Introduction
AlsoknownasMountAllen84001(ALH84001),itwastheworkbaseofgeologistRobertaScoreandagroupofhercolleaguesinAntarcticain1984.AblackrockfoundwhilewanderingaroundMountLunwaslaternamed"MountAllen84001"(ALH84001).ThestudyfoundthattherockwasfromMars.Itissaidthatthisisthefirstbio-fossilsamplefromotherworlds.InAugust1996,itfloodedthefrontpagesofnewspapersallovertheworld,makingtheworldasensation.
Originandorigin
Itwasasummerdayin1984,theweatherwasfineandthetemperaturewasbelow0℃.GeologistRobertaScoreencounteredsomething.Maybeitwillsoonprovetobethefirstcontactbetweenhumansandalienlife.AfterSkollfinishedaday'swork,heleftthestudiowithagroupofhercolleagues.WhenshedroveasnowmobilearoundtheirAntarcticworkbaseonMountAllen,shefoundablackrockinthesnow.Sheparkedthecarasideandsteppedforwardtoseewhathappened.Unabletoidentifywhatkindofrockitwas,Dr.SkollwrappeditupasusualandsentittotheMeteoriteProcessingLaboratoryattheJohnsonSpaceCenterinHouston,Texas.Itwaskeptinanitrogen-filledlaboratoryfor8years.ItwaslaterconfirmedtobefromMars.
MountAllen84001isameteoriteweighingabout1.75kgandthesizeofapotato.Apartfromcontainingfossils,thisrockisunremarkable.Itisgenerallybelievedthatit,likeotherrocksonMars,formedbetween4billionand4.5billionyearsago.Duringthattime,planetswithsolidcorescondensedandgraduallyevolvedintoadenserstateliketoday—celestialbodiesmainlycomposedofironandotherminerals(suchasquartzandothersiliconcompounds).Then,about36daysago,Between100millionand4billionyears,waterseepedintothegapsofrocksinliquidform.ItisbelievedthattherewasprobablyalargeamountofwateronMarsatthattime.Thiswaterwasfullofcarbondioxideintheatmosphere.Becauseofthis,thewatercouldremainintherocksintheformofcarbonates.Itisbelievedthatmicroorganisms(perhapslivinginthosewaters)contributedtotheproductionofthesecarbonates.Afterthemicroorganismsdie,theybecomefossilsintherocks,justlikethelimestoneontheearthpreservesalargenumberofbiologicalfossilsfromdifferentgeologicalperiods.Thisrockmayhavestayedsomewhereforbillionsofyears.Perhapsitispartofacliff,orpartofahugerockexposedontheground.Onedayabout16millionyearsago,ahugecometorasteroidcollidedwithMars.Afteraviolentimpact,ourrockseparatedfromthesurfaceofMarsandenteredtheouterspace.Inthefollowing16millionyears,MountAllen84001almostfloatedininterstellarspace.Duringthistime,aseeminglyinsignificantmammalonEarthgraduallyevolvedprimates.Afterafewmillionyears,itsplitintoseveralbranches,oneofwhichisHomosapiens.Afterthat,alongtimepassed.Whentheearlyhumansocietydevelopedagriculture,about7000to8000yearsbeforetheconstructionofStonehengeandPyramids,MountAllen84001enteredthesphereofinfluenceoftheearth’sgravityandfellinAntarcticaabout13000yearsago.Onthewasteland.Itstayedtherequietly,untilthatafternoonin1984,Dr.Skolearrangedforfatetodiscoverit.
Researchresults
ThefossilsoforganismsfoundinMountAllen84001,whicharebelievedtobeverytiny,almostinvisible,similartoprimitivebacteria.Thesefossilsareverysmall,thelargestonesareonly1/100thewidthofahumanhair,andmostofthemareonly1/10thesizeofthelargestones.Inotherwords,if1000suchorganismsarearrangedinastring,itwillbeaslongastheperiodafterthissentence.
