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1、高考英语《流行话题语境识词4500》Unit88IsThereLifeonMars素材Unit88IsThereLifeonMars? Formostofthe20thcentury,therehasbeenlifeonMars.Or,atleastinthemindsoftheEarthlings,whoinhabititsclosestplanetaryneighbor.Whenattheendofthe19thcentury,theAmericanastronomerPercivalLowellthoughthesawcanalsontheRed
2、Planet,hewasconvincedthattheywerepartofaplanetwideirrigationsystem,builtbyanadvancedMartiancivilization. Then,in1965,theMariner4spacecraftflewpasttheRedPlanet.Itrevealedadesolateworld;theprospectsforlifewerefadingfast.Allhopediedin1976,whentheVikingLanderofNasafailedtofindasingl
3、eorganicmolecule. TwentyyearsafterthelastmissiontoMarsfailedtodetectanysignsoflife,NasaispreparinganotherseriesofmissionstotheRedPlanettofindevidenceoflife.ThenewunmannedNasamissionstoMarsarearesultofthegrowingoptimismamongscientiststhatlifeexists,ordidexist,onMars.Thestrongeste
4、videnceinsupportoflifeonMarscomesfromthestudyofmicrobes.Scientistsarefindingmicrobesinmoreandmoreinhospitableconditions--inrockshundredsofmeterbelowtheEarth'ssurfaceandinvolcanicspringswellabovetheboilingpointofwater.AndtheseconditionsareremarkablysimilartoconditionsonMars.Theseo
5、rganismscanliveandmultiplywithoutoxygenorlightinextremetemperatures,usingonlyrocksandwatertosustainthemselves. ScientistsbelievethatconditionsonMarsaround3.8billionyearsagoweresimilartothoseoftheearlyEarth,whenprimitiveorganismswerespreadingthroughouroceans.Undertheseconditions,
6、itishighlyprobablethatlifemayhavearisenonMarsaswell. EveniflifedidnotarisennaturallyonMars,itdoesnotmeanthatitcouldnothaveexistedthere.LifeformscouldhavebeentransferredbetweentheEarthandMarsindebriscreatedbytheimpactofcometsandasteroidsonthesurfaceofthetwoplanets.Eventoday,about
7、500tonsofmaterialfromMarslandsonEartheveryyear.ItismainlyintheformofdustbutoccasionallyalargerchunkstrikestheEarth.Itisinthesechunksofrock,whichweremuchlargerandmorefrequentinthepast,thatlifeformscouldhavebeentransportedfromplanettoplanet.Buthowcouldtheselifeformshavesurvivedthei
8、rjourneythroughspacefullofdeadlycosmicra
