The Origin and Evolution of the Solar System The Graduate Series in Astronomy SeriesEditors: MElvis,Harvard–SmithsonianCenterforAstrophysics ANatta,OsservatoriodiArcetri,Florence The Graduate Series in Astronomy includes books on all aspects of theoretical and experimentalastronomyand astrophysics. The books are written at a level suitableforseniorundergraduateandgraduatestudents,andwillalsobeusefulto practisingastronomerswhowishtorefreshtheirknowledgeofaparticularfield ofresearch. Otherbooksintheseries DustintheGalacticEnvironment DCBWhittet ObservationalAstrophysics REWhite(ed) StellarAstrophysics RJTayler(ed) DustandChemistryinAstronomy TJMillarandDAWilliams(ed) ThePhysicsoftheInterstellarMedium JEDysonandDAWilliams Forthcomingtitles TheIsotropicUniverse,2ndedition DRaine DustintheGalacticEnvironment,2ndedition DCBWhittet The Graduate Series in Astronomy The Origin and Evolution of the Solar System M M Woolfson Department of Physics University of York, UK Institute of Physics Publishing Bristol and Philadelphia c IOPPublishingLtd2000 (cid:13) All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical,photocopying,recordingorotherwise,withoutthepriorpermission of the publisher. Multiple copying is permitted in accordance with the terms of licences issued by the Copyright Licensing Agency under the terms of its agreementwiththeCommitteeofVice-ChancellorsandPrincipals. BritishLibraryCataloguing-in-PublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary. ISBN075030457X(hbk) 0750304588(pbk) LibraryofCongressCataloging-in-PublicationDataareavailable SeriesEditors: MElvis,Harvard–SmithsonianCenterforAstrophysics ANatta,OsservatoriodiArcetri,Florence Publisher:NickiDennis CommissioningEditor:JohnNavas ProductionEditor:SimonLaurenson ProductionControl:SarahPlenty CoverDesign:VictoriaLeBillon MarketingExecutive:ColinFenton Published by Institute of Physics Publishing, wholly owned by The Institute of Physics,London InstituteofPhysicsPublishing,DiracHouse,TempleBack,BristolBS16BE,UK US Office: Institute of Physics Publishing, The Public Ledger Building, Suite 1035,150SouthIndependenceMallWest,Philadelphia,PA19106,USA TypesetinTEXusingtheIOPBookmakerMacros PrintedintheUKbyBookcraft,MidsomerNorton,Somerset Contents Introduction xv PART1 Thegeneralbackground 1 1 ThestructureoftheSolarSystem 3 1.1 Introduction 3 1.2 Planetaryorbitsandsolarspin 4 1.2.1 Two-bodymotion 4 1.2.2 Solarsystemorbits 6 1.2.3 Commensurableorbits 8 1.2.4 Angularmomentumdistribution 10 1.3 Planetarystructure 10 1.3.1 Theterrestrialplanets 10 1.3.2 Themajorplanets 12 1.3.3 Pluto 13 1.4 Satellitesystems,ringsandplanetaryspins 14 1.4.1 Classification 14 1.4.2 TheJoviansystem 15 1.4.3 TheSaturniansystem 18 1.4.4 SatellitesofUranusandNeptune 20 1.4.5 SpinsandsatellitesofMercury,Venus,MarsandPluto 23 1.4.6 TheEarth–Moonsystem 24 1.5 Asteroids 30 1.5.1 Characteristicsofthemajorasteroids 30 1.5.2 Thedistributionofasteroidorbits:Kirkwoodgaps 32 1.5.3 Thecompositionsofasteroids 32 1.6 Meteorites 35 1.6.1 Fallsandfinds 36 1.6.2 Stonymeteorites 37 1.6.3 Stony-irons 38 1.6.4 Ironmeteorites 38 viii Contents 1.6.5 Isotopicanomaliesinmeteorites 39 1.7 Comets 41 1.7.1 Typesofcometorbit 41 1.7.2 Thephysicalstructureofcomets 43 1.7.3 TheKuiperbelt 45 2 Observationsandtheoriesofstarformation 46 2.1 Starsandstellarevolution 46 2.1.1 Brightnessanddistance 46 2.1.2 Luminosity,temperatureandspectralclass 48 2.1.3 ThemotionsofstarsrelativetotheSun 