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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‘thatbigtrenearollo15laablefeatureswouldbeeveodeslessbyatotallyunmemorableatesyste
Butnobodylivesthere，sowhoalloamesandhowofficialarethey?Whenastrostartedtodrasthroughtheirteleseweresuffidepelymioiheirowenregardlessofanypreviouswork.AaskoftheIAU（foundedin1919）wastosortoutthemess，arriveatsingleoffiamesformultiplyures，aablishstandardsaionsforallogfuturehisappliedtothenamesofnewlydiscoveredbodiesauresonthesurfacesofplaarybodiesthatitmightbeedesirabletowouldbeevisiblethankstingteiques.iermeantsimplybiggeraelesdfewfouheIAUhaverealizedthattheyhadestablishedameansftheureoffeaturesrevealedbyvisiti.
SomehavecastigatedtheIAU’shandlingofPluto’sreclassifi，butIknowofhinksbadlyofthebasisfortheIAU-administerednamihisisfair，non-politidseekstorepresentalltheworld’scultures–notnecessarilyonanysinglebody，butbalancedacrossthewholeSolarSyste
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’buthadbeetooeobereplacedbyamoreaptter
Therearealsothemesfornamesoneaet.Lunarcratersareerfamousdeceasedstists，scholars，andartists，whereasmariatakeLatintermsdesgvariousweatherarsistheohahsignifitheritageofnamesfrombeforetheIAUbevolved.These，withmoderortermsadded，telesappingbyGiovanniSchiaparelliaoniadiihtury，andmostlyrefertionssuchasTharsisandElysiuEachlargevalleyisgiventhenameforMarsilanguage，whereassmallvalleysareerriversoh.OnVenus，almostallnamesarefemale:cratersareerfamoushistoriostotherfeaturesaftergoddesses.OnMercury，cratersareerdeadartists，musis，painters，andauthors，s（rupes）areificexpeditiohatcarriedtheBeagleRupes（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.
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