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Thiswebsitewouldliketoremindyou:Yourbrowser(AppleSafari4)isoutofdate.Updateyourbrowserformore security,comfortandthebestexperienceonthissite. × EncyclopedicEntry meteorite Forthecompleteencyclopedicentrywithmediaresources,visit: http://education.nationalgeographic.com/encyclopedia/meteorite/ MeteoritesarespacerocksthatfalltoEarth’ssurface. Meteoritesarethelaststageintheexistenceofthesetypeofspacerocks.Beforetheyweremeteorites,therockswere meteors.Beforetheyweremeteors,theyweremeteoroids.Meteoroidsarelumpsofrockormetalthatorbitthesun. MeteoroidsbecomemeteorswhentheycrashintoEarth’satmosphereandthegasessurroundingthembrieflylightupas “shootingstars.”Whilemostmeteorsburnupanddisintegrateintheatmosphere,manyofthesespacerocksreachEarth’s surfaceintheformofmeteorites. Dust-sizedparticlescalledmicrometeoritesmakeup99percentoftheapproximately50tonsofspacedebristhatfallsonthe Earth’ssurfaceeveryday.Somemeteorites,however,areaslargeasboulders. ThelargestmeteoritefoundonEarthistheHobameteoritediscoveredinNamibiain1920.TheHobameteoriteweighs roughly54,000kilograms(119,000pounds).TheHobameteoriteissobig,andsoheavy,ithasneverbeenmovedfrom whereitwasfound! MostmeteoriteslookverymuchlikerocksfoundonEarth,exceptmeteoritesusuallyhaveadark,burnedexterior.This exteriorisformedasfrictionfromtheatmospheremeltsthemeteoriteasitcrashestowardEarth.Knownasthermalablation, thisprocesscanalsogivemeteoritesaroughened,smooth,orthumbprintsurface.Thermalablationcreatesthesedifferent texturesduetodifferentchemicalspresentinthemeteorite. Meteoritescrashthroughtheatmospheresofallplanetsandmoonsinoursolarsystem.Someplanetsandmoonsdon'thave enoughatmospheretobreakapartmeteors,resultinginlargemeteorites.Theselargermeteoritescreatedeep,roundimpact cratersthatcanbefoundalloverourMoon,Mercury,andMars.In2005,thefirstmeteoritefoundonanotherplanetwas discoveredbyOpportunity,oneofNASA’sMarsroverspacecraft.In2014,Opportunity’ssisterspacecraft,Curiosity, discoveredameteoritethatwas2meters(7feet)wide,makingitthelargestyetdiscoveredonMars. TypesofMeteorites Morethan60,000meteoriteshavebeenfoundonEarth.Scientistshavedividedthesemeteoritesintothreemaintypes: stony,iron,andstony-iron.Eachofthesetypeshasmanysub-groups. StonyMeteorites Stonymeteoritesaremadeupofmineralsthatcontainsilicates—materialmadeofsiliconandoxygen.Theyalsocontain somemetal—nickelandiron.Therearetwomajortypesofstonymeteorites:chondritesandachondrites. Chondritesthemselvesareclassifiedintotwomajorgroups:ordinaryandcarbonaceous.Ordinarychondritesarethemost commontypeofstonymeteorite,accountingfor86percentofallmeteoritesthathavefallentoEarth.Theyarenamedforthe hardeneddropletsoflava,calledchondrules,embeddedinthem.Chondritesformedfromthedustandsmallparticlesthat cametogethertoformasteroidsintheearlysolarsystem,morethan4.5billionyearsago.Becausetheywereformedatthe sametimeasthesolarsystem,chondritesareintegraltothestudyofthesolarsystem’sorigin,age,andcomposition. 