solar system evolution

Solar System EvolutionA New Perspective

This completely rewritten new edition begins with an historical perspectiveof the place of the solar system in the universe. Evidence from meteorites isused to describe how the planets were formed and the giant planets are con-sidered in the light of the asteroids and why Pluto is not a planet.Explanations on why Earth and Venus turned out so differently, and howMars and Mercury are the survivors of many similar bodies, are also dis-cussed. The formation of the Moon in a giant impact leads to an assessmentof the importance of collisions and impacts in the solar system. It is con-cluded that our solar system is the end-product of many accidental andchance events. This leads to the philosophical discussion of whether planetslike our Earth are likely to be found elsewhere in the universe.

ROSS TAYLOR took degrees in chemistry and geology. He is Emeritus Professorand Visiting Fellow at the Australian National University, Canberra,Australia. He has received the premier awards of the Geochemical Society(Goldschmidt Medal) and of the Meteoritical Society (Leonard Medal), as wellas the Bowen Award of the American Geophysical Union and the GilbertAward of the Geological Society of America. Asteroid 5670 has been namedRosstaylor in his honor. He is a foreign Associate of the National Academy ofSciences of the United States of America and a Fellow of the AustralianAcademy of Science.

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Solar System EvolutionA New PerspectiveSECOND EDITIONAn inquiry into the chemical composition, originand evolution of the solar system

Stuart Ross TaylorThe Australian National University,Canberra, Australia






CAMBRIDGE UNIVERSITY PRESSCambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo

Cambridge University PressThe Edinburgh Building, Cambridge CB2 2RU, UK

Published in the United States of America by Cambridge University Press, New York

www.cambridge.orgInformation on this title: www.cambridge.org/9780521641302

© Cambridge University Press 1992© Stuart Ross Taylor 2001

This book is in copyright. Subject to statutory exceptionand to the provisions of relevant collective licensing agreements,no reproduction of any part may take place withoutthe written permission of Cambridge University Press.

First published 1992Reprinted 1994Second edition 2001This digitally printed first paperback version 2005

A catalogue record for this publication is available from the British Library

Library of Congress Cataloguing in Publication data

Taylor, Stuart Ross Solar system evolution : a new perspective : an inquiry into the chemical composition, origin, and evolution of the solar system / Stuart Ross Taylor. – 2nd ed. p. cm. Includes bibliographical references and indexes. ISBN 0 521 64130 6 1. Solar system. 2. Solar system–Origin. 3. Cosmochemistry. I. Title. QB501.T25 2001 523.2–dc21 00-068903

ISBN-13 978-0-521-64130-2 hardbackISBN-10 0-521-64130-6 hardback

ISBN-13 978-0-521-67566-6 paperbackISBN-10 0-521-67566-9 paperback






Contents

Preface to the First Edition xiiiPreface to the Second Edition xixAcknowledgments xxiiPrologue xxiii

Chapter 1 A brief history 1

1.1 The pre-Copernican view 11.1.1 The Greeks 11.1.2 Ptolemy 3

1.2 The Copernican Revolution 41.2.1 Tycho Brahe and Kepler 51.2.2 Clockwork systems 61.2.3 Galileo 61.2.4 Descartes and Newton 71.2.5 Kant 8

1.3 Laplace and his followers 91.4 Tidal theories 101.5 Solar accretion theories 121.6 Nebular theories 121.7 Notes and references 13

Chapter 2 The universe 17

2.1 The scale and structure of the universe 172.1.1 Titius–Bode Rule 172.1.2 Galaxies 192.1.3 Is the universe uniform? 212.1.4 Density 222.1.5 The age of the universe 23

2.2 Element synthesis 252.2.1 Deuterium 252.2.2 Helium 252.2.3 Lithium, beryllium and boron 262.2.4 Synthesis of the heavy elements 26

2.3 Notes and references 27

Chapter 3 Stars 29

3.1 Star formation 303.1.1 Pre-main sequence evolution 333.1.2 Binary and multiple stars 343.1.3 Red dwarfs 363.1.4 Brown dwarfs 36

v






3.2 Early violent stellar activity 383.2.1 T Tauri stars 383.2.2. FU Orionis stars 383.2.3 Bipolar outflows 39

3.3 The formation of the Sun 403.3.1 Solar composition 403.3.2 The solar corona 423.3.3 The fate of the Sun 42


Chapter 4 The solar nebula 47

4.1 The initial concept 474.2 Molecular clouds and interstellar dust 48

4.2.1 Cores 504.2.2 Interstellar dust 51

4.3 Presolar material 544.3.1 Diamond 554.3.2 Silicon carbide 56

4.4 Separation of nebulae 574.5 Dust disks around stars 58

4.5.1 Disks around pre-main sequence stars 584.5.2 HL Tauri disk 594.5.3 Disks around mature stars 60

4.6 Nebular collapse, nebular lifetime, and angular momentum transfer 614.6.1 Nebular lifetimes 624.6.2 The angular momentum problem 634.6.3 Nebular mass 64

