千米小说网

Chapter 3 Giant planets（第1页）

Chapter3　Gias

&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，hericcirishardertofathom。Apligfafluengthevisibilityofzohepoorlyuoodnatureandabuevertrapoundsaddcolourtothedectedtoresultfromphotochemicalreas。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’saxisrelativetotheSunisapparentfromthepatterm。Theregionarouhpolewasstillinsunlightin1998，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。