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Lecture1INTRODUCTIONTOEARTHMajorConcept(I)Theplanetsinoursolarsystem,includingEarth,begantoformoutofarotatingdiskofgasanddustabout4.6billionyearsago.Relatedorsupportingconcepts:Thepresentviewofthebeginningofthesolarsystemsuggeststhatacloudofgasanddustwasflattenedintoadiskbyrotation.Thedensityofthematterinthediskwasgreatestatitscenteranddecreasedtowardtheedges.Thesunformedatthecenterofthediskwherethetemperaturewashighestandtheplanetsformedatgreaterdistancesawayfromthecenter.Thecompositionsoftheplanetsdifferbecauseofanaturalsortingofelementsinthecloudwithdistancefromthecenter.ThetemperatureoftheearlyEarthwasprobablyquitehighdueto:a.thegravitationalcaptureofparticlesthatreleasedheatonimpact,b.theincreasedpressureontheinterioroftheplanetasitcontinuedtogrow,andc.thecaptureofradioactiveelementsthatgaveoffheatwhentheydecayed.Duringthefirstfewhundredmillionyears,theinternaltemperaturerosehighenoughtomeltironandnickel.Theseheavymetalssanktoformthecoreoftheplanetanddisplacedlightermaterialthatrosetoformchemicallydifferentiatedshells.MajorConcept(II)Earth’soceansoriginatedearlyintheplanet’shistory.Therearetwoproposedsourcesofthewaterintheoceans.Relatedorsupportingconcepts:Theoldestsedimentaryrocksfoundthatrequiredwaterfortheirformationare3.9billionyearsold.ThissuggeststhattherearebeenoceansonEarthforroughly4billionyears.TraditionallyscientistshavebelievedthatthewaterintheoceansandatmosphereoriginatedinEarth’smantleandwasbroughttothesurfacebyvolcanism.Mostmagmascontainfrom1%to5%dissolvedgasbyweight,mostofwhichiswatervapor.Ifweassumethatthecurrentrateofreleaseofwatervaporinvolcaniceruptionshasremainedessentiallyconstantforthepast4billionyears,thenthevolumeofwaterexpelled(putout)duringthattimewouldberoughly100timesthevolumeoftoday’soceans.ArelativelynewproposalisthatasignificantamountofwaterisaddedtoEarthfromspacewhensmallicycomets（彗星）enterEarth’satmosphere.Theearlyatmospheredidnothaveanyoxygen.Theoxygenreleasedfromtheinterioroftheplanetwouldhavecombinedwithmetalsatthesurfacetoproduceoxides.Itwasn'tuntillargeamountsofoxygenwereproducedafterplantsformedthatitbegantoaccumulateintheatmosphere.MajorConcept(III)Thereiscompellingevidencethattheremaybeotheroceansinthesolarsystem.Relatedorsupportingconcepts:TwoofJupiter’smoons,EuropaandCallisto,mayhaveoceansbeneaththeiricysurfaces.LiquidoceansonEuropamaybepossibledespiteextremelylowtemperaturesbecauseofheatgeneratedbyfrictionasitisdeformedbyJupiter’stide-producinggravitationalforce.Bothmoonshaveinducedmagneticfieldsindicatingtheyconsistpartlyofstronglyconductingmaterial.Thismaybeliquidwaterwithelectrolyticsalts.ThereisalsosomeindicationthatMarsoncehadanoceancoveringportionsofitsnorthernhemisphere.MajorConcept(IV)ThereareavarietyoflinesofindirectevidencethatallowustoestimateEarth’sageasbeingabout4.5to4.6billionyears.Relatedorsupportingconcepts:Earth’soriginalcrust,thatwouldhaveformedearlyinitshistory,hasbeendestroyedorcoveredbyactivevolcanismandtectonicprocesses.Thisisareflectionofthegeologicallyactivenatureoftheplanet.Consequently,theoldestsurfacerockswehavefound,roughly4billionyearsold,giveusaminimumageforEarthratherthananactualage.IndirectevidenceofEarth’sageincludes:a.thedatingofmoonrocksat4.2billionyears,b.thedatingofmeteoritesat4.5to4.6billionyears,andc.theoreticalcalculationsoftheageofthesunasbeingroughlythesameasthemeteorites.Wecandeterminetheageofrocksthatcontainradioactiveisotopesofelementsthatdecayatconstant,knownrates.Thisiscalledradiometricdating.Ifarockcontainsatomsofaspecificradioactiveisotope,halfofthemwilldecayintoanotherelement,calledthedaughterproduct,inaperiodoftimecalledthehalf-lifeoftheisotope.AnexampleofthistypeofprocesswouldbeUranium235anditsdaughterproductLead207withahalf-lifeof704millionyears.Arockthatinitiallycontained100atomsofU235andnoPb207wouldhavethefollowingnumbersofthetwoelementsaftertheindicatedelapsedtimes:Thereareseveraldifferentparent-daughterradioactivepairsthatoccurinrocks.Eachofthepairshasitsownspecifichalf-life,somequitelongandothersveryshort.Pairswithlonghalf-livesarebestfordatingveryoldrockswhilethosewithshorthalf-livesarebestforyoungerrocks.Ideally,anassigneddateforarockwillbebasedontheuseofmorethanoneradioactivepairsothatacheckcanbemadeonaccuracy.MajorConcept(V)Geologictimeisoftendifficulttograspbecauseofhowlongitiscomparedtoallofhumanhistory(tosaynothingoftheincrediblybrieftimespanofasinglehumanlife).RelatedorsupportingconceptsJustaswedividetimeintosegmentslikeyears,months,anddaystoidentifybothdurationandsomesenseofposition,wedividegeologictimeaswell.Thelongestdivisioniscalledaneon.Eonsaredividedintoeras,erasaredividedintoperiods,andperiodsintoepochs.Thegeologictimescaleandthehistoryofevolutionoflife-formsisgivenintable2.1.Theeras,inorderofmostdistantpasttomostrecenttime,are:a.Precambrian,b.Paleozoic(ancientlife),c.Mesozoic(intermediatelife),andd.Cenozoic(recentlife).Thedivisionsofgeologictimewereoriginallydecidedupononthebasisoftheappearanceanddisappearanceofspecificfossils.Thedevelopmentofradiometricdatingtechniquesallowedgeologists