9.2 billion years a.b. (after the bang) to the present day...
TRANSCRIPT
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9.2billionyearsa.b.(afterthebang)tothepresentday(13.7billionyearsa.b.)
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chapter5theemergenceoflife
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stromatolites
thenearestthingonearthtolivingfossils,stromatolites
arerocksmadefrommillionsofmicroscopiclayers
comprisedoftheremainsofbacteria.theycanstill
beseeninsharkbay,australia.
theMoon
thefamiliarnearsideoftheMoon.thedarkareas
arethemare,orseas–ancientlavabasinsfilledwith
darkmaterial–andthewholesurfaceispeppered
withcraters.
formationoftheMoon
twoplanetscollidedandtheircorescoalescedinto
asinglelargerplanet,earth.themantledebris
becametheMoon.
ataround4.6billionyearsago,theearthhadfinallyformedbutitwascompletelymolten,andbeforethesurfacecouldcooladramaticeventtookplacethatresultedintheformationoftheMoon.thefavouredtheoryatthepresenttimeis
thatthiswasduetoagiantimpact,whentheearthcollidedwithabody,perhapsaslargeasMars.thetwomerged,anddebriswasscatteredtoformtheMoon.thefactthattheMoonislessdensethantheearthshowsthattheactualcoresofthetwobodieswerenotinvolvedintheformationoftheMoonbutmergedtoformtheearth’spresentcore.
theroleoftheMoonourMoonappearstobespecial,andhasanessentialroleintheevolutionoflifeonourplanet.theMoonstabilisesthetiltoftheearth’saxis,whichiscurrently23degreesanddoesnotvarybymorethanadegree.iftheMoonhadnotbeenpresent,thistiltwouldhavevariedmarkedly,andtheclimaticsituationwouldbeverydifferent.comparethiswithMars,whichhasnocomparablesatellite;itstwoattendants,phobosanddeimos,aresosmallthattheirinfluenceisnegligible.thereisthusnostabilisingforce,andthetiltoftheMartianaxisvariesfrom11to35degreesoveracycleofroughly100,000years.theevolutionoflifedependsuponthelong-termstabilityofclimate.ifearth’saxisofrotationvariedwildlyoverashortperiod,thisstabilitywouldbelacking,andlifeasweknowitwouldnothavedeveloped.itseemsweoweourgratefulthankstotheMoonformakingourexistencepossible.
themostobviouseffectoftheMoonontheearthisinthetidesthatitraises.thefrictionthiscausesslowstheearth’srotation,andthisprocessiscontinuingtoday.anequallyimportanteffectistoincreasetheseparationbetweentheearthanditssatellite;thedistancebetweenthemisincreasingatarateof4centimetres(11/2inches)peryear.
asonemightexpect,theearthhasasimilareffectontheMoon,andtheearth’smassis80timesthatoftheMoon,soitsinfluencehasbeenevenmoreextensive.longagotheMoon’srotationwastidallysloweduntilitbecame‘captured’,orsynchronous,meaningthatitsperiodofspinbecameexactlyequaltoitsorbitalperiod.theresultisthattheMoonalwayspresentsthesamefacetotheearth.itisimportanttorememberthatalthoughtheMoonkeepsthesamefaceturnedtowardus,itdoesnotkeepthesamefaceturnedtowardthesun,andtheideathatthereisa‘darkside’totheMooniscompletelywrong.dayandnightconditionsontheMoonarethesameinbothhemispheres,apartfromthefactthattheearthwillneverbeseenfromtheavertedhemisphere.
thespeedoftheMoon’srotationquicklybecameaconstant,butthespeedwithwhichittravelsalongitsellipticalpathroundtheearthneverhas.followingtheusualtrafficlawsofthesolarsystem,theMoonmovesfastestwhennear‘perigee’,itsclosestpointtotheearth,andslowerelsewhere.thereforethepositioninorbitandtheamountofrotationbecomeoutofstep.theresultisthat,seenfromtheearth,theMoonseemstorocktoandfro.sometimesweseealittlearoundthemeanWestern‘limb’,andsometimesalittlearoundtheeasternedge.altogether,duetothisandothersmaller‘librations’,asthesewobblesarecalled,wecanexaminefromtheearthatotalof59percentoftheMoon’ssurface,althoughnevermorethan50percentatatime.itisonly41percentofthesurfacethatwecannotsee.
