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Chapter 4 Giant planets satellites and rings（第1页）

Chapter4　Giaellitesandrings

Ringsandalargefamilyofsatellitesarefeaturesontiaherearevariationsinemphasisahesimilaritiesbetweeneag-satellitesystemoutweightheirdifferences。

Riesystems

Mostoutersatellitesofeatplaraveliricorbits，usuallyiediretothespi。Furthermore，masarei〉30°relativetotheirplahetypicallyetric，retrograde，aureoftheirorbitsearheellites’，quiteapartfromthefactthat（beingatmostabout100kilometres，aenores，across）theyhavefartoolittlegravitytopullthemselvesintosphericalshapes。Theirregularsatellitesarethemostnumerousclass:atthelastt，Jupiterhad55，withorbitalsemi-majfrom105to400Jupiterradii;Saturnhad38，withorbitsfrom184to417Saturnradii;Uranushad9，167–818Uranusradii;aunehad6，223–1，954he‘regularsatellites’arethelargeonesinnear-circularprogradeorbits，muchclosertotheirplas，andwithverylowins。Jupiterhas4（theonesdiscalileo）whoseorbitalsemi-majefrom5。9to26。3Jupiterradii。Thesearesubstantialworlds，andgeologicallyhavemuorialplahoughofcoursetheydonotsatisfytheIAUdefinitionofaplaurnhas8（allbutonesiderablysmallerthanJupiter’s，andat3–59Saturnradii），andUranushas5（at5–23Uranusradii）。unehasesatellite，Tritat14hatwardedas‘regular’，exceptfradeorbit。Animportantcharacteristic，sharedbyallregularsatellites（ingTriton），isthattidalforceshavesuchagripoheyareinsynousro，sothat（liketheEarth’sMoohesamefacetowardstheirpla。

&ill，wefindirregular-shapedlumpsofdebristhatitisttodistinguishas‘is’。Thesehavecircular，prograde，equatorialorbits。Sodotheparticlesthatmakeuptherings，asomeis’orbitsliewithihereisprobablyaldiffereweenalargeringpartidasmalli。Jupiterhasonly4ks，butSaturnhas14，g7whoseorbitslieamosiregularsatellites。Uranushas13aune6。

&handnumbersvariesfromplaoplaurn’sbeingbyfarthemostspectacular，butiheirthiessishaensofkilometres。Mostly，theyareclosertotheirplahanadistanasthe‘Rochelimit’，aboundarywithinwhiylargebodyshouldberippedapartbytidalforces。Msareregardedasdebrisleftoverfromthetidaldisruptionofasatelliteoretthatstrayedtooclosetotheplasomelesssubstantialrirablysuppliedfromellitesbypartitedatospaceorthrownupbyimpacts。

Saturn’sringsaremadeofidreflectabout80%ofthesunlightfallietheirpromiappearahematerialinthemwouldsuffiakeabodyabout100kilometresicouldallbegatheredthindividualringparticleshavenotbeely，therateatwhigstheshadowoftheirplafallsashowsthatSaturn’srilyparticlesbetweenaboutatimetrearesirast，Jupiter’smuchlesssubstantialringsaremadelargelyofmietre-sizedparticlesthatarealsomuchlessreflethebrighticylumpsinSaturn’srierialatUraussunlightpoorly（likeJupiter’sriismostlytimetrestometresinsize（likeSaturn’sringmaterial）。

19。A5，000-kilometre-wideviewofpartsystem，seenby27July2009。Atthisscale，thecurvaturesrou（outofviewtht）isscarcelydisible。Therisuiclesaremostdenselypadblackspaceshhinparticle-freegaps。Pan，a28-kilometre-diametershepherdsatellite，beseenigap。AsingmostofthisgapinfluenarrowanddisuswithiheexallengthofPan’sshadowsthtisbecausethisimagewasrecordedwhentheSunlayveryclosetotheplaherings