4.7 Nebular structure and temperature 654.8 Notes and references 68

Chapter 5 Composition and chemical evolution of the solarnebula 73

5.1 CI chondrites and the primitive nebula 755.1.1 Comets as pristine samples of the nebula 78

5.2 Volatile elements 815.2.1 The rare gases 815.2.2 Oxygen isotopes 835.2.3 Chemical reactions in the nebula 87

5.3 Homogeneity or heterogeneity? 875.3.1 Heliocentric zoning 88

5.4 Gas loss from the inner nebula 895.5 Volatile-element depletion in the early inner nebula 91

5.5.1 Condensation or evaporation? 945.5.2 Water 995.5.3 Deuterium/hydrogen ratios 100


vi CONTENTS






Chapter 6 The evidence from meteorites 105

6.1 The most-ancient samples 1056.2 Refractory or calcium–aluminum inclusions (CAIs) 106

6.2.1 Composition 1086.2.2 Fremdlinge 1106.2.3 Rims 1116.2.4 FUN inclusions 112

6.3 The matrix of chondrites 1136.3.1 Composition 1136.3.2 Source of matrix 115

6.4 Chondrules 1166.4.1 Textures and cooling history 1176.4.2 Composition 1196.4.3 Complementary compositions 1216.4.4 Depletion mechanisms 1226.4.5 Oxygen isotopes 1236.4.6 Ages of chondrules 1246.4.7 Chondrule precursor material 124

6.5 Models for the origin of chondrules and CAIs 1276.5.1 Nebular origin of chondrules 129

6.6 Chondrites and other primitive meteorites 1326.6.1 Composition 1346.6.2 Hydration 1386.6.3 Enstatite chondrites 1396.6.4 Accretion temperatures for chondrites 139

6.7 Chronology 1406.7.1 Extinct radionuclides 1416.7.2 Sources of short-lived nuclides 1416.7.3 The beginning of the solar system, or T0 1446.7.4 Chondrite “ages” 1456.7.5 Rb–Sr and Mn–Cr isotopic systems 146

6.8 Fractionated meteorites and parent bodies 1486.8.1 Howardites, eucrites, and diogenites (HEDs) 1486.8.2 Ureilites 1506.8.3 Angrites and Angra dos Reis (ADOR) 1526.8.4 Aubrites 1526.8.5 Pallasites and mesosiderites 1536.8.6 Iron meteorites 153


Chapter 7 Building planets 163

7.1 The collapse of clockwork solar systems 1657.1.1 Planetary eccentricity 165

7.2 Differences between the inner and outer planets 1667.2.1 Forming giant planets 1687.2.2 Giant gaseous protoplanets 168

CONTENTS vii






7.3 Formation of the giant planets by the planetesimal hypothesis 1717.3.1 Significance of the asteroid and Edgeworth–Kuiper belts 1717.3.2 Early formation of Jupiter 1727.3.3 “Snow lines” 1737.3.4 Saturn, Uranus and Neptune 1747.3.5 Limits to growth 1757.3.6 Extrasolar planets 175

7.4 The inner solar system 1777.4.1 Accretion from dust or from a hierarchy of objects 1777.4.2 Evidence from cratering 1787.4.3 Planetary obliquities and spins 180

7.5 Planetesimals 1827.5.1 Fractionation in precursor planetesimals 183

7.6 Accretion of planetesimals 1857.6.1 Accretion of the inner planets 1877.6.2 Meteorites as planetary building blocks 189

7.7 The long-term stability of the solar system 1917.8 Notes and references 192

Chapter 8 The giant planets 197

8.1 Four giant cores 1988.1.1 Composition 2008.1.2 Heat flows 2018.1.3 Magnetic fields 202

8.2 Atmospheres 2038.2.1 Deuterium/hydrogen ratios 205

8.3 Origin of our giant planets 2058.3.1 On the usefulness of Jupiter 208

8.4 Planet X 2088.5 Extrasolar giant planets 209

8.5.1 Detection of extraterrestrial planets 2108.5.2 Orbital distribution 2118.5.3 Mass distribution 2138.5.4 Composition 2148.5.5 The pulsar planets 2168.5.6 Formation of extrasolar planets 216


Chapter 9 Satellites and rings 223

9.1 Miniature solar systems? 2239.2 Satellite classification 225

9.2.1 Regular satellites 2279.2.2 Captured satellites 229

9.3 Galilean satellites 2299.3.1 Io 2309.3.2 Europa 231

viii CONTENTS






9.3.3 Ganymede 2339.3.4 Callisto 233

9.4 Saturnian satellites 2359.4.1 Titan 236

9.5 Uranian satellites 2389.6 Neptunian satellites 240

9.6.1 Triton 2419.7 Planetary subnebulae 244

9.7.1 The cosmic junkyard 2479.8 Planetary rings 248

9.8.1 The jovian ring system 2499.8.2 Saturn’s rings 2499.8.3 The uranian rings 2519.8.4 The neptunian ring system 2529.8.5 Ring origin 254


Chapter 10 The refugees 263

10.1 Pluto and Charon 26310.1.1 Size, density and internal structure 26410.1.2 Atmospheres and surfaces of Pluto and Charon 26510.1.3 Origin of Pluto 26610.1.4 Origin of the Pluto–Charon pair 26810.1.5 Pluto and the solar-nebular composition 269

10.2 The Centaurs 26910.3 Comets 271

10.3.1 The Edgeworth–Kuiper Belt 27110.3.2 The Oort Clouds 27310.3.3 Comet Halley 27310.3.4 Formation sites for comets 27510.3.5 Deuterium/hydrogen ratios 27510.3.6 Cometary ices 27610.3.7 Comets from outside the solar system? 276