ourplanet–cradleforlifeatfirst,theearthwasmolten,andfartoohotforlifetoappear.itcooleddowngradually
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overroughly500millionyears,andformedasolidcrust.theoriginalatmospherewasmadeuplargelyofhydrogen,butthiscouldnotlast.theenergeticatomssoonleakedawayintospace,astheearth’sgravitationalpullwas,andis,tooweaktoretainthem.theremayevenhavebeenaperiodwhentheearthhadnoatmosphereatall,butthischanged.Volcanicactivitywouldhavebeenmuchmorecommon–andmuchmoreviolent–anderuptionsfrominsidetheglobesoonsentoutsufficientquantitiesofgasestoproduceanewatmosphere.ofcourse,thisatmospherewasverydifferentfromthatoftoday,notablyinitslackofoxygen.however,astheatmospherecooledwaterbegantocondenseandtherefollowedtheperiodwecouldcallthegreatrains,whichlastedforlongenoughtofillthelowerlyingregionswithwaterandproducethefirstoceans.
therewasalsoaperiodofbombardmentfrommaterialleftover,sotospeak,whentheplanetswereformed.thisisveryevidentwhenwelookatthelunarsurface,wherethecraterswereproducedbybombardmentduringthisperiod.ofcourse,theearthwasequallybombardedbuterosionhasremovedmostofitsscars.itisworthnotingthat,haditnotbeenforthecontinuingtectonicactivity,thecrashingtogetherofplatesandthesqueezingupofmountains,waterwouldhavecoveredthewholeofasmoothglobetothisday.thetectonicforcesaredrivenbyheatfromthedecayofuraniumandotherunstableheavyelementsdeepwithintheearth.thesematerialsmust,aswehaveseen,havecomefrompreviouscataclysmicstardeaths.somanydistanteventshavebeenresponsibleformakingitpossibleforthestagetobesetfortheemergenceoflife.
lifeemergedmuchearlierthanhasoftenbeenbelieved.thefirstorganismabletoreproduceitselfprobablyappearedabout4.3billionyearsago.theearliestevidenceoflife,attributedtothefirstorganismswhichwereclearlyprimitive,isamarkedriseinlevelsofoxygenintheatmosphere.thefactthatthepresenceofsignificantquantitiesofoxygenisanunmistakeablesignoflifegiveshopetothosescientistsdevelopingmissionstosearchforearth-sizedplanetsaroundotherstars;interstellartravelmaybesomewayoff,butwemaybeabletoseethesignatureoflifefromagreatdistance.theoldestevidenceoflifeyetdiscovereddatesfrom3.8billionyearsagoandisfoundinancientrocksfromtheislandofakiliainWesterngreenland.
theexactprocessbywhichlifearoseisstillunclear;contrarytopopularmyth,noonehasyetcomeclosetorepeatingthisfeatinthelaboratory.thetheory(unproven)goesthatchemicalreactionsweredrivenbyenergyfromsourcessuchaslightningstrikesandshort-waveradiationfromthesun.astimewentby,moreandmorecomplexmoleculeswereproduced,untileventuallyamoleculeappearedwhichcouldreplicateitself.theabilitytoreplicate,orreproduce,isfundamentaltoanythingthatwethinkofaslife.thereplicationwasnotperfect;eachgenerationbroughtwithitachanceofrandomvariations–errorsinthecopyingprocess.someoftheserandommutations,astheyarecalled,were
panspermia
thelatefredhoyle,andhiscolleaguechandra
Wickramasinghe,buildingonconjecturebythe
swedishscientistarrenhuis,maintainedthatcomets
coulddumpvirusesintotheupperatmosphere,
causingwidespreadepidemics.(Virusesarestrands
ofdnaorrnathatusetheapparatusoflivingcells
toreplicate.somebiologistsdisputethattheyare
actuallyaliveintheclassicsense.)againtherehas
beenlittlesupportforthisidea,andtheideahas
neverbeentakenseriouslybymedicalexperts.
probablythemostunusualserioustheoryin
connectionwithlifefromspacewasproposedbyno
lessaneminencethanfranciscrick,co-discoverer
ofthedouble-helixstructureofdna.Withthe
9.2billionyearsa.b.–present
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chemistleslieorgel,heputforwardthetheory
of‘directedpanspermia’,accordingtowhichlife
wasdeliberatelysenttoearthbybeingsfroman
advancedtechnologicalcivilizationfromfaracross
thegalaxy.itwaspointedoutthatthechancesof
micro-organismsbeingpassivelytransportedfrom
worldtoworldacrossinterstellarspacewereslight,
butthingswouldbedifferentifpreparationswere
made.differenttypesofmicro-organismcould
becarriedinaspaceshipanddepositedhereto
flourishanddevelop.Whenthetheoryappeared,it
isperhapsfairtosaythatscientistsingeneralwere
stunnedratherthanenthusiastic,butideasofthis
kindareextremelydifficulttodisprove.
