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Chapter 2 Rocky planets（第3页）

Atitsdeepestpoint，theflooris7kilometresbelowtherim（Earth’sGrandArizonaisonly2kilometresdeep），anditissowidethatifyoustoodoheoppositesidewouldbeoutofsightbeyondthehorizon。

&svastness，VallesMarineriswasnnizedbypre-SpaceAgetelescopicobservers。Thenotorious‘appedbytheItalianGiovanniSchiaparelliin1877，alyedbytheAmeriPercivalLowellwhountilhisdeathin1916thoughttheyweregiantwineeriMartians，areillusory。Theybearoanyofthemanygenuinears。Ofthese，examplesfedbyabraworkoftributaries（ingmanymugerversionsofthoseshowninFigure13）arelikelytohavebeensuppliedbyraierflowingalongothersprobablyleakedoutoftheground，possiblywhemelted。Thestreamlihe‘islands’whereelsdeboutotheplainsshowthemtohavebeeastrophicfloods。Robotiders（Viking1in1976andMarsPathfinderin1997）thattouinsuchpladanabundanpedtherebyflood>

&ehasnowmovedoioheyougullies，andnowfohowtheyareetheoryisthatwaterisrespoherecouldbereservoirsofliquidgroundwaterunderpressureiiansubsoil。e，suchasthecraterwallisbelowthewatertable，abarrierofithesoil（‘permafrost’）wouldnormallypreventitsescape。However，ifthebarrierweretempiveway，wateregushingout。Theliquidwouldable–itwouldbebothboilingandfreezingasitflowed–butitcouldtraversethelengthofohesegulliesbeforepletelyevap。Scepticsarguethatliquidflowisullies，andthattheybeexplainedasaresultofdryroches。

14。Twoviewsofthesame1。5-kilometre-wideareagtheinnerwallofa6-kilometre-diameterMartiancrater，reAugust1999（left）aember2ht）。Therimcutsacrossthetopleft，andtheflooristowardsthelht。Therearemanygullieserodedintotheslopeofinnerwall，oneofwhichseemstohaveflowedbetweees，gsomepaledebrisontothelowerslope

SomeMarsstistsseeevidenceflaciers，especiallyattheerodededgesofhighlahereisthesurfacetoday（exceptthatatthepoles），buttherroundrevealediionimagesfromorbitcouldbedebrisg（andinsulating）underlyingid-peingradardataobtainedfromMarsorbitlehis，whieofthereasonsiertoaccepttheregionshowninFigure10asadust-coveredfrozeherthanasalavaflow。

oonsuchasHadleyRille（Figure5）athwaysaainlynotcutbywater，andtheonlylunarwaterissmallquantitiesofitheregolithhepoles。Morethan200sinuouselshavebeenmappedonVenus，oneofwhietreslong。ItisveryuVenusexperiencedsuffitlyextremeclimategeforliquidwatertohaveexistedreoughtoerodetheseels，sotheytoowereprobablycutbylava。

Ihaveusednamesofsurfacefeaturesosseveraltimesalready:OlympusMons，VallesMarineris，HadleyRille，andsoon。Withoutsuames，Iwouldbereducedttothemas‘thebiggestvolars’，‘Mars’sgiant’，and‘thatbigtrenearollo15laablefeatureswouldbeeveodeslessbyatotallyunmemorableatesystem。

Butnobodylivesthere，sowhoalloamesandhowofficialarethey?Whenastrostartedtodrasthroughtheirteleseweresuffidepelymioiheirowenregardlessofanypreviouswork。AaskoftheIAU（foundedin1919）wastosortoutthemess，arriveatsingleoffiamesformultiplyures，aablishstandardsaionsforallogfuturehisappliedtothenamesofnewlydiscoveredbodiesauresonthesurfacesofplaarybodiesthatitmightbeedesirabletowouldbeevisiblethankstingteiques。iermeantsimplybiggeraelesdfewfouheIAUhaverealizedthattheyhadestablishedameansftheureoffeaturesrevealedbyvisiti。

SomehavecastigatedtheIAU’shandlingofPluto’sreclassifi，butIknowofhinksbadlyofthebasisfortheIAU-administerednamihisisfair，non-politidseekstorepresentalltheworld’scultures–notnecessarilyonanysinglebody，butbalancedacrossthewholeSolarSystem。

