation hypothesis, 66, 84, 85, 227, 228 rotational fission, 286 sediment ring, 230 Lunar palaeomagnetism, 185 Lunar Receiving Laboratory, 80 Lunar rock types anorthositic, 82, 90, 93, 102, 103, 173, 178, 207, 223, 289 anorthositic feldspar, 289 basalt, 82, 84, 89–91, 95, 98, 99, 101, 102, 113, 128, 135, 136, 150, 151, 154, 169, 176, 179, 201, 211–213, 223, 227–230, 235, 290, 295, 301 breccias, 103, 116, 148, 169, 229, 237, 238, 276 cumulates, 116, 150, 151, 170, 171, 173, 174, 291 feldspathic, 151, 179, 237, 293 ferroan anorthosites, 171, 173 igneous, 81, 82, 92, 106, 276, 290, 293, 296, 298 indigenous, 132, 229 mafic, 151, 170, 171, 295 magma, 12, 36, 82, 90, 91, 98, 99, 133, 135, 136, 148, 150–152, 169–171, 173, 174, 177–179, 196, 200, 201, 211–213, 224, 230, 237, 244, 263, 284, 291, 293, 294, 299 magnesium-rich, 171 norite, 106, 297 plutonic, 150, 151, 170, 171, 298 pristine, 132, 136, 142, 146–148, 170–173, 196, 237 pyroclastic, 83, 104, 298 silicates, 44, 66–68, 84, 85, 88, 98, 116, 118, 124, 127, 129, 147, 159, 168, 195, 196, 198–202, 216, 217, 225, 227, 228, 232, 237, 238, 244, 248, 253, 255, 256, 263, 265, 269, 284, 291, 293, 294, 296, 298 Lunar Sample Analysis Planning Team, 80, 82, 99, 113, 169, 170

307 Lunar Sample Preliminary Examination Team, 80, 93–95 Lunar Sample Review Board, 83 Lunar Science Conferences/ Lunar and Planetary Science Conferences Eighth, 126–129, 176, 234 Eleventh, 137 Fifteenth, 151, 197 Fifth, 117, 119 First (Apollo 11), 80, 83, 117 Fourteenth, 149, 263 Fourth, 113, 114, 117 Ninth, 131, 132 Second, 91, 93 Seventh, 122, 124, 126, 170, 171, 198, 210–212, 285 Sixth, 121, 122, 185, 186, 189, 190 Tenth, 133, 135, 198 Third, 99, 169 Thirteenth, 146, 186, 187 Twelfth, 144 Lunar Science Institute (LSI), 77–80, 83, 91, 96, 99, 104, 105, 111, 113, 115–117, 119, 131, 135, 141, 170, 171, 181, 276, 278 Lunar structure asthenosphere, 113 core, 57–59, 67, 84, 88, 93, 99, 106, 112, 118, 120, 128, 129, 132, 143, 148, 153, 154, 158, 169, 174, 182–185, 187, 189, 190, 193–195, 197, 201, 207, 213, 216, 217, 219–221, 225, 227–231, 234–237, 244, 253, 255, 256, 258, 259, 264–267, 270, 284–286, 291, 295–297 crust, 12, 15, 16, 23, 38, 89, 90, 95, 99, 103, 106, 113, 115, 132, 143, 148–151, 170, 171, 176, 180, 200, 201, 210, 227, 295 lithosphere, 36, 37, 113, 114, 181, 294 mantle, 62, 63, 84, 85, 88, 97, 118, 120, 123, 128, 129, 132–134, 148, 153, 158, 168, 173, 174, 176, 177, 195, 197, 207, 227–229, 237, 238 mascons, 79, 90, 93, 106, 114, 129, 180, 296 regolith, 83, 103, 293, 299 Lunar surface features basins/mare, 13, 16, 26, 37–41, 46, 51, 58, 77, 79, 81, 82, 89–91, 93–96, 102–104, 106, 113, 134–136, 143, 154, 155, 173, 180, 190, 201, 208, 212, 223, 224, 238, 247, 279, 290, 291, 293, 295, 296, 298, 300

