ARTEMIS Science Objectives

33 Pages·2011·3.34 MB·English

https://ntrs.nasa.gov/search.jsp?R=20110015549 2019-03-30T11:02:27+00:00Z
SpaceSciRev
DOI10.1007/s11214-011-9777-9
ARTEMIS Science Objectives
D.G.Sibeck·V.Angelopoulos·D.A.Brain·G.T.Delory·J.P.Eastwood·
W.M.Farrell·R.E.Grimm·J.S.Halekas·H.Hasegawa·P.Hellinger·
K.K.Khurana·R.J.Lillis·M.Øieroset·T.-D.Phan·J.Raeder·C.T.Russell·
D.Schriver·J.A.Slavin·P.M.Travnicek·J.M.Weygand
Received:25July2010/Accepted:8April2011
©USGovernment2011
Abstract NASA’s two spacecraft ARTEMIS mission will address both heliospheric and
planetary research questions, first while in orbit about the Earth with the Moon and sub-
sequently while in orbit about the Moon. Heliospheric topics include the structure of the
Earth’smagnetotail;reconnection,particleacceleration,andturbulenceintheEarth’smag-
netosphere,atthebowshock,andinthesolarwind;andtheformationandstructureofthe
(cid:2)
D.G.Sibeck( )
Code674,GSFC/NASA,Greenbelt,MD20771,USA
e-mail:[email protected]
V.Angelopoulos
IGPP,UCLA,LosAngeles,CA90095,USA
D.A.Brain·G.T.Delory·J.S.Halekas·R.J.Lillis·M.Øieroset·T.-D.Phan
UCB,Berkeley,CA,USA
J.P.Eastwood
ImperialCollege,London,UK
W.M.Farrell·J.A.Slavin
NASA/GSFC,Greenbelt,MD,USA
R.E.Grimm
SWRI,Boulder,CO,USA
H.Hasegawa
ISAS,Sagamihara,Japan
P.Hellinger·P.M.Travnicek
AstronomicalInstitute,Prague,CzechRepublic
K.K.Khurana·C.T.Russell·D.Schriver·P.M.Travnicek·J.M.Weygand
UCLA,LosAngeles,CA,USA
J.Raeder
UNH,Durham,NH,USA
D.G.Sibecketal.
lunar wake. Planetary topics include the lunar exosphere and its relationship to the com-
positionofthelunarsurface,theeffectsofelectricfieldsondustintheexosphere,internal
structureoftheMoon,andthelunarcrustalmagneticfield.Thispaperdescribestheexpected
contributionsofARTEMIStothesebaselinescientificobjectives.
Keywords ARTEMIS·Moon·Reconnection·Particleacceleration·Turbulence·Wake·
Lunarsurface·Lunarcore·Dust·Electricfields·Crustalanomalies
1 Introduction
ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s
InteractionwiththeSun)isNASA’sfirstdualspacecraftmissiontotheMoon.Bymission
design and both efficient navigation and flight operations during the primary phase of the
THEMIS mission from February 2007 to September 2009 (Angelopoulos 2008), the out-
ermost two THEMIS spacecraft P1 and P2 found themselves with fuel reserves sufficient
toundertakeaseriesofoverfortymaneuvers,includingmultiplelunarapproachesandfly-
bys, that enabled them to reach and parallel the Moon during trans-lunar injection phases
fromOctober2009throughSeptember2010.FromOctober2010toJanuary2011,thetwo
spacecraft were in Lissajous orbits at the Lagrange points of the Earth-Moon system that
straddledtheMoonandwereseparatedbydistancesontheorderof∼20Earthradii(R ).
E
FromJanuarytoJuly2011,theirLissajousorbitswereseparatedby5to20R onthesame
E
sideoftheMoon.InJuly2011theyenteredlunarorbits(oneprograde,theotherretrograde)
with separation distances ranging from 500 km to 20 lunar radii (R ). Note that 1 R is
L E
6371km,while1R is1737km.