10.4 The asteroids 27710.4.1 The asteroid belt 27810.4.2 Kirkwood Gaps 27910.4.3 Zonation in the asteroid belt? 28010.4.4 Asteroid families 28110.4.5 The Apollos, Atens, and Amors (near-Earth asteroids) 28310.4.6 The spectrophotometric paradox 28510.4.7 The “missing olivine” problem 28610.4.8 Individual asteroids 28610.4.9 Other asteroid belts? 28810.4.10 Origin of the asteroid belt 289

10.5 Asteroidal and cometary dust 29110.5.1 CP and CS particles 29210.5.2 The zodiacal cloud 293

CONTENTS ix






10.5.3 Zodiacal dust and the search for extrasolar planets 29310.5.4 The terrestrial record 29410.5.5 Dust as a solar-system sample 294


Chapter 11 The survivors: Mercury and Mars 301

11.1 Mercury 30111.1.1 Internal structure 30211.1.2 Surface composition 30311.1.3 Evidence for planetary contraction 30511.1.4 Bulk composition 30611.1.5 Origin of Mercury 306

11.2 Mars 30911.2.1 Shape and surface topography 30911.2.2 Tharsis 31011.2.3 Obliquity 31111.2.4 Martian interior 31211.2.5 Martian (SNC) meteorites 31311.2.6 Bulk composition 31611.2.7 Accretion and evolution 31911.2.8 Plate tectonics? 31911.2.9 Crust and dust 32011.2.10 Atmosphere and water 32411.2.11 Life on Mars? 32511.2.12 Phobos and Deimos 327


Chapter 12 The twins: Venus and the Earth 331

12.1 Venus 33112.1.1 Crustal composition 33412.1.2 Impact craters and the age of the surface 33612.1.3 Heat loss and rates of volcanism 33812.1.4 A one-plate planet 33812.1.5 The geological history of Venus 33912.1.6 Bulk composition 34012.1.7 Primitive oceans on Venus? 34112.1.8 A close relative of the Earth? 342

12.2 The Earth 34312.2.1 Formation and early history 34312.2.2 An early crust? 34512.2.3 The oceanic crust 34712.2.4 The continental crust 34912.2.5 Comparison with other planetary crusts 35112.2.6 The terrestrial mantle 35312.2.7 Mantle composition 35612.2.8 The siderophile-element paradox 35812.2.9 The terrestrial core 360

x CONTENTS






12.2.10 The source of terrestrial water 36112.2.11 The atmosphere 362


Chapter 13 The Moon 369

13.1 Some physical properties 37013.2 The lunar crust 372

13.2.1 The highlands 37213.2.2 The maria 37713.2.3 Lunar cratering 379

13.3 Lunar mantle and core 38213.4 Lunar bulk composition 383

13.4.1 Volatile elements 38313.4.2 Vanadium, chromium and manganese 38413.4.3 Depletion of siderophile elements 38613.4.4 Refractory lithophile elements 386

13.5 Evolution of the Moon 38813.5.1 Ancient lunar magnetic field 38913.5.2 Atmosphere 390

13.6 Hypotheses of lunar origin 39113.6.1 Pre-Apollo ideas 39113.6.2 Large-impact model for the origin of the Moon 39313.6.3 Eucrites and the Moon 395


Chapter 14 The role of impacts 401

14.1 A reluctant conversion 40114.2 Mechanics of cratering 403

14.2.1 Types of craters 40714.2.2 Large basins 409

14.3 Surface histories of the planets and satellites 41214.3.1 The early intense bombardment 41514.3.2 Basin ages 416

14.4 Lunar cataclysms? 41714.5 The impactor population in the early solar system 42014.6 Cratering flux since the heavy bombardment 422

14.6.1 Periodic extinctions 42314.6.2 The Torino scale 42414.6.3 Venus 42414.6.4 Mars 426


Chapter 15 Epilogue: on the difficulty of making Earth-likeplanets 431

15.1 The plurality of worlds? 43115.1.1 Nebular constraints 43215.1.2 Consequences of the early formation of Jupiter 433

CONTENTS xi






15.2 The random nature of inner planet formation 43415.2.1 Effects of impacts on biological evolution 436

15.3 Is the continental crust of the Earth unique? 43815.4 Synopsis 43915.5 Notes and references 442

Name index 445Subject index 452

xii CONTENTS






Preface to the First Edition

The true dimensions of the terrestrial globe were revealed mostly in thefifteenth and sixteenth centuries, principally through the technicaldevelopment of truly ocean-going vessels and the magnetic compass,which enabled the exploration of the oceans. Even with these advan-tages, the real extent of the Great Southern Continent or terra australisincognita had to await the voyages of Cook in the eighteenth century.This flood of new geographic knowledge replaced the medieval view ofthe world (although Flat-Earth societies still persist). Such understand-ing was gained only very slowly. A detailed knowledge of the topographyof the ocean floors and our understanding of their composition andorigin has been obtained only a little ahead of our radar pictures of thesurface of Venus.

Our exploration of the solar system is at a similarly heroic stage. Thedistant points of light, barely resolvable in telescopes, have beenrevealed through the use of space vehicles, the latter-day equivalent ofthe Portuguese caravels, as separate worlds, with an astonishingamount of diversity.