1of6 Ordinarychondritescanbeclassifiedintothreemaingroups.Thegroupsindicatethemeteorite’squantityofiron.TheH chondritegrouphasahighamountofiron.TheLchondritegrouphasalowamountofiron.TheLLgrouphasalowamount ofironandalowamountofmetalingeneral. Carbonaceouschondritesaremuchmorerarethanordinarychondrites.Astronomersthinkcarbonaceouschondritesformed farawayfromthesunastheearlysolarsystemdeveloped.Astheirnameimplies,carbonaceouschondritescontainthe elementcarbon,usuallyintheformoforganiccompoundssuchasaminoacids.Carbonaceouschondritesalsooftencontain waterormaterialthatwasshapedbythepresenceofwater. Likeordinarychondrites,carbonaceouschondritescanbemoreminutelyclassifiedbasedontheirmineralcomposition.All groupsofcarbonaceouschondritesaremarkedwithatwo-orthree-lettercodestartingwithC.Carbonaceouschondritesare oftennamedafterthefirstspecimenofthattyperecovered.TheCIgroup,forinstance,isnamedaftertheIvunameteorite, whichcrashedintoTanzaniain1938.CImeteoriteshaveahighamountofcarbon,aswellasclays.Carbonaceous chondritescanalsobenamedaftertheplacewherethefirstspecimenofthetypewasfound.TheCVgroupisnamedaftera meteoritethatcrashednearthecityofVigarano,Italy,in1910.ThemostfamousCVmeteoriteisprobablytheAllende meteorite,whichfelltoEarthnearPueblodeAllende,Chihuahua,Mexico,in1969.TheAllendemeteoritehasthousandsof tinychondrulesmadeofthemineralolivine.TheAllendemeteoritealsohasgrainsofaspecialkindofcarbon—diamonds. Thesediamondsareactuallyolderthanthesolarsystem,andastronomersthinktheywereproducedasblastmaterialfroma nearby,ancientsupernova. Achondritesdonotcontainthelavadroplets(chondrules)presentinchondrites.Theyareveryrare,makingupabout3 percentofallknownmeteorites.Achondritesformfromthebrittleouterlayerofanasteroid,whichissimilartoEarth’scrust. Therearemanyclassificationsofachondrites.The“primitiveachondrite”group,forinstance,hasaverysimilarmineral compositiontochondrites.LunarmeteoritesareachondritesthatcrashedtoEarthfromtheMoon,whileMartianachondrites crashedtoEarthfromourneighborplanet,Mars. Veryfewmeteorites,onlyabout0.2percent,comefromMarsandtheMoon.TheseachondritesaretheresultsofMarsand theMoon’sownmeteoriteimpacts.LargemeteoriteshitthesurfaceofMarsandtheMoon,blastingoffbitsofrock.These rockbitsrarelymaketheirwaytoouratmosphereasmeteorsandevenmorerarelyhittheEarth’ssurface. IronMeteorites Ironmeteoritesaremostlymadeofironandnickel.Theycomefromthecoresofasteroidsandaccountforabout5percentof meteoritesonEarth. Ironmeteoritesarethemostmassivemeteoriteseverdiscovered.Theirheavymineralcomposition(ironandnickel)often allowsthemtosurvivetheharshplummetthroughEarth’satmospherewithoutbreakingintosmallerpieces.Thelargest meteoriteeverfound,Namibia’sHobameteorite,isanironmeteorite. Stony-IronMeteorites Stony-ironmeteoriteshavenearlyequalamountsofsilicateminerals(chemicalsthatcontaintheelementssiliconand oxygen)andmetals(ironandnickel). Onegroupofstony-ironmeteorites,thepallasites,containsyellow-greenolivinecrystalsencasedinshinymetal. Astronomersthinkmanypallasitesarerelicsofanasteroid’score-mantleboundary.Theirchemicalcompositionissimilarto manyironmeteorites,leadingastronomerstothinkmaybetheycamefromdifferentpartsofthesameasteroidthatbrokeup whenitcrashedintoEarth’satmosphere. MeteoriteImpactCraters MeteoritescrashthroughtheEarth’satmospherewithtremendousforce.Thelargestmeteoritesleaveenormousholesinthe groundcalledimpactcraters. Thebest-preservedimpactcraterintheworldistheBarringerMeteoriteCrater,nearWinslow,Arizona.There,morethan 50,000yearsago,ameteoriteweighingabout270,000metrictons(300,000tons)slammedintotheEarthwiththeforceof2.5 milliontonsofTNT.Theimpactblastedahole1kilometer(.6miles)wideandabout230meters(750feet)deep.The fragmentsleftofthespacerockshowthatitwasanironmeteorite. 