L
Observationsfromlunarorbitoverasuchwiderangeofinterspacecraftseparationsare
ideal for addressing a number of longstanding heliophysics objectives, including defining
thecharacteristicsofreconnectionandturbulenceinthesolarwindanddistantmagnetotail,
the nature of particle acceleration at the Earth’s bow shock and interplanetary discontinu-
ities,andtheelectrodynamicsofthesolarwind’sinteractionwiththeMoon.However,by
carefuloptimizationofmissionparameters,inparticularperiseleneandorbitalinclination,
thefully-instrumentedandfully-functionalARTEMISspacecraftcanalsobeemployedto
addressasetofplanetaryresearchobjectivesincludingthecompositionandmagnetization
of the lunar surface, the effects of electric fields on dust in the vicinity of the Moon, and
the structure of the lunar interior. With orbits that will remain stable for over a decade,
ARTEMISwillprovideobservationsofthelunarenvironmentoversolarcycletimescales,
offermeasurementsthatoverlapandenhancethoseofNASA’sLunarReconnaissanceOr-
biter(LRO)andLunarAtmosphereandDustEnvironmentExplorer(LADEE)missions,and
providecrucialsolarwindobservationsforcorrelativestudieswithotherNASAmissionsto
theEarth’smagnetosphereandionosphere/thermosphere.
Angelopoulos(2011,thisissue)providesamissionoverviewforARTEMISthatsumma-
rizestheoverallscientificobjectives,orbits,instrumentation,missionandscienceoperations
thatareexpandeduponinindividualpapersinthiscompendium.Thispaperdescribeshow
theabovemissionelements willenabletheARTEMIS team toreach scientific closure.In
terms of organization, Sect. 2 describes ARTEMIS planetary objectives while Sect. 3 de-
scribestheARTEMISheliophysicsobjectives.Section4presentsconclusions.
ARTEMISScienceObjectives
2 ARTEMISPlanetaryScienceObjectives
TheMoonholdscriticalinformationregardingtheoriginofthesolarsystem.Becauseitis
devoidofplatetectonics,volcanismandsurface-alteringatmosphericprocesses,theMoon’s
surfacehasrecordedthe4.5billionyearsofsolarsystemhistorymorepurelythananyother
planetary body. Understanding the lunar surface and the stratification of the lunar interior
providesawindowintotheearlyhistoryoftheEarth-Moonsystemandcanshedconsider-
ablelightontheevolutionofterrestrialplanetssuchasMarsandVenus.Thesurfaceiscon-
stantlysputteringionsanddust,whichthenenterandcirculatewithinthelunarexosphere
beforeescapingintothesolarwind.Ions‘picked-up’bythesolarwindgainsimilarveloc-
ities but mass-dependent energies as they follow mass-dependent trajectories. ARTEMIS
will use its energy-angle spectroscopic capability and its electric and magnetic field sen-
sorstodeterminethemotion,source,composition,andfluxofexosphericions.Thesurface
electricfieldsthatloftandtransportdustparticlescanberemotelysensedusingelectronre-
flectometryormeasureddirectlyfromaltitude.ARTEMISwilluseitselectromagneticfield
measurementstodeterminetheforcesactingondustpopulationsandcausetheiraccelera-
tionanddepositionorlossinthelunarenvironment.
Apollo-era data indicates that the Moon formed via the impact of a Mars-sized object
withtheearlyEarth,andlaterdifferentiatedintoprimarycrust,mantleresiduum,andpos-
sibly a small iron-rich core. Radial profiles for the Moon’s temperature and composition,
and their lateral variability today, hold important clues regarding the history of lunar dif-
ferentiation. Broadband((cid:3)10mHzto10Hz)electromagnetic soundingwillimproveour
knowledgeofthesestatevariablesforthecore,mantle,andcrust.Thesensitivemagnetome-
tersonARTEMISwillapproachtheMoononorbitsthatbringthem,oneatatime,towithin
100 km of the lunar surface, i.e. to altitudes close enough to detect the core response to
varyingexternaldriversbutdistantenoughtominimizeperturbationsfromcrustalanoma-
lies.ARTEMISalsoprovidessensitivehorizontalelectricfieldmeasurementsfrom1–10Hz
thatwillfurtherenablemagnetotelluricinvestigations.