The information has been rapidly and widely disseminated, elec-tronic media having superseded the printing press of the Renaissance.Everyone is informed of the striking new discoveries. Although noEldorados have emerged, the pictures reveal a plurality of worldsunimagined by the Elizabethans. Every satellite has turned out to differin some significant feature from its neighbor: “. . . the sense of noveltywould probably not have been greater if we had explored a differentsolar system” [1].

This comment on the jovian system reveals a fundamental truth.Lurking behind the photographs and radar images is a new observation,uncomfortable for Homo sapiens. What combination of circumstancescould reproduce in some other planetary system the detail observed inour own solar system or lead to the assembly and evolution of a clone ofthe Earth? Whether one contemplates the battered face of Mercury, thecrumpled crust of Venus, our own water-sheathed planet, the vastdeserts and gigantic landscapes of Mars, or the profusion of distinct icysatellites of the giant planets, the uniqueness of the individual planetsand satellites informs us of a sober fact: The system is unlikely to beduplicated in detail elsewhere.

Other planetary systems doubtless exist. That they would resembleours in any but the broadest detail is only a remote possibility. Too manychance events have occured to bring our system to its present condition.Jacques Monod [2] remarked on the uniqueness of the evolutionary path xiii






for life and of the apparently unsurmountable difficulties of duplicat-ing our version of intelligent life elsewhere. The planets and satellitesare rich in diversity and the difficulty of producing clones of our presentsolar system makes duplication as unlikely as the possibility of findingan elephant on Mars.

Despite the cornucopia of new information about the solar system,it is curious how little effect this has had on the development of theo-ries for its origin. The Apollo and Luna data from the Moon had almostno impact on theories of the origin of the solar system, or indeed of theMoon, until comparatively recently. The reason for this is partly theunusual circumstances of lunar origin and evolution and the very largeamount of new data from separate fields that had to be assimilated.

The complexity of the solar system is not in accord with theories thatstart from some simple initial condition. Such hypotheses do notpredict the diversity of the present system. Thus, while it is tempting tolook for grand unified theories to explain our solar system, the basicapproach is wrong and the trail is false. Too many chance events occur.Among the many singular events that determined the outcome, a fewexamples may be cited: the size of the fragment of the molecular cloudthat became the solar nebula; the formation of the Sun as a singlerather than a double or multiple star; the huge size of Jupiter and itscontrolling influence on the asteroid belt and Mars; the random accre-tion events that made the Earth larger than Venus; the unique collisionsthat produced the bone-dry Moon and the high density of Mercury (a redherring for grand theories); the differences among the satellite systems;and the hundred other curious features of the solar system such asHyperion tumbling chaotically until the end of time. All are the resultof events that might readily have taken a different turn.

Scientists and philosophers have been attempting to find an expla-nation for the origin of the solar system since the question was firstraised by the Greeks, but a universally acceptable scenario has yet toappear. A major reason for this failure to solve one of the oldest scientificquestions has lain in the approach adopted. Another planetary systemwill differ in numbers and sizes of planets and satellites. Thus, the questfor a universally applicable solution to the origin of the system is notfruitful. Attention is better directed to explaining how the variousaspects of the system came about.

In this book I have attempted to provide this and to describe ourcurrent understanding of the origin and evolution of the solar system.In this respect, I have followed the path of most previous investigatorsin trying to account for the existence of the planets, satellites, asteroids,and comets. The formation and evolution of the Sun as a quite normalG-type star is well enough understood, so in this account it is mainly anoffstage companion to the discussion. The book is biased toward a geo-chemical point of view because of my field of interest. Cosmochemistryand geochemistry are not, however, without their difficulties, andthose readers who wish to find a simple statement of the composition ofthe Earth will be disappointed to learn that many problems yet remain.I have, nevertheless, been unable to resist treading among other areas,

xiv PREFACE TO THE FIRST EDITION






conscious that such a path is fraught with as many perils as thoserecorded by John Bunyan (1628–1688) in The Pilgrim’s Progress.

The subject is rendered very difficult by the wide variety of contraryopinions held among investigators and the many alternative explana-tions offered for the same set of data. Sometimes the data are suspect. Itis thus surprising to find that the students of cratering frequently dis-agree on the numbers of craters involved. One might have hoped that atleast the database was agreed on. However, it is a common error tounderrate the difficulties of subjects outside one’s experience. Thediversity of opinion is only exceeded by the strength with which theopposing views are held and defended, supporting the thesis that sub-jectivity is more common in science than hoped for [3]. This has notmade my task easier in trying to reach some conclusions about thestatus of our understanding of the solar system.

Readers may complain that I have chosen to cover too broad a canvasand that the scope of the book should be restricted to areas of immedi-ate competence. Thus, they may question why topics as disparate asChiron, comets, crustal development, calcium-aluminum inclusions,chondrules, and Copernican Revolution, and lunar cataclysms are alldiscussed.

The problem is that detail in one area may provide a crucial con-straint in another. If one paints with too broad a brush, error may creepin. Grand theories are useless if they cannot explain the minute details.Thus, the concept of giant gaseous protoplanets condensing from frag-ments of the nebula was viable until it was demonstrated that Jupiterand Saturn were not of the solar composition required by the theory.The rare-gas isotopic abundances in the terrestrial planets rule out thepopular concept of late-accreting veneers of carbonaceous chondrites.Consequently, an apparently minor detail from one topic may negateextensive work from another, just as the presence of the europiumdepletion in lunar mare basalts removed the model, developed fromexperimental petrology, that these lavas were derived from a primitiveunfractioned lunar interior.

