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Chapter3 Gias
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&hebodiesthatdomiheSolarSystem–providedyouthinkitissizethatmatters，andarewilliheSuhefiasareillustratedtosthelowerhalfofFigure3，showinghowpreheheirsizeovershadowstheterrestrialplaheviewofUranusisfromtheHubbleSpaceTelesorbitabouttheEarth，whereastheiasareasseenbyvisitiheirdominationbymassisnotquites，becausetheyarelessdeheterrestrialplaer’sdensityisonly24%oftheEarth’s，andSaturnisevenlessdenseandwouldfloatifdroppedintoasuffitlylarge（ahetical）bucketofwater.AllofthemhaveriorialplahoughournandUranusaresuffitlypromiobevisibleihslooksolid，theyaremadeofmyriadspartidareextremelyinsubstantial.Theyarediscussed，alongwiththegiaellites，ier.
By，thesizeofagiaismeasuredfromthetopofitsclouds.Theseotheplaroposphere，abovewhicharelargelytraandprogressivelylessdenselayersclassifiableinthesamewayasfortheEarth’satmosphere.Thebaseofagiaroposphereishardtodefineandhasneverbeenexploredevenier，wherein1995aryprobereleasedbytheGalileospacecraftreachedadepthof160kilometresbelowthecloud-topsbeforepressure（22atmospheres）aure（153°C）putpaidtoit.Probably，thetroposphereofeatplamergesseamlesslyintoafluidiemperaturesandpressuressohighthatthereisnodistiweengasandliquid.ly，thereisnosolidsurfacethatahumaandupon.
Basicdatafiasaregivehepolardiametersquotedtherearelessthaorialdiameters，becauserapidrateofrotatioable2）flattenstheirshapes.Jupiter’spolardiameteris6.5%less，andSaturhanitspolardiameter.Thedifferenlyabout2%forthelessgassyandmoreslUraune（ahaerrestrialpla）.
Interiors
Thereisostudytheinteriiaweuseatmospheriposition（99%hydrogenandhelium）andeneralktheSolarSystemasawholeismadeoftostructamodelthatistwithitsmeasureddensity，andwiththeinteriorpressuresthatweferfromthis.Belohere，eatplahaveazoingmostlyenmolecules（H2）aoms（He），iisbettertocall‘fluid’ratherthaher‘liquid’aseous’.Attheverytre，thereisprobablyaroercore，ofaboutthreeEarth-massesierandSaturn，andaboutoh-massinsideUraune.Surroundingtheihereoughttobeanoutercoreof‘iposedofunknownproportionsofwater，ammonia，aingtoabouttwicethemassoftheEarthier，maybesixEarth-massesiurn，twelveinsideUranus，andfifteeukheseouterandinneroltenorsolid，becausealthoughweestimatethepressure（astaggering50milliohetreofJupiter），wedonotkionandhaveonlyavagueideaofthelikelytemperature（rangingfrominexcessof15，000°thetreofJupitertoabout2，200°Cattheeofune’score）.ofhowmaterialsbehaveundersuchextremesissketgwhethermetallicoulddiffereheroktowardstheanihecoresofUrauevenbeuedmixturesofidrock.
Table5 Basicdatafiaethatthemassuhousandtimesbiggertharialplaable3
Agforthecoresleaveslittlemorethah-massforthehydrogeeriorsofUraune，prisi6，000kilometresthick.Hasgiants’JupiterandSaturnhavemuchdeeperenvelopesenandheliumsurroundingtheirexcessof300ah-masses，respectively.Hydrogeomodelthaniceorrodstistsareprettytthatatpressuresgreaterthanabout2millionatmospheres，hydrogenatomsaresqueezedsotightlytogetherthatelesarenolongeredaboutspes.Iheyareablethaseaenthatbehaveslikeamoltehisfreedomofeleakes‘metalli’aductorofelectricity.Ashellofmetalli（withsomeheliumdissolvedinit）surroundingJupiter’scoreprobablyatsforabout260Earth-masses（80%ofJupiter’stotalmass），whereasaroundSaturhoughttopriseamoremodest41Earth-masses（justover40%ofSaturn’stotalmass）.Figure17illustratesthefulliructureofJupiter.
