Detection of Intact Lava Tubes at Marius Hills on the Moon by SELENE (Kaguya) Lunar Radar Sounder


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Detection of Intact Lava Tubes at Marius Hills on the Moon by SELENE (Kaguya) Lunar Radar Sounder
Series Title:
Geophysical research letters
Creator:
Kaku, T.
Haruyama, J.
Miyake, W.
Kumamoto, A.
Ishiyama, K.
Nishibori, T.
Yamamoto, K.
Crites, Sarah T.
Michikami, T.
Yokota, Y.
Sood, R.
Melosh, H.J.
Chappaz, L.
Howell, K.C.
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American Geophysical Union
Wiley
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Lava tubes ( lcsh )
Lunar geology ( lcsh )
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Intact lunar lava tubes offer a pristine environment to conduct scientific examination of the Moon's composition and potentially serve as secure shelters for humans and instruments. We investigated the SELENE Lunar Radar Sounder (LRS) data at locations close to the Marius Hills Hole (MHH), a skylight potentially leading to an intact lava tube, and found a distinctive echo pattern exhibiting a precipitous decrease in echo power, subsequently followed by a large second echo peak that may be evidence for the existence of a lava tube. The search area was further expanded to 13.00–15.00°N, 301.85–304.01°E around the MHH, and similar LRS echo patterns were observed at several locations. Most of the locations are in regions of underground mass deficit suggested by GRAIL gravity data analysis. Some of the observed echo patterns are along rille A, where the MHH was discovered, or on the southwest underground extension of the rille.
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Volume 44, Issue 20
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7 p.

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DetectionofIntactLavaTubesatMariusHillsontheMoon bySELENE(Kaguya)LunarRadarSounderT.Kaku1,2 ,J.Haruyama1 ,W.Miyake2 ,A.Kumamoto3 ,K.Ishiyama1 ,T.Nishibori1, K.Yamamoto4,SarahT.Crites1,T.Michikami5,Y.Yokota1,6 ,R.Sood7 ,H.J.Melosh8,9 , L.Chappaz10 ,andK.C.Howell81InstituteofSpaceandAstronauticalScience,JapanAerospaceExplorationAgency,Sagamihara,Japan,2Departmentof MechanicalEngineering,GraduateSchoolofEngineering,TokaiUniversity,Hiratsuka,Japan,3DepartmentofGeophysics, GraduateSchoolofScience,TohokuUniversity,Sendai,Japan,4NationalAstronomicalObservatoryofJapan,Sendai,Japan,5FacultyofEngineering,KindaiUniversity,Osaka,Japan,6FacultyofScience,KochiUniversity,Kochi,Japan,7Departmentof AerospaceEngineeringandMechanics,CollegeofEngineering,TheUniversityofAlabama,Tuscaloosa,AL,USA,8Schoolof AeronauticsandAstronautics,PurdueUniversity,WestLafayette,IN,USA,9DepartmentofEarth,Atmospheric,and PlanetaryScience,PurdueUniversity,WestLafayette,IN,USA,10AstroLabs,Pasadena,CA,USAAbstractIntactlunarlavatubesofferapristineenvironmenttoconductscienti cexaminationofthe Moon ’ scompositionandpotentiallyserveassecuresheltersforhumansandinstruments.Weinvestigated theSELENELunarRadarSounder(LRS)dataatlocationsclosetotheMariusHillsHole(MHH),askylight potentiallyleadingtoanintactlavatube,andfoundadistinctiveechopatternexhibitingaprecipitous decreaseinechopower,subsequentlyfollowedbyalargesecondechopeakthatmaybeevidenceforthe existenceofalavatube.Thesearchareawasfurtherexpandedto13.00 – 15.00°N,301.85 – 304.01°Earound theMHH,andsimilarLRSechopatternswereobservedatseverallocations.Mostofthelocationsarein regionsofundergroundmassde citsuggestedbyGRAILgravitydataanalysis.Someoftheobservedecho patternsarealongrille A ,wheretheMHHwasdiscovered,oronthesouthwestundergroundextensionof therille.1.IntroductionLunarlavatubesareimportantfromvariousscienceperspectivesandprovidepotentialsitesforfuturelunar baseconstruction(Coombs&Hawke,1992;Haruyamaetal.,2012;Hörz,1985;Oberbecketal.,1969).Since theinsidesoflavatubesareshieldedfrommeteoritebombardment,cosmicradiation,orparticleimplantation,theyareexpectedtobeinpristinecondition,anenvironmentwithpreservedlavacomposition,textures, andevenvolatiles.Carefulexaminationoftheinteriorcanaddinsightconcerningtheevolutionaryhistoryof theMoon.Theradiationandmeteoritebombardmentthatdisturbsthegeologicrecordatthesurfaceofthe Moonalsomakesitaharshplaceforhumansandinstruments;thus,theinsideofanintactlavatubewouldbe thesafestplaceontheMoonfromanexplorationperspective.CoombsandHawke(1992)investigatedlunar surfacesegmentsassociatedwithrillestoinfertheexistenceofintactlavatubesunderthesurface.However, theLunarOrbiterandApollophotographstheyusedwerenotconclusivebecausetheimagecoveragebythe photographswaslimitedandmostwerenadirobservations.Noplausibleevidencefortheexistenceofintact lavatubesontheMoonwasreportedinthe20thcentury. In2009,alarge,deepholewasdiscoveredinthelunarMariusHillsinimagedataacquiredbythe SelenologicalandEngineeringExplorerforSELENETerrainCamera(TC)with10m/pixelresolutionfroman orbit100kmabovethelunarsurface(Haruyamaetal.,2009).Bothdiameteranddepthare50m (Haruyamaetal.,2012,2016;Robinsonetal.,2012).TheMariusHillsHole(MHH)islocatedinrille A ,which wasonceproposedasaprimaryinvestigationstationforApollo(Elston&Willingham,1969;Greeley,1971; Karlstrometal.,1968).Haruyamaetal.(2009)hypothesizedthattheholewasaskylightthatappearedto beanopeningintoalavatube. Later,higherresolutionnadirandobliqueangleobservationsperformedbytheLunarReconnaissance OrbiterNarrowAngleCamerawith50cm/pixelresolutionfromanorbit50kmabovethelunarsurfaceconrmedthattheMHHisaskylightopeningintoalargespace(Robinsonetal.,2012;Wagner&Robinson,2014); the ooroftheholeextendedatleastseveralmeterseastwardandwestwardunderaceilingoftwoother KAKUETAL.INTACTLUNARLAVATUBEDETECTIONBYLRS10,155 PUBLICATION S GeophysicalResearchLettersRESEARCHLETTER10.1002/2017GL074998KeyPoints:  Adistinctiveechopatternwasfound intheLRSdataobtainedaroundthe MariusHillsHole(MHH),apossible skylightofalavatube  Aroundanarea(13.00 – 15.00°N, 301.85 – 304.01°E)aroundMHH,similar LRSechopatternswerefoundat severallocations  Thelocationsexhibitingtheecho patternareconsistentwithmass de citssuggestedbytheGRAIL gravitydataanalysis SupportingInformation:  SupportingInformationS1 Correspondenceto: T.Kaku, 7bemm027@mail.u-tokai.ac.jp Citation: Kaku,T.,Haruyama,J.,Miyake,W., Kumamoto,A.,Ishiyama,K.,Nishibori,T., … Howell,K.C.(2017).Detectionof intactlavatubesatMariusHillsonthe MoonbySELENE(Kaguya)lunarradar sounder. GeophysicalResearchLetters , 44 ,10,155 – 10,161.https://doi.org/ 10.1002/2017GL074998 Received18JUL2017 Accepted24SEP2017 Acceptedarticleonline17OCT2017 Publishedonline25OCT2017 ©2017.AmericanGeophysicalUnion. AllRightsReserved.

