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21.A42-kilometre-wideclose-upofpartofthearaChaionofEuropa，where‘melt-through’fromtheunderlyingohasallowedraftsoficetodriftapartbeforethearearefroze.Sunlightisfrht
InFigure21，‘ball’terraiedbythereatprocessthataffectsEuropa.Thisis‘melt-through’，asiureofbrokesdescribedas‘derafuturechaion，theoesunusuallywarm–maybebecauseofsilicatevolisontheofloor–andthebaseofthesurfaceshelloficegraduallymelts，sotheiesthiually，meltihesurfadrafts（orfloes）oficebreakawayfromtheexposededgesoficeshelfanddriftintotheexposedoyexposedwaterwouldrefreezeprettyquidperhapsitisbettertothinkofkilometre-thickiudgingtheirwayintoaseacoveredbyicyslush，ratherthanintotrulyopehesummerthawofpatheEarth’sAr.Ih-westpartofFigure21，youseethewaymasinallyfittedtogether，becausetheyhavenotdriftedfarapartandtheir‘ball’texturesbematched.
&emporaryheatexcessdiesaway，theorefreezesasceasetodrift.Theiceoftherefrozenseasurfaeaththeraftsbeginstothi.Whentherefrozenareaissuffitlythidbrittle，newcracksmayopen，aionof‘ball’texturebegihewhion.Ihereisayoungcrakedbyanarreoherside，runningdiagonallyacross.Itlooksunremarkablewhereitcrossesrafts，butyoutellthatitmustbeyoustherefrozensealyiherafts.
Ifthisstoryiseveelycorretherearesht-provokingimpliicalreaswiththeunderlyingroaketheosalty–thoughthemostabundantdissolvedsaltmightbemagnesiumsulfateratherthansodiumchlorideasih’soysutidallyheatedrockprovidesahabitatforlifeequivalenttowherelifeisbelievedtohavebeguh.Lalightisnohindrahe‘primaryproducers’atthebaseofthelocalfoodwouldderivetheirenergyfromthechemicalimbalancesuppliedtotheoatsubmarisprings（hydrothermalvents）.Suchlifeisdescribedasthetic，asopposedtophotosyiEarth’sofloors，thehottestventsarecalled‘blacksmokers’becauseoftheplumeofmetalsulfideparticlesthatformswhefluidmixesintotheowater.Theseventsaresurprisingoasesoflife，whereunitiesanisms（ineasadvancedasshrimpsandcrabs）feedoicmicrobesthatgaintheirenergybygdioxideihane.Iflifeaninsuchasetting，whynotalsoonEuropa?
LifesealedbelowicethatisnormallykilometresthickwouldbeextremelygtEuropalaodrillormeltahhtheiordertolaunarieinona‘blae.However，suambitiousmissionmaynotbenecessaryiftheridgeseithersideofayoungcrackarebuiltofslushsqueezedupfromtheo.Whileacra，itcouldprovideaosyiclifesutsor（morereasonably）marinealgae.Likelifeohesecouldhaveevolvedfromtheticforebears.Radiatiohetopfewtimetresoftheexposedwaternuniherewouldbeenoughsunlightforphotosyheres.Ifthereareprimaryproduts，algae）livingoffsunlight，therecouldbeaheTofindout，thefirststepistoieasamplefresqueezedoutofacrack.
&bigNASA-ESAcollaborationinouterSolarSystemexploratioobeamissioersysteItsprimarygoalwillbetoverifytheexistenceofEuropa’sogietratingradaraheamountoftidalflexing（whichwouldbeonlyaboutametreihick’igonbedrock，butabout30metresfora‘thiingonano）.Sadly，alabeplatedyet，buttherewillatleastbehigh-resolutiorosorbittenicmoletheridgematerial.
Enceladus
ItwouldbemucheasiertofindbiomarkersifEuropanicecouldbesampledwithouthavingtogodowntothesurfaceladus，asatelliteofSaturhatopportunity.Itisonly504kilometresierandhastoolowadensitytouchrock.Voyagershowedittelittleworld，heavilycrateredinpartsbutelsearentlylacraters.Thehigher-resolutioransmittedbyi，whianorbitalsurveyoftheSaturnsystemin2004，showsasurfaceyfamiliesofcracks（thoughratheruhe‘ballionsofEuropa）.Italsodiscoveredjetsoficycrystalsventingtospaearthesouthpole（Figure22）.Fortunately，icarriedamassspeeteriudyofioralparticles，sothespacecraft’strajectorywasadjustedtoallowitththeplumeandples.Thesewerefoundtowater，methane，ammonia，onoxide，anddioxide.Thererobablysomesimpleahoughthatisachemicaltermdenotingsliheraimplyabiolin.Iftheplumeshadbeenknownirumeedfordetearkersmighthavebeenini’spayload.