moresuccessful,survivinglongerorreproducingmoreeasilythanothers,andsoweremorelikelytoformthenextgeneration.thiscompetitionbetweenslightlydifferentformsisattheheartofwhathasbecomeknownasevolution.thelongstatelyprocesswhichmusthaveledfromthesesimplereplicators,nomorethancomplexmolecules,tothevastvarietyoflifethatweseearoundustoday,hadbegun.
theearliestknownfossilsareofbacteria.theseorganismsprobablylivedinthehotoceansfoundonearthatthistime.Wecanbefairlyconfidentabouttheirage,becausegeologicalmethodscantellustheageoftherocksinwhichtheremainsoftheseprimitiveorganismsarefound.inrocksofthistimewealsofindwhatarecalledstromatolites,
Volcano
earth'searlyoxygen-freeatmosphereisbelievedtohave
formedasaresultofaperiodofprolificvolcanicactivity.
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blacksmoker
Moreproperlyknownashydrothermalvents,black
smokersaresitesoveramilebelowthesurfaceof
theoceanwheresuperheatedgasesareescaping
fromfissuresintheearth’scrust.theyarefoundin
mid-oceanridges,andsurprisinglyahugevariety
oforganismssurroundthem,thrivingdespitehigh
temperatures,acompleteabsenceofsunlight,andhigh
acidity.thesecreaturesformafoodchaincompletely
independentofenergyfromthesun,butthatisinstead
basedonchemicalsemergingfromtheventitself.
9.2billionyearsa.b.–present
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theemergenceoflife 113
rock-likestructuresbuiltbyblue-greenalgae,alsoknownascyanobacteria;stromatolitestoodatebacktoaround3.5billionyearsago,and,remarkably,sometypessurvivetoday,notablyinpartsofthenorthernterritoryofaustralia.duringtheearlystagesofearth’shistory,cyanobacteriaplayedamajorroleinproducingfreeoxygen,andthisbegantheprocessofproducingabreathableatmosphere.
Wehavefoundthatlifeisextremelyversatile;someformsareamazinglytolerant,andcanthriveinthemostunlikelyplaces.forexample,oneofthefirstplaceslifemayhaveappearedisaroundwhataretermedhydrothermalvents,oftenknownasblacksmokers.thesearefissuresinthefloorsoftheoceansthatleakouthot,acid-richwaterfrombelow;theeffluviaareoftenblack–hencethenickname.thetemperatureofthewateremergingfromthesefissures,atleastamileunderthesea’ssurface,maybeashighas400°c.Waterisabletoreachthistemperature,higherthanitsnormalboilingpoint,duetothepressure,whichis25timesthatduetotheatmosphereatgroundlevel.remarkably,thefissuresareteemingwithspecializedlifeformssuchastubeworms,shrimpsandevenclams,whichsurviveinanenvironmentasacidicasvinegarthat
colonyoftubeworms
relativesofthesecreaturesliveneartheblacksmokers
inextremelyhostileconditions.theyhavenomouth
orstomachandsurvivebyabsorbingchemicalsinthe
waterthroughtheirskin.
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inspirationalhistory
itwasabookbypatrickMoore,theearth,intheschool
library,thatintroducedbriantotheamazingstoryof
thetrilobites,andinspiredhimtoalife-longpassionfor
astronomy.therewereonce15,000speciesoftrilobite,
andbythetimetheywerewipedoutbythepermian
extinction250millionyearsago,theyhadroamedthe
earthfor300millionyears.bycomparison,wehumans
havesofarbeenaroundforlessthan200,000years.
thetrilobites’nearestlivingrelativeisthehorse-shoe
crab.thisparticulartrilobiterecentlyfounditswayto
anatureshopinnewyork,forsalealongwithafine
selectionofmeteorites,dinosaurbones,andotherclues
toourdistantpast.
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theemergenceoflife 115
wouldbeinstantlyfataltomostotherformsofsealife,andwithoutreceivinganyenergyfromthesun.