308 Crisium, 35, 41, 79, 223, 295 Humorum, 79, 296 Imbrium, 5, 16, 20, 21, 24, 35, 37–41, 43, 46, 79, 94, 95, 99, 102, 106, 113, 135, 143, 154, 173, 223, 238, 289, 290, 293, 296, 297, 300 Nectaris, 79, 296 Oceanus Procellarum, 91, 92, 223 Orientale, 113, 135, 223, 296 Serenitatis, 35, 41, 79, 103, 104, 173, 223, 296 Sinus Iridum, 41, 300 Smythii, 223 Tranquillitatis, 79–82, 292 central cones of craters, 12 craters, 1–5, 7–13, 19–21, 23, 25, 26, 28–42, 46, 47, 51, 65, 104, 289, 294, 298 Anaxagoras, 29, 289 Aristarchus, 11, 29, 290 Copernicus, 10, 29, 43, 91, 102, 106, 291 Dolland, 101 Kepler, 29, 294 Plato, 29, 298 Schickard, 13, 299 Shorty, 104 Tsiolkovsky, 223 Tycho, 5, 29, 32, 33, 37, 106, 296, 301 domes, 3, 12 formations, 36, 82, 121, 129, 157, 159, 178, 181, 184, 195, 200, 213, 232, 244, 245, 251, 269 Cayley, 102 Descartes, 101–103, 292 Fra Mauro, 94, 95, 102, 293 graben, 103, 154, 293 grooves, 13, 24, 43 highlands, 47, 51, 77, 82, 90, 91, 93, 95, 101–103, 106, 113, 115, 116, 128, 132–135, 143, 146–148, 150, 151, 153, 155, 169–172, 179, 200, 229, 230, 237, 238, 289, 292, 299, 301 mare, 15, 176, 201, 290, 297 maria, 26, 41, 43, 46, 51, 54, 82, 98, 101, 113, 155, 210–212, 223, 224, 238, 295, 301 mountains, 3, 13, 23, 36 Alps, 13, 16, 36, 43, 289 Apennine Front, 95 Apennines, 5, 6, 13, 16, 43, 95, 99, 101 Carpathians, 16, 290 Caucasus, 16, 43, 291 pits, 7, 9, 13, 25, 26

Index rays, 5, 8, 15, 29, 37, 102, 107, 301 seas, 3, 15, 16, 28, 29, 35–37, 79, 142, 145, 175, 289, 290, 295, 296 terraces, 15 terrae, 47, 51 thrust faults, 179, 301 troughs, 12 uplands, 20, 47, 51, 95, 115, 150 walled basins, 13 walled planes, 13 wrinkle ridges, 3, 301 Lunar Surveyor, 77 Lunesimals, 253 Lunokhod 2, 182 Lyttleton, Raymond Arthur, 89 M MacDonald, Gordon J. F., 52, 54, 72, 81, 94, 179, 180, 208, 213, 215 Mädler, Johann Heinrich, 7, 13, 19 Magma, 12, 36, 82, 90, 91, 98, 99, 133–136, 150–152, 169–171, 173, 174, 177–179, 196, 200, 201, 211–213, 224, 230, 237, 245, 263, 284, 293–295, 298, 299 Magma ocean, 90, 91, 133, 134, 136, 150–152, 169–171, 173, 174, 177, 178, 200, 201, 213, 224, 230, 237, 245, 263, 284, 295 Magmatic differentiation, 128, 295 Magmatism, 135, 150, 151, 170, 212 Magnesium number (mg#), 168, 295 Magnesium ratio, 295 Magnetic paleointensities, 185–188 Malcuit, Robert J., 223–225 Mallet, Robert, 10 Manned Spacecraft Center, 77, 80, 113 Mantle, 62, 63, 84, 85, 88, 97, 118, 120, 123, 128, 129, 132–134, 148, 153, 158, 168, 169, 173, 174, 176, 177, 182, 194–198, 200, 201, 205–207, 216, 219, 223–231, 234–238, 243–248, 251, 253, 254, 256, 265–270, 285, 286, 294, 295, 297, 298 Margulis, Lynn, 280 Marshall, Clare, 173 Marvin, Ursula B., 21, 39, 40, 126, 127, 170 Mass-fractionation line, 121, 122, 235 Masursky, H., 102 Matsui, Takafumi, 177, 178 Mayeda, T. K., 121, 122, 235 Mayer, Robert, 23 McCammon, C. A., 227 McCord, Thomas, 280 McGetchin, Thomas R., 131 McKinnon, William B., 217, 218 McSween, Harry, 280, 290, 292, 296, 297, 299

