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In-situMicroCTInstrumentfortheNorthPolarLayeredDepositsofMars
R.W.Obbard([email protected])1,P.Sarrazin1,2,N.T.Vo3,K.Zacny4,S.Byrne5
SeventhInternationalConferenceonMarsPolarScienceandExploration(#6078)Ushuaia,Argentina�Tuesday,14January2020
1-SETIInstitute,MountainViewCA,USA,2-eXaminArt,LLC,LosAltos,CA,USA,3-DiamondLightSource,Didcot,Oxfordshire,UK,4-HoneybeeRobotics,Pasadena,CA,USA,5-Lunar&PlanetaryLaboratory,UniversityofArizona,Tucson,AZ,USA
BackgroundandIntroductionTheNorthPolarLayeredDeposits(NPLD)areamulti-kilometerthicksequenceofdusty-icelayersthoughttorecordpreviousclimateconditionsmuchlikeEarth’sicesheets(Figure1).Decipheringthispolarrecordhasbeen,andremainstoday,amajorgoalofMarsresearch[1].A1mcoreoftheMarsNPLDsampledbyMISTwouldprovideinformationaboutapproximately1000Martianyearsofclimatehistory.OurobjectiveistodevelopasamplingandanalysissystemtocharacterizetheporosityanddistributionofdustintheMarsNPLDwithhigherresolutionandtogreaterdepthsthanispossiblewithcurrenttechniques.BasedonX-rayattenuation,X-raymicrocomputedtomography(microCT)providesnondestructivethree-dimensionalvisualizationandcharacterizationofinternalfeaturesofmultiphasematerialswithspatialresolutiondowntoseveralmicrons.MicroCTisalreadyusedtoanalyzemicron-scaleporosityandsedimentdistributioninterrestrialicesheets[2]andseaice[3].
ConceptDemonstrationAbreadboardprototypewasbuiltusingacommercialmicrofocusX-raytube(60µmspotsize),astepper-motorbasedsamplerotationstageandanX-raycamerabasedonaAPS-CformatCMOSsensorfittedwithaCsIscintillatoronfiberopticsplate.Paraffinwaxwasusedasanicesimulantfortestsatroomtemperature,with0.8mmAlspheresembeddedtorepresentinclusions.X-rayattenuationimageswerecollectedatangularstepsof0.9°overafullrevolution(400angularpositions).Theseprojectionswerecombinedtoformasinogram.Imagepreprocessingincludedflat-fieldcorrection,ringartifactremoval,andbeamhardeningcorrection[4].Adedicatedcalibrationmethodwasdevelopedtocomputeinstrumentgeometryparametersthatcouldn’tbemeasuredwithhighprecisiononthebreadboard.ReconstructionwasdoneusingaNVIDIATeslaK20CgraphicscardwithaGK110graphicsprocessingunit(GPU)and5GBmemory.3DreconstructionsoftwarewasbasedonaFilteredBack-Projection(FBP)method.Resultswerecomparedtothosefromacommercialinstrument(Fig.3).Next,thebreadboardsystemwastestedina-15°Cfreezer.Thesamplewasa25mmdiameterrodoficewithlargesandgrains.AgainresultswerecomparedtothosefromacommercialdesktopmicroCT,thistimeat-15°C(Fig.4).
InstrumentConceptMicroInSituTomography(MIST)isacoupledcoringandmicroCT-analysissystem(Fig.2).Acoringdrillproducesasamplecore2.5cmindiameterand0.5–1minlengthandcapturesitwithinanX-raytransparentcarbonfibertube.Asthistubeiswithdrawnfromthesurface,aminiaturizedmicroCTsystemrotatesarounditcollectingX-rayattenuationimages.ThemicroCTsubsystemisbasedonacone-beamgeometrywithasimplearchitecturecombiningamicrofocusedX-raytube,acorescanningstage,andanX-rayimagesensor.
FutureWorkWeareimprovingthebreadboarddesignwithanewsourceandabetterdetector.WenextplantodesignandbuildatransportablebrassboardofthemicroCTsubsystemwhichwewilltestatNASAAmes.Wewillthenfabricateafullscale(1mlong)augerandtestitwiththeHoneybeeTRIDENTdrillina-15°Ccoldroom.
Figure1.MicroCTreconstructions(each1-2cm3)ofrelevantEarthsystems.(Left)Tephraparticles,color-codedbysize(5-40μm),inice(madetransparent)from3149mdepthintheWestAntarcticIceSheet(Right)Porosity(brinechannels)inseaicefrom130cmdepth,RossSea,Antarctica
Figure2.(Left)MISToperationalconcept.(Middle)Breadboardcorerdesign,showingtheaugertubeand(nested)breakofftubeintegratedwiththeHoneybeedrilltestbed.(Right)ArchitectureoftheanalyticalheadofMISTmicroCTsubsystem.
References1. Byrne,S.(2009)AnnualReviewofEarthandPlanetarySciences,372. Iverson,N.etal.(2017)NatureScientificReports,7:114573.Obbard,R.W.,G.Troderman,andI.Baker(2009)J.Glaciol.,55,1944.Vo,N.T.,AtwoodR.C.,andDrakopoulos,M.(2018)Opt.Exp.,26,28396-28412
Figure4.Reconstructed2Dimagesofsandgrainsiniceimagedat-15°C.(Left)frombreadboardinstrumentoperatedat25kV.(Right)Forcomparisonanimageatapproximatelythesamedepth(butdifferentmagnification)producedwithaSkyscan1173atMontanaStateUniversity.(Ringartifactsaretheresultofinappropriateimagecorrectionsfortheearlydataset.)
AcknowledgementsMISTisfundedbyNASAgrantPICASO17_2-0094.Thisworkhadits inceptionatthe“UnlockingtheClimateRecordStoredwithinMars’PolarLayeredDeposits”studyattheW.M. Keck Institute for Space Studies (Pasadena, CA). N. T. Vo developed thereconstruction technique for a misaligned cone-beam system. We also thank theorganizers of the 2018 CAMERAWorkshop at Los Alamos National Laboratory fortheirwelcomeandsuggestions.
Figure3.Reconstructed2Dimagesoficecoresimulant(waxrodwith0.8mmAlspheres).(Left)frombreadboardinstrumentoperatedat25kV.(Right)Forcomparison,animageatapproximatelythesamedepthinthesampleproducedbyacommercialdesktopmicroCTinstrument(Skyscan1173).NotethepositionsnotonlyofthetwoAlspheres,butalsoofthevoids.
