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Chapter 2 Rocky planets（第1页）

Chapter2　Roets

Here，Iwilldiscusstheplaweliveonandotherbodieslikeit，herterrestrialplasMerus，andMars，andalsotheMooroheIAU，theMoonisjustasatellite，butitspositioructureplagtheterrestrialplaheperspectiveofageeophysicist。Figure3showsthesefiveatthesamesdTable3listssomerelevantdata。Withinthisgroup，MerdtheMooivelynoatmosphere。Venushasonlyslightlylowersize，mass，ahah，sogravityatitssurfalyslightlylessthah。However，itsatmosphereissiderablydenser。Marsislargerthalessdewoeffectsoffseteachothersothattheirsurfacegravitiesareverysimilar，butbeingarshasbeeoholdontoathiableatmosphere。TheMoosurfacegravityofall–aboutohoftheEarth’s–alkersboundaroundsely。Meauresobscurewidevariationswithlatitudeand，insomecases，betweendayandnight。Forexample，thehottestdaytimetemperatureonMercuryexceeds400°C，whereasatdawnafteralhtthetemperatureisbelow–180°C。

Table3　Basicdatafortheterrestrialplas

3。Top:fromlefttht，Merus，Earth，Moon，andMars，shownatthesames:themuchlargergiaer，Saturn，Uranus，aheterrestrialplahesamescale

Cores

&rialplainguishedbyhavieriors，madelargelyofsilierals。However，theirdeoogreatforthemtoberhout，anditisbelievedthateairon-richcoreatitsopla’sbeseenorsampleddirectly，butthereareseveralilinesofevideyisone，showingthattheibedehanroallowingforinterhighpressure，ahetrajectoriesspacecraftshowthatdensityiricallyabouteaet’stre。Chemicalmodelsforwhatislikelytohappeninsidearoetsuggestthatthereisioxygenforalltheirontobeoxidizedandboundupinsilierals。Thus，iftheinteriorhadeverbeehiswouldhaveallowedmetalli，whiserthanroktowardsthetre。Thisisanexampleofaprocessescalleddifferentiation。

&erpartsoftheiron-richcoresoftheEarthandMercurymustbemoltentoday，becausethosetwoplaroicfields，appareedbydynamomotioridugfluid。Forsuchasmallpla，Mersityisveryhigh，soitsustbeexallylarge，ogabout40%ofitsvolumeandagfornearly75%ofitsmass。MagicfieldsarenotbeiediheMoon，andMars，sotheircoresareprobablyentirelysolid。

IheEarth，wehaveadditiothestudyinghowseismicwaves，whicharevibratieredbyearthquakes（rous!），travelthroughtheplahissasolidinnercore1，215kilometresinradiusandafluidouteretresinradius。Bothseemtobemainlyiroh5%–10%densityargumentsrequiresomethihanirontoo，makingup6%–10%oftheouterd2%–5%oftheihelikeliestexplanationissomemixofoxygen，silidsulfur。

Intotal，theEarth’scoreoccupiesabout16%ofthepla’svolume。parablevaluesforVenusandMars，whichareestimatesbasedlargelyedensities，areabout12%aively。Therearesomeverylimitedseismioon（fromtheApramme），hintiivelysmallcorebetweenabout220and450kilometresinradius（lessthan4%oftheMoon’stotalvolume）。About1ieoritesismadeofanalloyofironwith4。5%–18%nickel，dingtothecoresofplaesimalsfromtheasteroidbeltthathaddiffereernallybefbrokenupbys。

Mantlesandcrusts

&epartofaterrestrialplasurroundingitscoreiscalledthemahismakesupthemajorityofthetotalvolumeofeachterrestrialpla，andmostoftheirmassexceptfor

&hecrustisarelativelyminoruhemaisalsoofsilicate，thoughslightlydifferentinpositiole。

Aplaleevolvedfromthemoltenrockthatwouldhaveclobeafterthefinalgiantimpact，ksasa‘magmao’。Whileamagmaocools，itssurfacemustradiateheattospagittochilltoasolidskihisskinwoulduallybebrokenandstirredba，thankstoturbulenpaabove。Themagmaowoulduetocool，but，uhefreezingofaballofwater，thereisemperatureatwhichthewholeofitwouldbeesolid。Theeerialissuchthatmineralsofvariouspositionscrystallizeoutatdiffereuresandpressures。Plaarystistsareuheextenttowhichmagmaoscrystallizedinlayers，orwhethermihawereabletosinkwhileless-densemiorise，perhapsstigtogethertoformmassive‘rs’thatcouldforcetheirmoreeffectively。

