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1、CallistoandGanymedeFrom1996to1999,theGalileospacecraftpassedthroughtheJoviansystem,providingmuchinformationaboutJupiter'ssatellites.Callisto,theoutermostofJupiter'sfourlargestsatellites,orbitstheplanetinseventeendaysatadistancefromJupiteroftwomillionki
2、lometers.LikeourownMoon.Callistorotatesinthesameperiodasitrevolves,soitalwayskeepsthesamefacetowardJupiter.Itsnoontimesurfacetemperatureisonlyabout-140°C,sowatericeisstableonitssurfaceyear-round.Callistohasadiameterof4.820kilometers,almostthesameasthat
3、ofMercury.Itsmassisonlyone-thirdasgreat,whichmeansitsdensitymustbeonlyone-thirdasgreataswell.ThistellsusthatCallistohasfarlessoftherockymetallicmaterialsfoundintheinnerplanetsandmustinsteadbeanicybodythroughmuchofitsinterior.Callistohasnotfullydifferen
4、tiated,meaningseparatedintolayersofdifferentdensitymaterials.AstronomerscantellthatitlacksadensecorefromthedetailsofitsgravitationalpullontheGalileospacecraftduringseveralverycloseflybys.Thisfactsurprisedscientists,whoexpectedthatallthebigicymoonswould
5、bedifferentiated.Itismucheasierforanicybodytodifferentiatethanforarockyone,sincethemeltingtemperatureoficeissolow.Onlyalittleheatingwillsoftentheiceandgettheprocessstarted,allowingtherockandmetaltosinktothecenterandtheslushyicetofloattothesurface.YetCa
6、llistoseemstohavefrozensolidbeforetheprocessofdifferentiationwascomplete.LikeourMoon'shighlands,thesurfaceofCallistoiscoveredwithimpactcraters.Thesurvivalofthesecraterstellsusthatanicyobjectcanformandretainimpactcratersmitssurface.Inthinkingoficesofarf
7、romtheSun,itisimportantnottojudgeitsbehaviorfromthatofthemuchwarmericeweknowonEarth;atthetemperaturesoftheoutersolarsystem,iceonthesurfaceisnearlyashardasrock,andbehavessimilarly.IceonCallistodoesnotdeformorflowlikeiceinglaciersonEarth.Callistoisunique
8、amongtheplanet-sizedobjectsofthesolarsysteminitsabsenceofinteriorforcestodrivegeologicalevolution.Thesatellitewasborndeadandhasremainedgeologicallydeadformorethanfourbillionyears.Ganymede,anotherofJupiter'ssatellitesandthelargestinourso