Finally,ARTEMISwilluseitscomprehensiveparticleandfieldsensorstostudytheinter-
actionofthesolarwindwithlunarcrustalmagneticanomalies.InitialKaguyaobservations
have already provided significant new information on an ion sheath, electron heating and
solar wind reflection around magnetic anomalies at 100 km (Saito et al. 2010), but wave
propertiesandsolarwindparticleflowaroundthestrongfieldregionremainpoorlyunder-
stood.ARTEMISwillstudythemagneticanomaliestoinferpropertiesoftheancient,seed
magneticfieldandtodeterminetheaccessibilityofthesolarwindtothesurfaceandtheef-
fectithasonlunarsurfaceageing.Electricfieldandplasmawavedata,togetherwithionand
electronmeasurementsinthevicinityofthemini-magnetosphereandshock-likestructures
(e.g., Halekas et al. 2006a) that form around thecrustal anomaly (the first comprehensive
plasmameasurementsattemptedattheMoon)promiseexcitingnewsciencewithpossibly
significantramificationsforplanetaryevolution.
2.1 ExosphericIonsandPlasmaPick-up
FortyyearsafterthefirstApollolandings,thecompositionandstructureofthelunarexo-
sphere remain poorly understood. A number of studies have used ground-based measure-
mentstoinvestigatetheexistence,extent,variabilityandlikelysourcesofeasilyobserved
exospheric species such as sodium and potassium (Potter and Morgan 1988; Tyler et al.
1988;Mendilloetal.1991,1999;Wilsonetal.2003,2006).However,thebehaviorofthese
speciesmaynotberepresentativeofother,moreabundant,species(PotterandMorgan1988;
FlynnandMendillo1993).
D.G.Sibecketal.
Fig.1 Aschematicshowingthetrajectoriesofrecentlypicked-upionsandtheexpectedmeasurementsof
thefluxes(topleft)andcomposition(topright)ofthepicked-upions
Insitumeasurementsofpickupionsofferanappealingcomplementarymeansofprobing
surfaceandexosphericpropertiesandprocessesattheMoon(e.g.HartleandThomas1974;
Cladisetal.1994;YokotaandSaito2005;HartleandKillen2006;HartleandSittler2007;
Saitoetal.2010).Newlycreatedions,producedbysolarwindsputtering,photostimulated
desorption,micro-meteoriodimpactvaporizationorionizationofexosphericgasesaregen-
eratedatrelativelylowenergies(0.01–10eV),butimmediatelyfeeltheeffectofsolarwind
magnetic and electric field (Stern 1999;Wurtzet al. 2007).The‘picked-up’ ionsare then
acceleratedincycloidaltrajectorieslikethatshowninFig.1.
InsitumeasurementsbyioninstrumentsonthespaceplasmamissionsAMPTE,WIND,
andGEOTAILconfirmedthepresenceoflunarheavyionsfardownstreamfromtheMoon
in the solar wind and in the distant magnetotail (Hilchenbach et al. 1991, 1993; Kirsch
etal.1997;Malletal.1998),butreliedonfortuitousconfigurationsofthesolarwindand
ARTEMISScienceObjectives
IMFandwerenotabletoprovidecomprehensiveinformationonexosphericcomposition,
sourcerates,orvariability.Further,thesestudiescouldnotdistinguishbetweenlunarions
originatingatthesurfaceandthosecreatedintheexosphere.
Morerecentstudieshaveshownthatitispossibletoseparatethesignaturesofsurfaceand
exosphericions,usingmeasurementsmadeataltitudessmallerthanthetypicalgyroradius
ofnewlyionizedlunarspecies.Forexample,theupperleftinsetinFig.1showstheresult
of a computation by Yokota and Saito (2005) of the expected differential fluxes of Na at
∼100 km altitudes resulting from both surface sputtering and exospheric photoionization.
Thesurfaceionsarenearlymonoenergetic,whiletheatmosphericionsspreadoutinenergy
duetotheextendedsourceandlowobservationaltitude.Athigheraltitudes,theionswould
be accelerated still further, resulting in a much more monoenergetic spectrum and much
higher ion energies for both surface and exospheric sources (with little reduction in flux
unlesssignificantscatteringoccursinoneiongyroperiod).Atdistanceswithintwogyroradii
from the source, one may discriminate between ions with different masses by comparing
observed energies and gyrophases with those predicted for the cyclical motion shown in
Fig.1,thoughdistinguishingbetweensurfaceandexosphericionsbecomesmoredifficult.
