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GEOPHYSICALINVESTIGATIONOFTHEMOONR.C.Weber,C.R.Neal,B.Banerdt,S.Kedar,N.Schmerr,M.Panning,M.Siegler
LunarScienceforLandedMissionsWorkshopJan.10-12,2017
• Constraintsonthesepropertiesarisefromgeophysicalobservations:Ø SeismologyØ HeatflowØ LunarlaserrangingØ MagneticinductionstudiesØ Gravityfield
ThePlanetaryDecadalSurveyrecognizesgeophysics asahigh-priorityscienceobjectiveforafuturelunarlandedmission.Why?
• Interiorproperties(layering,composition,seismicvelocityanddensity,presenceofpartialmelt,corestate:liquid vs.molten)provideimportantconstraintsonlunarformationandevolutionmodels,aswellaspossibleindicatorsofanearlydynamoformagneticfieldgeneration.
Wieczorek etal.(2006)Rev.Mineral.Geochem.60,221– 364.
• GRAILlunargravitymissionmappedtheMoon’sgravityfieldinextremedetailØ Shallow(crustal)structuretightlyconstrained,butstilltiedto(uncertain)ground-truthseismicestimates
fromtheApollolandingsites• Re-analysesofApolloseismicdatafoundevidenceforcorereflections,includingthepresenceofapartialmelt
layerabovetheliquidoutercoreØ Differingperspectivesonwhetherapartialmelt layerisrequiredtosatisfyavailableconstraints(gravity,
seismic,geodeticconstraints,EMsoundingdata,phase-equilibriummodels,dissipation)
Recentadvancesinlunargeophysics
Wieczorek etal.(2013)Science 339,671-675
Weberetal.(2011)Science331,309-312
• Apollo seismicdata&GRAILrevolutionizedourunderstandingoftheMoon’sinterioranditsformation/evolution.Nevertheless,manyquestionsremainregardingtheamount/distributionofseismicity,aswellasthedetailedstructureofthecrust,mantle,andcore.
• ILNstudy:seismometersweredeemedthemostimportantinstrument
• FutureseismicmissionØ Wideraperturenetwork(betterlocationsforbetterinteriormodels)Ø FarsidecoverageØ Bettersensitivity(although theywereverysensitive,theApollo
seismometerLSBwasabovebackgroundnoiselevel)Ø Widerfrequencyrange(lowerfrequenciesarelessaffectedby
scatteringandsufferlessattenuation)
Whygoback?
• Anetworkofgeophysical“nodes”(atleast4)operatingcontinuouslyforanextendedperiodoftime– atleast2(ILN),4(LUNETTE),or10(LGN)years
• Scienceobjectivesforeachnode:
TheLunarGeophysicalNetwork– Whatisit?
Seismometer Heatflowprobe Retroreflector Magnetometer• Understandthe
currentseismicstateanddeterminetheinternalstructureoftheMoon
• Measuretheheatflowtocharacterizethetemperaturestructureofthelunarinterior
• Increaseabilitytodeterminedeeplunarstructureandconducttestsofgravitationalphysicsbyinstallingnext-generationlaserrangingcapability
• Useelectromagneticsounding tomeasuretheelectricalconductivitystructureofthelunarinterior
Lunarseismology– whatisit?
• Seismometersarestationaryinstrumentsthatmonitorthegroundforseismicshakinginducedbynaturaltectonismandmeteoriteimpacts.
• Idealdeploymentconditions:Ø GoodgroundcouplingØ Continuousdatacollection/transmissionØ Longevity(severalyears)
Lunarseismology– whatisit?
Datarecordedbyseismometerscanbeusedtostudy:
Ø Thetransmissionofseismicenergythroughtheplanet,allowingrecoveryofstructuralpropertiesandlayering(crust,mantle,core)
Ø Theamountanddistributionofseismicity(groundshaking),whichcanilluminatetectonicprocessesandquantifypotentialthreatstofuturelandedassets.
Lee-LincolnscarpattheApollo17landingsite
Lunarseismology– typesofseismicity
Knownlunarseismicityconsistsoffourprimarytypes:
1. Shallowmoonquakes (largebutrare“tectonic”eventssimilartointra-platequakesonEarth)
2. Deepmoonquakes(smallbutfrequenteventsthatoccurmonthlyaccordingtothetidalcycle)
3. Meteoriteimpacts(muchmorefrequentthanonEarthduetolackofatmosphere)
4. Thermalevents(localizedcrackingofrocksandregolithmovementinducedbylargeday/nighttemperatureswings)
5. Artificial impacts(e.g.,lunarmodules,SaturnIVBstages)
and
Landingsiteselectionforoptimalgeophysicalobservations
Seismometersareoftenpackagedwithothergeophysicalinstruments.Jointconsiderationoflanding siterequirementsmaximizessciencereturn.
Seismology Heatflow Laserranging EMsounding
• Detectthemaximumnumberofsources
• Multipleinstrumentswith ageographicallywide(global) footprint,includingobservationsfromthefarside
• Land>200kmawayfromboundaries betweenregionswithdisparatethermalproperties
• Increasethe footprintofcurrentnetworkofretroreflectors
• NearsidesitesonlytotakeadvantageofEarth-basedlasers
• Co-locationwithotherexperiments.
• Studies ofcrustalmagnetismandplasmainteractionstudieswouldbenefitfrommeasurementsatsitesofstrongDCmagnetizationand/oralbedoanomalies
Generalsiteconsiderations:fewboulders,slopes<10°,nearlobatescarps/thrustfaultstotestongoing seismicity
Previouslandingsitesuggestions
Landing sitessuggestedbypreviousmissionconceptLUNETTE:ALunarGeophysicalNetwork
Previouslandingsitesuggestions
Landing sitessuggestedbypreviousmissionconceptILN:TheInternationalLunarNetwork
Landingonthefarside
Requiresorbitalrelayforcommunications,butenablesobservationstoaddresskeyunknownsregarding:• Farsideseismicity• Globaldichotomies
Ø CrustalthicknessØ Marevolcanism&relatedgeochemicalsignatures
Synergywithgeothermalmeasurements
Obtainingmeasurementsbothinsideandoutside ofthePKTisalsoimportant forunderstandinggeothermalheatproductionandinteriorthermaldistribution.
TheApolloHeatFlowExperimentswerebothwithinareasdominatedbyThorium-richcrust;aheatflowprobeoutsidethisregionwouldmoretightlyconstrainmantleheatproductionandcomposition.
ThePKTcouldalsocreateahotmantleregion,whichmaybedetectableinbothheatfluxandseismicmeasurements.
Apollo15Apollo17
Conclusions
Seismologybenefitsscience• Modern,broadband scientificinstrumentationandinnovativeroboticdeploymentmechanismsarecurrently
underdevelopmentbyseveralU.S.andinternationalteams• NASA’sNewFrontiers5AOwilltargetaLunarGeophysical
Network• Favoredlanding sitesareeitherinternalorexternaltothe
Procellarum KREEPTerrane(notatboundaries), closetothelunarlimbs,andonthefarside
Seismologybenefitsexploration• Seismichazardcanalsobeassessedwithsmall,lightweight
accelerometersthatcaneasilybeaccommodatedinlanderstructuralcomponents
• Anylanderatanylocationonthelunarsurfaceshouldhaveaseismometer!
JPLLUNETTEmissionconcept