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Chapter4 Giaellitesandrings
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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），themeicalpropertiesaingbehaviouroftheiceareveryalogoustohowrockbehavesintheinnerSolarSysteInotherwords，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.
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