Kaguyafindingshaverecentlyadvancedourunderstandingofthesolarwind’sinteraction
withthelunarsurfaceaswellasthenear-Moonwake.Kaguya’sinsituionmassspectrome-
trywasusedtoconfirmthepresenceofNa+inadditiontoHe+,C+,O+,K+,andAr+inthe
sunlitlunarexosphereataltitudes∼100kmwhentheMoonwasinthesolarwind(Yokota
etal.2009)andEarth’smagnetosphere(Tanakaetal.2009).Someionsoriginatedfromthe
exosphere,othersfromthesurface.Otherspeciesmaybepresentaswell.Theheavyionflux
varies with solar zenith angle but not with solar wind flux or meteor shower occurrences,
suggestingastabledriverforthesputteringprocess.
Kaguyaobservationsalsoindicatethatfrom0.1to1%ofthesolarwindprotonsincident
upontheMoonarereflected.Solarwindconvectionelectricfieldsacceleratesomeofthese
protons to speeds triple those of the solar wind (Saito et al. 2008). The pickup ions find
access not only to the high latitude wake, but also areas deep within the low latitude and
lowaltitudewakethroughfullykineticprocesses.Knowledgeofthesolarwindelectricand
magneticfieldssufficestoreconstructobservedprotonspectrafromdaysidereflectedproton
sources.
TheESAandSSTinstrumentsonARTEMISwillmeasuretheenergyanddirectionof
pickupionparticledistributions,whiletheEFIandFGMinstrumentswillmeasuretheambi-
entelectricandmagneticfields.Withthisinformation,researcherswillbeabletobacktrack
observedpickupionstoeithersurfaceorexosphericsources.Furthermore,theywillbeable
toroughlydeterminetheionmass,sinceboththeionenergyandthesizeofthecycloidaltra-
jectoriesscalewithmass.ARTEMIScanthereforeusepickupionmeasurementstoremotely
probe theproperties of the surface and the exosphere. Measurements from the ARTEMIS
probemeasuringthelunarexospherecanthenbecombinedwithobservationsfromtheother
ARTEMISprobetodeterminetheresponseoftheexospheretosolarwinddrivers.
Although ARTEMIS will generally be further away from the Moon than Kaguya, the
largegeometricfactoroftheESAtotalioninstruments(afactorof∼10greaterthanthose
onKaguya)willenablesensitivemeasurementsofthepickupionsunderstablesolarwind
conditionsoverdistancesrangingfrom100kmtoseveralthousandkmfromthelunarsur-
face.Whenappliedasafunctionoflunarphasethetechniquewilldeterminethedependence
ofthelunarexosphereonlunarlongitudeundergoingillumination,therebyprovidingtheion
compositionversusselenographiclongitude.Sincethepickupiontrajectoriesmeasuredby
ARTEMIScanbeback-traced,thelatitudinalexosphericiondistributioncanalsobedeter-
mined.
D.G.Sibecketal.
A second method of indirect detection of lunar ions may also be possible using
ARTEMIS measurements. Pickup ions form ring distributions that can generate electro-
magneticioncyclotronwaves.ItmaybepossibletodetectthesewavesusingtheARTEMIS
vectormagneticfielddata.Wavesfrompickupionshavebeenidentifiedintheplasmaen-
vironmentsofmanysolarsystembodies,includingVenus,Mars,andeventhemoonsofthe
giantplanets(Russelletal.1990;Kivelsonetal.1996;Patersonetal.1999;Grebowskyetal.
2004;Doughertyetal.2006;RussellandBlanco-Cano2007;Delvaetal.2008),andtheir
presenceorabsencecanbeusedtoconstrainthelocaldensityofpickupions,andtherefore
thesourcestrength.
Finally,synergisticmeasurementsmadebyARTEMISandotherorbitingspacecraftcan
beusedtoconstrainexosphericproperties.TheLunarReconnaissanceOrbiter(LRO)mis-
sioncarriestheLyman-AlphaMappingProject(LAMP)ultravioletinstrument(Gladstone
etal.2010),whiletheupcomingLunarAtmosphereDustEnvironmentExplorer(LADEE)
missioncarriestheNMSforneutralmassspectrometryandtheUVSultraviolet/visualspec-
trometer(Deloryetal.2009).Thesemissionsthusmeasuregasesbeforeionization,while
ARTEMISmeasuresthempost-ionization.BycoordinatingARTEMISmeasurementswith
thosefromtheseandotherspacecraft,wecangreatlyadvanceourunderstandingofthelunar
exosphereanditscouplingtothesurfaceandthespaceenvironment.