&ernalstructureofthegiaillbeevolvihepossibleexofUranus，theyallradiatemoreheattospatheyretheSun.Jupiterissomassivethatitcouldstillbeleakingoutasignifitofprimordialheattrappedwithisformation，butforSaturhisheatexcessshowsthatheatmustactuallybebeihin.Thediscyistete，sointerionmaystillbe.Thesettlihaerialinwards（allowiogrowwhilethesurroundingshellbeesthipurer）wouldvertgravitatioialeoheat.Suefromggrowthofcores（orinnercores）or，forSaturnonly，fromilingofheliumdropletsiallilayer.
17.Cut-awaydiagramshowingtheproposedinterhiheprincipaltroposphericcloud-topzht）as（dark）arelabelled
Atmospheres
position
Intrasttothereasoionaboutgiaerioftheiratmospheresdrawmoreoio.Thepositionofdthelayersbemeasuredbyopticalspectroscopy，whichisthestudyofhowsunlightofdifferehsisabsorbedatvariousdepthswithimosphere.Inaddition，theaveragemolecularmassateachdepthbedetermiofrefraexperiencedbyradiosignalstransmittedbyaspacecraftwhileitdisappearsfromviewbehi.Also，theGalileoentryprobemadevariousmeasuremeer’satmitsdest.Table6paresthechemipositionofthefiamospheres.Inadditiontothespecieslistedthere，eaallertracesofethyne（C2H2），Jupiterhasethene（C2H4），aurnbothhavephosphine（PH3），onoxide（dgermane（GeH4）.
&layerofuousUrauneisofmethaicles.ItistoowarmformethaionatJupiterandSatureadammoideoformthetopmostclouds.Thesetopcloudlayersareabout10kilometresthick，belowwhichthe‘air’probablybeesclearagain.Calssuggestthatier，thereshouldbeasedlayerofadeofammoniumhydrosulfide（NH4HS）about30kilometresbelow，andathirdcloudlayer，thistimeofwater（iceatthetop，liquidletsbelow）about20kilometreslowerstill.TheGalileoentryprobefoundprobableammoniumhydrosulfidecloudsatabhtdepthbutdidnotfindanywater-iesaythem;otherssaythattheprobepeoagapbetweendisuouswater-iceclouds.ThesamecloudlayersareexpectedatSaturn，butspacedaboutthreetimesfurtherapartbecauseofSatury.AmmcloudsareexpeethanecloudsofUraune.
Table6 Gasesdetetheatmospheresofthegias，showingthemeasuredproportionmadeupbyeach
&mosphericpressureatthetopofJupiter’sammoniacloudsisafactoroftwoorthreelessthanthesea-levelatmosphericpressureoh，whereasoplaoppressureisclosetoEarth’ssea-levelpressure.
Cir
Aglobalpatternofdsrunniheequatorisvisibleohroughasmalltelesilarpatteredlessdramatitheias.Smustplaysomeroleiionofthis，visible，partoftheiratmospheres，butitappearstobepoweredmostlybyiobetrolledbytheirrapidrotation.
Traditionally，thedarkbaoas‘belts’aerveningbrightbandsas‘zohehemaisandzonesonJupiterareindiFigure17.Becausethereisnosolidsurfacetoactasaframeofreferendspeedsongiasaremeasuredrelativetotheplaeofrotation.Ohecloud-topwindblowstotheeastatupto130metresperseostoftheEquatorialZoedgesoftheNorthaorialBeltssharethismotion，butthewindspeeddedultimatelyreverseswithdistanceawayfromtheequatoracrosseatiltheTropiesarereached，wherethewiiain，andsooedreversalsacrosseadzohepions.
InJupiter’szomosphereism，leadingtosationofammoniacloudshighup，wheretheynaturallyappearbright.versely，iheatmosphereismostlysinking，drawiopslower，sotoadepthwheretheylookdarker.Localexstothispatterifiedohegeneralrulezonesandsisscarcelyseemstoholdatalloplas，hericcirishardertofathoApligfafluengthevisibilityofzohepoorlyuoodnatureandabuevertrapoundsaddcolourtothedectedtoresultfromphotochemicalreas.Jupiter’svarioushuesofyellowandredcouldbecausedbysulfur（releasedphotochemieitherhydrogensulfideorammoniahydrosulfide），phosphorous（fromphosphine），orhydrazine（N2H4，madephotochemionia）.
