问题标题：
介绍地球的解说词
问题描述：
介绍地球的解说词
牛立军回答：
TheEarth'ssurfaceisveryyoung.Intherelativelyshort(byastronomicalstandards)periodof500,000,000yearsorsoerosionandtectonicprocessesdestroyandrecreatemostoftheEarth'ssurfaceandtherebyeliminatealmostalltracesofearliergeologicsurfacehistory(suchasimpactcraters).ThustheveryearlyhistoryoftheEarthhasmostlybeenerased.TheEarthis4.5to4.6billionyearsold,buttheoldestknownrocksareabout4billionyearsoldandrocksolderthan3billionyearsarerare.Theoldestfossilsoflivingorganismsarelessthan3.9billionyearsold.Thereisnorecordofthecriticalperiodwhenlifewasfirstgettingstarted
71PercentoftheEarth'ssurfaceiscoveredwithwater.Earthistheonlyplanetonwhichwatercanexistinliquidformonthesurface(thoughtheremaybeliquidethaneormethaneonTitan'ssurfaceandliquidwaterbeneaththesurfaceofEuropa).Liquidwateris,ofcourse,essentialforlifeasweknowit.TheheatcapacityoftheoceansisalsoveryimportantinkeepingtheEarth'stemperaturerelativelystable.LiquidwaterisalsoresponsibleformostoftheerosionandweatheringoftheEarth'scontinents,aprocessuniqueinthesolarsystemtoday(thoughitmayhaveoccurredonMarsinthepast).
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