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Chapter 2 Rocky planets（第2页）

Mercury

MercuryismuchlesswellknownthantheMoon。UnderhalfofitwasimagedbyNASA’sMariner10in1974–5，aheeduntilNASA’sMESSENGERprobebeganaseriesoffly-bysin2008。Thisrevealeddetailssuffiostpeople’sscepticismovertheextentofvolerple，ihesmoothterrainiandfillingthe120-kilometre-diameterbasinjustabhtoftreisedasvolic。PreviousdhtehatMercurylacksthetrastinalbedo（reflectedbrightweenpalerhighlandsahatmakesthemariasoobviousohisseemstobebecausethemineralsmakingupMercury’slavasuchlessironthanistypiar（arial）basalts。PlainsformedbylavaprobablystitutethemajorityofMercury’ssurfaareoldenoughtodatebacktotheeraofthelateheavybombardmentandaredeered，eraerssuperimposedonthem。

MESSENGERimagedafewvoltsandalsoetre-sizedblotebright，somedark–thatmaybesitesofparticularlyyoungexplosiveeruptions。HowloayedvolicallyactiveislikelytoremainuilaspacecraftachievesorbitaboutMerdreagessystematidiail。ThefirstcewillbewhenMESSEheorbitalphaseofitsmissionin2011，andiftheissueremaishouldbesettledbytheEuropeanSpacy’sBepiissionthatisduetoarriveatMer2020。Atpresent，itissafetosaythatextensiveareasoflavawereemplagaperiatleast4to3billio，andpossiblyextendingtowithibillionyears。Sugdurationofvolicaerotanticipated，afromthesamemysteriousheatsourcethatkeepspartofitsolten。

Venus

VenusismuchbiggerthassizeandmasswouldsuggestalmostasmuchradiogeproduastheEarth，andhenceasimilarlevelofvolicactivity。However，beuslacksplateteioperatesratherdifferently。

Venushasadelyosphere，whichkeptthesurfaysteryuntilitbecamepossibletostudyitbymeansure6sheofpartofVenusobtainedbyNASA’sMagellanprobe，edalmosttheebetween1990and1994。Radarimagesareassembledbyplexanalysisoftheecedbarespooaualstringofradarpulsesbeamedatthesurfaostpurposes，youtreatradarimagesliketheblad-whiteopticalimagesthattheyresemble，thoughiureisainlybyhhthelocalsurfaceis，ratherthanitsalbedoi。

Figure6typifiesmuus。Itshowsnumerousindividuallavaflows–sher（brighter）aher（darker）–thatflowedfromwesttoeastaage。ThelobatepatternoftheindividualflowscloselyresemblesthatoflavaflowsohandMars，butwhichishardtodisoonandMercurywheretheflinshavebeedegradedbyimpacts。

Aswellaslavaflabouthalfitssurfaushasmanyclearlyidentifiablevoloes。Figure7shole。Inthebadisa5-kilhvolowithgentlyslopingflanksofthekindkhasa‘shieldvolo’thatresultsfromrepeatederuptionofbasaltthroughasi。Someindividuallavaflowsbemadeoutontheflanks。Nooneissurehowlongagothisvoldotherslikeitlasterupted。Therehavebeenintriguinghintsbutnoproofofretorpresent-dayavoloessuchasthis，ahertoosmallforreliablecrater-gstatistics。Thisparticularvoloisbuiltupoerrainunitthatissmoother，exerousfracturesupothefroundisprobablyuhtlavaflowsimmediatelytoitsleft。

6。A500-kilometre-wideMagellaofVenus。Theareaismostlylava，fedfromasouretreswestoftheimage，butih-easterissedterraiheVenus。Runningfromnorthtosouthiheimageisamouedandfracturedterrainthatisbreachedbythelavaflows