Index Melosh, H. Jay, 264–268, 274, 285 Merrill, R. B., 121, 127, 172, 186, 211, 212 Metamorphism, 115, 124, 170, 292, 296 Meteorite impact, 83, 90 Meteorites achondrites, 122, 128, 143, 226 aubrites, 254 carbonaceous chondrites, 121, 122, 128, 291 chassignites, 235 chondrites, 92, 143 CI chondrites, 168 C1 meteorites, 236 enstatite chondrites, 226, 254, 285 eucrites, 147, 168, 206, 235 irons, 25, 122 mesosiderites, 122 nakhlites, 235 ordinary chondrites, 121, 122, 229 pallasites, 122 shergottites, 168, 235, 300 SNC, 235, 255, 300 Meteoroid impacts, 97 Meteoroids, 97, 113, 290, 296 Meydenbauer, Albrecht, 6, 13, 14, 19, 33, 41 Michel, F. Curtis, 280 Mitler, H. E., 118–120, 122 Mizuno, Hiroshi, 220, 221 Moments of inertia, 98, 106, 112, 150 Moonlets, 13, 21–24, 84, 118, 129, 138, 244, 286 Moonquakes, 97, 99, 113, 179 Morozov, Nikolai Alexandrovich, 27–30, 33, 37, 41 Mueller, Steven W., 217, 218 Munk, Walter, 52, 208 N Nakagawa, Yoshitsugu, 160 Nasmyth, James, 11–13, 15, 44, 45 Neison, Edmund, 11–13, 19 Newsom, Horton, 193–195, 197, 198, 229 Nordmann, John Charles, 176 Norman, Marc, 173 O O’Connell, Richard, 280 O’Keefe, John A., 86–89, 111, 112, 128, 129, 232 Öpik, Ernst Julius, 30–33, 65–67, 84, 100, 118, 120, 122, 216, 278 Orowan, Egon, 199, 256

309 P Paine, Thomas O., 80 Paleomagnetism, 189, 190 Paleozoic era, 52 Paneth, Friedrich, 127 Papike, James, 172, 280 Parmentier, Marc, 153 Peale, Stanton, 269, 280 Pepin, Robert O., 118, 119, 131, 279, 280 Phillips, Roger J., 124, 136, 137, 143, 148, 151, 161, 167, 169, 181, 188, 194, 196, 214, 219, 235, 236, 238, 247, 250, 253, 260–262, 264, 266, 267 Phinney, Robert A., 104 Phinney, W. C., 170, 171 Planetary bodies (other than the Earth and Moon) Callisto, 207, 248 Charon, 232 Chiron, 248 Ganymede, 207 Jupiter, 144 Mars, 143, 164, 255 Mercury, 255 Neptune, 70 Phoebe, 248 Pluto, 232, 248 Rhea, 248 Saturn, 146 Tethys, 146 Titan, 207 Triton, 207 Uranus, 69 Venus, 162 Vesta, 147, 235, 292 Planetesimals, 8, 43, 46, 63, 64, 67–70, 84, 118–120, 122–124, 128–130, 134, 136, 138, 141, 143, 144, 146–148, 153, 154, 158–160, 165–168, 174, 176–178, 198, 199, 206, 216, 217, 219–222, 228, 230, 231, 244, 245, 247–249, 251–255, 258, 275, 298 Planetoid, 59, 118, 119, 144 Planetology, 29 Plate tectonics, 81, 151, 154, 189, 274, 301 Podosek, Frank A., 203 Poincaré, Henri, 56–58 Porco, Carolyn, 215, 216 Prinz, M., 115 Proctor, Richard Anthony, 9–11, 13, 19, 26, 36 Proto-Earth, protoearth, 8, 46, 51, 55, 59, 65, 89, 122, 129, 138, 153, 177, 216, 220, 231–238, 252, 257–259, 263–267, 270, 283, 285, 286

