千米小说网

Chapter 1 The Solar System（第3页）

longagothishappened，beeraieredies。Wemeasureradioactivedecayproduthemtoworke，articularlymemorablenumber:4。567billiohemost‘primitive’meteorites，whicharefragmentsofsmallplahatneversufferedheatiion，arecalled‘adrites’adireditionsintheearlySolarSystem。

&o，shadbeeiallyamatterofetesimalsreachedabout10kilometresihegreatergravitationalpullestoneswasabletomakeitselffelt。Thesesufferedmorefrequents，srowthoutpacedthatoftheothers。Withiensofthousahelargestplaesimalshadgrowntoathousandorsokilometresacross，gobblingupmostofthesmalleroheprocess。

Theselargeplaesimalsaredignifiedbyaaryembryos’。MaybeafewhundredwereformedintheinnerSolarSystem。Theywouldhavebeenmassiveenoughfravitytopullthemintosphericalshapes。Theymayhavebeeernallyftooccur，allowingirontosinkinwardstoformadistihatislargelyimmaterialbecauseofe。

&aryembryoswerewhattheterrestrialplasformedfrom。Nowthatthemajorityofthesmallstuffwasgone，signifitgrowthcouldhappewoembryossmashedtogether。Suchaisreferredtoasa‘giantimpadliberateseelymeltthemergedbodyformedbytheagineasphereofmlowiexceptforafewraftsofchilleditssurface，withdeepinsidea‘rain’ofirolinginwardsthroughthesilicatemagmatoatothetraldyoushouldhaveapiyourmindthatveysthestateofaplaaryembryoihofagiantimpact。

Thisassumesthattheimpaashbothbodiestosmithereens，butiablyaamountofdebriswillbethrownouttospaceasejethe。Itprobablytookabout50millioobuildupah-sizedplabyserialgiantimpactsbetweearyembryos。Becauseoftherahesandtheplex‘familytree’ofgiantimpactsbetweehemselveshadbeenfiantimpacts，itismeaardanysingleembryoearlyintheprocessas‘theproto-Earth’or‘theproto-Venus’。

&heorbitravitatioheyouroostirtheroetesimalsiricorbits，sothatmutualswereoftentooviolenttoallowgrowthbya。Instead，fragmentationwasae，seplahatmighthaveeventuallycollidedtoproduceafifthterrestrialplaogrowhere。Today，inthatregion，wefindmostoftheasteronlyatinyfraassthatohere。Jupiterscatteredthemajorityintomarkedlyetricorbits，sothateventuallymostcollidedwithJupiteria，orwereejetheSolarSystementirely。

OpinionisalsodividedovertherelativeratesofplaarygrowthierpartsoftheSolarSystem，anditisuherJupiterformedbeforeoraftertheEarthandVeheygrewbyembryo–embryos，Saturn，Uranus，ahavegrowhanJupiterbecausesshouldbelessfrequentwithingdistaheSun。

SgofgasfromthenebulawastermiheSuTauri’phase，arTTauriwhidergoingthisprocesstoday。Forperhaps10milliorongoutflowofgasfromthestar，calledthe‘TTauriwind’，blowsawayalltheremaininggasanddust。AlikelyreasonforUraunehavingproportiohaplahattheytrow，leaviocollectgasbeforetheTTauriwindputaheprocess。

Migratis

Afurthermatterofdebateswaysinwhichorbitsgeovertimeaowhichthishappeicularlyamoplailthesolarnebulaersed，gravitatioioweeerialaiendgraduallytodecreasetheradiusoftheirorbits，gembryosandyouomigrateinwards。Afternebulardispersal，gravitatioiosandsmallerbodiescouldhaveplayedaneveiesuggestaperiodofhalfabillionyearsorsowhegiawasdeflegtheorbitsofoutlyingietesimalsinwards，wheretheywouldbelikelyeventuallytobeherinwardsbyiiogia，andsoonuntiltheypassedoughtoJupiterforJupitertofliwards。Theseout-flungietesimalscouldbetheinoftoday’sOortCloud。JupitermusthavemovedfraallyclosertotheSuimeitflungabodyoutwards，butverselytheiaswouldhavebeewardseaeofthemsofiwards。ThisstoryhasJupitermigratinginwards，whileSaturn，Urauwards。ItiseveUraunesrovidinganopportunityforUranus’saxistobeetippedoverintoitspreseoday’sThatsurvivedbeyondthezoclearduri>

