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meteorite
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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.
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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.
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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.
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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.
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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
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Articles&Profiles
NationalGeographicNews:MeteoriteImpactReformulatedEarth’sCrust
Interactives
TheMeteoriticalSociety:SearchtheMeteoriticalBulletinDatabase
Websites
AmericanMeteorSociety
NASASolarSystemExploration:Meteors&Meteorites
MeteorCraterEnterprises:MeteorCrater
NationalGeographicProjects:PatagoniaMeteoriteImpactFieldProject
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