Buildingonwhathadalreadybeonpractiarfeatures，IAUureassignsasingleuocraters，whereasmostotherfeaturesaregivenanameplusaLatiortermthatdekiis。Thus‘OlympusMons’means‘OlympusMountain’，tellingyouimmediatelythatthisfeatureisamountainnamedOlympus。houghhatOlympusMonsisavolo，thedesdoesnotsaythis。Dessiionallyavoidiion（whichmayturnoutt）andsticktodes。

oortermsthatyoumaymeetare:chasma（adeep，eloeep-sideddepression），fluctus（aflow-likefeature），fossa（long，narroression），mensa（flattoppedpromihcliff-likeedges），planitia（low-lyingplain），planum（highplainorplateau），rupes（sdvallis（brangvalley）。Ohereisalsomare（pluralmaria）whislatesas‘sea’buthadbeetooeobereplacedbyamoreaptterm。

Therearealsothemesfornamesoneaet。Lunarcratersareerfamousdeceasedstists，scholars，andartists，whereasmariatakeLatintermsdesgvariousweatherarsistheohahsignifitheritageofnamesfrombeforetheIAUbevolved。These，withmoderortermsadded，telesappingbyGiovanniSchiaparelliaoniadiihtury，andmostlyrefertionssuchasTharsisandElysium。EachlargevalleyisgiventhenameforMarsilanguage，whereassmallvalleysareerriversoh。OnVenus，almostallnamesarefemale:cratersareerfamoushistoriostotherfeaturesaftergoddesses。OnMercury，cratersareerdeadartists，musis，painters，andauthors，s（rupes）areificexpeditiohatcarriedthem。BeagleRupes（Figure4）iserHMSBeagleonwhichCharlesDarwinvoyagedwhileamassiionsthatiheoryofevolution。

Atmospheres

&sbirth，eachterrestrialplahavedevelopedanatmospherewheninternalgasesleakedoutviathemagmao。Theseprimitiveatmospheresdonotsurvivetoday，thoughthegasesesvoloesshowwhattheymayhavebeeheMoonandMercuryhavetoolittlegravitytohangontoagasblahe‘atmospheres’thatyouesfihem，withpressuresmuchlessthanabillioh’satmosphericpressure，sistlargelyofstrayatomskhesurfacebymieteoriteandicrayimpact。Sosparsearetheseatomsthateachismorelikelytodriftoffihahahisdefihe‘exosphere’ofaplaismerelythetezoatmospheres，butisallthattheMoonandMermuster。

&rravityofthemoremassiveterrestrialplahemtohangontogasmoreeffectively，althoughthedensityandchemipositionhaveevolvedoutnitioofnumerousprotheearlydays，themoreactivesolarwindmayhavestrippedawaymostinalatmosphere，butthesewouldbereplenishedbyvolimpoihatshthsolarultravioletlightsplitmoleculesofourintenandoxygen。Hydroge，aospace，whichmakesthis‘photodisso’ofwateranirreversibleprousandMarshavebothlostmuchihisway。Thepositio-dayatmospheresofVeh，andMarsaresummarizedinTable4。

Havibyultravioletlight，atmosphericmolebihers，byseriesofreasdescribedas‘photochemistry’。Thisoccursespeaplahermosphere’whisabout100kilometresabovethesurfaedbecausethislayerisheatedbythesolarultraviyusediingmoleculesapartthemofsomeoftheireles。Thelatterprocessisization，andions（mainlyofoxygehanddioxideforVenusandMars）besuffioerreaospheretomakeariduglayerreferredtoasthe‘ionosphere’。WhenassplasmafromtheSuhisdistortsthemagicfieldandusualtstoflowintheiobadlydisruptradiounidevencausepowerfailures。

&hepresent-dayatmospheresofterrestrialplas，showingtheabuhesixmostongasesexpressedasapertageofthetotalnumberofmolecules（waterisveryvariableimosphere），andthesurfacepressurerelativetoEarth

Whehebaseofthetroposphereisheated，itmustexpand，whichmakesitbuoyant。Itwilltheobereplacedbycolderairdisplaabove。Thisisanotherexampleof（whichyoupreviouslymetlale），andiswhatdrivestheweatheroh，Venus，andMars。Thepatternofcirisdifferentineachcasebecauseitalsodependsonsuchfactorsastheplaion（slowforVeeofrotatiomosphere（muchfasterthanthatoftheplahecaseofVeroposphere），andtheday–uredifference（highforMars，smallforVenus）。Figure15showsthecharacteristicciraboveVenus’ssouthpole。Intrast，spiralstormsystemsimospheretendtobegiropics。

