9.2 billion years a.b. (after the bang) to the present day...
TRANSCRIPT
9.2billionyearsa.b.(afterthebang)tothepresentday(13.7billionyearsa.b.)
chapter5theemergenceoflife
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
110
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
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.
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
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.
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.
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.
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
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.
118
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
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.
120
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
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.
122
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
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.