Worldwide,thefossilrecordallowsustotracetheevolutionoflivingcreatures.generallyspeaking,lifeevolvedratherslowly;foralongtimeitwasconfinedtothesea,andonlyduringwhatistermedthedevonianperiod,around400millionyearsago,didlifespreadtotheland–plantsfirst,thenarthropods(suchasinsects,spidersandcrustaceans)andvertebrates.theplantgrowthonlandcontinuedtoproducechangestothecompositionoftheatmosphere.plantssurviveusingphotosynthesis,whichremovescarbondioxidefromtheairandusesittobuildfoodintheformofsugarmolecules.awasteproductofthisprocessisoxygen,whichplantsreleaseintotheair.
graveyardofthedinosaursthegreatestcatastropheinlife’shistoryoccurredattheendoftheeraknowntogeologistsasthepermian,250millionyearsago.thepermianlastedforaroundsixtymillionyears,andseemstohavebeenatimeofwidespreaddeserts.Mostoftheworld’slandmasseswerejoinedtogetherinavastcontinent,whichhasbeencalledpangaea.itseemsthatthispermianextinction,oftenknownmorepoeticallyasthe‘greatdying’,wasthegreatestinhistoryandwipedoutmostoflifeonearth.this,ofcourse,canbeestablishedbythefossilrecord,butthereisnocraterlefttoguideusastothecauseofthedisaster.instead,wemustdependoncertaincarbonmoleculesknownasfullerenes.thesemoleculesformacage-likestructure,mostoftenintheshapeofaball,andinsidethiscagesingleunreactiveatomsaretrappedatthetimeofformation.theheliumandargonfoundinfullerenesattheendofthepermianseemtohavecomefromspace,producedintheatmosphereofastarthatexplodedasasupernovabeforethesunwasformed.thesechemicalsmaybetheremainsofameteoritethatcarriedmaterialleftoverfromthebeginningofthesolarsystem.itissuggestedthat,asaconsequenceoftheimpact,theremayalsohavebeenhugeamountsofvolcanicactivity,coveringtheentirelandsurfaceinlavatoadepthofthreemetres(ninefeet).itisthereforefarfromsurprisingthat90percentofallmarinespeciesand70percentoflandvertebratesfailedtosurvive.
reptilesbegantoappearthroughoutthepermian,andwecometotheageofthedinosaurs,someofwhichwerehugeandferocioushunterswhileothersweresmallandherbivorous(planteaters).onesmallharmlessdinosaur,nolargerthanacanary,hasbeennicknamedthetweetieosaurus.
dinosaursruledtheworldforalmost200millionyears(incomparison,humanbeingshavebeenonearthforlessthan200,000years),butthen,attheendofthegeologicalcretaceousperiod,65millionyearsago,thegreatdinosaurssuddenlyvanished.yettheextinctionmaynothavebeentotal;itnowseemscertainthatsomeofthesmallerspeciessurvivetothepresentdayintheformoftheirfeatherydescendants,thebirds.thedepartureofthedinosaursmayhavebeenagoodthingfromourpointofview,becauseitmeantthatmammalscoulddiversifyfromsmallshrew-likeanimalsintothewidevarietyofspeciesweseetoday.theapesthatevolvedintheMioceneperiod(25to5millionyearsago)areourdirectancestors.
investigationofthecauseofextinctionsisapopularpursuit,andopinionsvary.sofarasthedemiseofthedinosaursisconcerned,thefavouredtheoryoftodayisthatalargemeteoritestrucktheearth,throwingupacolossalamountofdust,andcausingglobaldevastation–ithasevenbeenclaimedthatthesiteoftheimpacthasbeenidentified;
chicxulubbasin
thechicxulubcraterinMexicomaybethesiteofthe
impactthatendedthereignofthedinosaurs.
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lightcurveartwork
Marsexpress
formanyyears,peoplehavespeculatedaboutthe
possibilityoflifeonMars,whichistheonlyworld
inthesolarsystemthatisnottoounliketheearth.
itismuchsmaller,sothatithaslostmuchofthe
atmosphereitmayoncehavehad,anditiscolder,
becauseitisaround44millionmilesfurtheraway
fromthesun,buttherearenotoxiccloudsorlethal
radiationzones.resultsobtainedin2004bythe
Marsexpressorbiter,andthetwoUsroversspirit
andopportunity,provethatsaltyseasoncecovered
partsofthesurface,andpresumablyconditions
thenweresuitableforlife.theMartianengineers
invokedbypercivallowellamerehundredyears
agotoexplainthecanal-likefeatureshethought
hesawonthesurfacehavebeenbanishedtothe
realmofsciencefiction,althoughmanypeople,
exploringthesurfaceofMars
inthisreconstruction,nasa’srovervehicle,spirit,
standsproudlyonthesurfaceofMars,halfwayup
husbandhill.thisrobotgeologist,alongwithitstwin,
opportunity,hasprovidedthebestevidenceyetthat
therewasonceliquidwaterontheplanet’ssurface.