&mospherealsodiffersfromitsneighboursintheplexityofitslayering。AtVenusandMars，temperaturedecreasesrapidlywithaltitudeiroposphere，thendecreasesmoreslowlywithaltitudeina（ing）layercalledthemesosphere，andthehaltitudeihermospherebecauseoftheultravioletabsorptiohisuerrestrialplasinhaviendingfromabout10to50kilometresaltitude，betweenitstroposphereandmesosphere，wheretemperatureihaltitude。Thisisthestratosphere，whichiswarmedbytheabsorptionof230–350-re-wavelengthultravioletphotons（towhichthethermosphereandmesospherearetra）byozonemolecules。Ozenatomsboundinamolecule（O3），asopposedtotwooxygenatoms（O2）whichiswhatisusuallymeao‘oxygen’，andisassembledfromoxygenbyphotochemicalreashigherimosphere。

15。The‘eye’ofVenus’s2，000-kilometre-diametersouthped24hoursapart。Thedotihesouthpole。Theseimages，rethemid-ihecloud-topsabout60kilometresabovethesurface。Thetreofthe‘eye’isearingbrighter），showihecloudsaredraarmer，deeperlevels

Manypeopleareawareofthe‘holeintheozohe‘gree’，buttendtoflatethemastwinatege。However，thetwoareverydifferent。Theozonelayeroly）iratosphere，andiswhere230–350-ravioletlightisabsorbed。Thisisveryimportantforourselvesandothersurfaglife，becauseifitissuchradiationdgeicdamage。Ittakessurprisinglylittleozoobeaives。Ifyougatheredalltheozoisdispersediosphereintoasisea-levelitwouldbeonlyabout3millimetresthick。Thisisafragileveil，sowheninthe1970sand1980sitbecameapparentthatoverAntarcticathestratospherehadlostperhapshalfitsozoherewassiderable，andtalkofa‘holeintheozohemairacedtoreavolvingindustrialchemicalscalledchlorofluoros（CFCs）which，asaowbeeheirformeruseinaerosolspraysassotheyotleakimosphere。TheAntarehole’andalesseroheArctiowstabilized。Depletionofozoneisonlyafewpertoutsideofthepions，aableoverthetropics。

Thereisweerationandmeanglobaltemperature。Abadlydepletedozonelayerwouldmakelifeu，buthaslittletodowithclimategel。Thetropospherictemperatureofaplarolledbyhoweffectivelythelohereabsorbsiion。Thisisbecausevisiblesunlightwarmstheground，andthewarmedgrousiioureoftheatmospheredependsontwofactors:theheatitpitactwiththeground，andhowmuchoftheoutgoiionitabsorb。

Mostgasspeciesaretraoiion，butmolesistingoftwoormoredifferesabsorbiestrongly。Thusnitrogen（N2），oxygen（O2），andargon（Ar）donotabsorbiour（H2O），dioxide（CO2），sulfurdioxide（SO2），ahane（CH4）do。Byanalogyingwarmthinsideagreehisiscalledthe‘greehereisanaturalgreeimospheresofVeh，andMars。Thanksmostlytoitsenormousloadofdioxide，theatmreeoainsitssurfacetemperatureanimpressive500°Cabovewhatitwouldotherwisebe。ourandtheEarthbyabout30°dgreenhofMars，whichhasatenuousdioxide-richatmosphere，isonlyabout6°C。

&h’sgreekeepsthetemperaturewithithelifethathasevolvedhere。Mediatedbylifeitself，thestrengthofthegreehasgedtokeepthetemperatureirange。Fourbillio，theSunwasonlyabout70%asbrilliantasitistoday，sotheEarthwouldhavebeenverymuchcolder，haditsatmospherebeeoday’s。However，before4billio，itrobablymostlydioxide，and100timesdeoday，sothegreewouldhavebeehankstoprimitivealgae，thedioxidethaddecreasedtoabout10timesitspresehalfabillio，sreemusthavedeedten（O2）firstappearedsometimebetweenabout2。7and2。2billo，aabout170%ofitspreseioween250and200millio。Clearly，lifeohhasbothinfluendbeedfromthepositiomosphere。

Againstabadradualdethenaturalgree，whiterbalaheslowwaxingoftheSun’sluminosity，therehavebeenseveralexsih’sclimate。IceAges，whenmuextremecasesall）ofthesurfacewaterwasfroze-khesearetrollednotsomuchbytheatmosphereasbyvariatiooftheaxisaricityoftheorbit。SimilareffectsprobablyexplaiigesinessofMars’ssurfae。