&heless，theplaheirmutualgravitationalpullsareuouslygfiguration。Chaostheorysaysitisthereforeimpossibletopredietarypositiohanafewmillionyearsahead。However，itbeshownthattheSolarSystemissuffitlystablethatislikelytocollideorbeejethefewbillionyears。robablysafeforatleast5billionyears，whiastroheSuiahewanderingsofMarswillbetheleastoftheproblemsfayfarfutureEarthlings。

Whyallthesatellites?

Bynow，youshouldnotbesurprisedthatthereisnhtforwardahequestioellitesssidetheirplasorwereacquiredlater。Thelargeprogradesatellitesofthegiaheeasiesttoexplaihoughttohaveformedwithinacloudofgasanddustsurrouplagrew，ratherlikeaminiatureversionofthesolarinyprogradesatellitesoresiiplasareprmentsersatellitesthatcametoodbrokeapart。Theoutersatellitesofgiasaremradeorbits，andtheseareprobablycapturedbodiesthatbegaNOs，oretnuclei。

&ically，itisalmostimpossibleforaplaureapassingvisitorintoorbitaboutitself。Aninallerbodywillbeslahepullofitsgravity，butit’teasilybesloweddowobecapturedintoorbit。However，ifthevisitorisadoubleobjeeofthepairbecapturedbytransferofmomentumtotheothermember，whichwillscootawayeveheenter。Atlyfavouredexplanatiourogradesatellite，Triton，isthatTritonwasformerlyhalfofadoubleTNOthatstrayedeptuhisseemsplausible，becauseseveralTobetwinbodies。Mindyou，itleavesuheissueofwhysomanyTNOs（aeroidstoo）havesatellitesiplace。

&h’sMoonhasadiffereioohavesedfromdebristhrownintoorbitabouttheEarthbythefinalembryo–embryooftheseriesbywhichtheEarthgrew。ThetwotiesofMars（PhobosandDeimos）areasteroids，tureintoclosecircularorbitsisood。

saeringtimescale

Thereisawell-uoodhierarchyofcratermiesdependingoer，androducedexperimentallyandinodels。OntheMoon，cratersfrommicroscopicsizeupto15kilometresacrosshavesimpleboes。Thenuptoabout140kilometresdiameter，cratersdonotbeedeeperbuthaveflatfloors，andusuallyatralpeakformedbyrebouelyafterex。ThereisaniceexampleopofFigure2。Largercratersmayhaveagroupoftralpeaks，aersbiggerthan350kilometrestaketheformoftwoormoretrigs。Thetransitionsfromoherhtlysmallerdiametersorravity。

&h’sgrecordispoorlypreserved，becauseitisawhereprocessesthateraseorburycratersalmostkeeppacewiththerateatwhichcratersareformed。FortuhevasttratterrainsurvivingontheMoonallowustotthedensityofimpactsurfaceswhoseagesareknown，thankstodatablesamplesreturheAprammeofmannedlunarlandings，supplementedbyafewSovietunmanurnmissions。BythismeahedateofthelateheavybombardmentandalserateatwhichghasaffectedtheMooheEarthmusthavebeehesamefluxofimpactorsasitssatellite，andtherearegoodreasohatthisisalsoareasonableapproximationforMerus，andMars。gcratersisthusthebestwaywehavetoestimateagesoarysurfaiftheabsoluteageisindoubt，itisusuallysafetoassumethatasurfacewithalowersityisyouhahahighersity。

2。A470-kilometre-wideviewofaheavilycrateredregionoftheMoon。Mostofthesecratersdatefromabout3。9billiaers。Eachcraterwasformedbytheimpactofabodyabout20to30timessmallerthaeritself。PartsoftheEarthwouldonilar