2of6 MorethanahundredimpactcratershavebeenidentifiedonEarth.PerhapsthemostfamousistheChicxulubCrater,in Yucatan,Mexico.TheChicxulubCratercanbeidentifiedonland,beneathdozensofmetersofsediment,althoughabouthalf ofthefeatureissubmergedintheGulfofMexico.ItisoneofthelargesimpactcraterseverdiscoveredonEarth. Despiteitssize,theChicxulubCraberisfamousforanotherreason.Manyscientiststhinkthelargemeteoritethatcreated theChicxulubCrater—measuringroughly10kilometers(6miles)wide—triggeredtheextinctionofthedinosaursandother animalandplantlife65millionyearsago. VOCABULARY Term PartofSpeech Definition ablation noun removalofmaterialfromthesurfaceofanobject,includingmelting,evaporation,or erosion. account noun oralorwrittendescriptionofevents. achondrite noun typeofstonymeteoritecontainingnohardeneddroplets(chondrules). aminoacid noun nutrientcontainingcarbon,hydrogen,oxygen,andnitrogenthatiscriticalforalllife. ancient adjective veryold. approximately adjective generallyornearanexactfigure. asteroid noun irregularlyshapedplanetarybody,rangingfrom6meters(20feet)to933kilometers (580miles)indiameter,orbitingthesunbetweenMarsandJupiter. astronomer noun personwhostudiesspaceandtheuniversebeyondEarth'satmosphere. atmosphere noun layersofgasessurroundingaplanetorothercelestialbody. boulder noun largerock. boundary noun lineseparatinggeographicalareas. brittle adjective fragileoreasilybroken. carbonaceous chondrite noun typeofstonymeteorite(chondrite)thatcontainstheelementcarbon,usuallyinthe formoforganiccompoundssuchasaminoacids. chondrite noun typeofstonymeteoritecontaininghardeneddroplets,calledchondrules,ofsilicate minerals. chondrule noun smalldropletofsilicatemineralfoundinstonymeteorites. clay noun typeofsedimentaryrockthatisabletobeshapedwhenwet. composition noun arrangementofthepartsofaworkofartinrelationtoeachotherandtothewhole. core noun theextremelyhotcenterofEarth,anotherplanet,orastar. crust noun rockyoutermostlayerofEarthorotherplanet. crystal noun typeofmineralthatisclearand,whenviewedunderamicroscope,hasarepeating patternofatomsandmolecules. debris noun remainsofsomethingbrokenordestroyed;waste,orgarbage. diamond noun typeofcrystalthatispurecarbonandthehardestknownnaturalsubstance. dinosaur noun verylarge,extinctreptilechieflyfromtheMesozoicEra,251millionto65million yearsago. 3of6 discover verb tolearnorunderstandsomethingforthefirsttime. disintegrate verb tofallapartanddisappear. dust noun microscopicparticlesofrocksormineralsdriftinginspace.Alsocalledcosmicdust orspacedust. element noun chemicalthatcannotbeseparatedintosimplersubstances. embed verb toattachfirmlytoasurroundingsubstance. enormous adjective verylarge. exterior adjective,noun ontheoutsideoroutdoors. extinction noun processofcompletedisappearanceofaspeciesfromEarth. fragment noun pieceorpart. friction noun forceproducedbyrubbingonethingagainstanother. gas noun stateofmatterwithnofixedshapethatwillfillanycontaineruniformly.Gas moleculesareinconstant,randommotion. harsh adjective extreme. impact noun collisionorcrash. impactcrater noun circularsurfacedepressionmadebytheimpactofameteorite. indicate