310 Proto-Moon, protomoon, 86, 100, 101, 118, 119, 174, 181, 213, 215–217, 232, 238, 244, 263 Protoplanets, 153, 199, 220, 261, 262, 264, 265, 268, 269 Proxmire, Senator William, 133 Q Quaide, William L., 276, 277, 279 R Radioactive heating, 46, 54, 89, 90, 98, 113 Radioisotope dating, 81, 299 Rammensee, Werner, 127, 128 Ranger missions, 79 Rare-earth elements, 91, 92, 94, 173, 179 Rasmussen, Kaare L., 198, 199 Refractory elements, 86, 88, 89, 93, 116, 128, 134, 135, 147, 159, 168, 179, 194, 200, 207, 235, 236, 299 Reid, A. M., 115 Reynolds, John H., 203–205 Ridley, W. I., 115, 116 Ringwood, Alfred E. “Ted”, 66, 67, 83–86, 120, 128, 129, 132, 195, 196, 226–230, 235, 236, 247, 265, 278, 294 Roche, Édouard, 59, 65, 67, 71, 72, 85, 100, 101, 118, 153, 217, 221, 258, 259, 263, 299 Roche limit, 221, 299 Rubey, William Walden, 78, 83, 96 Rubincam, David Parry, 233 Runcorn, Stanley Keith, 187, 189, 190, 238 Ruskol, Evgenia L., 64, 65, 199, 254–256, 278 Russell, Christopher T., 183, 189 Russell, Henry Norris, 59, 86 Ryder, Graham, 147, 148, 173 S Safronov, Victor Sergeevich, 68–70, 124, 136, 160, 167, 216, 249, 278 Saul, John, 154 Schmidt, Otto Yulyevich, 60–65, 69, 216, 278 Schmitt, Roman A., 92, 116 Schopf, Thomas J. M., 145 Schröter, Johann Hieronymous, 3, 5 Schubert, G., 149 Schultz, Peter, 26, 154, 155 Scott, Eugene H., 218 Secular resonance, 154, 299 Seifert, Stefan, 195, 196 Seismometers, 96, 114 Seitz, Frederick, 77 Selenography, 3, 12, 25, 299