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holes,MareTranquillitatisHoleandMareIngeniiHole(Haruyamaetal.,2010).Theheightfromthe oortothe ceilingexceeds15mforMHH.However,becauseobliqueobservationsdonotpenetratefarbeneaththerim ofthehole,thefullextentofthesubsurfacespacearoundtheMHHremainedunknown. Gravitymeasurementsprovideoneprobetoexplorethedistributionofsubsurfacestructures(AndrewsHannaetal.,2013;Bills&Ferrari,1977;Brattetal.,1985;Thurber&Solomon,1978;Yamamotoetal.,2016). Theexistenceofalavatubeextendingafewtoseveraltensofkilometersinlengthwouldaffectgravityconditionsandbedetectableasa “ massde cit. ” ThetwinGravityRecoveryandInteriorLaboratory(GRAIL) spacecraftwerelaunchedin2011(Zuber,Smith,Lehman,etal.,2013;Zuber,Smith,Watkins,etal.,2013). TheGRAILrepresentationoftheMoon ’ sgravity eldisasphericalharmonicexpansion,currentlyreported todegreeandorder1,200(Chappazetal.,2017).Developingmethodsofgradiometryandcrosscorrelation toisolatethetargetsignalofmassde citsfromtheGRAILgravitydata,Chappazetal.(2017)detectedseveral locationsofhorizontallyextendedmassde cits.Alavatubeextendingafewtoseveraltensofkilometers wouldbealargeenoughvoidtocreateamassde citdetectableintheGRAILdata.Amassde citsignature followsrille A andisfarlargerthanexpectedfromthetopographictroughalone,whichimpliestheexistence ofalargevoidspacebeneaththevisiblesurfaceextending60kmtothewestoftheMHH.Someofthemass de citsdetectedbyChappazetal.(2017)couldbecausedbylongopenlavatubes(Chappazetal.,2017). Geologicallyplausiblealternativesourcesofcaveincludelargehighlyfracturedfaultsorpartiallycollapsed lavatubes. Anothermethodtoexplorethesubsurfacestructureusesground-penetratingradarsystems(e.g.,Miyamoto etal.,2005;Phillipsetal.,1973;Seuetal.,2004;Seuetal.,2007).Campbelletal.(2009)investigatedtheMarius HillsusingEarth-basedradarwith12.6cmand70cmwavelengthsandcharacterizeddomesbythehighcircularpolarizationratiosoftheradardata.The12.6cmradarsignalspenetrateuptoameterortwo,andthe 70cmdatareachaboutafactorof5timesgreaterdepth.However,theradardatawerenotappropriateto detectfeaturesdeeperthanafewtensofmeters. TheLunarRadarSounder(LRS),anactiveradarsounder,wasinstalledonSELENE.Theoperationfrequencyof theLRSis4 – 6MHz(around60mwavelength),andtransmissionpoweris800W.Subsurfacestructuresat depthsofafewhundredmeterstoafewkilometershavebeeninvestigatedusingLRSdata(e.g.,Ono etal.,2009;Oshigamietal.,2012;Oshigamietal.,2014).However,mostpreviousworksfocusedonthestructureoflaterallyextensiveit.Inthepresentstudy,weexamineindetailtheLRSechodatare ectedfromafew tensofmeterstoafewhundredmeters ’ depthtocon rmtheexistenceofunderlyingintactlavatubes.2.MethodsWeusedradarechodatafromtheLRSonboardSELENEtoinvestigatetheexistenceofundergroundlava tubesatdepthsofafewtenstoafewhundredsofmeters.TheLRSconsistsoftwosetsofdipoleantennas withatip-to-tiplengthof30mtransmittingelectromagnetic(EM)wavesandreceivingechoesfromthe MoonornaturalradiowavesfromtheEarthandotherplanets(e.g.,Jupiter).TheLRSactivelytransmits frequency-modulatedcontinuousEMwavessweepingfrom4to6MHz(around60mwavelength)in 200 stotheMoon.Thebandwidthof2MHzleadstotherangeresolutionof75minvacuum(dielectricconstant is1)andasmallerresolutioninthegrounddependingonthedielectricconstant.Thetransmission intervalofLRSis50ms,whichcorrespondsto75monthegroundintheSELENE ightdirection (Kobayashietal.,2012;Onoetal.,2009).TheLRStransmissionpowerof800Wisdesignedtodetectsubsurfaceboundariesevenatdepthsofafewkilometers.TheechoesofEMwavestransmittedbyLRSantennasare