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Chapter 4 Giant planets satellites and rings（第2页）

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，therecouldbeahem。Tofindout，thefirststepistoieasamplefresqueezedoutofacrack。

&bigNASA-ESAcollaborationinouterSolarSystemexploratioobeamissioersystem。ItsprimarygoalwillbetoverifytheexistenceofEuropa’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