Orbitalresooaplexgravitatioweenringsandtheisthatthem（Figure19）。Thoseareoftencalled‘shepherdsatellites’beesweepyofthegapsintherings，andothersform，deform，andmaintainnarrswithorbitsjustwithinorjustbeyondtheirown。

Ingeneral，ringsoccurclosertotheirplaheregularsatellites，butSaturioninthatitalsohasadiffuseofdark，dustymaterialtredarouofPhoebe，osiirregularsatellites。Thematerialinthisring，whichwasdis2009usingaspace-basedielescope，ispresumablybeingsuppliedfromPhoebeitobeuood。

Remarkablesatellites

&imewheymueexpethelargestoftheouterplaobedrearyobjetice-balls，heavilypock-markedbyimpacts，theywouldrecordtheouterSolarSystem’sbombardmenthistory，butwouldbeoferestunlessyouwaudymutualorbitalevolution。Thatwasthestail2March1979，eale，wattheUyofia，published（withtaperpointingoutthattheexact2:1orbitalresoweenJupiter’siGalileaes，IhttoresultinsomuchtidaldistortionofIo’sshapethatitsinteriorshouldbemolteesofdensityplusspectrosalysisoftheirsurfaces，itwasalreadyknownthatIohasarockyliketheothersatellitesthatarepredomiosuggestamolteninteriorinsidearockybody（wherethemeltiureissomuchhigher）articularlyboldstep。FewmighthavebelievedthisclaimifVoyager1hadnotflownpastafewdayslaterandtrauresofexplvoloes，toppedby30heruptionplumes。

AlthoughtidalheatingofIoisbyfarthestrohesameprocessaffectsvariousothersatellites，andmanymnsofaepisodesoftidalheating。Thismakesthemvaried，andintriguis。Theydon’tmindthatinmostofthemonlytheadeofrock，surrouhitleofice，sachemicallydistincticycrustatthesurfadertheloeraturesprevailierSolarSystem（rangingfrom–140°CforJupiter’ssatellitesdownto–235°eptues），themeicalpropertiesaingbehaviouroftheiceareveryalogoustohowrockbehavesintheinnerSolarSystem。Inotherwords，thosebodieshaveboththebehaviourandstructureofaterrestrialplahroplainthedisteadofrothedmantle。

IoisaioninbeiharockydmantlesurroundinganirondwouldbeclassifiableasaterrestrialplawastheSuninsteadofJupiter。Europaisahybrid，havingastructurelikeIoburiedbelow100–150kilometresofice。HereIdescribebothofthoseaheothersatellitesthatfasatememost，tratiiveexamples，thougheveer-pockedice-ballshaveturobemthanthedullglobesfined。

Io

Ioisonlyslightlybigger（3，642kilometresier）ahanourownMoowocouldhardlybemoredifferent。Io’sterrainisresurfacedsorapidlybyvolicprocessesthatnotasicrateristobeseethattheeffectravityinfogstrayprojewardsmustmeanthatIoisstruckmoreoftenthanJupiter’sheavilycrateredsatellitesGanymedeandCallistothatorbitbeyondEuropa。WheVoyager1colourclose-upsofIowerestudiedin1979，itsyellowhueledmanytosupposethatthelobatelavaflowsthatcssurfaceweremadeofsulfur。However，itisIo’svolismismolteerial（true‘rock’）。Thetemperaturesretheheartvoltsarewellinexcessof1，000°C，despitetheintensedtheactiveareas。ThegasthatescapestodriveexplosiveeruptionssuchasthoseinFigure20ismostlysulfurdioxide（whereasowouldbemostlyour），andbothsulfurandsulfurdioxideseonthesurfaceas‘frost’thatimpartsthecolourtoIo。

IolieswithiedpartifinedbyJupiter’smagheradiationthereissoiNASA’sGalileomissiontrollersdidnotallowthespaakerepeatclosefly-bysofIo，soonlyasmallfraofIo’sglobewasimagedwellenoughtoshowdetailsbelowafewhuresihemostdetailedimages，thepixelsareoresadevenonthosenoimpactcratershavebeenfound。