22.TwoiimagesofEnceladus.Left:anoverexposedtview，sholumeextendi100kilometresabht:anobliquevieartofEbyseveralfamiliesofcracks，likethosefromwhichtheplumeiskninate.Afeactcraters（toosmallfertosee）shoarticularregionisprobablyive
Almostly，tidalheating（drivenby2:1orbitalresour-oeDioheationaheimpetusfortheplumes.However，edEobesoadthisisparticularlybafflissimilar-sizedneighbourMimasisaypicalcrateredice-ballshowingnohistoryofactivity.ItisuEnceladushasaglobalohiddessurface，buttheremaybepodsofliquidwaterbeheplumesources.Liquidwaterisgoodforlife，buttheavailabilityofhinEnceladusissurelymuchmorerestriwithinalargebodysuchasEuropa，soEnceladusdoesnotseemsuisinghabitat.
Titan
TitanisSaturn’sohatrivalsJupiter’sGalileaesinscale（5，150kilometresier）.Voyagershoweditonlyasafuzzyeball，because–aloneamoes–ithasademosphere.Thisis97%ismadeopaquebymethasphotochemicalderivativesthatturosphereintoaitanhasadmantlemadeofice（mostlywater-ice）ogtheouterohirdofTitan’sradiusandarockycore.Therecouldbeanironinnerwhichcase，tobalaheaverageglobaldensity，thebaseoftheitlewouldhavetobedeeper.Titan’srotationperiodisaffectedbyseasonalwinds，showingusthatthelithospheremustbedecoupledfromtheinterior，mostlikelybyaninterhisostlywateroramixtureofwaterandammonia（whiremainliquidatasiderablylowertemperaturethaer）.Mostmodelsplaceitasalayerwithile，ratherthansituatedimmediatelyontopoftheinternalrock.
TheimissioheproblemofseeingthroughtoTitan’ssurfathreeways:itobtaiusableimagesofthesurfaenarrowbandsofthenear-irumwherethesmogisleastopaque，itusedimagingradarliketheMagellaertoseethegrouiveofditcarriedalandingamedHuygens，thatprovidedimagesfrombelowtheclparachutedesttothesurfa’ssurfacegeologicalprocessesrevealedbythisarrayofimagingteiquesaresuperbaomanyoftheprocessesthatoEarth.Thecrustisdominantlywater-ice，veryrigidandroitsbehaviourinTitan’s–180°Csurfaeorestoronasandyplaihpebbles.ItlookedlikeMarsexceptthatbothsandandpebblesweremadeofice.Thesandcouldhavebeenwind-blown，andindeedradarimagesrevealvastfieldsofwind-blownsaherpartsofTitahepebblesmusthavebeentransportedbyflowingliquid，whiTitan’satmospheripositionaure，mustbemethane（CH4）orethane（C2H6）.Asitdesded，Huygenssawbrangdrainageeartothelandingsite，andradarimagingrevealsplexvalleysystemsinmanyiinhighlahe‘bedrock’oficycrustisexposedanddrainingintolowlandbasiaccumulates.Betterthanthat，itfouhaedliquidmethahpoles（Figure23）.Somelake-bedsweredry，andothershadshalles，anditislikelythattheyvaryseasonally.Titanisclearlygeologicallyactive.Afewdeeplyerodedimpactcratershavebeenreized，andtherearesomesitesofsuspe’，whereicy‘magma’iseruptedanalogouslytoterrestriallavaflows.
23.A1，10mosaicofiradarimages，an’shedarkareasarelakes，thelargestofwhichexceeds100，000squarekilometres，20%biggerthanLakeSuperiorinNorthAmeridritiageelsbeseehelakes.Linesoflongitudehavebeenadded;blankareasareunimaged
&owhiaoributetothesgandresurfagofTitan’ssurfaowisclearthaterosionofbedrothiscase，ice，ofcourse）folloortaioaremajorplayers.RainfallonTitanmustsistofdropletsofmetha，likeraih，ihegroundandfeedsspringsthatsupplystreamsaheethachemicallywiththeicy‘bedrock’，itserosivepower，therateatwhichitevaporatesbaosphere，andhowloherebefoutagaiaibefaethanologiccyclethatmimicsEarth’shydrologiccycle.Marshadrainfall，rivers，andlakeslongago，butTitaherplacewheretheyoccurtoday.Oneday，wewillseoexploreTitahhly–perhapsingaballoontodriftbelowthesmog，withvariablebuoyancysoittouiingplaces.Suchamissionplethelakefluid，andobtainpicturesofwavesbreakinghlyalienshore.
MirandaandAriel
Althoughpresent-dayonTitanremainstroversial，aotbedoubtedontwoofUranus’sfiveregularsatellites，ArielandMirahesurfacetemperatureis–200°C.ItseffebeseebackbyVoyager2，whichflewthroughtheUranussysteminJanuary1986.