Circularorellipticalpatternsoftricfracturestermed‘ae’，ofwhichmorethan300havebeeifiedonVehoughttoshareaii-ribasinsoftheMooheyythingfrom200metrestoover2，000kilometresadareusuallyassociatedwithsomeformofvolism。Probablyeaarksasitewelliheastheleimpihebaseofthelithosphere。aewheretheplumeisstillpreseedasverybroaddomes，whereasolderones，edbyamantleplume，havesagged。Thesagging，inparticular，explairicfractures。

7。puter-gehree-dimeiveviewshowingthevolohiswasmadebydrapingaradarimageoveramodelofthetopographyobtainedbyradaraltimetry。Verticalscaleisexaggeratedtenfold。Bothsetsofdatawerecollectedbythe　Magellaheimpacttherightfroundis23kilometresacross

ImpactVenusaremoreonthasiderablylessabundantthanontheMoonandMercury（youwillnotfindanyiherearetwofactorsatplayhere。Craterslessthanabout3kilometresacrossareefromVesdemosphereshieldsthesurfaallimpactors。Hercratersareformedbyobjectsgtoomuergytobeaffectedbytheatmosphere。Theirladaobeexplaiheyouhesurface，whichworksoutetobebetweenabout500millionand700millioherearesofterraiobeverymuugerthanthegle。

&aionforthisinthe1990srettymuchthewholeplawasresurfayofvolismthatbegan500–700millio，andlastedhaensofmilliohiswouldbetwithVenus’slackofplateteigtomostheatfromthedeepermarappedbelowthelithospheritilmuchoftheuppermostastheed。Eventually，theselithospherewouldfouhebuoyantmagmafrombelowwoulderupt。Somethingsimilarcouldhaveoccurredhalfadozentimessinuswasformed，acouldhappenagainwithi100millionyears。

Thismodelgflobalvolismhasretlybeenged，onthegroundsthatthegstatistiotruleradualprressivelysmallerareascouldhavebeenresurfacedbylavasatrahroughoutthepasthalfbillionyears。

&h

&h，voldplatetebiheiandthuspreventmajorastheemperatureexsofthekindpostulatedforVenus。Onlyaboutohirdoftheheatgehelithosphereleaksoutbyostoftheheatisveyedtothetopofthelithospherebyeruptionatmid-es（whereerialisaddedtplates）and，toasmallerexteionsatvoloesabovesubduesandatvarious‘hotspots’abovemaheasthenosphereiscooledbythereincorporationintoitoftheold，coldpartsoflithosphericplatesatsubdues。

&wegettoaVenus-likevoliccatastropheiswheensofmilliiohousaresacrossisburiedbytheeruptionofuptotenetresofbasaltlava。Thisisknownasa‘floodbasalt’。The‘DeTraps’ofIndia（66millioheBrito-Arcticfloodbasalts（Greenlah-westernBritishIsles，57millionyears），andthebiaRiverfloodbasalts（ernUSA，16millionyears）areamoer-knoles。Thesemajorbutrareeventscouldbecapableofiingsomuiotablysulfurdioxide，plusfisofvoliownas‘ash’，imlobalclimatecouldbeseverelyaffected。Figure8sholeoflavaflowsoh，forparisonwiththeimagesfromotherplas。

Thewayinwhiohprobablydiffersmostfromtheotherplahatexpansionofgaswithinrisingmagmatendstomakeasubstantialproportioionsexplosiveihisisfortworeasoisthatrecycledwater，dioxide，andsulfurdioxideesgupwardsabovesubduesaddsgreatlytotheleakagefromthedeeperinteriorases，sotheEarthhasmasavailabletodriveeruptionsexplosively。Thesedisthattheexisteialcrustfacilitatesgeionofmagmawithahighersilitthanbasalt。Thesesilicarichmagmasaremorevisbasalt，smentmoreeasily。Classic‘picture-book’steeplyioessutFujiinJapaoh，becausetheyaresymptomaticofrelativelysilidpartlyexplosiveeruptions。