chicxuluboffthecoastofMexico,wherewecandetecttheerodedtractsofavastcrater.theevidenceisbasedmainlyonthefactthatrockslaiddownatthisperiodoveralargeareacontainmorethantheexpectedamountofiridium–anelementcomparativelyrareonearthbutcharacteristicofmeteorites.Wecannotbecertaintheimpactwipedoutthedinosaurs,butthetheoryiswidelysupported.
ithasbeenusefultospendsometimeindiscussionoflifeonearth,becausethequestionmustnowbeaskedwhetherthissequenceofeventshasbeenduplicatedelsewhere.if,elsewhere,thereisanearth-likeplanetorbitingasun-likestar,therehavebeensuggestionsthatweshouldexpecttofindsomekindoflife,eventhoughwehavenoideahowlifestarted.butwewillneverbesureuntil–if–wedetectsignalsfromanother
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civilization.thesearchison;however,alldirectsearchesforsignalsfromotherintelligentbeings(seti–thesearchforextraterrestrialintelligence)havesofardrawnablank.
istherelifeonMars?Whatfactorsdoweneedtoconsiderwhencalculatingtheoddsofsucceedinginoursearch?onepointmustbeclearedupatonce:wearediscussinglifeasweknowit.alllifeofthekindwecanunderstandisbasedupononetypeofatom,theatomofcarbon;onlyitcanlinkupwithenoughotheratomstoformthecomplexatomgroupsormoleculesthatareneeded.itfollowsthatlife,whereveritexists–here,onMars,oronaplanetinadistantgalaxy–mustbecarbon-based.WorldssuchastheairlessMoonmustberejected
Marsexpressrover
theeuropeanspaceagency'sMarsexpress
probecarriesastereocamerathatproducesthree-
dimensionalimages.
includingscientists,weresorrytoseethemleave
thestage,butlowlyorganismscouldexistthere
evennow.itisnotunreasonabletosupposethatif
lifeappears,itwillevolveasfaraslocalconditions
permit.itmaywellbethatlifeappearedonMars
andbegantoevolvebuthadlittleopportunityto
diversifybeforeconditionsdeteriorated.
itwasrecentlyclaimedthatevidenceofmicro-
organismshadbeenfoundinrocksthoughtto
havebeenthrownfromMarstoearthbythe
impactofahugemeteoriteonthesurfaceofMars.
however,thereisdoubtastotheaccuracyofthe
conclusions,anditwouldtakeasampleofthe
Martiansurfacebroughtbackbyaprobe,showing
clearsignsoflife,toallowustoconcludethatlife
willappearwhereverconditionsarefavourable.
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outofhand.inoursolarsystem,perhapsonlytheearthissuitedtocomplex,intelligentlifeforms.thecounter-argument,ofcourse,isthatwemaybecompletelywrong,andthattherecanbeintelligentbeingswhosebodiesarebaseduponatomsofgold,andwhobreatheinatmospheresofsulphuricacid.beingsofthiskind(beMsorbug-eyedMonsters)aremuchlovedbyscience-fictionwriters,fromhgWellsonward,butiftheydoexistthenthewholeofourmodernscienceiswrong,andthisseemswildlyimprobable.
Wehaveatleastestablishedthatmanystarsdoindeedhaveplanetarysystems,butforaplanettobearlifethereareseveralconditionsthatmustbefulfilled.(again,letusstressthatweareconsideringonlylifeofthekindwecanunderstand.onceweentertherealmofcompletelyalienlifeforms,speculationbecomesendlessand,fornow,weintendtorestrictthediscussiontocarbon-basedlife.)theplanetmusthaveanatmospherethatcontainssufficientfreeoxygen;itmusthaveasolid(orpossiblyliquid)surface;theremustbeanadequatesupplyofwater;afairlyequabletemperature;andalongperiodoverwhichconditionsdonotchangedramatically.alltheseconditionsarefulfilledbytheearth,butbynootherbodyinthesolarsystem.