2.2 SurfaceCharging,ElectricFields,andDust
Althoughonemightthinkofthelunarenvironmentasessentiallydormant,itisinfactvery
active electrically. Since the Moon has only a tenuous exosphere and no global magnetic
field,itssurfaceliesdirectlyexposedtotheimpactofsolarUVandX-raysaswellassolar
windplasmaandenergeticparticles.Thiscreatesacomplexanddynamiclunarelectricen-
vironment,withthesurfacetypicallychargingpositiveinsunlightandnegativeinshadow
topotentialsthatvaryoverordersofmagnitudeinresponsetochangingsolarillumination
andplasmaconditions.Thesepotentialshavebeenmeasuredbyinstrumentsonthesurface
andbyLunarProspectorinorbit,butwestilldonotfullyunderstandthenear-surfaceenvi-
ronment,especiallytheroleofsecondaryandphotoemittedelectrons,andthestructureof
theplasmasheathatthesurface.
The largest observed lunar potentials typically occur on the nightside, in the absence
ofphotoemission,whereambientplasmacurrentsprimarilydrivesurfacecharging(Manka
1973;Stubbsetal.2007).Usingmeasurementsoftheangulardistributionofreflectedelec-
tronsandacceleratedsecondaryelectronsfromthesurface,LunarProspectorprovidedes-
timates of negative nightside lunar surface potentials of ∼−100 V or less in the wake
and magnetospheric tail lobes (Halekas et al. 2002a, 2008c), and occasionally as high as
−2–4kVinthemagnetosphericplasmasheet(Halekasetal.2005,2008c)andduringSEP
events(Halekasetal.2007).However,allLunarProspectormeasurementswerefundamen-
tally handicapped by a lack of spacecraft potential measurements and the spectrometer’s
rathercoarseelectronenergyresolution.ARTEMIS’shighresolutionfullplasmaandelec-
tricfieldmeasurementswillenableresearcherstomeasurelunarsurfacepotentialsfromor-
bitaccuratelyandprecisely,aswellasdeterminetheirresponsetoexternalinfluences.The
excellent energy resolution of the electron spectrometers on ARTEMIS will also enable a
betterunderstandingoftheimportanceoflunarphotoelectronsandsecondaryelectrons,and
mayallowustodeterminewhetherthepotentialintheplasmasheathvariesmonotonically,
ornon-monotonicallyassuggestedbyrecentsimulations(PoppeandHoranyi2010).
Near-surfaceelectricfieldslikethosedescribedabovecanloftmicron-sizedchargeddust
grains(Nitteretal.1998).Thehorizon‘glow’observedbycamerasonthelandedSurveyors
ARTEMISScienceObjectives
5, 6 and 7 at lunar twilight was attributed to sunlight scattering by dust electrostatically-
levitated just above the lunar surface by electric fields at the terminators (Criswell 1973;
Rennilson and Criswell 1974). Astronauts in Apollo’s command module also witnessed
a twilight horizon glow extending 10’s of kilometers above the lunar surface which was
suggestedtoresultfromsurfaceejectionofsubmicrongrainstohighaltitudes(McCoyand
Criswell1974;McCoy1976).LunarProspectorhasconfirmedthepresenceofstrongand
dynamicnear-surfaceelectricfieldsthatarecapableofloftingsmallchargedgrainstohigh
altitudes (10’s of km) in a dust ‘fountain’ effect (Stubbs et al. 2006). These near surface
electric fields may be strongest at the terminator—the starting location of the lunar wake
(Farrelletal.2007).
The nature of the wake plasma discontinuity at the terminator remains unknown. Ide-
ally,thereshouldbea‘perfect’plasmadiscontinuityattheterminatordividingflowingsolar
wind plasmafrom aperfect plasmavoid. Dueto their higher thermal velocities, electrons
shouldmigrateintotheplasmavoidfurtherthanions,therebycreatingaverylargeambipo-
lar E-field just above the lunar surface. This initial plasma expansion E-field could drive
dusttransportattheterminator(Farrelletal.2007).However,plasmadiscontinuitiestendto
dissipatebyradiatingplasmawavesatgroupvelocitiesthatexceedthespeedofthediscon-
tinuity.TheseoscillatorywavesandtheirrelatedULF/VLFelectricfieldsmaydiminishthe
ambipolareffectandassociateddustlofting.
