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Mercury（水星）In1974,forthefirsttime,asU.S.spacecraftpassedwithin400milesoftheplanetMercury.Thenameofthespacecraftwasmariner10.Therewerenopeopleonthespacecraft,buttherewerecamerasthatcouldtakeclearpicturesfromalongdistance.Whatthepicturesshowedwasinteresting.TheyshowedthatMercury'ssurfacewasalotlikethesurfaceoftheMoon.ThesurfaceofMercuryisfilledwithhugeholescalledcraters.AlayeroffinedustcoversMercury.This,too,islikethedustontheMoon.ThereisnolifeoneitherMercuryortheMoon.Venus（金星）Formanyyears,nooneknewmuchaboutVenus.Whenpeoplelookedthroughtelescopes,theycouldnotseepastVenus'clouds.Longage,peoplethoughtthecloudscoveredlivingthings.Spacecraftradarhasshownthisisnottrue.Venusistoohotforlifetoexist.ThetemperatureonVenusisabout900degrees!Rememberhowhotyouwerelasttimeitwas90degrees?Nowimagineitbeing10timeshotter.Nothingcouldexistinthatheat.ItisalsoverydryonVenus.Forlifetoexist,watermustbepresent,becauseoftheheatanddryness,weknowtherearenopeople,plantsorotherlifeonVenus.Earth（地球）Oneplanetinoutsolarsystemcertainlysupportslife—Earth.Ourplanetisthethirdplanetfromthesunandittakes365days,orayear,toorbitthesun.This
rotationmakesitpossibleformostofourplanettohavefourseasons—winter,spring,summerandfall.Besidesbeingabletosupportlife,ourplanetisuniqueinanotherway—Earthis75%coveredbywater.Nootherplanethasthatmuch,ifanyliquidonitssurface.Thisliquidanditsevaporationhelpprovidethecloudcoverandourclimatepatterns.Earthhasonenaturalsatellite—themoon.Scientistsandotherexpertsallovertheworldhavecreatedandsentintoorbitothersatellitesusedforavarietyofpurposes—communication,weatherforecasting,andsoon.Mars（火星）TheU.S.hassentmanyunmannedspacecraftstoMarssince1964.(Unmannedmeanstherewerenopeopleonthespacecraft.Thatiswhyscientistsknowalotaboutthisplanet.Marshaslowtemperatures.ThereisnowateronMars.Thereisonlyagascalledwatervapor.ThereisalsoiceonMars.ScientistshavealsolearnedthatthereisfogonMarsintheearlymorning!DoyourememberwhenyoulastsawfoghereonEarth?ScientistssaythefogonMarslooksthesame.AsonEarth,thefogoccursinlow-lyingareasoftheground.AnotherinterestingthingaboutMarsisthatitisverywindy.Thewindblowsupmanyduststormsonthisplanet.Aspacecraftcalledmariner9wasthefirsttotakepicturesofduststorms.Later,theunmannedVikingspacecraftlandedonthe
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