however,theremaybeotherlessobviousrequirements.forexample,afairlyregularalternationofdayandnightseemsadvantageous.ifonehemisphereoftheplanetisinpermanentdarkness,andtheotherpermanentlyilluminatedbythesun,violentwindswouldensue,rainwouldbeimpossibleandtemperatureswouldhardlybeconduciveto
coprateschasma
thestereocameraontheeuropeanspaceagency’s
Marsexpressspacecraftallowsscientiststoreconstruct
theviewofMarsfromanyperspective,likethis.
coprateschasmaislocatedinthemiddleoftheValles
Marineris,aseriesofcanyonsthatdwarfearth’sgrand
canyon.
9.2billionyearsa.b.–present
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theemergenceoflife 119
life;freezingononesideandboilingontheother.ofcoursetheremightbeafavourablezoneontheterminator(theboundarybetweenthedarkandlightsides).
letusfocusontemperature.aroundastarthereisaregionknownasthehabitable,or‘goldilocks’zoneorecosphere,whereaplanetwillbeneithertoohotnortoocoldforlifetoexist.neitheroftheorbitsofVenusorMarsliewithintheecosphere;Venusistooclosetothesun,andtoohot,andMarsistoofaraway,andtoocold.onlyourplanetmovescomfortablyinthemiddleofthezone;theearth,likebabybear'sporridge,isjustright.astarlessluminousthanthesunwillhaveitsecospherecloser-in;withamorepowerfulstar,theecospherewillbefurtherout.Manyrequirementsareselfevident,andruleoutmanystarsascandidatesforplanetarysystems;astronglyvariablestar,forexample,wouldgiveanyplanetamostchangeableclimate.
Wehavealreadyseenthatourgalaxyhasaroundahundredbillionstars,anaveragenumberforalargegalaxy.itseemslikelyfrompresentdayobservationsthatmostsinglestarshaveplanets,leavinguswitharoundfortybillionsolarsystems.howmanyoftheseplanetsliewithintheirstar’secosphere?ontheevidenceofourownsolarsystem,theonlyonethatwehavesufficientknowledgeof,wemightsurmisethateachsolarsystempossessesoneplanetwithinitsecosphere.Weshouldruleoutplanetsaroundstronglyvariablestars,however,sowemightbeleftwithtwentybillionsuitablyplacedplanets.howmanyofthesewillberocky?thisisanewproblem–aswehaveseen,othersolar
christmasonanalienworld
oneoftheplayfulvisionsofwhatlifemightbelikeon
anotherplanetbygertrudeMoore,patrick'smother.
factandfiction
renownedforhisscientificwriting,patrickhasalso
writtenmanyscience-fictionnovels.
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systemsseemtohavegasgiantsinthegoldilocksregion.itisdifficulttodeterminetheprobabilityoffindingarockyplanetinasuitableplace,butoutofthe120orsosystemsknown,30havenogiantplanetintheway,sousingthatfractionasouroptimisticestimatewehavefivebillionplanetsonwhichwethinkconditionscouldallowlifetoform.onhowmanyofthesediditform?toanswerthisquestion–perhapsthemostdifficultofall–weneedtoknowandunderstandtheexactmechanismbywhichlifestarted.itisfairtosaythatbiologistshavenoexperimentallytesteddetailedtheories,andthismakesreducingtheprobabilitytoasinglenumberverydifficult.iftheprobabilityisevenoneinathousandbillion,findingjustourcivililizationwithinourgalaxyseemsanamazingfluke.if,assomebelieve,itisclosertooneinahundred,thenwewillhavemanymillionsofpromisingplanetstosearch.thisproblemisoneofthemainreasonsthediscoveryoflifeonMarswouldbesoimportant–iflifebeganindependentlytwiceinthesamesolarsystem,theoddsofitbeginningthroughoutthegalaxymustbegreatlyimproved.eventhen,though,ourproblemswouldnotbeover!
