unit6--Life-Beyond-Earth--summary

Somethingastonishinghashappenedintheuniverse.Therehasarisenathingcalledlife.Lifehasawayofbeingobviousandyetit'snotoriouslydifficulttodefineinabsoluteterms.Wesaythatlifereplicates.Theremaybenoscientificmysterysotantalizingandyetsoresistanttoananswer.Extraterrestrialliferepresentsanenormousgapinourknowledgeofnature.scientistshavediscoveredabewilderingamountofcosmicturf,andyettheystillknowofonlyasingleinhabitedworld.Weallhaveoursuppositions,ourscenarios.someastronomerestimatedthattherearemanytechnologicalcivilizationsinourgalaxyalone.Andothersastronomerwhothinkswemayaswellbealoneinthisgalaxyifnotintheuniverse.Alltheestimatesarehighlyspeculative.ThefactisthatthereisnoconclusiveevidenceofanylifebeyondEarth.Absenceofevidenceisnotevidenceofabsence.Evenifweconvinceourselvesthattheremustbelifeoutthere,weconfrontasecondproblem,whichisthatwedon'tknowanythingaboutthatlife.Despitetheenvelopingnebulaofuncertainties,extraterrestriallifehasbecomeanincreasinglyexcitingareaofscientificinquiry.Thefieldiscalledexobiologyorastrobiologyorbioastronomy—everyfewyearsitseemsasthoughthenamehasbeenchangedtoprotecttheignorant.Whateverit'scalled,thisisascienceinfusedwithoptimism.Wenowknowthattheuniversemaybeaswarmwithplanets.Inthepastdecadeorganismshavebeenfoundthrivingonourownplanetinbizarre,hostileenvironments.IfmicrobescanliveintheporesofrockdeepbeneaththeearthorattherimofascaldingYellowstonespring,thentheymightfindaplacelikeMarsnotsoshabby.Marsisinthemidstofafull-scaleinvasionfromEarth.Inthecomingyearsprobeswillalsogoaroundand,atsomepoint,intoJupiter'smoonEuropa.Thaticyworldshowsnumeroussignsofhavingasubsurfaceocean—andcouldconceivablyharboradark,coldbiosphere.Thequestforanalienmicrobeissupplementedbyacontinuingefforttofindsomethinglarge,intelligent,andcommunicative.SETIhasnotyieldedaconfirmedsignalfromanaliencivilizationin40yearsofexperiments,butthesignal-processingtechnologygrowsmoresophisticatedeachyear.Theoptimistsfigureit'sonlyamatteroftimebeforewetuneintherightchannel.ButIthinkExtraterrestriallife,bydefinition,isnotconvenientlylocated.Butthereareothertruthsthatsustainthesearchforalienorganisms.Oneisthattheuniverselookshabitable.Anotheristhatliferadiatesinformationaboutitself.lifeisnotlikelytoremainforeverintherealmoftheunknown.Contactwithanaliencivilizationwouldbeanepochalandculturallychallengingevent,butexobiologistswouldsettlegladlyforthediscoveryofatinyfossil,amereremnantofextraterrestrialbiochemistry.Wealsoknowthatafunctioningecosystemdoesnotrequiresunlightorphotosynthesis.Evenmorecomplexlifecanadapttohostileplaces.Asscientistsstruggletofindatraceoflifesomewhereelseintheuniverse,thereexistsformanypeopleamoredramaticsituation,oneinwhichextraterrestriallifeisn'tmicrobialandslimybutratherintelligent,technological,andlurkinginourmidst.Ithinkit'sfairtosay,however,thatflying-sauceralienslackscientificstature.Iftheyinsistonbeingsojumpy,iftheyinsistonabductingpeopleinthemiddleofthenightwhennooneelsecanverifytheirpresence,theyhavenorighttoenterareputablenaturalhistorymuseum.Asmanywritershavenoted,Thealiensareanextrapolationofmodernastronomyandengineering,buttheyalsopossesssomeancienturgetocometoEarthandmesswithhumanbeings.Whatmakesthemsointriguingisthatevenscientistswillconcedethatalienbeingscouldverywellbeouttheresomewhere.Manyscientistsdon'twonderwhyaliensarebuzzingtheEarthinflyingsaucers—theywonderwhytheyaren't.Humanscouldtheoreticallycolonizethegalaxyinamillionyearsorso,andiftheycould,astronautsfromoldercivilizationscoulddothesame.OurcontemporaryculturedidnotinventthisideaoflifebeyondEarth.Morethan2,000yearsagotheGreekphilosopherMetrodorusofChioswrote,Itisunnaturalinalargefieldtohaveonlyoneshaftofwheat,andintheinfiniteUniverseonlyonelivingworld.Inthefinalyearsofthe9thcentury,LowellrevealedthediscoveryofhundredsofcanalsandarguedthattheseweretheartificialcreationsofanintelligentMartiancivilization.Infact,hewrote,theMartianswouldcertainlyhavetobesuperiortous.Hereasonedthattheirglobe-spanningengineeringprojectswerefarbeyondourowncapabilitiesandthattheabilityofaraceofcreaturestoliveinharmonyoverthewholeofaplanet.WhentheVikinglandersdescendedtotheMartiansurfacein1976,theyfoundnocompellingsignoflifeandindeeddiscoveredthatthesurfacecontainsnotraceoforganicmolecules.Themoodchangedinthe1990s.BiologistsweredetectingorganismsinsuchexoticenvironmentsonEarththattheywereinspiredtolookanewattherestofthesolarsystemaspotentiallyhabitable.TheyalsodiscoveredsignsthatlifeappearedearlyintheEarth'shistory.Intriguingly,ataboutthetimelifearoseonEarth,Marswasamuchmorehospitableplanetthanitistoday.ImagesoftheMartiansurfaceindicatethattheplanetoncehadflowingriversandperhapsanocean.LifecouldevenhavestartedonMarsandspreadtoEarthaboardameteorite.In1996ateamofthreeNASAscientistsbasedinHoustonannouncedthatthispotato-sizerockALH84001,foundinAntarctica,containedwhatappearedtobeMartianfossils.ThediscoverywasproclaimedatanunforgettableNASApressconferenceinWashington,D.C.,onAugust7,1996.Everyonerealizedthehistoricalgloryofbeingrightaboutthesepurportedmicrofossils—a