8。A70-kilometre-wideviewfromspagthe‘oon’lavafieldinIdaho，USA。Thesourceoftheflowswasaseriesoffissuresheruggedhighlah-west。parethelobateformofthelavaflowswiththeflowsonVenusinFigure6

Mars

paredtotheEarthandVenus，Marshasrelativelyfewvoloes。However，theirsmallnumbersarepensatedbylargesize。MajsebasalticshieldvoloesotheTharsisregion（muchofwhiFigure9）andtheElysiumregion。OlympusMoTharsisvoleasuringabout600kilometresacrossitsbaseand24kilometresfromtoptobottom，whichmakesitthelargestvoltheentireSolarSystem。TherearetworeasonswhyMarshassuchbigvoloes。ThefirstisthatMarsisa‘o’。Itslithosphereisanintactshell（asiivelystationarywithrespecttotheunderlyihenosphere。Uh，whereplatesdriftarouomaplume-fedvoloesarecarriedawayandtheirmagmasupplyafteronlyafewmillionyears，amantleplumeonMarssuppliesmagmatothesamespotihospherefastheplumeremainsaonsmayhavebeguructedmorethanabillio。Wehavenowaytotell，becausewelydate（bycraterg）osedatthesurfacetoday，and’tseetheolder，buried，iheedifice。Thereareseveralcalderasatitssummit，whosefloorsaredatedataround100to200milliotheyoulavaflowsontheflaobeonlyabout2millionyearsold，anditislikelythatOlympusMoagaihervoltheTharsisregioelyolder，andareprobably。

9。A3，000-kilometre-widemosaiagesshowingseveralenormousshieldvolars。OisOlympusMons，theSolarSystem’slargestvoltheright-haharsisTholus，andrunnifromthetreofthese，alihree:PavonisMons，Ass，aholus

Thesearshassuchbigvoloesis‘becauseit’。Ithasaclithosphere，abouttwiceasthickastheEarth’s。IfyoutransplantedOlympusMohorVeivelythinlithosphereswouldsagbeheload，andthevolowouldloseheight。

High-resolutionimagesrevealdetailsoflavaflowsontheplaihelargevoldiherregionsofMars。However，therearealsardedbysomeasvolicthathavearousedsiderabletrure10shole。mentsofimpactejearshavebeehasmeteorites。Theyarealleitherbasaltiorecoarselyeis，spanniallizatioendingfrom4。5billiooasyoungas160millionyears。WeferthatigneousrockmakesupthebulkoftheMartiancrustatdepth，eventhetractsofsurfacehaveaveofvariouskinds。

10。A50-kilometre-wideimageofatroversialareaofMarsobtainedbyESA’sMarsExpressorbiter。Somesaytheplatysurfaceisalavaflowwithafracturedgcrust。Othersseethisasbrokenpaowdust-covered）onthesurfaceofafrozewoimpactcratersareolderthaysurfadtheirrimswerehighenoughtopreveeriorsfromfloodiersareactuallyciroutliareforeshortehisobliqueview

Surfaceprocesses

&hahering

Volismisdrivenfromi，butplaarylandsbesculptedjustasmuchbyprocessesthatotiallyatthesurfaabodythatisairlessandthereforeuedfromspace，byfarthedomiihesurfaeesaes。Fragmeerial（‘ejecta’）thrownoutfromcratersblahesurfacetoadepthofseveralmetres，andsiteswheresolidbedrockisvisibleatthesurfacearerare（Figure11）。Thelunarsoil，knolith’，inwhichtheApolloastroheirfootprintsisposedofgrainsmostlyafraillimetreinsize，prisisofcrystal，tinybitsofrodglassyspherulesthatarefrozesofmeltgeheheatoftheimpact。Regolithisuallyrearrangedoyofscalesbyexofddispersalofejecta，apropaing’。OnMercury，eedsarefaster，regolithgrain-sizeisexpectedtobeaboutohatolith。Ifthereisnoatmosphere，solarultravioletlightreachthesurface，whereitmay，overtime，breakchemiipad（ifthereisicfield）chargedpartithesolarwindalsoaffectsurfacechemistry，soairlessbodiesexperieeofprocesses，collectivelydescribedas‘spaceweathering’，thatslowlyalterthepositionofthesurfaple，thebondslinkingiroomsbebroken，allowioesdleavingsubmicroscopietal，anophaseiron’。