&helargerofthetwo（1，158kilometresiisaplexglobe，whoseoldestcrateredterrainisumerousfaultsboundinghigh-standingblocks.Mostofthefaultsdefi-flooredvalleysofthekihedes‘chasma’.However，ratherthanpreservingdhlahefloorsofmostchasmatahavebeencoveredbysmoothmaterial，oratleastbysomethingthatappearssmoothatthe1-kilometreresolutionoftheVes.
Probablyipast（morethan2billio），tidalheatiofAriel’ssurfadtheeffusionoficlava.Thiscoveredthefloorsofthechasmata，andinplabeseenspreadihemtopartlyburysomeolderimpactcraters.AtthedistanusfromtheSun，itsiceisexpeoreplexcocktailthalysaltyidoellites.Themostlikelymelttobeextractedbypartialmeltingisa2:1mixtureofwaterandammonia.Thisisliquidat–100°dsoeratedbymuodestheatingthanwouldberequiredtomeltpure
Individual‘lava’flowsalsobeseenonMiranda，whius’ssmallestregularsatellite（472kilometresier）.Forsuybody，ithasaremarkablydiversesurfaorevariedeventhanEnceladus，althoughthenumberofsuperimposedimpactcratersshowsthatitslastactivityrobablybillionso.Voyager2sawonlyhalftheglobe.Halfofthatisheavilycratered，butisunusualinthatmostcraters（theolderohprofiles，asiftheyhavebeenmantledbysomethingfallingfromabove，andoersarepristiherhalfoftheimagedareaprisesthreesharp-edgedterrainunitsdescribedasae.Eachisdifferent，buttheyallplexridgedortonallypattererrain，ingfeaturesidentifiedasiclavaflows（probablywater-ammonialava，asonAriel），andpockedbypristiersequivalenttothoseintheheavilycrateredterrain.
&heoryaboutMirandathateaarepresentsafragmentfrlobalbreak-upaionhasbeendisostlikely，theaearesitesof，ofwhilythehasehasleftreizableflow-liketratlingoftheoldertheterraiheaydemonstrateexplosiveeruptiicypartitospaeofwhichsettled，sosubduepre-existingtopography.Whenandwhythishappeknow.WeareuofindoutuhermissiontoUranus，whiotlikelybeforemid-tury.
Triton
Tritouellite（2，706kilometresisouterpartisicy，butitisdeohaveasubstantialrockyVoyager2flewpastin1989，itfoundpolarcapsoffrenice（previouslydetectedspectroscopiEarth）.Likethears’spolarcaps，theseprobablyshrinkinsummer，bysublimatiohaing，addienttoTriton’sthiableatmospherewhichismadelargelyeable‘bedroingTriton’scrustappearstobeamixtureofmethane，dioxide，dioxide，aheremaybeammoniatoo，whichisalmostiicalspectroscopy.
&imagesofTritonhavearesolutionofabout400metresperpixel.Theyrevealageologiplexsurfadthepolargvariouslandformsthatmayhavebeeedically（Figure24）.Impactcratersoccureverywhere，butnotinvastnumbers，anditispossiblethatmuchofthesurfaceislessthanabillionyearsold.Tritonisalsoremarkablefeysersthateruptthroughthepolarcap，loftingdarkparticlestoaheightofabout8kilometres.Thereareafewhigh-altitudeadeencrystals，analogoustocirrusourohere.
OhpolarcapwasseenbyVoyager2，becausemostofthenorthernhemispherewasion’sseasonsarepeculiar，resultingfromabinatioune’s29.6°axialtiltaddedtothe21°inofTriton’sorbit.Furthertothis，Trion’sorbitalplaneprecessesaboutune’saxissothatafullseasoooune’s164-yearorbitalperiod，buttoa688-yearcycle，with164-yearsubcyclessuperimpthefullcycle，thesubsolarlatitudeesbetween50°northand50°south.ByVoyager2madeitsfly-by，TritonroagextremesoutherheSunearly50°south，separtofthenorthernhemispherewasindarknessandotbeseesouthernpolarcapshowedsignsofbei，anditssublimationtogaswasverifiedbyobservatiohin1997shohericpressurehaddoublediyearssiheVoyagereneaheuhpolarcapr，asatmehefrigidsurface.
24.Amosaiagesga2，00ionofTriton.Southistowardsthetop，andsunlightistheupperright.Theraggededgeofthesouthponallyacrossthetopoftheimage.Long，narres（sulci）maybefissureswhereicicymagmated.Thesmoothplainsandbasinsiareprobablyexpansesoficlava.Thedimpledareaireaiscalled‘teloupeterrain’，byvisualanalogytotheskiitsinisunknown
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