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