&oviewlookingacrossHadleyRille，takenbyApolloastrohe2-metre-thitallayerrunningatheleftisarareexampleofbedrock（probablyalavaflow），hereexposedonasteepslope。Everywhereelseiscinsizefrombouldersdowntodust

lahasanatmosphere，o，a，impareachthesurfaceathighspeed。Forexample，imosphere，stonyasteroidslessthanabout150metresiobeshearedapart。Theresultisaresmallenoughtobesloweddownbyfri，sobythetimetheyreachthegroundtheyhavelostalmostalltheiriy，anddonotformcraters。Meteoriticdust，whiieteoritesbutalsmentsfriallyablatedfrermeteorites，settlestothegroundatanaverageacrateof0。1–1mmpermilliohisdustmakessuytributioalrateofsedimentationthatitistotallysedbyothersedimeondeepofloorfarfromland。

Erosionandtransport

Impaingaside，processesthatwearafragmentsarewind，flowingwater，andmovingice（glaciers）。Wateralsodissolverchemicalweatherissolutionbyearelsewhere，beiedihisappliesespeciallytosaltdeposits，andalsotomanyformsofaterocks。However，olimestone（calciumate）isfmentsoftheshellsainganimportantbiologicalstepinturningdissolvedate（ordissolveddioxidegas）intosolidmaterialthaterock。

&ormsonMarsarefamous，havieles1809。Atperihelion，whenMarsreythanataphelion，windsinexetresperseliftsomuchdusthighimostofthesurfaceisobscuredforseveralweeks。SometimeslittlebeseenphotherthaofOlympusMons。Becauseofthecloudsthatoftehere，thisoftenlookswhite，hesformernameofNyxOlympiowsofOlympus），whichwasrevisedwhenimagesfromspacecraftshoasreallygoingon。ManysigionofwindonMarsbeseenfromround（Figure12）intheformofsanddunesandalsosmallerwihesurfacedust。Someofthedunesactivelysculptedbythewind，butothershaveprobablyheirformformillionsofyears。Wind-bloentoferosiononMars。Theloherisitymeansthataableoftranspsandgrainshastobeblowingmuchfasterthah，andsomeexposedlayersofrockhavebeecuriouslysculptedbyabrasion。

12。Somesanddunes，needingonlytreeforsfact，thispicturewastakenbyNASA’sOpportuhesurfaars，lookingobliquelydownfromtherimofatoafieldofduerfloor。Thevisibleareaisabout100metresacross

&hlings，flowingwaterisusuallythemostfamiliaragentfortranspsediment–inariver，oraswavesoheSolarSystem，hahtlyhassurfaditionsallowingliquidwatertobestable。Venusisfartoohot，andalthoughtheureonMarscreepwellabove0°C，itsatmosphereissotenuousthaticeatthesurfasdirectlytovapourratherthaihereisabuhatwateroncefliousvolumesacrossthesurfaars（Figure13）。MarshassufferedextremesofclimateatleasttheequalofEarth’s，andbillionsoitsatmospherewassuffitlydeopermitrainfallandcatastrophig。ThelargestintheSolarSystem，VallesMariheValleysoftheMarierbeingdisagesreturheprobeMariner9in1971），isa4，00riftsysteminitiatedbyfragofthecrust，butwidenedbyerosioerflhit。

13。Aseriesofeast–westfracturesatteststothetears’sVallesMarinerisplex，ofwhilyafraiscoveredbythis800-kilometre-wideview。helyinoitfromthesouth，whichshowtheroleplayedbyfloideningthemain