Index Selenology, 38, 299 Sevier, John R., 134, 135 Shaler, Nathaniel Southgate, 25, 26 Sharpton, Virgil “Buck”, 26 Shervais, John W., 200, 201 Shock lithification, 300 Siderophile elements, 88, 98, 111, 116, 128, 129, 132–134, 148, 170, 193–195, 197, 206, 207, 228, 229, 237, 258, 300 Silicates, 44, 66–68, 84, 85, 88, 98, 116, 118, 124, 127, 129, 147, 159, 168, 195, 196, 198–202, 216, 217, 225, 227, 228, 232, 237, 238, 244, 248, 252, 253, 255, 256, 263, 265, 269, 284, 291, 293, 294, 296–298, 300 Silver, Leon T. (Lee), 131, 280 Simonds, Charles H., 170, 171 Singer, S. Fred, 52, 83, 213–217, 225 Sinus Iridum, 41, 300 Slattery, W. L., 259, 260, 264 Smith, Joseph V., 89–91, 118, 128, 169 Snowbird Conferences, 144, 145 Soderblom, Laurence, 280 Solar nebula, 62, 67, 68, 83, 85, 111, 118, 119, 122, 128, 147, 153, 158, 194, 216, 217, 226, 258, 269, 291, 300 Solar wind, 68, 107, 127, 147, 184, 189, 206, 228, 300, 301 Solid-body tides (bodily tides), 208, 209 Solomon, Sean, 179–182, 280 Sonett, Charles P., 264–266, 268 Spallation, 265, 300 Spin angular momentum, 52, 213, 289 Spurr, Josiah Edward, 38, 39 Stevenson, D. J., 168, 189, 245, 262–264, 274, 283, 284 Stewart, Sarah, 286 Stochastic processes, 245, 300 Stoeckley, T. R., 224 Stomatolites, 207 Strangway, David, 111, 112, 116, 279, 280 Stratigraphy, 21, 300 Struve, Otto, 39 Swindle, Timothy D., 203, 205 T Taurus-Littrow Valley, 103 Taylor, G. Jeffrey, 90, 148, 169, 170, 173, 174, 273–275 Taylor, Lawrence A., 200 Taylor, S. Ross, 79, 80, 117, 132, 133, 137, 146, 194, 200, 203, 205, 207, 228, 283, 284, 287 Tectonic, 30, 32, 36, 73, 106, 179–181

Index Theia, 284 Thermal history of the Moon, 46, 73, 83, 106, 180, 275 Thermal Remanent Magnetization (TRM), 185, 186, 301 Thiersch, August, 13, 19, 37 Thiersch, Heinrich, 13, 19, 37 Thompson, A. C., 168, 245, 263 Thompson, Elihu, 24 Thompson, William, 58 Tilghman, Benjamin Chew, 24, 25 Trace elements, 116, 127, 133, 168, 174, 179, 199, 234, 291, 297, 301 T-Tauri phase, 84, 228, 301 Turcotte, Donald L., 175–178 U Universities Space Research Association (USRA), 77, 78, 119, 136, 279, 280 Urey, Harold Clayton, 41, 45, 46, 60, 67, 68, 70, 71, 79, 90, 93, 94, 97, 98, 114, 115, 129, 150, 176, 262, 275 V Vail, S. M., 248–251 VanArsdale, William, 208–210 Vanyo, James P., 207, 208 Venera spacecraft, 146 Vesuvian-type volcanoes, 9, 19, 20 Veverka, Joseph, 280 Vogel, T. A., 224 Volatile elements, 77, 82–86, 89, 93, 98, 116, 118, 128, 137, 147, 148, 159, 168, 197, 200–203, 206, 219, 227, 228, 234, 236, 270, 283, 291, 301

311 W Wakita, Hiroshi, 91, 92 Walker, David, 149–151 Walker, Robert M., 280 Wänke, Heinrich, 126–128, 132, 227, 234–239, 278 Ward, W. R., 122–125, 129, 167, 168, 243, 245, 246, 257, 263, 264, 270, 274 Warner, J. L., 170, 171 Warren, Paul H., 170–174, 198, 199, 255, 256, 274 Wasson, John T., 170–173, 198, 199, 255, 256 Webb, James Edwin, 77, 279 Wegener, Alfred, 33–37 Weidenschilling, Stuart J., 243, 252–255 Wells, John West, 52–54, 207, 208 Wetherill, George West, 69, 123–126, 159–168, 176, 190, 221, 222, 249, 255, 269, 276, 291, 296 Wilhelms, Don E., 41, 60 Wise, Donald U., 56, 57, 59, 86, 89, 232, 286, 287 Wood, John A., 40, 82, 83, 89–91, 93, 116, 118–120, 122, 124, 133, 157, 169–171, 176, 226, 275, 276, 280 Y Yoder, Charles F., 126, 182, 183, 210, 285 Z Zvjagina, E. V., 124