The reverse of this particular coin is that the mere accumulation ofthe staggering amount of detailed observations in the solar system is oflittle use unless there is some unifying concept; the discovery of thePeriodic Table was needed to make sense out of the bewildering array ofproperties of the chemical elements.

Although this book treads into far more nebulous territory than, forexample, Lunar Science: A Post-Apollo View [4], it might be recalled that thestate of our understanding of the Moon at that time was also beset withmany misconceptions and false trails, a particularly good examplebeing the many ingenious but erroneous explanations advanced toexplain the europium anomaly [5].

It therefore behooves workers in this interesting field to consider allthe details if they wish to avoid error, unless they want to make all theplanets out of CI carbonaceous chondrites, or the Moon from theEarth’s mantle. In this context, I had hoped, somewhat naïvely whenbeginning this task, to conclude with definitive compositions at least

PREFACE TO THE FIRST EDITION xv






for the terrestrial planets. What has emerged in reality is that even themajor-element compositions of the inner planets are poorly con-strained. A solution will have to wait until the problems of the momentsof inertia of Mars and Mercury, the nature of the lower mantle of theEarth, and that of the light element in the core, among many others, areresolved.

However, it is easy to become fascinated with the curious landscapesof Miranda and Triton, the large crater on Mimas, the enormous layereddeposits in Valles Marineris, the crumpled terrain of Venus, the rare gasabundances in the atmospheres of the terrestrial planets, the KirkwoodGaps in the asteroid belt, the relationship of Pluto and Charon, the com-position of Comet Halley, or that of interplanetary dust. While contem-plating all these marvels, it is not difficult to become lost among thetrees, forgetting that the principal purpose of this book it to understandthe origin of the system, and to integrate all these wonders into a coher-ent explanation.

Thus, it is not my intention to provide a detailed travelogue orCook’s Tour of the solar system, tempting though that option may be,since there is already a multitude of good technical books and paperscovering both the detail and overviews of the solar system [6]. I assumethat most readers are familiar with the spectacular images and I havenot attempted to reproduce many of these or provide much mathe-matical treatment. This is available in the material referenced and itseems pointless to fill up pages with equations of use only to a few spe-cialists.

Rather, I have attempted a commentary on the problems of its originand evolution. I had orginally planned to start the book with a descrip-tion of the present solar system, and proceed from there into more neb-ulous regions, moving backwards in time into the unknown. I waswisely persuaded to begin instead with the solar nebula, and to bringthe story forward in chronological order, a decision which has provenrobust. However, as one of the editors of Meteorites and the Early SolarSystem remarked, “. . . good reasons could be found for placing everychapter before every other chapter” [7].

In general, I have attempted to reference the most recent reviews andtexts; the subject is moving so rapidly that older material quicklybecomes of historical interest. In many areas, the subject has comeclearly into focus only in the past two or three years as the avalanche ofdata from the past two decades has been assimilated. Accordingly,much of the older literature is now of historical, rather than scientific,interest [8]. Literature coverage extends to April 1990, with a few lateradditions.

The deluge of new information and the emergence of some agree-ment on the general scenario of solar system origin and evolution hasturned the subject in a few years from one having the status of a hobby,“. . . pursued by eccentric, elderly gentlemen who fight with oneanother’s theories of the origin of the solar system” [9] to one in whichquite detailed scientific questions can be posed. In this context, I havebeen selective in giving more attention to those studies that fall within

xvi PREFACE TO THE FIRST EDITION






the general theme of the planetesimal hypothesis, rather than to givean overview of all competing ideas. Here I have attempted to produce abroad survey pointing to the many questions of interest that can beaddressed and the regions in which our understanding is inadequate.Even in those areas such as the surface geology of Mars, where we havemuch basic knowledge and understanding, many significant questions(for example, the relative importance of sedimentary vs. igneousprocesses, or the early climate and role of ice) remain obscure. Only asample return, followed by a manned mission, will supply the answersto these particular questions.

The wealth of new detail leads to the complaint, common to manyscientific disciplines, that there is now too much information for anyindividual to comprehend. In this view, it was simpler in past ages whenthe corporate body of knowledge was so much smaller.

However, it is doubtful that this is true; scientists in the Renaissancehad to deal with a staggering burden of topics such as astrology,alchemy or numerology, on which Newton wasted so much of his time.In any subject where there are no unifying principles, as in chemistrybefore the Periodic Table revealed an essential simplicity or biologybefore evolution cast its revealing light, it is impossible for an individ-ual to be an expert in every nook and cranny. Nevertheless, given somegeneral principles of physics and chemistry as a guide, it is possible todiscern the forest, even through the thickest underbrush, just as thearchitectural unity and splendour of a great building may be glimpsedfrom a distant prospect.

Among the other intellectual baggage that has been discarded, thedifferences between the relative ages of the universe and the solarsystem have clearly distinguished their origins; we no longer have toaccount for them together. The age of the solar system, obtained frommeteorites as 4560 m.y., is only about one-quarter of the age of thevisible universe. This fact – a relatively recent discovery – separates theorigin of the Earth and solar system from that of the universe. Thisknowledge has become so ingrained in thinking that it is surprising torecall that as recently as 1950, the age of the Earth, established by iso-topic dating of rocks, approached and sometimes exceeded the astro-nomical estimates for the age of the universe.