ARTEMISwillexaminethewakeflank/discontinuityasafunctionofdistancefromthe
terminator,providingenoughpassesundervaryingsolarwindandIMFdirectionstoprovide
auniqueevaluation oftheeffectiveness oftheambipolarforces insurfacedustlofting.In
essence,ARTEMISwillcrossthediscontinuityatvariousstagesofitsevolution,providing
detailedobservationsoftheplasmaexpansionanddiscontinuitydissipationprocesses.For
similarsolarwindandIMFconfigurations,thewakeflankcanbeconsideredtimestationary,
andpassagesthroughtheflankatvariousaltitudesareequivalenttoexaminingthedisconti-
nuitydissipationprocessasafunctionoftime.Theinitialplasmaexpansionprocessisnot
onlyfundamentallynewscience,butalsocrucialtounderstandingsurfacedustloftingfrom
E-fieldsattheterminator.
2.3 InternalStructure
Electromagneticsoundingencompassesawidevarietyofinductivemethodsusedtosense
the interior of the Earth, the Moon and other planetary bodies (Schuster and Lamb 1889;
Grant and West 1965; Sonett 1982; Khurana et al. 1998; Simpson and Bahr 2005). Suit-
ablesignalsforplanetary-scalesoundingsarisefromthesolarwind,magnetospheres,iono-
spheres,andlightning,dependingontheplanetaryenvironment(Grimm2009).Thesetime-
varying electromagnetic source fields induce eddy currents in planetary interiors, whose
secondaryelectromagneticfieldsaredetectedatthesurface.Thesecondaryfieldsshieldthe
interioraccordingtotheskin-deptheffect,whichisexploitedbyelectromagneticsounding
by using impedance measurements over a range of frequency to reconstruct conductivity
overarangeofdepth.
Electromagnetic induction studies performed during Apollo gave an indication of the
deepelectrical conductivityprofileandlimitedtheradiusofanystronglyconductingcore
tolessthanabout20%ofthelunarradius(e.g,Sonettetal.1972;Dyaletal.1974;Hood
et al. 1982; Hood and Sonett 1982). Laboratory conductivity-temperature measurements
wereusedtotranslatethelunarconductivityprofiletoaselenothermthatisroughlyequiv-
alentwiththebackground,globalheatflowderivedbyWieczoreketal.(2006).Theouter
500kmwaspoorlyconstrainedduetothelimitedmagnetometerbandwidth(higherfrequen-
ciesareneededtoresolveshallowerdepths).
D.G.Sibecketal.
Fig.2 TheARTEMIS
spacecraftprobethelunar
interiorindividuallybythe
magnetotelluricmethodand
jointlyusingthemagnetic
transferfunctionmethod.The
investigationwillprovide
constraintsoncoresize,onthe
compositionofthelowermantle,
andonthenatureof
upper-mantleheterogeneity,
particularlytheanomalous
ProcellarumKREEPTerrane
Inspiteoftheseadvances,significantquestionsremain.TheInternationalLunarNetwork
ScienceDefinitionTeamReport(1998)identifiedthreenewinvestigationsforelectromag-
neticsounding:(1)determinethestructureoftheouter500km(uppermantleandcrust)that
maycontainkeyinformationonearlylunarevolution,specifically,theextentofthemagma
oceanandtheoriginofmajorprovinces;(2)providetighterboundsontheconductivityof
thelowermantle(500–1400kmdepth),thatmayconstraintraceamountsofH O(Saaletal.
2
2008;GrimmandMcSween2009);and(3)determinewhethertheMoonhasacore,andis
itmetalormoltensilicate.
Thefundamentalquantitythatmustbemeasuredinanyinductionexperimentisthefre-
quencydependentelectromagneticimpedanceZ.Themostimportantconstraintisthattwo
knownquantitiesarealwaysnecessary(Grimm2009),e.g.,Ohm’sLawZ=V/I.Apollo-
eralunarstudiesusedthedistantly-orbitingExplorer35todeterminethesourcemagnetic
field,andcomparedthistoApollo12surfacemeasurementsthatrepresentthesumofsource
and induced fields. This is the Transfer-Function technique (see Fig. 2). Alternatively the
Magnetotelluric Method can provide a complete sounding from individual platforms us-
ingorthogonalhorizontalcomponentsoftheelectricandmagneticfields(e.g.,Vozoff1991;
SimpsonandBahr2005;GrimmandDelory2008).Theplane-waveimpedancesdetermined
bytheMagnetotelluricMethodareextendabletosphericalgeometrywhenskindepthsbe-
come comparable to the planetary radius (Weidelt 1972; see also Schubert and Schwartz
1972).