observedwithdifferenttimedelayscorrespondingtothedistancestore ectors,suchassurfacecraterwalls and/orsubsurfaceboundaries.TheobservedLRSechodataarecomposedofre ectionsfromthesurfaceand subsurfaceboundaries.WeusedLRSechodatathatwereprocessedusingtheSyntheticApertureRadar(SAR) algorithm(Kobayashietal.,2012).AsaresultofSARprocessing,thesignalislimitedtoreturnsfromthesurfaceandsubsurfacedueeast-westofthesubspacecraftpoint.ThesyntheticaperturesizefortheLRSecho datausedis5km.ThedatasetweusedistheSounderSARimage(power)processedusingasyntheticapertureof5kmandisarchivedintheSELENEDataArchivewebsite(http://l2db.selene.darts.isas.jaxa.jp/index. html.en). Figure1plotsatypicalLRSechopowerdatapro leinamareregion(13.715°N,304.010°E).ThetimedifferencesofreceivedechoeswereconvertedtocorrespondingdepthswheretheLRStransmittedEMwaves GeophysicalResearchLetters10.1002/2017GL074998 KAKUETAL.INTACTLUNARLAVATUBEDETECTIONBYLRS10,156

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transmittedbytheLRSwerere ected.Thedielectricconstantusedhere forconvertingtime-to-depthisunity(1),forvacuum.Thetruelocation ofthere ectorsisthereforeshallowerthantentativelygivenwiththe vacuumdielectricconstantvalue.Thelargestechopeak(redpoint;normalizedto0dB)isfromthenadirsurfaceoftheMoon.Afterthelargest echopeak,theobservedsignallevelgraduallydecreasesduetoadsorptionandscatteringofEMwavesintheundergroundand nallyfallstoa noiselevelof 34dB(greentriangle),weakerthanthenoiselevelof about 25dB.3.ResultsWe rstinvestigatedtheLRSdatafromaSELENEorbitapproachingthe MHH(14.100°N,303.262°E)andfoundanechopatternwithtwopeaksat locationT1,whichis1.2kmeastand0.3kmsouthfromtheMHH. Figure2plotstheLRSechopowerasobservedatlocationT1versus thesubsurfacedepthfromwheretheLRSradarpowerwasre ected. Herewesetthedielectricconstant( )tothatofvacuum,whichis1,to calculateanupperlimitforthedepth.The rstechopeak(redpoint;normalizedto0dB)isfromthesurface,andthesecondpeak(bluesquare) mustbefromthesubsurfaceboundary.Priortothesecondechopeak,thereceivedechopowerdecreased precipitouslywithtimetoanoiselevelof 28.1dB(greentriangle).Afterthesecondechopeak,thereceived echopowerdecreased(orangepentagon).Thepurplediamondmarksthethirdechopeak.InFigure2,the depthofthegreentriangleis125mandthatofthebluesquareis250m.Whenweassumeadielectricconstantof4orhigher,amoreappropriatevalueforlunarsubsurfacematerials(Onoetal.,2009),weobtaina morerealisticdepthof62.5morshallowerforthegreentriangleand125morshallowerforthebluesquare. ThesubsurfaceboundarydetectedatT1(bluesquare)maybeaceilingora oorofacave,suchasanunderlyinglavatube. Weexpandedthesearchareato13.00 – 15.00°N,301.85 – 304.01°EaroundtheMHH,inanefforttoidentify locationswithsubsurfacecaveswheretheechopatternsexhibitthefollowingthreecharacteristicfeatures similartothoseoflocationT1: 1.Theechopowerreturnedfromthesubsurfaceregionsshallower than125mdecreasesmorethan25dBcomparedtothesurface echopowerlevel(i.e.,inFigure2,echopoweroftheredpointechopowerofthegreentriangle > 25dB),whichindicatestheexistenceofacaveorahomogeneousrockbulgewherenochangeof dielectricconstantinthesubsurfaceregionoccurs,andthus,there isnore ectionofsoundingEMwaves. 2.Thereisasecondechopeak(forexample,thebluesquarein Figure2)atadepthoflessthan225mfromthesurface,as observedatlocationT1,whichindicatestheexistenceofaboundarywithachangeofdielectricconstantsuchastheceilingorthe oorsurfaceofalavatube. 