One is comforted on this journey by the steady convergence ofscientific ideas toward some kind of consensus, as new facts areacquired. Science is in this way distinct from most other human activi-ties, which display the opposite tendency of divergence with time, aprocess most clearly revealed by the multitude of religious and philo-sophical systems.

Stuart Ross TaylorHouston, TexasDecember 1990

PREFACE TO THE FIRST EDITION xvii






Notes and references

1. Smith B. A. et al. (1979) Science, 204, 951.2. Monod J. (1974) Chance and Necessity, Collins, 187 pp.3. Mitroff I. (1974) The Subjective Side of Science: A Philosophical Enquiry into the

Psychology of the Apollo Moon Scientists, Elsevier, New York, 329 pp.4. Taylor S. R. (1975) Lunar Science: A Post-Apollo View, Pergamon, New York,

372 pp.5. Ibid., pp. 154–159.6. There are so many good texts that it would be invidious to choose among

them. A good beginning is Morrison D. and Owen T. (1987) The PlanetarySystem, Addison-Wesley, Reading, Massachusetts, 600 pp.

7. Kerridge J. F. (1988) in Meteorites and the Early Solar System (eds. J. F. Kerridgeand M. S. Matthews), Univ. of Arizona, Tucson, p. xv.

8. The fate of most scientific work is to be forgotten. The typical scientificpaper has a half-life of five years, forming a curious contrast to the domi-nant egos of most workers in the field. The correct is incorporated into thegeneral body of knowledge; the erroneous is mostly ignored.

9. Wetherill G. W. (1988) in LPI Tech. Rpt. 88–04, p. 81.

xviii PREFACE TO THE FIRST EDITION






Preface to the Second Edition

My purpose in writing this book is to enquire into the solar system andhow it came to be. So much progress has been made in the past decadethat this book has been completely rewritten from the first edition. Theseven large chapters in that edition have been restructured into fifteensmaller ones that deal more readily with the increased flood of infor-mation.

As in the first edition, I have tried to place the solar system in thebroader context of the universe. My excuse for venturing into fields suchas cosmology is to reinforce the point that the solar system is a relativenewcomer in the universe and came about through a fortunatesequence of chance events. A secondary purpose is to try to overcomethe narrow and potentially hazardous specialization that is endemic inscience and that I talk about in the Prologue. I have attempted to educatemyself in fields remote from my own through discussions with manycolleagues, listed in the Acknowledgments.

The book does not follow the usual descriptive arrangement of start-ing with Mercury and marching stolidly out through the giant planetsto Pluto. Instead, the unconventional arrangement I have adopted herehas arisen naturally as I have tried to explain how the system arose fromthe solar nebula and why the various bodies happen to be where theyare. The result is that there are many associations that may at first sightseem surprising.

I begin with a brief historical outline that traces the development ofthinking about the solar system. This is followed by comments aboutthe place of the solar system in the universe, the formation of stars andof the solar nebula. The important and unique information provided bymeteorites on the early evolution of the nebula next sets the stage forconsidering the formation of the planets themselves. Although longconsidered as an inevitable process that would produce the solarsystem as a result of some general law, this view of the formation of“clockwork” planetary systems has been superseded by a recognitionthat random events have predominated. This accumulation of planetsfrom short-lived disks around stars separates the planetary scientistfrom the astronomer, who deals with the simpler processes thatproduce the stars from the collapse of gas clouds. Early violent solaractivity swept away the gas and volatile elements from the innernebula, out to the vicinity of Jupiter. This local increase in density so farfrom the Sun enabled that planet to form a large core quickly enoughto capture the fleeing gas by gravitational attraction. Disks aroundyoung stars are short-lived, so that giant Jupiter was the earliest planet xix






to form, followed by Saturn and a little later by the ice giants, Uranusand Neptune, who formed just too late to collect gas. The giant planetsare here lumped together rather than separating the gas giants fromthe ice giants as is customary. These massive bodies probably formedmuch closer together than where they now find themselves and theirpresent masses are the result of chance rather than the consequence ofsome pre-ordained plan. At this point, the problems of forming thegiant extrasolar planets are considered. A multitude of satellites of thegiants formed close upon their parents.

Left over after these major events that shaped the system were theicy bodies in the outer parts of the nebula, where the amount of mater-ial was too small to produce more planets. Pluto and the comets are thefamous examples, while Uranus and Neptune, migrating outwards,swallowed up much of the material so that the solar system seems to ter-minate abruptly at the orbit of Neptune. Meanwhile Jupiter, on thesunward side, swept up so much material that only a depleted zone ofrubble, now the asteroid belt, was too scattered by the giant to form aplanet. For the next 50 or 100 million years, the rocky rubble left over inthe inner solar system collected itself into larger bodies, of which Marsand Mercury are survivors. Earth and Venus were more fortunate, theEarth in particular acquiring the Moon from a chance collision withsomething bigger than Mars. Collisions dominated the growth of all theplanets as their variable tilts and rotation rates testify. So the formationof the Earth, that turned out wildly different from its twin, Venus,demonstrates the importance of chance events in making planets andemphasises the difficulties of forming a duplicate of our planet else-where, a message reinforced by the startling diversity among the extra-solar planets.