With these considerations, we have defined an electromagnetic sounding investigation
for ARTEMIS. Transfer Function soundings can be performed when the Moon is in the
Earth’s magnetotail, both probes are in daylight, one probe is near periapsis (∼100 km),
andtheotheratleastseverallunarradiiaway(includinguptoapoapsis∼19,000km).This
ensures that the distant probe measures only the primary field while the periapsis probe
measuresthesumoftheprimaryandsecondaryfields.Onlythefluxgatemagnetometersare
necessary. This geometry contrasts with the free-solar-wind environment used for Apollo
TransferFunctionsoundings.Here,thedaysideconfiningplasmalayerwouldhaverequired
thesecondmeasurement(primary+secondaryfield)tobemadeatornearthesurface,well
belowtheARTEMISperiapsisaltitude.Measurementswithbothspacecraftinthewakeare
also possible, although it may be more difficult to separate the relevant signals in the hot
tenuousplasmainthisshadowedregion,wherespacecraftchargingmayreachextremes.
Theprobeperiapsesof100kmrestricttheminimuminvestigationdepthtoafewhun-
dredkmorgreater.Theymayglimpsesomeuppermantlestructurebutwillcertainlyprobe
the lower mantle, especially given that measurements may be made during some tens of
minutesaroundperiapsis.Closestudyoffield-linedeflectionovermanyperiapsespassages
maybeusedtodetectthecore(Russelletal.1982;Hoodetal.1999).
ARTEMISScienceObjectives
ARTEMISwillconductthefirstextraterrestrialtestsoftheMagnetotelluricMethod,us-
ingbothelectric(EFI)andmagnetic(FGM)recordsduringperiapsispassages.Whilethese
measurementsmaybepossibleunderavarietyofconditionsandgeometries,theyarelikely
bestconductedinsunlightwhileinthemagnetotail.Asthesearethesameconditionsfavor-
ingTransferFunctionsoundings,resultsshouldbeidentical:discrepancieswillhighlightthe
kindsofdataprocessingand/orauxiliaryinformationneededtoextracttheelectromagnetic
partoftheelectric-field.
With its 1.5-yr mission at low periapses, ARTEMIS will have both data redundancy
(signal integration) and geographic coverage around the equator. The ARTEMIS mission
willadvanceourunderstandingofthedeeplunarinteriorandprovideabaselinefornext-
generationlunarandplanetaryelectromagneticsounding.
2.4 MagneticAnomalyInteractions
Mapsforlunarcrustalmagneticfieldsremainsignificantlyundersampledandthecausesof
lunarcrustalmagnetizationremainuncertain.Localenhancementsinthecrustalmagnetic
fieldcanshieldthelunarsurfacefromspaceweatheringcausedbyionsputteringandgen-
eratemini-magnetosphereswhoseeffectscanbefeltfarfromthelunarsurface.ARTEMIS
willprovidethecombinationofinsituandremoteobservationsneededtosubstantiallyim-
proveourglobalpictureofcrustalmagnetismandthendetermineitseffectsonitsimmediate
environment.
2.4.1 CausesofCrustalAnomalies
Patternsofcrustalmagnetizationpreservearecordofplanetaryevolution.Thermoremanent
magnetizationinthepresenceofadynamomagneticfieldisthedominantprocessbywhich
the terrestrial crust has been magnetized, as is likely the case for Mars (Connerney et al.
1999). Alternating ‘stripes’ of magnetized seafloor crust at mid-ocean ridges were crucial
in confirming geodynamo polarity reversals and the plate tectonics hypothesis (Vine and
Matthews1963;VineandWilson1965).Theorigin(s)oflunarmagnetismarefarlesscer-
tain: lunar sample measurements indicate the possible existence of a lunar dynamo from
3.6–3.9billionyearsago(Cisowskietal.1983)withanorderofmagnitudedecreasebefore
andafterthatperiod.
