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).