Somefossilsareoval,somearetubular,butsurprisingly,theyareverysimilartosomefossilsofbacteriaandothermicroorganismsonearth.Infact,theyaresosimilarinappearancetothebacteriaontheearththattheywereconsideredbacteriawhentheyfirstappeared.Onthedaytheywerediscovered,whenastaffmemberoftheNationalSpaceAgency’staskforcefromtheJohnsonSpaceCenter,EverettGibson,tookhomethefirstphotosofthemicroorganismsandplacedthemonthekitchentable,hesaidThewifeofabiologistimmediatelyasked:"Whatkindofbacteriaarethese?"
So,whatevidencecanbeusedtosupportthesealmostinvisible"smallspots"isthefirsttimewehumanshaveencountereditWhataboutthethesisofextraterrestriallife?
ThefirstpieceoffavorableevidenceisthatthisrockisindeedfromMars.Scientistsarequitesureofthis.BecausethegascompositioncapturedinthemeteoritestructureisconsistentwiththecompositionoftheMartianatmospheremeasuredbytheVikingprobe20yearsago.Themostimportantcomponentinthismixedgasiscarbondioxide(CO2).ABritishresearcherstudiedmeteoritesamplesshortlyafterthenewswasreleased.Hesaid,"The'obvioussimilarities'makepeoplebelievethatitreallycamefromMars.WerecognizedthecharacteristicsoftheMartianatmosphere,anditsmaincomponentsareCarbondioxide.Ifthereisnohighconcentrationofcarbondioxideinameteorite,itisnotaMartianmeteorite."
Nowthatweknowwherethemeteoritecomesfrom,thenwhyshouldwesaythatthesebiologicalfossilsandgeophysicalstructuresarealsoDoesitoriginateoutsidetheland?Animportantpieceofevidenceisthattherocksurfaceispreservedincarbonatesedimentsandresemblesfossilsofsingle-celledorganisms.Carbonatesthemselvesarecarbon-basedcompounds.Theyareoftenorganicstructures,buttheydonotnecessarilyhavebiologicalorigins.Thosecell-likestructuresaresosmallthateventhemostadvancedmicroscopestodaycan’tseetheircellwallsclearly.Theyarecalled"nano(meter-level)bacteria"(namedfromtheprefix"nano",whichmeansthatthefollowingobjectmeasuresonebillionthofameter,thatis,10-9meters).
Thethirdpieceofevidenceisinorganiccompoundsinthesediments—iron-containingcrystals.Mainlyironoxideandironsulfide.Bacteriaontheearthcanmakesuchcompounds.However,thesedepositsfoundonMountAllen84001reallypuzzledscientists.Onearth,somerarebacteriamakeironoxideintheformofmagnetite(asitsnamesuggests,thissubstanceismagnetic).Itisspeculatedthattheyrelyonthismagnetismtoadjusttheirorientationrelativetotheearth'snaturalmagneticfieldandusethistohelpguidethedirectionofaction.However,itisabitunusualthatmicrobesonMarsalsoproducethismagneticsubstance.SincethemagneticfieldofMarsdoesnotexceed0.2%(ie1/50)oftheearth'smagneticfield,whydothesemicroorganismsontherocksproduce"magnetite"?
Thereisanotherlastpieceofevidence,perhapsthemostpowerfulevidencethatthefossilsinthismeteoriteoriginatedfromoutsidetheearth.ScientistsdiscoveredlargeorganicmoleculesinthismeteoritethathaveneverbeenseeninotherMartianmeteorites.Thenameofthesemoleculesis"polycyclicaromatichydrocarbons"(orPAHforshort)-aclassofoilycompoundsthatcanbefoundincommonsubstancessuchascoalandnaphthalene(themaincomponentofmothballs).PAHcanbetransformedfromthecorpsesofbacteriaontheearth,whichmakesiteasytothinkthatthesemoleculesinMountAllen84001mayhavebeencontaminatedbygroundpollutioninAntarctica.However,theresultsofthestudyindicatethattheconcentrationofPAHinsidethemeteoriteishigherthanonthesurface.ProfessorRichardZare,theheadofthe84001chemicalanalysisteamatMountAllen,commentedonthisdiscovery:"Inanycase,thisisthefirsttimethathumanshavediscoveredorganicmoleculesrelatedtoMars."