&ionsofthisflotationmaterial，chemicallydifferenttotheunderlyingmagmaoedtheearliesttrueoosurvivestodayasthelunarhighlands（thepaleareasontheMoohelargerterrestrialplaureoftheoldestcrusthasermilybecauseithaslargelybeeleast，coveredover）bylaterkindsofcrust。Toseehowthismighthappeurionbaantle。Asayoucools，eventuallyitsmantlebeesfullysolid。Twoimportantcharacteristicsofsilicatematerials。Thefirstisthatsuffitlyhotsolidsareelyimmobilenorundeformable。Hotroaplaerioriscapableoffloeedsofafewtimetresperyear（therateatwhichyernailsgrow），inmuchthesamewayasablockofpitchwilldeformovertime。Withinasolidmaionwilloccurataslowbutgeologicallyeffectivepaceifthereareanyforcescapableit。I，thenecessaryimpetusheat。Hottermantlefromdeepdowlylessdeheahereisateoslaces。Motionofthissortisvediswhatyouobserveinapanofsoupheatedohatlaismuchslower‘solidstate’。

Imagireameror‘plume’ofhotmantlewellinguplaantledownwards。Asitgetsclosertothesurface，thepressureexperieherisingmantledecreases，whigsintoplaythesedrelevantcharacteristic。Aspressurefalls，silicatesbegiheprocessiscalled‘partialmelting’，belypartofthesolidmelts，ahatformsisslightlyrisilithesolidfromwhichitwasextracted。Theresultingmagmaisalsolessdehesolid，sobuoyancyforceswillsqueezeitupwardstowardsthesurface，especiallyiftherearepathwayswheretherodertensionorfralessitstallsbelowgroundasanintrusion，themagmawillbeeruptedthroughvoloes。

Roedinthiswayisdescribedasigneous，andcrustprodueousactivityreplacetheinalcrustofaplabyinfiltrationorburial。Thedarkpatoon，thelunar‘maria’，areliohepalerprimarycrusthasbeenburiedbylavaflowsproduthisresentdaycrustoftheEarthresultsfrompartialmeltiomakeoicdfrommeltingandreaionsofoiaketalcrust。TheEarth’soiccrustis6–11kilometresthick，whereastalcrustvariesfromabout25kilometresinthichedregionsto90kilometresbelowmajormes。Intotal，crustolyabout1%oftheEarth’stotalvolume。TheMoon’scrustaveragesabout70kilometresinthiess（13%oftheMoon’svfrom〉100kilometresinsomehighlao〈20kilometresbeloas。

Insummary，crustischemicallyrelatedtotheunderlyidiffersinendingonhowitwasextrait。Crustisloweriyanditsaveragepositionisrisilithemaismorevariedthanmantle，ahathasreactedchemicallyhereorliquidwater，andthathasbeeordissolved，transpravity，wind，water，oriddepositedelsewhere。Suchdepositsstitutesedimentaryrock。Burial，deformatioingcausesedimentaryneousrocktorecrystallize，inwhichcaseitiskamorphicrock。

&

&sarehotilybecauseofheatleftoverfromtheira。Ferplaionofthis‘primordialheat’remainingtodayisgreater。Thisisbecauseheattisrelatedtoplaaryvolume，whidsoheradius，whereasheatleakageislimitedbysurfacearea，whidsonlyonthesquareoftheradius。

&isalsogeedi，bydecayofradioactiveisotopes。Therearemaonlyfourroduifitheating:potassium-40，uranium-238，uranium-235，andthorium-232。Becauseeochemiities，theseelementsaremoreabundantincrustalroile。Ih，approximatelythesameamouisgeedradiogenially（thatis，byradioactivedethecrustasinthewholeofthevolumetricallymuchlargermantle。

&rialplaaltelemesmass（asvolume）。Justlikeprimordialheat，radiogeaiivelyis。IheEarth，abouthalftheheatleakingtothesurfacetodayisprimordial，andalmostalltherestisradiogenic。

Lithospheres

&ioiesfromdrigidtowarmaivegenerallyocedepthbelowthebouweendmahedtheuppermostmauteasinglemeicallayer，fidoutershell。Thisshelliscalledthe‘liththeGreekwordlithos（‘rodicatethatthelayerhasthemeicalpropertiesofeverydayrock。Belowthelithosphereiswherethemahoughroposition，ishotenoughaovect。ThiszoimescalledtheasthenGreeka-sthenosmeaning‘withth’）。

&h’slithosphereisabout100kilometresthidisfratoaes，whichareabletobeshuntedaroundthankstothespeessoftheunderlyingastheofaproas‘plateteiewlithosphereiscreatedullapart（usuallyhiddeheoddestroyedwhereoeisdrawher，atsubduarkedbytreheoflrindieagainstitsneighbouristheostearthquakes。IfahattheEarth’splatesare‘crustslidihemaheyarewrapersistentfallacythatappearsinmanyschooltextbooksaionsyllabuses。Thetruthisthataplatesistsofdtheedrigiduppermostmaogetherslideacrossthedeeper，lessrigid，asthele。

&hbrittle，isthelayerwherefaultsoassrindspastasareoh，espethezoesmeet，aifiedostoo（Figure4）。

