Asymmetric Distribution of Lunar Impact Basins Caused by Variations in Target Properties

AsymmetricDistributionofLunar

ImpactBasinsCausedbyVariations

inTargetProperties

KatarinaMiljković,1*MarkA.Wieczorek,1GarethS.Collins,2MatthieuLaneuville,1

GregoryA.Neumann,3H.JayMelosh,4SeanC.Solomon,5,6RogerJ.Phillips,7

DavidE.Smith,8MariaT.Zuber8

MapsofcrustalthicknessderivedfromNASA’sGravityRecoveryandInteriorLaboratory(GRAIL)

missionrevealedmorelargeimpactbasinsonthenearsidehemisphereoftheMoonthanonitsfarside.

Theenrichmentinheat-producingelementsandprolongedvolcanicactivityonthelunarnearside

hemisphereindicatethatthetemperatureofthenearsidecrustanduppermantlewashotterthanthat

ofthefarsideatthetimeofbasinformation.UsingtheiSALE-2Dhydrocodetomodelimpactbasin

formation,wefoundthatimpactsonthehotternearsidewouldhaveformedbasinswithuptotwicethe

diameterofsimilarimpactsonthecoolerfarsidehemisphere.Thesizedistributionoflunarimpact

basinsisthusnotrepresentativeoftheearliestinnersolarsystemimpactbombardment.

Progressinunderstandingimpactbasinson

theMoonhasbeenhamperedbythesim-

plefactthatthereisalackofconsensuson

thesizeofthelargestbasins(1–3).Fromanim-

pactphysicsperspective,themostrelevantmetric

forthesizeofabasinisthediameterofitstran-

sientcavity,butasitsnameimplies,thisstructure

isshort-livedanditsdiameterisnoteasilyes-

timatedfromsurfacemeasurements(4).Mostim-

pactbasinsonthenearsidehemisphereofthe

Moonhavebeenfilledbylavaflows,hidingim-

portantmorphologicalcluesthatcouldbeused

fordeterminingthesizeofthetransientcavity.

Otherimpactbasinshavemultiplerings,anditis

unclearwhichofthese,ifany,mostcloselyap-

proximatesthetransientcavity.Becausetheim-

pactprocessexcavateslargequantitiesofcrustal

materialandupliftsmantlematerialbeneaththe

basincenter,analternativemetricforthesizeofa

basinisthediameteroftheregionofcrustalthin-

ning(5–7).High-resolutiongravitydataobtained

fromNASA’sGravityRecoveryandInteriorLab-

oratory(GRAIL)mission(8)haveprovidedglob-

almapsofcrustalthicknessontheMoon(9)that

allowforanunambiguousdeterminationofthe

regionofcrustalthinningforallimpactbasins

withdiametersgreaterthan200km.

GRAILgravitydatashowthatlateralvaria-

tionsintheMoon’scrustalthicknessaredomi-natedbyimpactbasinsrangingindiameterfrom

~200to2000km(9).Approximatelyhalfofthose

basinsformedintheImbrianandNectarianpe-

riods,from~3.7toperhaps4.2billionyearsago

(Ga)(10,11)(tableS1).Thesoleexceptionisthe

SouthPole–Aitkenbasin,whichistheoldestand

largestimpactstructureontheMoon,andwhich

wedonotconsiderfurtheronthegroundsthatit

likelyformedduringamuchearlierepochthan

theotherbasinsforwhichvariationsincrustal

thicknesshavebeenpreserved.Wequantifythe

sizeoflunarimpactbasinsbythediameterDof

theregionofcrustalthinning(1).Thereare12ba-

sinsoneachhemispherewithdiametersgreater

than200kmandcrustthinnedtoafewkilometers,

asresolvedbyGRAIL(Fig.1).Althoughtheto-talnumberofbasinsisequalonthetwohemi-

spheres,theirsizedistributionishighlyasymmetric

(Fig.2).Whereasthereareeightbasinsonthe

nearsidehemispherewithdiametersgreaterthan

320km,onlyoneofthissizeisfoundonthe

farside,andthisbasin(Orientale,94°W,20°S)

straddlesthewesternlimboftheMoon.Simu-

lationsoftheMoon’simpactbombardmentby

near-Earthasteroidsshowthatthedifferencein

crateringratebetweenthenearsideandfarside

hemispheresshouldbelessthan1%(12)fora

largerangeofimpactconditions.Withauniform

crateringrate,thereislessthan2%probability

thateightbasinswithdiametersgreaterthan

320kmwouldformonthenearsideandonlyone

suchbasinonthefarside(fig.S1).

TheMoonshowsmajorgeologicaldifferences

betweenthenearsideandfarsidehemispheres.

Thenearsideisdominatedbythecompositionally

uniqueProcellarumKREEPTerrane(PKT),which

ishighlyenrichedinheat-producingandother

incompatibleelements[potassium,rare-earthele-

ments,andphosphorus(KREEP)]thatlikely

formedduringthelatestagesofmagma-ocean

crystallization(13,14)(Fig.1).Morethan99%

byareaoftheMoon’sexposedbasalticlavas

eruptedonthelunarnearside;thisconcentration

hasbeenattributedtohigherthanaveragenear-

sidemantletemperatures,atleastinparttheresult

ofthehighconcentrationofheat-producingele-

mentsinthenearsidecrustanduppermantle

(15).Theevidenceforviscousrelaxationoftopo-

graphicreliefofnearsidebasins(16,17)andthe

presenceofmarebasaltsextendingbeyondthe

confinesofthesurfaceareaofthoriumenrich-

ment(whichdefinesthePKT)suggestthathigher

thanaveragesubsurfacetemperaturessurrounded

1InstitutdePhysiqueduGlobedeParis,SorbonneParisCité,UniversitéParisDiderot,Case7011,LamarckA,5,35rueHélèneBrion,75205Pariscedex13,France.2DepartmentofEarthSciencesandEngineering,ImperialCollegeLondon,SouthKensingtonCampus,LondonSW72AZ,UK.3SolarSys-temExplorationDivision,NASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA.4DepartmentofEarth,Atmospheric,andPlanetarySciences,PurdueUniversity,WestLafayette,IN47907,USA.5DepartmentofTerrestrialMagnetism,CarnegieInstitutionofWashington,Washington,DC20015,USA.6Lamont-DohertyEarthObservatory,ColumbiaUniversity,Palisades,NY10964,USA.7PlanetaryScienceDirectorate,SouthwestRe-searchInstitute,Boulder,CO80302,USA.8DepartmentofEarth,AtmosphericandPlanetarySciences,MassachusettsInstituteofTechnology,Cambridge,MA02139,USA.