50 2.1.4 Themassesofstars 51 2.1.5 TheHertzsprung–Russelldiagramandmain-sequencestars 52 2.1.6 Thespinratesofstars 54 2.1.7 Evolutionofstarsawayfromthemainsequence 54 2.2 Theformationofdenseinterstellarclouds 59 2.2.1 Denseinterstellarclouds 59 2.2.2 HeatingandcoolingintheISM 59 2.2.3 Thepressure-densityrelationshipforthermalequilibrium 62 2.2.4 Jeans’stabilitycriterion 63 2.2.5 Mechanismsforformingcooldenseclouds 65 2.3 Theevolutionofproto-stars 72 2.3.1 TheHayashimodel 72 2.4 Observationsofstarformation 75 2.4.1 Infraredobservations 75 2.4.2 Radio-waveobservations 75 2.5 Observationofyoungstars 77 2.5.1 Identifyingyoungstellarclusters 77 2.5.2 Age–massrelationshipsinyoungclusters 78 2.6 Theoriesofstarformation 79 2.6.1 Starsandstellarclusters 79 2.6.2 Ageneraltheoryofstarformationinagalacticcluster 80 2.7 Planetsaroundotherstars 95 2.8 Circumstellardiscs 98 3 Whatshouldatheoryexplain? 100 3.1 Thenatureofscientifictheories 100 3.1.1 Whatisagoodtheory? 100 3.1.2 Theacceptanceofnewtheories 101 3.1.3 ParticularproblemsassociatedwiththeSolarSystem 102 3.2 Requiredfeaturesoftheories 103 3.2.1 First-orderfeatures 103 3.2.2 Second-orderfeatures 104 3.2.3 Third-orderfeatures 106 Contents ix PART2 Settingthetheoreticalscene 109 4 Theoriesupto1960 111 4.1 Thehistoricalbackground 111 4.1.1 Contributionsoftheancientworld 111 4.1.2 FromCopernicustoNewton 113 4.2 Buffon’scomettheory 117 4.3 TheLaplacenebulatheory 118 4.3.1 Somepreliminaryideas 118 4.3.2 ThenebulamodelofSolarSystemformation 119 4.3.3 Objectionsanddifficulties 120 4.4 TheRochemodel 121 4.4.1 Roche’smodificationofLaplace’stheory 121 4.4.2 ObjectionstoRoche’stheory 122 4.5 TheChamberlinandMoultonplanetesimaltheory 124 4.5.1 Theplanetesimalidea 124 4.5.2 TheChamberlin–Moultondualistictheory 125 4.5.3 ObjectionstotheChamberlin–Moultontheory 126 4.6 TheJeanstidaltheory 127 4.6.1 Adescriptionofthetidaltheory 127 4.6.2 Thetidaldisruptionofastar 129 4.6.3 The break-up of a filament and the formation of proto- planets 130 4.6.4 ObjectionstoJeans’theory 131 4.7 TheSchmidt–Lyttletonaccretiontheory 133 4.7.1 TheSchmidthypothesis 133 4.7.2 Lyttleton’smodificationoftheaccretiontheory 134 4.7.3 Theproblemsoftheaccretiontheory 135 4.8 ThevonWeizsa¨ckervortextheory 136 4.8.1 Thebasicmodel 136 4.8.2 ObjectionstothevonWeizsa¨ckermodel 137 4.9 Themajorproblemsrevealed 137 4.9.1 Theproblemofangularmomentumdistribution 137 4.9.2 Planetformation 138 4.9.3 Implicationsfromtheearlytheories 139 x Contents PART3 Currenttheories 141 5 Abriefsurveyofmoderntheories 143 5.1 Themethodofsurveyingtheories 143 5.2 TheProto-planetTheory 144 5.3 TheCaptureTheory 146 5.4 TheSolarNebulaTheory 149 5.5 TheModernLaplacianTheory 151 5.6 Analysingthemoderntheories 155 6 TheSun,planetsandsatellites 156 6.1 Surveyingextanttheories 156 6.2 FormationoftheSun:dualistictheories 156 6.2.1 Themagneticbrakingofsolarspin 158 6.2.2 Thesolarspinaxis 162 6.3 FormationoftheSun:monistictheories 163 6.3.1 Removingangularmomentumfromacollapsingnebula 163 6.4 Formationofplanets 169 6.4.1 PlanetsfromtheProto-planetTheory 169 6.4.2 PlanetsfromtheCaptureTheory 171 6.4.3 PlanetsfromtheSolarNebulaTheory 184 6.4.4 PlanetsfromtheModernLaplacianTheory 192 6.5 Formationofsatellites 195 6.5.1 SatellitesfromtheProto-planetTheory 196 6.5.2 SatellitesfromtheModernLaplacianTheory 198 6.5.3 SatellitesfromtheCaptureTheory 198 6.6 Successesandremainingproblemsofmoderntheories 204 6.6.1 TheSolarNebulaTheory 