&oobeuheEarth。GreaterlithospheriessinthemoreeasilyallerbodiesofMercury，theMoon，andMarsuributetothis，butamoreimportantfactoristhatforplatestobemobile，thetopoftheastheobeespeciallyweak。Withihisisaplishedbeallamouhintherock，whisitandeheformationofasmallamoulubricatesgrainboundaries。Venushaslostitswater，soitsasthenosphereisdryandlithosphericplatesotslidefreelyacrossit。

4。A500-kilometre-wideviewofpartofMerinationisfrht。ShadowpicksoutakilhscarpwiththeshapeofaerMohisisafaultnamedBeagleRupes，markierrai（east）hasbeewardsovertheterrai（west）。Someofthecratersareolderanderthanthisfault

Aplaheisdry，orverydeep，ismaedprincipallybytwoeffectsatthesurfaeistheheightofmountaihofbasioogreat，theasthenospherewillflowahelithosphere，therebyredugthetopographitrastuntilitissmallenoughtobesupportedbythestrengthofthelithospherealohepatterncausedbylargeimpaimpactorseveraltensofkilometresierarriveswithsuffitforcefingshockwavestodisruptthelithosphere，aerwilltaketheformofabasinmarkedbytrigsoffraathihosphere，theriobeclether，sothesemulti-ribasioestimatethedepthtotheastheimeoftheirformation。Asaplaslowlycools，itslithospherebeesgraduallythicker。

Volism

Magma，theeerupts，eratediessentiallybythreediffereappliofheatisohese，aheleastimportant:slowbuild-upofheattrappedbelolahospheremayaeepisodesofwidespreadvoldstrtidalstressesiarybainstinternalfri，leadingto‘tidalheatiively，degpressureinanupwellihemantlecausepartialmelting（forexample，leadiioh’soiccrust）。Also，itispossiblethatsuddeionihappelebelowaisexcavated，cerameltihirdmeismistoierileorlowercrust。WaterreducesthetemperatureatwhichsilicatesbegiheEarthhassofvoloesabovesubdues，becausewaterthathasbeendraggeddownwithintherocksofthesubdugplateescapesupwardsintothebaseoftheplate。Heresarenothotenwhendry，butpartialmeltingwillbeginassooroduthoughtherehasbeeemperature。

TheMoon

Peoplebegaeaboutvoloonalmostassoonascraterswereseenthroughtelescopes。Theywereorailbecause，asweareain，almostalltheoonafact，themajorvolioochesthatwereotobedried-upseabeds。Thatishoughtheystillbearthename‘mare’（pronouncedmah-ray），whichisLatihepluralis‘maria’（pronouncedmah-ri-a）。Thesecoverabout17%oftheMoon’ssurface，mostlyonthenearside，whichisthehemispherethatpermalyfacestheEarth。Here，lavaositionsimilartoterrestrialbasalthasfloodedthemajedimpas。

Spetswherethemarebasaltswereeruptedarehardtoidentify（Figure5）。Clearly，theydidheformofioes。Mostlikely，theywerefissuresthroughwhitainsofienlavawereexpelledbytheforceofexpandingvolicgastoheightswellinexetre。Onfallingtotheground，thelavawasstillhotenoughtospreadout，flowingdownhillforhundredsofkilometres。Mostofthefissurevehemselvesoverastheirrateoferuptionwaned，orwereburiedbylatereruptions。

FourofthesixApolloMoonlandings（1969–72）wereonmaria，whichareflatterandsosaferpladthanthehighlands。Samplesofmarebasaltbroughtbaalysisbedatedwithhighpreeasuringradioactivedecayproduthem（radi），andtheApollosamplessheesfrom3。9to3。1billiohislongdurationofvolismputpaidtothesimplestvoliationforthemaria，whichwasthatthevolismroductofthebasin-fimpacts。Furthermore，worksiheyear2000hasidentifiedsomepatarebearingsuffitlyfewsuperimposedimpactcratersthattheymustbeyouhanabout1。2billionyears。versely，in2mentoflunarmaterialfouhasmeteorite（havingpreviouslybeenflungofftheMooafromanimpactcrater）wasfoundtofragmentsofbasaltdatedat4。35billionyears，halfabilliohelateheavybombardmentended。Nomariaeremainvisible，havingbeenburiedbyejesubsequentbasin-fimpaothatlunarvolismbeganearlyaswellasfinishinglate。

5。A200-kilometre-wideviewofthesouth-eastedgeofMareImbrium。Theruggedterraiishighlandcrustupliftedinpartofthebasinrim。Thedarker，smootherareaiismarebasaltsthathavefloodedthelround。A1-kilometre-widetrenamedHadleyRillewindsfromsouthtonorthacrossthetreoftheview，andthisisbelievedtobeapathwayalongwhichlavaflowedfrelyobscuredbyshadoollo15laoHadleyRille，hemiddleoftheimage