verb todisplayorshow. integral adjective veryimportant. ironmeteorite noun rock,madeofironandnickel,thathascrashedtoEarthfromoutsidethe atmosphere. lava noun moltenrock,ormagma,thateruptsfromvolcanoesorfissuresintheEarth'ssurface. lunar adjective havingtodowithEarth'smoonorthemoonsofotherplanets. mantle noun middlelayeroftheEarth,madeofmostlysolidrock. Mars noun fourthplanetfromthesun,betweenEarthandJupiter. massive adjective verylargeorheavy. metal noun categoryofelementsthatareusuallysolidandshinyatroomtemperature. meteor noun rockydebrisfromspacethatentersEarth'satmosphere.Alsocalledashootingstar orfallingstar. meteorite noun typeofrockthathascrashedintoEarthfromoutsidetheatmosphere. meteoroid noun small,rockybodytravelingaroundthesun. micrometeorite noun dust-sizeparticleofspacedebristhatburnsupasitentersEarth'satmosphere. mineral noun inorganicmaterialthathasacharacteristicchemicalcompositionandspecific crystalstructure. minute adjective verysmallamount. Moon noun Earth'sonlynaturalsatellite. moon noun naturalsatelliteofaplanet. 4of6 nickel noun chemicalelementwiththesymbolNi. olivine noun typeofsilicatemineral. orbit verb tomoveinacircularpatternaroundamoremassiveobject. organic compound noun chemicalsubstancethatcontainstheelementcarbon. pallasite noun typeofstony-ironmeteoritecontainingolivineorperidotcrystalsembeddedinan iron-nickelwebbing. particle noun smallpieceofmaterial. planet noun large,sphericalcelestialbodythatregularlyrotatesaroundastar. plummet verb tofallsharply. primitive adjective simpleorcrude. quantity noun amount. relic noun mementoorsurvivingobjectofthepast. rock noun naturalsubstancecomposedofsolidmineralmatter. rover noun vehiclethatremotelyexploresaregion,suchasthesurfaceofamoon,planet,or othercelestialbody. sediment noun solidmaterialtransportedanddepositedbywater,ice,andwind. silicate noun mostcommongroupofminerals,allofwhichincludetheelementsilicon(Si). solarsystem noun thesunandtheplanets,asteroids,comets,andotherbodiesthatorbitaroundit. spacecraft noun vehicledesignedfortraveloutsideEarth'satmosphere. specimen noun individualorganismthatisatypicalexampleofitsclassification. stony-iron meteorite noun rock,madeofnearlyequalpartsmetalandsilicateminerals,thathascrashedto Earthfromoutsidetheatmosphere. stony meteorite noun rock,madeofsilicateminerals,thathascrashedtoEarthfromoutsidethe atmosphere. submerge verb toputunderwater. sun noun staratthecenterofoursolarsystem. supernova noun sudden,violentexplosionofamassivestar. texture adjective,noun physicalortactilecharacteristicsofasubstance. thermal adjective havingtodowithheatortemperature. TNT noun (trinitrotoluene)chemicalcompoundoftenusedasapredictable,reliableexplosive materialwithrelativelysafehandlingproperties. tremendous adjective verylargeorimportant. trigger verb tocauseorbeginachainofevents. ForFurtherExploration 5of6 Articles&Profiles NationalGeographicNews:MeteoriteImpactReformulatedEarth’sCrust Interactives TheMeteoriticalSociety:SearchtheMeteoriticalBulletinDatabase Websites AmericanMeteorSociety NASASolarSystemExploration:Meteors&Meteorites MeteorCraterEnterprises:MeteorCrater NationalGeographicProjects:PatagoniaMeteoriteImpactFieldProject ©1996–2015NationalGeographicSociety.Allrightsreserved. 6of6