IfIo’spresent-dayrateofvolismisrepreseheloheiredmahavebeeimesover。gofoldersurfacesbylavaflowsandfall-outfromeruptioingtoagloballyaveragedburialrateofaetresperyear，obscuresimpactcraterstoorapidlyfora。IfIoeverhadanouterlayerofiicactivityhaslongsincevaporizedit，allowingittobelosttospace，becauseIo’sgravityistooweaktoholdontoouras。laceforavolologisttovisit，ifonlytheharshradiationedidnotmakeIo’ssurfaceshlyinimiaion。

20。Top:partofthetviewofIoseeo-boundNewHorizonsmissionpassingJupiterinMarch2007。TheplumefromavoltatasitecalledTvashtarthenight-siderises300kilometressothatitsupperpartisinsunlight。AglowbeseenatitssourdtheshadowedlowerpartoftheplumeisfaintlyilluminatedbylightrefleJupiter。Bottom:A250-kilometre-wideviewofTvashtarseeyearsearlierbytheGalilehtisfromtheleft。Thedarkestmaterialisretlavaflows，abrightstreakheupperleftisilavabeiedfromavolicfissure

Europa

Europa（3，130kilometresier）ismyfavourite。Vesfromfly-bysin1980and1981showeditssurfaglikeacrackedeggshell，withveryfewimpactcraterstobeseeidalheatingwassomehowrefashioningEuropa’sicyouterlayer，thoughnotsorapidlyasIo。Higher-resolutionimagingbytheGalileomissionrevealedaplexsurfacehistory，aoanunusuallybittertroversy。Itwasalreadywellknoa’ssurfaceispredominantlywater-idtheglobe’soveralldensityshowsthatitsicycarapacehastobeabout100–150kilometresthick，adenser，roterior。However，densityargumentsotdistiweensolididliquidwater。Thesurfaceiceisstrongahankstoitslowtemperature。Thetroversythatemergedwasoverthestateofthe‘ice’belowthesurface。Wasitfrozenallthewaydowntotherockorwasthelowerpartliquid，cappedbyafloatingiceshell?

&errequiresagreaterrateofiidalheatiheexoticeptofaglobaloofliquidwaterbelowtheice。SofarasIamed，evidenagessuchasFigure21makesitclearthattheierallythin，ores，andsomustbefloatingonwater。However，forseveralyearsofGalileo’sorbitaltouroftheJupitersystem，apowerfullobbygroupontheimagiedintryihesurfacefeaturesasaresultofprocessesdrivenbysolid-stateihickicelayer。

WhatisnowthegenerallyacceptedbasiseologyisbestexplaioFigure21。Thisshh-standing‘rafts’ofidedby100-metrecliffs。Thesurfacesoftheraftsarecharacterizedbyapatternesandgriyofdires。Betweehetextureismorejumbled，andlacksaclearpattern。TherearelargeexpansesofEuropa（beyion）thathavenotbeeoraftsahesurfaisuedridgesaheraftsinFigure21areclearlybrmentsofthissortofterrain。Theridgeaerheopeningandgofcracks，probablyonatidaltwithEuropa’s3。6-dayorbitalperiod。Globally，onlyafewcrackswouldbeactiveataime。Whenanactivecraed（toawidthofperhapsjustametreorso），waterisdrawnupfrombeloorarilyexposedtotheofspaceatthetopofthecracksimultaneouslyboilsaprettysoonbeescoveredbyslush。Whentheeslushissqueezedoutontothesurfaiheclosedcraexttimethes，theridgeissplit，aobymoreslushwhenthecrackclosesagain。Afeeningandgwouldsufficetosurrresofthesizewesee。Eventually，eachcracksealspermaanewcrackwillbegiesomewhereelse，aernisbuiltup，givingtheridgeaerrainuchofEuropa，ahathasbeeofaball。

21。A42-kilometre-wideclose-upofpartofthearaChaionofEuropa，where‘melt-through’fromtheunderlyingohasallowedraftsoficetodriftapartbeforethearearefroze。Sunlightisfrht