3.Theecholevelofthethirdlargestpeak(forexample,thepurplediamondinFigure2)issmallerthanthatofthesecondpeak;herewe assumethatthethirdechopeakshouldbeweakerthanthesecond one,andanystrongthirdechoshouldberegardedasthatfroman off-nadirsurfacepoint. Figure3presentstheresultsthatmarkvariouslocationscorresponding tothethreeaforementionedcharacteristicfeaturesinthevicinityofthe MHH.Thebackgroundofthe gureisanimagefromtheSELENETC.The graylinescorrespondtotheLRSobservationtracks.Thecircularpoints ontheselinesindicatelocationswhereechopatternssatisfythe Figure1. TypicalLRSechopowerversussubsurfacedepth.TheLRSdata wereobservedatamareregion(13.715°N,304.010°E).Thedielectricconstantusedhereforconvertingtimetodepthisunity(1),asifitwerea vacuum.Thelargestechopeak(redpoint;normalizedto0dB)isfromthe nadirsurfaceoftheMoon,andsubsequentlyobservedechoesgradually decreaseinpowertothenoiselevelof34dB(greentriangle),weakerthan thenoiselevelofabout25dB. Figure2. TheLRSechopowerversusthesubsurfacedepthatlocationT1 (14.100°N,303.262°E).TheLRSdatahaveanechopatternwithtwopeaks atT11.2kmeastand0.3kmsouthoftheMHH.Thedielectricconstantused forconvertingtimetodepthisunity(1),appropriateforvacuum.The rst echopeak(redpoint;normalizedto0dB)isfromthesurface,andthesecond one(bluesquare)mustbefromasubsurfaceboundary.Priortothesecond echopeak,thereceivedechopowerprecipitouslydecreasedwithtimetoa noiselevelof 28.1dB(greentriangle).Thisechopatternwithtwopeaks andasubstantialechodecreasebetweenthemimpliestheexistenceofa cave,suchasanunderlyinglavatube(seetext).Afterthesecondechopeak, thereceivedechopowerdecreased(orangepentagon).Thepurplediamond marksthethirdechopeak. GeophysicalResearchLetters10.1002/2017GL074998 KAKUETAL.INTACTLUNARLAVATUBEDETECTIONBYLRS10,157

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conditionofthethreecharacteristicfeaturespossiblysuggestinga cave,basedontheechopatterndeterminedatlocationT1.More than80locations,candidatesitesofpossiblesubsurfacecaves, weredetectedinthissearcharea.Thecolorsofthecirclesdenote thepowerdifferencebetweenthe rstandsecondechopeaks ( Prb:apointindicatedbyaredcircle-apointbyabluesquarein Figure2).Somecandidatesitesforthepresenceofacaveexhibit strongsecondechopeaks;thelowerthe Prbvalue,themorelikely thepresenceofasubsurfacelavatube(forinstance,atcirclepoints withblacktoredcolorsinFigure3).Someofthecandidatecave sitesarealignedalongrille A (T2)andonapossibleextensionofrille A (T3andT4).ThelocationoftheMHH(awhitestarinFigure3)is indicatedbyanarrow.4.DiscussionWefoundseveralcandidatesitesforintactlavatubecaveswherea largesecondpeakofradarechotransmittedbytheSELENELRSwas observedwithanintervalcharacterizedbyaprecipitousdecrease inechopowerafterthelargestechofromthenadirsurface.Some ofthecandidatesiteswereatrille A ,whereaskylightholeofa possiblesubsurfacelavatubehadbeendiscovered(Haruyama etal.,2009). Therearetwointerpretationsoftheechopatternwithtwopeaks andasubstantialechodecreasebetweenthem.The rstinterpretationisthatthesecondechopeakisfrom theceiling( boundary)orthe oor( boundary)ofalavatube,andtheechofromthe oorofthelavatube wasburiedinthepeakofechofromtheceiling;thus,noadditionalechoisseen.Thedecreaseinechopower beforethesecondechopeakdemonstratestheexistenceofalargedenserocklayerthroughwhichtheradar passedwithoutanysigni cantre ections.Thelavatubemustbelocateddeeperthan75mfromthesurface, andtheheight(from