Colourvariationsarelessprourn’satmosphere，aernofzosislesspromi.However，windspeedsarehigher，ingwindsinexetrespersedprevailingfor10°eithersideoftheequator.
RotatingstormsystemsarewellknownonbothJupiterandSaturfamousisJupiter’sGreatRedSpot.ThisbeseeninFigure3，asaraddlingthebouheSouthEquatorialBeltaropie.Itextends26，000kilometresfromeasttoiralstrudtakingaboutsixdaystorotateanticlockwise.Ithasbeeelescopicobservatio1830.SmallerstormsbemadeoutatavarietyofsbothJupiter（lookaloemperateBeltinFigure17）andSaturn.Aboutonceevery3summerinitsnortherurobedisfiguredbyagiantstormsystemthatbeginsasawhitespotor，butwithinamonthspreadtlraduallyfadingfromview.
&erandSaturnhaveayellowishusauneappearbluey-green.Thisisbecauseweseetheircloud-thadepthmethanegas，whichpreferentiallyabsorbsthelonger（red）wavelengthsoflight.
The82.1°CaxialtiltofUraremeseasonalvariations.Forexample，wheheotohavevisitedUrain1986，thesouthpolewasinfullsunlightandmostofthenorthernhemispherewassufferingdecadesofdarkssouthernhemispherelookeddisappointinglyblandontheVes，butastheUranianyearprogressedandtheSuoriseaoverawiderraudes，theglobebegaoresembletheias（Figure18）.In2007，Urahroughitsequinox，ahpole，followedgraduallybytherestofthesouthernhemisphere，begantodriftintolong-termdarkness，whichwillpeakwithsouthernmidwinterin2028.
&unewasrevealediailduringVoyager2’s1989fly-by，itresembledablueversionofJupiter.Therewasevenagiantstormsystemintheformofadarkspotjustsouthoftheequator，whichwasdubbedthe‘GreatDarkSpot’isfamousJovian.However，itprovedtobeshorter-lived，andhadvanishedby1994.UerandSaturorialtuneblowswest（oppositetotheplaion），asbeseewarddriftoftheGreatDarkSpotrelativetothesmaller，moresoutherlyspotinFigure18.
&ospheres
&hegiashasastroicfield.The‘magieune，whichistheeasureofaplaarymagicfield，is25timesgreaterthah’s.Uranus’sis38times，Saturn’sis582times，andJupiter’sis1，949timesgreater.Togehesefields，eaetmustazoridugfluidundergoingsomekindofvectivemotioerrestrialplahmagicfields（MerdEarth），theexplanationisafluidshelloftheirirohemagicfieldsofJupiterandSaturnareprobablygehemetallilayer，stirredintomotios’relativelyrapidrotation.PressuresaretoolowformetalliinUrauicfieldsarehardertoatfor，butareprobablycausedbymotioridug‘ice’oftheiroutercores.
18.Top:UraheHubbleSpaceTelesAugust1998（left）andJuly2ht）.Theorientatio’saxisrelativetotheSunisapparentfromthepatterTheregionarouhpolewasstillinsunlightin1998，buttheaxishadbeeotheSunby2006.High，brightcloudsareapparehinthe1998image，whichalsoshsaheiheringswereedge-onandinvisiblein2006，butinsteadweeularsatellites（Ariel）anditsshadow.Bottom:ttuneseenbyVitsapproa1989，reostexaeplaatireatDarkSpotandassociatedwispsofhigh，brightnitrogencirruscloudsarepromihegeneralbaure，andasmallerdarkspotfurthersouth
Animportantceofaplahaviicfield（liestoMerdtheEarthtoo）isthatitstheplainsideazoowhieticfieldliheSunotusuallypehiszoneiscalledthepla’s‘maghepathsedpartithesolarwiororolledbytheSuicfield，uhe‘bowshock’ofapla’smagosphere，whichdivertsthempastthepla.
Chargedpartigetthroughsometimes，especiallybyleakingbackuptheloail，down-Su.hesebeelledalongfieldliowardsthetopoftheatmosphere，wheretheirarrivalcausesglowsintheskycalledaurorae，wellkhandobservedalsoourn.
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