Clouds

Clhlyreflective，sothecloudierahemyisreflecteddirectlybatospace。However，acloudyskyiheabilityofarapheatfromthesunlightthatdoesreachthegrouofglobaltemperatureisplex。TheunbrokencloudsofVesaveditssurfabeingthhlycookedbythegree。

wheureandpressuremakeitfavourableforsometoftheatmospheretoseasliquiddropletsoricepartithecaseoftheterrestrialplatisusuallywater。AlthoughwaterisonlyasmallfraofVenus’satmosphere，thereisenoughtoformauouslayerofcloudatthetopofitstroposphere，betweenabout45and65kilometresabovethesurface。There，oursesasdropletsabout2mietresacross。Theseremainsuspeoosmalltofall，andaredescribedasaerosoldroplets。Atmosphericsulfurdioxidedissolvesiheyturnintosulfuricacid。However，ifaellyouthatit‘rainssulfuriVenus’，theyarewrohedropletsaredraabout45kilometresbyatmosphericcir，theheatcausesthemtain，andtheyheeraindrhtofallground

Polardos

Aswellasgtoformospheristituentsmayseaseithericeorliquidatthesurface。TheEarthistheorialplaohaveostoday，whichofcoursearemadeofoles，waterisfrozentoformpolarcaps。TheyouVenusmayhaveenjoyedabriefepoosclobe，beforetheevaporatedour（subsequentlylostbyphotodisso）addedtreeleadiparchedsituation。

However，Marsisdifferent。Avast‘OusBorealis’ogthewholeofthelow-lyingnorthernplainsabout3。8billiowasihe1990s。Althoughthatremaiious，manytthelikelihoodoflakesoeobecalled‘seas’atthetimewhenelssuchasthoseinFigure13wereflowing，andsomefrozenrelicsmayevensurvive，buriedbydust（Figure10）。However，thereisnodoubtthaticeexistsatthesurfaars’spolarcaps（Figure16）。Theseahafringeofdioxidefrostthatgrowsandtraally。

16。1，500-kilometre-wideimagesofMars’snorthernpolarearlyspri）andhighsummer（right）。Iofthedioxidefrosthassublimed（turnedfromicetovapour），leavingonlytheresidual，‘perma’capofwater-ice

&h’sandMars’spolarteracthere。Theyare，i，depositsofgasesthathave‘frozenout’oftheatmosphere，eitherfallingfromthecloudsassnoworgdiretothegrouemperaturerises，materialfromthepolarcapsisreturmosphere，eitherbymeltingaion（forwateroh，andprobablyMarsi）orbysublimilyfromiars，fordioxideaoday）。

EquilibrialiketheseotoairlessbodiesliketheMoonandMerdsopolarottobeexpected。Hthe1990sitwasradarsigedwithunusualstrengthfrompermalyshadioershepolesofbothbodies。Thiswouldbetwithwater-icedispersedasgrainswithih。Apossibleexplanationisthatthefloorsofthesecratersaresocoldthatanystraywatermoleculesthatwahemtehesurfa‘coldtraps’。Thiswaterbepartofthesebicouldhavebeeerbyimpaets。FindingasupplyofwaterontheMoonisofgreatimportanceifahumany，orevenjustapermalyoccupiedbase，isevertobeestablishedthere。Thepolesareclearlythebestbet，andin2009waterwasediaplumecreatedacecraftwastoapermalyshadowedpolarfraredspectraobtaiherspacecraftrevealedwaterandhydratedmineralsdispersedihacrions，iratihopesthattheMoonmightnotbesowhollyinhospitableasformerlybelieved。

&weeninterior，surfadatmosphere，andthepohem，isextremelyimportah’s‘hydrologiccycle’isthemostfamiliarexample。Itisnotasianarrayofiedloops。Ierintheosevaporatestoformdlaterprecipitatesoutasrainorsuallyfindsitswaybatotheos（viariversorseasonalpolarcaps）。Waterbedrawerior（deeplyatsubduoreshallowlybyinfiltrationoftheground）andre-emergeviavoloes。Italsoreactchemicallywithrock（chemicalweatherioredwithihereisalsoanimportant‘cycle’withloopspassimospheridioxide，livingplantsandanimals，dissolvedariones，hydrodeposits，volidsoon。

Marsissuretohavesimilarcycles，thoughagmoresporadidoverdifferenttimesdwithdiffereiveimportancesforeachloop。ThereareprobablyevenslowervolvingdioxideandsulfurdioxideonVenus，inwhichtheatmosphereweathersthesurfacerocks，whituallybeeburiedbylavaflowstodepthsatwhichthegasesareliberatedondescapebahvolts。Untilloredaheplexitiesandtimesulti-loopedaedcycles，ofwhatmakeseaet‘tick’willremainnaive。