oortoceiling)ofthelavatubemustbesmallerthan75m.Thesecondinterpretationis thatthesecondechopeakisfromthe oor( boundary)ofalavatube,whereastheechofromtheceiling( boundary)ofthelavatubewasburiedinthepeakofechofromthesurface.Thedecreaseinechopower beforethesecondechopeakmeanstheexistenceofacavethroughwhichtheradarpassedwithoutany re ections.Thecavemustbelocateddeeperthan75m,andtheheightofthelavatubemustexceed 75m.Itseemsmoreplausiblethatthesecondechopeaksofthecandidatesitesarefrom oors,because theprecipitousdecreaseinechopowerpriortothesecondechopeakismostconsistentwiththepresence ofvacuumspace.Dielectricconstantsofanylavaformingtheceilingofalavatubewouldbelesshomogeneousthanthatofvacuumspace;vacuumspaceyieldsamoreconsiderabledecreaseinechoes(Figure4). Weinvestigatedwhetherornotnearbygeologicfeatures,suchaswallsoflargecraters,mayhaveproducedoffnadirEMsurfacere ectors.Barswerecenteredonlocationswhereprominentsecondechopeaksareobserved toseeifotherfeaturesmaycorrespondtothesecondecho.ThebarsarepresentedinFigure3.Thehalfof widthsofthesebarsare6.990kmaroundT1,5.850kmaroundT2,6.993kmaroundT3,and5.640kmaround T4.WeusedthedataforwhichSARprocessing(Kobayashietal.,2012)wasperformed.Therefore,besidesthe surfaceecho,onlyechoesfromsubsurfaceboundariesand/orfromthedueeastand/orduewestsurface shouldremain.Thepossiblelocationsoftheoff-nadirsurfacere ectorswith Prb < 10dBaremarkedwith XonthebarsinFigure3.However,notopographicfeature(e.g.,craterwalls)thatcouldcausealargeoff-nadir surfaceechohasbeenfoundatthelocationsindicatedbyXmarks.Thus,thesecondechopeaksobservedare probablyfromsubsurfaceboundaries,notfromanyEMwavere ectorsonthesurface. Forthesamesearchregion(13.00 – 15.00°N,301.85 – 304.01°EaroundtheMHH),wecomparedtheresultof Chappazetal.(2017)basedonthetwinGRAILspacecraftdata.Figure5indicatesthelavatubecandidatesites assuggestedbytheLRSdataoverlaidonacross-correlationBouguergravitymapChappazetal.(2017)developed.Thecoldcolorsonthemapareconsistentwithmasssurplus,whereasthehotcolorscorrespondto massde cits(i.e.,low-densityspaceorvoidssuchascaves).Mostlavatubecandidatesitessuggestedby Figure3. Thesearchledtotheidenti cationoflocationswhereLRSechopatterns satisfytheconditionofthreecharacteristicfeaturesassociatedwithlocationT1. ThebackgroundimageisfromtheSELENETC.Thegraylinescorrespondtothe LRSmeasurementtracks.Thecolorofthecircularpointsdenotesthepowerdifferencebetweenthe rstandthesecondechopeaks( Prb).Thebarsaroundthe candidatesitesindicateequidistancescorrespondingtoportionswherere ectors yieldingthesecondechopeaksshouldexist.ThehalfofwidthsofbarsaroundT1 are6.990km.AsaresultofSARprocessing,echoesfromsubsurfaceboundaries, and/orechoesfromre ectorsatdueeastorduewestoff-nadirsurfaceremain(X marksonthebars).ThelocationoftheMHH(awhitestar)isindicatedbyanarrow. GeophysicalResearchLetters10.1002/2017GL074998 KAKUETAL.INTACTLUNARLAVATUBEDETECTIONBYLRS10,158

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LRSdataarealsoapparentasLRSdatacorrelatewiththeareaof massde cits(redtoyellowregions)inthecross-correlation Bouguergravitymap.Prominentmassde citsareseenonrille A andonanextensiontothesouthwestoftherille(redregions). Therefore,thelava owsthatformedrilleAmayhave owed towardthesouthwestbeyondthevisibleendoftherillethrough atubeorroofedoverchannel. Somecandidatesitesareinareasofmasssurplus.Thecircular pointsofthesesitesareindicatedbygreentoblue,andtheradar echopeaklevelsforthesitesaresmall.Therefore,thesitesmay