204 6.6.2 TheAccretionTheory 205 6.6.3 TheModernLaplacianTheory 205 6.6.4 TheCaptureTheory 206 6.6.5 TheProto-planetTheory 207 7 Planetaryorbitsandangularmomentum 209 7.1 Theevolutionofplanetaryorbits 209 7.1.1 Round-offduetotidaleffects 209 7.1.2 Round-offinaresistingmedium 210 7.1.3 Bode’slaw 214 7.1.4 CommensurabilityoftheJoviansatellitesystem 215 7.1.5 Commensurabilityofplanetaryorbits 216 7.2 Initialplanetaryorbits 221 7.2.1 TheAccretionandSolarNebulaTheories 222 7.2.2 TheProto-planetTheory 223 7.2.3 TheCaptureTheory 223 Contents xi 7.3 Angularmomentum 225 7.3.1 AngularmomentumandtheProto-planetTheory 225 7.3.2 AngularmomentumandtheModernLaplacianandSolar NebulaTheories 227 7.3.3 AngularmomentumandtheCaptureTheory 228 7.3.4 AngularmomentumandtheAccretionTheory 229 7.4 ThespinaxesoftheSunandtheplanets 229 7.4.1 SpinaxesandtheSolarNebulaTheory 230 7.4.2 SpinaxesandtheModernLaplacianTheory 232 7.4.3 SpinaxesandtheAccretionTheory 232 7.4.4 SpinaxesandtheProto-planetTheory 233 7.4.5 SpinaxesandtheCaptureTheory 234 8 Aplanetarycollision 237 8.1 Interactionsbetweenproto-planets 237 8.1.1 Probabilitiesofinteractionsleadingtoescape 237 8.1.2 Probabilitiesofinteractionsleadingtoacollision 242 8.1.3 Numericalcalculationofcharacteristictimes 243 8.2 TheEarthandVenus 244 8.2.1 Aplanetarycollision;generalconsiderations 245 8.2.2 AcollisionbetweenplanetsAandB 246 9 TheMoon 251 9.1 TheoriginoftheEarth–Moonsystem 251 9.1.1 Thefissionhypothesis 251 9.1.2 Co-accretionoftheEarthandtheMoon 254 9.1.3 CaptureoftheMoonfromaheliocentricorbit 255 9.1.4 Thesingleimpacttheory 256 9.1.5 TheEarth–Moonsystemfromaplanetarycollision 261 9.2 The chemistry of the Earth and the Moon and formation of the Moon 263 9.2.1 PossiblemodelsofMoonformation 265 9.3 ThephysicalstructureoftheMoon 267 9.3.1 Hemisphericalasymmetrybybombardment 269 9.3.2 AcollisionhistoryoftheMoon 271 9.3.3 Mascons 272 9.3.4 Masconsandbasaltsinmarebasins 274 9.3.5 VolcanismandtheevolutionoftheMoon 276 9.3.6 Calculationsofthermalevolution 278 9.4 Lunarmagnetism 282 9.4.1 Adynamotheory 284 9.4.2 Theinductionmodeloflunarmagnetism 285 9.5 Summary 293 xii Contents 10 Smallerplanetsandirregularsatellites 294 10.1 Introduction 294 10.2 Mars 295 10.2.1 Marsaccordingtoaccretiontheories 296 10.2.2 Marsaccordingtotheplanet-collisionhypothesis 296 10.2.3 TheMartiancrust 298 10.2.4 TheCOM–COFoffset 300 10.2.5 PolarwanderonMars 302 10.3 AgeneraldescriptionofMercury 303 10.3.1 Mercuryandaccretiontheories 305 10.3.2 MercuryandtheCaptureTheory 306 10.4 Neptune,PlutoandTriton 307 10.4.1 EncounterscenariosfortheNeptune–Triton–Plutosystem 308 10.4.2 CommentsontheNeptune–Triton–Plutosystem 311 10.5 Irregularsatellites 313 10.6 Summary 314 11 Asteroids,meteoritesandcomets 316 11.1 Asteroidformation 316 11.2 Meteorites 317 11.2.1 Stonymeteorites 318 11.3 Stonyirons 322 11.4 Ironmeteorites 324 11.5 Informationfrommeteorites 325 11.6 Isotopicanomaliesinmeteorites 326 11.6.1 Oxygenisotopicanomalies 327 11.6.2 Magnesiuminmeteorites 328 11.6.3 Neoninmeteorites 330 11.6.4 Anomaliesinsiliconcarbidegrains 331 11.6.5 Thedeuteriumanomaly 332 11.7 Explanationsofisotopicanomaliesinmeteorites 332 11.7.1 Aplanetarycollisionoriginforisotopicanomalies 334 11.8 Comets—ageneralsurvey 354 11.8.1 NewcometsandtheOortcloud 357 11.9 Theinner-cloudscenario 364 11.10Kuiper-beltobjects 366 11.11Cometsfromtheplanetarycollision 367 11.12Ideasabouttheoriginandfeaturesofsmallbodies 368