Atpresehishitannuallybyabout10，reaterthan1kilogram，butmostofthosearetoosmalltosurvivepassagethroughtheatmosphere，wheretheyareheatedandwornawaybyfri。Theyearlysupplyof1，000-kilogrammeteoritesisonlyabout10，aervalbetweenimpaetre-diameterobjects（whichwouldproduceaetresacross）isabout5，000years。Impaetreierarriveatrandomaboutonceevery20throughtheatmosphereasifitwereingthegroundwithundiminishedspeed，andfacraterperhaps20kilometresacrer，aing，impactsareeve。

&sasabodesoflife

&hwerenotatafortabledistaheSun，youwouldhisbook，becauselifemaynothavebeeestablishedandweothaveevolvedhereevenifithad。Stiststalkofa‘habitablezone’arouar，atadistahesurfacetemperatureowouldbenortoocoldforlife。ByanalogywithGoldilocks’preferenceforbabybear’spe（whosetemperaturewas‘justright’），thehabitablezoimescalledthe‘Goldiloe’。Inthistext，‘habitable’meacouldsustainlifeofasimplemicrobes。Itdoesnotimplythattheewouldbeinhabitablebyhumans。

Becauseourkindofliferequireswater，thehabitablezoneisusuallyequatedwiththedistanastaratwhichthesurfacetemperatureofaplawouldeoexistinliquidstate。Thedensityandpositionofaplamosphereihesurfacetemperature，butthemaintrolisheatrethestar。ThehabitablezoheSuedtoextendfromabout0。95toabout1。5AU。TheseestimatesputVenus（0。72AU）wellinwardoftheihehabitablezone，andMars（1。52AU）atitse。TheSun’soutputhasprobablyilysiswerefiablezowardsovertime，soMarswouldseemtobeapoordidateforlife，butitisnotahopelesscase。

Theideaofahabitablezonedefinedbyplaarysurfacetemperaturehasbeencriticizedastoonarrow。Thereareceswhereheatgehimayprovideanealableforlife，althoughthesurfacemayseeminhospitable。Eveh，wekremanismslivingbelow0°Corabove100°C。Thus，evenifalllifeis，likethatoh，basedonsofoleddepeerasasolvent，thereareseveralplatheSolarSystemwhereitcouldexist（thoughoheEarth，whereitisk）amanymillionsofhabitableplaceselsewhereinthegalaxy。Ishallreturntothatthemehebook。

&ion

Telescopesareveryuseful，forexampletomeasurethetemperatureandpositionofapla’ssurfadatmosphere。ThepolariarswerecorretifiedbyWilliamHerschelaslongagoas1781。Jupiterissuffitlylargeaitsbemohfairlymodesttelescopes。However，thisbookwouldbedulleraivewereitnotforhalfaturyofspaceexplorationrobesfromEarthhavevisitedeveryplaheSolarSystem。SovietprobesreachedtheMoonin1959，andtwelveAmeriautswalkedonitssurfa1969and1972。UnmannedAmeriASA）aprobesflewpastVenusandMarsinthe1960s，aandsoftlandingsdurihefirstJupiterandSaturnfly-byswereinthe1970s，ahergiaswerevisitedinthe1980s。Siheterrestrialplashavebeenexploredbyinglycapableorbiters，roboticrovershavecrawledaartiansurfaplexorbitaltoursofbothJupiterandSaturnhavebeenachieved。

HighlightsintheyearsaheadioEarthofsamplescollears，asteroids，as，andarenewalofhumaheMoon。TheUSAandRussiaareheonlyspacepowers。TheEuropeanSpaesolotoMarsaoSaturnjointlywithNASA，andwillsoohJapan。TheJapaprobestotheMoonandtoanasteroid，anddIndiahaveeachreachedtheMooifically，therehasbeenmuchostprobesstrumentstributedbyseveralnations），butitisuhereisalsonatioake，togetherwithlierterests。