I have attempted to summarise the literature through to the end of1999 with some outliers into the 21st century. Readers will observe thatthere is a bimodal distribution in the dates of the papers, a commonfeature in second editions, that first peak toward 1990 and then toward1999. As in the first edition, I have tried to refer to the more recent worksfor two reasons. Firstly, reference to them will in turn enable the readerto refer back to the previous literature on the topic. A more seriousreason is that the subject is moving so rapidly that published papers ontopics such as the extrasolar planets, the source of water on the Earth orthe origin of Uranus and Neptune in our own system rapidly becomedated as new information appears. Thus in many areas, papers morethan a couple of years old have already entered the realm of past history,interesting only in reflecting the thinking of the period.

However, the purpose of references in a book such as this is also torefer to primary sources, so that I have included a number of the cita-tions from the first edition for this reason. I have excluded all referencesto conference abstracts and other grey literature except in a very fewunavoidable cases where the information is not available elsewhere. Ihave followed the convention adopted in the first edition of listing ref-erences by number in order of appearance in each chapter, rather thanbreakup the continuity of the text with names and dates. I list first

xx PREFACE TO THE SECOND EDITION






authors only, followed by et al., except for two-author papers, as it ispointless in a work such as this to provide lists that on occasion extendto 30 names. With over 1300 references, often to fields remote from myspeciality, it is inevitable that I have missed some relevant sources. Myapologies for any such oversights.

I have continued to include tables of relevant data, but two develop-ments have altered the more pressing need that I perceived for present-ing such material in the first edition. One is the appearance of ThePlanetary Scientist’s Companion (Oxford University Press, 1998) that con-tains much relevant information. The second is the vast amount of up-to-date data available on the Internet that include, for example, lists ofextrasolar planets, centaurs and transneptunian objects. Both thesedevelopments have made data tabulations less crucial, particularly as agap of a year or so exists between compiling data and its appearance inbook form. In many cases, as with the basic petrological data on mete-orites, little has changed in the decade since the previous edition. Inothers, ranging from updated information on the Galilean satellites tothe appearance of the extrasolar planets, the data are new.

This great increase in the availability of comprehensive data-setsthus frees the author to concentrate on understanding rather thanmerely cataloguing the material. The task of the author, however,differs from that of the compiler. He has a responsibility to the reader topresent his own, perhaps biased, interpretation rather than merelyto tabulate previous efforts. As one of my colleagues memorablyremarked, I prefer to make my own mistakes than to repeat the errors ofothers. Thus there is still a need for critical evaluation of the mass ofavailable data. This applies particularly to geochemistry where theproblem of sampling planetary bodies remains formidable. With thisexcuse, I have given here my own assessment of such debatable issues asthe composition of the terrestrial mantle and crust, the Moon and thatof the primitive solar nebula.

Stuart Ross TaylorCanberra

PREFACE TO THE SECOND EDITION xxi






Acknowledgments

My wife Noël has provided continuous help and support over manyyears, without which I could not have undertaken this work.

The Department of Geology, Australian National University, has pro-vided facilities through a Visiting Fellowship that have enabled thisbook to be written. I also thank the Department of Nuclear Physics,Research School of Physical Sciences, at this University for assistance. Ithank Clementine Krayshek for drafting the figures and tables and theResearch School of Earth Sciences at this University for providing thisservice.

I am grateful to the following scientists for discussions, comments,supplying preprints or reprints and generally assisting me in thisendeavour:

Craig Agnor, Richard Arculus, Pawel Artymowicz, Neta Bahcall, GiborBasri, Willy Benz, Rick Binzel, Alan Boss, Adrian Brearley, Roland Buser,Paul Butler, Natalie Cabrol, Al Cameron, Robin Canup, Pat Cassen, ChrisChyba, Bob Clayton, Harold Connolly, Dale Cruikshank, Kimberly Cyr, DonDavis, Thérèse Encrenaz, Tezer Esat, Paolo Farinella, Maria Firneis, MikeGaffey, Daniel Gautier, Brett Gladman, Guillermo Gonzalez, JitendraGoswami, Ron Greeley, Richard Grieve, John Guest, Alex Halliday, GregHenry, Roger Hewins, Munir Humayun, Bill Irvine, Rhian Jones, AmirKahn, Bill Kaula, Gero Kurat, Harold Levison, Mike Lipschutz, JackLissauer, Gunter Lugmair, Jonathon Lunine, Renu Malhotra, Geoff Marcy,Ursula Marvin, Francesco Marzani, Michel Mayor, Lucy-Ann McFadden,Bill McKinnon, Scott McLennan, Andy McWilliam, Patrick Michel, ThierryMontmerle, Andrew Nelson, Horton Newsom, Marc Norman, Ulrich Ott,Herbert Palme, Jim Papike, Jon Patchett, Arch Reid, Kevin Righter, SaraRussell, Alex Ruzika, Ed Scott, Frank Shu, Chip Shearer, Alex Shukolyukov,Dave Smith, Ann Sprague, Jeff Taylor, Larry Taylor, Mark Thiemens, ChrisTinney, Allan Treiman, David Trilling, Virginia Trimble, Stéphane Udry,Don VandenBerg, Harri Vanhala, Meena Wadhwa, Malcolm Walter, DavidWark, Jerry Wasserburg, George Wetherill, Don Wilhelms, John Wood,Tetsuo Yamamoto, Ed Young and Maria Zuber.