Therearemanyhypothesesforthesourceoftheambientfieldandthemagnetizationpro-
cess.Steadymagnetizingfieldsofbothexternal(solarorterrestrial)andinternal(lunardy-
namo)originhavebeenproposed,ashavetransientfieldsgeneratedbyimpacts(forreviews,
see Fuller 1974, and Wieczorek et al. 2006). Thermoremanent magnetization is likely the
dominantprocessforigneouslunarsamples;however,shockremanentmagnetizationmay
besignificantinlightlymetamorphosedbreccias,whichcarrythestrongestandmoststable
remanentmagnetizationofalllunarsamplesbecausetheycontainmoremetallicirongrains
(likelyproducedbymeteoriticimpacts)(Fulleretal.1974;FullerandCisowski1987).Mare
basalts,ontheotherhand,containlessnanophaseironandgenerallyhaveweakerremanent
magnetization(Colemanetal.1972).
With samples from only a handful of lunar landing sites, orbital magnetic measure-
ments have been relied upon to study the global pattern of crustal magnetism. Two types
ofdatahavebeenused:magnetometerandelectronreflectionmeasurements.Magnetome-
ter data provide vector information but are limited by spacecraft altitude and the fact that
lunar crustal magnetic fields at satellite altitudes are typically very weak: ∼2 and ∼3 or-
ders of magnitude weaker than the terrestrial and Martian cases respectively (Connerney
D.G.Sibecketal.
etal.2004).Electronreflectionmeasurementsinvokemagneticmirroringandusemagne-
tometerobservationstogetherwithelectronenergyandangulardistributionsmeasuredfrom
orbit to estimate the magnetic field magnitude at the lunar surface (Anderson et al. 1975;
Linetal.1976).Electronreflectiondatahavethevirtueofbeingparticularlysensitivetothe
weakestcrustalfields(Halekas2003;Mitchelletal.2008).
Electron reflection and magnetometer measurements from the Apollo 15 and 16 sub-
satellites(Hoodetal.1981;Linetal.1988)andtheLunarProspectorspacecraft(Hoodetal.
2001;Halekasetal.2001;Mitchelletal.2008)haveenabledglobalmappingoflunarcrustal
fields.However,themapisseverelyundersampled,withsignificantnoiseandpixelization
clearlyvisible(seeFig.3),accountingformuchuncertaintyingeophysicalinterpretation.
ARTEMISESA,withbetterenergyresolutionthanLunarProspectororApollo,will,along
with ARTEMIS magnetometer measurements, enable more accurate correction for theef-
fectsofelectrostaticpotentialsbetweenthelunarsurfaceandthespacecraft(Halekasetal.
2002b;Mitchelletal.2008)andthereforeprovidemoreaccuratecrustalfieldestimates.The
twoARTEMISprobeswillmakemanyhundredsoflowaltitude(<400km)electronreflec-
tion measurements within 10◦ (20◦ if optimized) of the equator on every periselene pass,
resulting in better spatial coverage and higher fidelity crustal field maps in these regions
thancurrentlyexist.
Existingmapsshowweakerfieldsoverthemarebasaltscomparedwiththehighlands(in
agreementwiththeaforementionedsamplestudies)andrevealthatthelargestareaofstrong
crustalfieldslieinregionsdiametricallyopposedtotheImbrium,Serenitatis,Crisiumand
Orientale impact basins (see Fig. 3). This led to the hypothesis that crustal magnetization
is associated with basin-forming impacts. According to this hypothesis, the hypervelocity
(>10km/s)impactsthatformsuchlargebasinsproduceaplasmacloudthatexpandsaround
theMoon,compressingandamplifyinganypre-existingambientmagneticfieldatthean-
tipodalpoint(HoodandHuang1991),wherethefocusingofseismicenergyandimpactof
basinejectamayresultinsubstantialshockremanentmagnetization(HoodandArtemieva
2008). A key element of this hypothesis is that it does not require a global dynamo field,
althoughrecentlyWieczorekandWeiss(2010)haveshownthatthesimplestexplanationfor
some of the strongest anomalies like Reiner-Gamma involves a steady, dynamo magnetic
field.BettermapsenabledbyARTEMISintheequatorialregionswillhelptoelucidatethe
mechanismofantipodalmagnetizationbyallowingustobetterconstrainthecharacteristics
oftheremanentcrustalremanentmagnetization.
Fig.3 Impactbasinrims(whitecircles,forImbriumtheinteriorringisalsoshown)andtheirantipodes
(blackcircles),superimposedonamapoftotalsurfacemagneticfieldstrengthaveragedover5×5◦ bins
(Mitchelletal.2008)