nothaveanylargecavesunderthesubsurface,ortheymaycontain somevoidssurroundedbyalargemasssurplus.Wenotethatan areain13.5 – 13.8°N,302.5 – 302.8°Econtainsmanycandidatesites, someofwhichareSELENELRSdatawithalargesecondechopeak (indicatebyredpoints).Manylavatubesorcavernousvoidslikely existbelowthesurfaceinthisregion.5.ConclusionsWeinvestigatedtheLRSdatatodetectsubsurfaceintactlavatubes. WefoundacharacteristicfeatureinLRSdataindicatingalarge Figure4. TwointerpretationsoftheechopatterninLRSdataatlocationT1(14.100°N,303.262°E).The rstinterpretationis thatthesecondechopeakisfromtheceiling( boundary)andthe oor( boundary)ofalavatube.Thedecreaseinecho powerdemonstratestheexistenceofalargedenserocklayer.Thelavatubemustbelocateddeeperthan75mfromthe surface,andtheheight(from oortoceiling)ofthelavatubemustbesmallerthan75m.Thesecondinterpretationisthatthe secondechopeakisfromthe oor( boundary)ofalavatube,whereastheechofromtheceiling( boundary)ofthelava tubewasburiedinthepeakofechofromthesurface.Thedecreaseinechopowerbeforethesecondechopeakmeansthe existenceofacave.Thecavemustbelocatedshallowerthan75m,andtheheightofthelavatubemustexceed75m. Figure5. Thesearchidenti edlocationswhereLRSechopatternssatisfytheconditionofthreecharacteristicfeaturesassociatedwithT1andoverlaidthemona cross-correlationBouguergravitymapChappazetal.(2017)developed.Thecold colorsonthemapareconsistentwithmasssurplus;thehotcolorscorrespondto massde cits(i.e.,lowdensityspaceorvoidssuchascaves).Prominentmass de citsareobservedonrille A andonanextensionsouthwestoftherille(red). Mostotherlavatubecandidatesitesnotonrille A arealsointheBouguermass de cits(redtoyellow). GeophysicalResearchLetters10.1002/2017GL074998 KAKUETAL.INTACTLUNARLAVATUBEDETECTIONBYLRS10,159

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secondechopeakafteraprecipitousdecreaseinechoclosetotheMHH,apossibleskylightofanundergroundlavatube.SeverallocationsaroundtheMHH(13.00 – 15.00°N,301.85 – 304.01°E)exhibitsimilarcharacteristicfeaturesintheLRSdata.Sincetherearenopossiblefeaturesonthesurfacethatcouldcauseasecond echopeak,theselocationsarecandidatesitesforthepresenceofundergroundlavatubesorcavernousvoids. Wenotethatmostofthesecandidatesitesareatlocationsconsistentwithamassde citonthecrosscorrelationBougueranomalymapbasedonGRAILdata.Inparticular,somelavatubecandidatesitesare alongrille A ,inwhichtheMHHwasdiscovered,andonapossibleundergroundextensionsouthwestof therillewherethecross-correlationBouguermapindicatesalargemassde cit.Inconclusion,wehaveidenti edanintactlavatubeatMariusHillsontheMooninSELENELRSdata.ReferencesAndrews-Hanna,J.C.,Asmar,S.W.,Head,J.W.III,Kiefer,W.S.,Konopliv,A.S.,Lemoine,F.G., … Zuber,M.T.(2013).Ancientigneousintrusions andearlyexpansionoftheMoonrevealedbyGRAILgravitygradiometry. 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Zuber,M.,Smith,D.E.,Lehman,D.H.,Hoffman,T.L.,Asmar,S.W.,&Watkins,M.M.(2013).GravityRecoveryandInteriorLaboratory(GRAIL): Mappingthelunarinteriorfromcrusttocore. SpaceScienceReviews , 178 ,3 – 24.https://doi.org/10.1007/s11214-012-9952-7 Zuber,M.,Smith,D.E.,Watkins,M.M.,Asmar,S.W.,Konopliv,A.S.,Lemoine,F.G., … Yuan,D.N.(2013).Gravity eldoftheMoonfromthe GravityRecoveryandInteriorLaboratory(GRAIL)mission. Science , 339 ,668 – 671.https://doi.org/10.1126/science.1231507 GeophysicalResearchLetters10.1002/2017GL074998 KAKUETAL.INTACTLUNARLAVATUBEDETECTIONBYLRS10,161


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