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PLANETARY SCIENCES American and Soviet Research Edited by Thomas M. Donahue with Kathleen Kearney Trivers David M. Abramson Proceedings from the US-USSR Workshop on PlanetaIy Sciences January 2-6, 1989 Academy of Sciences of the Union of Soviet Socialist Republics National Academy of Sciences of the United States of America National Academy Press Washington, D.C. 1991

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NOTICE: The project that is the subject of this report was approved by the officers of the National Academy of Sciences and the Academy of Sciences of the USSR on January 12, 1988. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance. This report has been reviewed by a group other than the authors according to procedures approved by a Report Review Committee consisting of membem of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distin- guished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it lay Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Frank Press is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Robert M. White is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identity issues of medical care, research, and education. Dr. Samuel 0. Thier is president of the Institute of Medicine. The National Research Council was organized lay the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined By the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Frank Press and Dr. Robert M. White are chairman and vice chairman, respectively, of the National Research Council. Library of Congress Catalog Card No. 90-62812 International Standard Book Number 0-309-04333-6 Copies of this report are available from: Soviet and East European Affairs National Research Council 2101 Constitution Ave., N.W. Washington, D.C. 20418 Additional copies are for sale from: National Academy Press 2101 Constitution Ave., N.W. Washington, D.C. 20418 S216 Printed in the United States of America

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Foreword The Academy of Sciences of the USSR and the National Academy of Sciences of the United States of America sponsored a workshop on Planetary Sciences at the Institute for Space Research in Moscow, January 2-6, 1989. The purpose of the workshop, which was attended by Soviet and American scientists, was to examine the current state of our theoretical understanding of how the planets were formed and how they evolved to their present state. The workshop assessed the type of observations and experiments that are needed to advance understanding of the formation and evolution of the solar system based on the current theoretical framework In the past, models of the formation and evolution of the planets have been just that, models. They essentially portrayed possibilities: that events could have transpired in the way depicted without violating any known constraints and that, if they did take place, certain consequences would follow. Now, we may be advancing beyond that stage to the point where it may be possible to settle fairly definitely on certain scenarios and exclude others. Assessment of the present state of theories and the observational base will help determine the extent to which this is the case. This workshop focused on the present status of observational and theoretical understanding of the clearing of stellar nebulae, planetesimal formation, and planetary accretion; the evolution of atmospheres; the rela- tionship of still existing primitive bodies to these topics; and the relationship of ground-based and in situ measurements. As the papers presented at the workshop and published in this volume show, astronomical observations are now at hand that will reveal the sequence of events occuring in circumstellar disks with sufficient precision to define the models of planetary formation. Moreover, theories for the origin of the solar system have reached a point where it is now possible, . . .

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indeed it is essential, to examine them in the broader context of the origins of planetary systems. Until recently, it has not been possible to select among a range of feasible scenarios the one that is most likely to be correct, in the sense that it satisfies observational and theoretical constraints. We are now advancing to that stage. Similarly, theories for the formation of planetary atmospheres are becoming more sophisticated and respond to an increasingly complex set of observational data on relative abundance of atmospheric species and the state of degassing of the interior of the Earth. But, as the papers presented here will demonstrate, there is still a way to go before all the questions are resolved. The topics discussed at this workshop were timely, and the debate and discussion were full and informative. The participants learned a great deal, and the scientific basis for cooperation in planetary sciences was strength- ened appreciably. The Soviet hosts extended their usual thoughtful and gracious hospitality to the American delegation, and the entire experience was memorable and rewarding. T.M. Donahue Chairman, NAS/NRC Committee on Cooperation with the USSR on Planetary Science 1V

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/~kno~iedgements ~anda1 suppon Tom ~ ~r me top and p[o~db~s ~ At acted. ~ uans~don of me S-e1 presenmbons ~r IS pubU=l~n by ~ghl USA ~ also a~l~ged a~ appareled. v

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Contents FOREWORD Thomas M. Donahue ACKNOWLEDGEMENTS 1. THE PROPERTIES AND ENVIRONMENT OF PRIMITIVE SOLAR NEBULAE AS DEDUCED FROM OBSERVATIONS OF SOLAR-TYPE PRE-MAIN SEQUENCE STARS Stephen E. Strom, Susan Edwards, Karen M. Strom 2. NUMERICAL TWO-DIMENSIONAL CALCULATIONS OF THE FORMATION OF THE SOLAR NEBULA Peter H. Bodenheimer 3. THREE-DIMENSIONAL EVOLUTION OF EARLY SOLAR NEBULA Alan ]? Boss FORMATION AND EVOLUTION OF THE PROTOPLANETARY DISK Tamara ~ Ruzmaikina and RIB. Makalkin 5. PHYSICAL CHEMICAL PROCESSES IN A PROTO- PLANETARY CLOUD Avgusta K Lavrakbina vii . . . 111 v 1 17 31 44 61

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6. MAGNETOHYDRODYNAMIC PUZZLES IN THE PROTOPLANETARY NEBULA Eugene H. Levy 7. FORMATION OF PLANETESIMALS Stuart] Weidenschilling 8. FORMATION OF THE TERRESTRIAL PLANETS FROM PLANETESIMALS George ~ Wetherill 9. THE RATE OF PLANET FORMATION AND THE SOLAR SYSTEM'S SMALL BODIES Motor S. Safronov 10. ASTROPHYSICAL DUST GRAINS IN STARS, THE INTER- STELLAR MEDIUM, AND THE SOLAR SYSTEM Robm D. Gehrz 11. LATE STAGES OF ACCUMULATION AND EARLY EVOLUTION OF THE PLANETS Audrey ~ Amazed and G.~ Pechemikova 12. GIANT PLANETS AND THEIR SATELLITES: WHAT ARE THE RELATIONSHIPS BETWEEN THEIR PROPERTIES AND HOW THEY FORMED? David ~ Stevenson 13. THE THERMAL CONDITIONS OF VENUS Vladimir 1V Zharkov and V.S. Solomatov 14. DEGASSING James C. G. Walker 15. THE ROLE OF IMPACTING PROCESSES IN THE CHEMICAL EVOLUTION OF THE ATMOSPHERE OF PRIMORDIAL EARTH Lev Al Mukhin and M.~ Gerasimov 16. LITHOSPHERIC AND ATMOSPHERIC INTERACTION ON THE PLANET VENUS Vla~lislav ~ Molkov .. . V111 70 82 98 116 126 143 163 174 191 203 218

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17. RUNAWAY GREENHOUSE ATMOSPHERES: APPLICATIONS TO EARTH AND VENUS James ~ Hasting 18. THE OORT CLOUD Leonid S. Marochnik, Lev M. Mukhin, and Roald Z. Sagdeev 19. THE CHAOTIC DYNAMICS OF COMETS AND THE PROBLEMS OF THE OORT CLOUD Roald Z. Sagdeev and G.M. Zaslavskiy 234 246 259 20. PROGRESS IN EXTRA-SOLAR PLANET DETECTION 270 Robert ~ Brown APPENDIX I List of Participants APPENDIX II List of Presentations IX 289 292

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