Robin Brett, US Geological Survey, Tezer Esat, Australian NationalUniversity, Chris Koeberl, University of Vienna and Scott McLennan, SUNY,Stony Brook have continued to be actively helpful. Brian Mason and the lateLouis Ahrens, Gene Shoemaker and Harold Urey all contributed to settingme on this path. It has been a pleasure to work with Cambridge UniversityPress during the production of the book. I am grateful to Dr Brian Watts formeticulous copy-editing that has materially improved this book. I thank DrSimon Mitton for his continuing encouragement and support.xxii






Prologue

A principal task in writing in the first edition of this book was toexamine the series of events that led to the formation of the solarsystem. The conclusion, so greatly illuminated by the previous threedecades of planetary exploration by spacecraft, was that random eventshad predominated in the construction of the great variety of planetsand satellites. Thus it was unlikely that duplicates might be found else-where. This was in contrast to earlier views that the solar system was asorderly as a clock and that, given sufficient computer power, one mightsimulate the construction of such a clockwork system from first princi-ples according to the laws of physics and chemistry.

In the 1992 edition, I commented that “the . . . common occurrenceof disks around young . . . stars strengthens the case for the existence ofother planetary systems. If so, would they resemble our own? Would wesee something like the Galilean satellite system of a few equal-sizedplanets, systems with one giant planet, or a single brown dwarf com-panion?” After contemplating the satellite systems around our giantplanets, I concluded that “no simple sequence of reproducible eventshas occurred in our solar system. Other planetary systems . . . will be dif-ferent in detail to our own. What their satellites might look like is onlyfor bold spirits to predict” (p. 251).

This concept was reinforced in dramatic fashion in 1995 by theunequivocal discovery of extrasolar planets. Now that the question ofthe existence of other worlds, that has been around for 24 centuriessince Democritus, has been answered, I wish to take a different tack inthis edition. Nature is able to make a variety of planets in surprisingorbits so that the problem posed by Lucretius and most dangerously byGiordano Bruno has moved on to the matter of their habitability. Isthere life out there and what are the chances both of finding it and com-municating with the “little green men” that are so desperately hopedfor by many of the human inhabitants of this planet?

Here I have tried to enquire into this question by studying the detailin our planetary system and making some comparisons with the newones that are being discovered daily. What emerges once again is therandom and stochastic nature of events leading to the formation ofplanetary systems and the immense difficulties in constructing a dupli-cate of the Earth. Added to this problem of forming habitable planetsare the biological problems, both of the origin of life and its evolution.It thus becomes exceedingly difficult to be optimistic about the possi-bilities of finding duplicates of the Earth or any extraterrestrial life thatremotely resembles Homo sapiens. xxiii






In a work of this scope, it is impossible to be expert in the manydiverse fields that I cannot resist straying into. My excuse for wanderinginto regions as remote from my speciality as cosmology, where a merestudent of rocks has little business, is that science has become danger-ously fragmented. This is understandable. The difficulties of becomingexpert even in some tiny corner of science are well enough known. Weall tend to get into our own particular foxholes, secure in our under-standing of alkenes, atomic particles, ants, andesites or asteroids.

This leaves the way open for salesmen of simplistic solutions thatpurport to explain everything in terms that a young child could follow.Of these, creationism in its various incarnations represents the mostserious threat. The whole endeavor of science and ultimately the verysurvival of our civilized society that has arisen from the study of thenatural world in the past four centuries is at risk. One writer has com-mented that “though science is stronger today than when Galileo knelt beforethe Inquisition, it remains a minority habit of mind and its future is very muchin doubt. Blind belief rules the millennial universe, dark . . . as space itself” [1].

One is reminded of what has been called “the failure of nerve” of theancient world. Following the scientific advances of ancient Greece andthe major progress at the Museum and Library (Mouseion) atAlexandria, there followed a retreat into the comfort of myths. Much ofthe great library at Alexandria, that had found shelter in the Temple ofSerapis from civil wars in the 3rd century AD, was destroyed by aChristian mob in 391 AD. The remaining books were burnt in 646 AD bythe Arab conquerors of the city [2]. Book burning seems to be a popularand widespread habit, judging from more recent examples that haveincluded both Nazi Germany and the Red Guards of China.

There followed a retreat from objective truth into medieval modesof thought as the Dark Ages began. The stage was set by Saint Augustine(354–430 AD) who ordained that as God had created everything, “it isnot necessary to probe into the nature of things” as the Greeks haddone. This attitude still survives.

Will faith once again smash science, as happened in the ancientworld? This seductive pathway is not without its difficulties. As HarveyBrooks (1971) has remarked “if the modern era has created social and culturalconditions in which the enterprise of science is no longer viable, it has sown theseeds of its own disintegration and decay, to be followed by the disappearance ofa large fraction of the world’s present population and a decline in the materialconditions of human life. It is a mere detail whether this will come about firstthrough some ecological disaster, through the decay and demoralisation of thetechnological structure, or through a military holocaust” [3].

References

1. Ferris, T. (1997) The New Yorker, April 14, p. 31.2. Forster, E. M. (1925) Alexandria, A History and a Guide, Peter Smith, London

(considered the best guide book ever written).3. Brooks, H. (1971) Science, 174, 21.

xxiv PROLOGUE






solar system evolution

Documents