Wemustthenaskhowlikelyitisthatlife,oncebegun,willformintelligentbeingsthatwecouldcommunicatewith.somebiologistsbelievethatoncelifehasstarted,intelligenceisinevitable,whileothersargueequallyconvincinglythatintelligencelikeoursisatrueone-off.howmanyoftheseintelligencescouldwedetect?theymusthavereachedorsurpassedaleveloftechnologyachievedbythehumanraceonlyinthelasthundredyears.andthenwemustwonderhowlongacivilizationcapableofcommunicationwilllastbeforebeingdestroyed,eitherbynaturaldisasterorbyitsownfolly.inourowncaseatpresent,thelatterseemsmorelikely.Wehavereachedthepointwheretheuncertaintiesarebiologicalratherthanastronomical,andweawaitfurtherdevelopments.now,rememberwehavesofarconsideredonlyourowngalaxy–oneofbillions.thethoughtthatinallofthisvastnesswecouldbealoneisliterallyawe-inspiring.
ifintelligenceexistselsewhere,arethereanywaysofhavingameaningfulinteraction?Wecandismissmodern-typespacecraftwithoutfurtherado.evenifwecouldtravelatthespeedoflight,reachingeventheneareststarthatmightbeattendedbyplanetswouldtakeyears,andaccordingtoeinstein'stheoryofrelativity,travelatthefullspeed
projectozma
the84-foot(25.9metre)radiotelescopeatgreen
bank,WestVirginia,wasusedbyfrankdrakeand
histeamin1960tocarryoutthefirstsearchfor
extraterrestrialintelligence.
Jodrellbank
the250-foot(76m)radiotelescopewasbuiltat
Jodrellbankbybernardlovellin1957,andhas
subsequentlybeeninvolvedinsetiprojects.
cetianderidani
twonearbystarsseemedparticularlygood
candidatesascentresofplanetarysystems:tauceti
andepsiloneridani,bothofwhichareeasilyvisible
withthenakedeye.bothareofsolartype,though
lessluminousthanthesun.tauceti,atarangeof
11.9light-yearshas40percentoftheluminosity
ofthesun,whileepsiloneridanilies10.7light-
yearsawayandhas30percentofsolarluminosity.
thesewerethetargetstarsforthefirstexperiment
forseti,carriedoutbyfrankdrakeandhis
teamin1960.theinvestigators‘listenedout’at
awavelengthof21cm(1420megahertz),the
frequencyofemissionssentoutfromthecloudsof
coldhydrogenspreadthroughoutthegalaxy,but
theresultswerenegative.drakeformulated
thefamous‘drakeequation’tocalculatethe
probabilityoflifeexistingelsewhereintheUniverse.
Unfortunately,asdrakehimselfwasthefirstto
pointout,theformulacontainstoomanyunknowns
forfirmconclusionstobedrawn.theexperiment
wasknownofficiallyasprojectozma,afterthefake
wizardinfrankbaum’sclassicchildren’sbook,the
Wizardofoz.
taucetihasturnedouttobesomethingofa
disappointment.likethesun,itisayellowdwarf
star,itsmassis80percentofthesun’s,and
itssurfacetemperatureismuchthesame.its
ecosphereshouldextendbetween0.6and0.9
9.2billionyearsa.b.–present
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theemergenceoflife 121
astronomicalunits(1a.U.istheaveragedistance
oftheearthfromthesun),sothatifitreplaced
thesuninoursolarsystem,Venuswouldbe
comfortablyplaced.butwhentaucetiwasstudied
withtheJamesclerkMaxwelltelescopeinhawaii,
themostpowerfultelescopeofitstypeintheworld,
thingsbegantolooklesspromising.thestaris
associatedwithadiskofdebris,andsoanyplanets
orbitingtauceti,wouldbesubjecttoconstant
bombardmentfromasteroidsofthekindwebelieve
wipedoutthedinosaurs.Withsomanylarge
impacts,itseemsunlikelylifecouldhavesurvived.
epsiloneridani,ontheotherhand,isvery
differentinasmuchasitreallyisthecentreofa
planetarysystem.in1998adustdiskaroundit
wasdiscovered,ataboutthesamedistanceas
thatbetweenoursunandtheKuiperbelt;there
areclumpsinit,whichsuggestthepresenceofa
planet.in2000alargeplanetwastrackeddown
bythewobbletechnique;itsmassisrathergreater
thanthatofJupiter,andithasaveryeccentricorbit.
athirdplanethasbeensuspected,furtheroutand
muchmoremassive.alltheseareunsuitedtolife,
butthereseemstobenodustclosetothestar,
andithasbeensuggestedthatanymaterialthere
hasbeensweptupbyearth-sizedplanets.the
ecosphereoferidaniliesatarangeofjustover
halfthedistancebetweentheearthandthesun.
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Voyager2
launchedin1977,Voyager2flewpastUranus(1986)
andneptune(1989),returningthefirsteverclose-up
imagesoftheseouterplanets.itisthefirstman-made
objecttoleavethesolarsystemaltogetherandhead
forthestars.itcarriesgold-platedgramophonediscs
containingimagesandsoundsfromearth,incaseit
encountersalienlife.therecordsincludeaschematic
diagramshowingearth'spositioninthegalaxy,sothat
anyrecipientsoftheseinterstellargiftswillbeableto
thankusinperson.
theanthropicprinciple
inmoderncosmologytherehasbeenanattemptto
examinewhatareknownas‘anthropicarguments’.
thesearebasedontheso-calledanthropic
principle,whichstatesthattheUniversemustbe
thewayitis,becauseifitweretobeanydifferent,
wewouldnotbeheretoobserveit!togiveatrivial
example,iftheUniversewerethesizeofanatom,
creaturescomplexenoughtobeconsciouscould
notexist.sophisticatedversionsofthisargument
havebeendeveloped,andusedtosupportthe
hypothesisthatwereallyareunique.however,itis
difficulttoseehowsuchtheoriesmaybetested.
oflightwoulduseupaninfiniteamountofenergy–whichisanotherwayofsayingthatitsimplycannotbedone.obviously,usingourrocketswouldinvolveajourneylastingforcenturies,anddevicessuchasspace-arks,wheretheoriginaltravellersdieearlyinthevoyageandonlytheirdescendentssurvivetomake‘planetfall’,arefornowlikelytostayassciencefiction.interstellartravelwillrequireatechnologicalbreakthrough,whichmaycometomorrow,withinayear,withinacentury,withinamillionyears–ornever.Untilitdoes,fromamaterialpointofviewweareconfinedtothesolarsystem.
sofarascommunicationisconcerned,wehaveasyettriedonlyonemethod:radio.radiowavestravelatthesamespeedaslight,andthereforethetimeoftravelbetweenourselvesandthenearestpromisingstarsamountstoonlyafewyears.Moreover,radiocommunicationoveragoodmanylight-yearswouldbepossiblewithourpresentequipment.iftherewereastronomerslivingon,say,aplanetorbitingthestartauceti,11light-yearsaway,theycouldpickupsignalsofthestrengthwearecapableofsending.similarlywecouldpickupsignalsfromthem.
Wemightexpectanyartificialtransmissiontobebaseduponmathematics;afterallwedidnotinventmathematics–wemerelydiscoveredit.Variousingenioussystemshavebeentargetedandcodedmessageshavebeentransmitted,notonlytoε (epsilon)eridanibuttomanyotherstars.contactmustbeslow;ifwesendamessagetoepsiloneridaniin
9.2billionyearsa.b.–present
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theemergenceoflife 123
triton
triton,neptune’slargestsatellite,wasthelastobject
tobephotographedbyVoyager2beforeitleftthe
solarsystem.Voyager2hadtravelled5.6billionmiles
in12yearstoreachit.
2006,itwillarrivetherein2017,sonoreplycanbeexpectedbefore2028,makingquick-firereparteeratherdifficult.butitisameasureofourchangedattitudethatexperimentsofthiskindareconsideredworthattempting.ifnoreplyiseverreceived,itmayindicatethatwearetryingthewrongexperiment,thatthereisnotechnologicalcivilizationwithinreasonablerange,orthatmankindreallyisunique.
Wecannotsendspacecrafttootherstars,butamoreadvancedcivilizationmightwellbecapableofinterstellartravel.Wehaveyettobeconvincedbyanystoriesofflyingsaucers,alienabductionsandinvadersfromalphacentauri,butwemustrememberthatoursisanewandnodoubtprimitivetechnology.ithasbeensuggestedthatweshoulddoourbesttoremainundetected,andevenrecallthefewprobessuchasVoyager2thatarenowleavingthesolarsystempermanently,butthiswouldbeillogicalevenifitwerepracticable(whichitisnot).perhapsweshouldbecomfortedbythewordsofpercivallowell;‘acivilizationcapableofreachingearthwouldhaveleftwarfarbehindandwouldcomeinpeace’.inanycaseitistoolateforusto‘keepsilent’.Wehavebeenbroadcastingsinceabout1920,sotoanyonewithinarangeof80light-yearsweare‘radionoisy’.
Weknowthatlifeonearthhasalimitedfuture,andthateventuallytheincreaseintheluminosityofthesunwillrenderourworlduninhabitable.Wemustlookahead,soletusconsiderthefutureoftheUniverse.