A variety of radars designed to explore the hidden structures and properties of the Solar System's planets and bodies

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structures and properties of the Solar System’s planets

and bodies

Valérie Ciarletti

To cite this version:

Valérie Ciarletti. A variety of radars designed to explore the hidden structures and properties of

the Solar System’s planets and bodies. Comptes Rendus Physique, Centre Mersenne, 2016, 17 (9),

pp.966-975. �10.1016/j.crhy.2016.07.022�. �insu-01368065�

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JID:COMREN AID:3334 /SSU [m3G; v1.186; Prn:15/09/2016; 15:29] P.1 (1-10) C. R. Physique•••(••••)•••–•••

Contents lists available atScienceDirect

Comptes

Rendus

Physique

www.sciencedirect.com

Probing matter with electromagnetic waves / Sonder la matière par les ondes électromagnétiques

A

variety

of

radars

designed

to

explore

the

hidden

structures

and

properties

of

the

Solar

System’s

planets

and

bodies

Une

diversité

de

radars

conçus

pour

révéler

et

caractériser

les

structures

cachées

des

petits

corps

et

planètes

du

système

solaire

Valérie Ciarletti

LATMOS/IPSL,UVSQUniversitéParis-Saclay,UPMCUniversitéParis-6,CNRS,78280Guyancourt,France

a

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t

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c

l

e

i

n

f

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Articlehistory: Availableonlinexxxx Keywords: Radar Spacemissions

Electromagneticwavepropagation

Mots-clés :

Radar

Missionsspatiales

Propagationdesondesélectromagnetiques

Sincethe veryﬁrstobservationsoftheMoonfromtheEarthwithradarin1946, radars aremoreandmorefrequentlyselectedtobepartofthepayloadofexplorationmissionsin theSolarSystem.Theyare,infact,abletocollectinformationonthesurfacestructureof bodiesorplanetshiddenbyopaqueatmospheres,toprobetheplanetsubsurfaceoreven torevealtheinternalstructureofasmallbodycometnucleus.

AbriefreviewofradarsdesignedfortheSolarSystemplanetsandbodies’explorationis presentedinthepaper.Thisreviewdoesnotaimatbeingexhaustivebutwillfocusonthe majorresultsobtained.Thevarietyofradarsthathavebeenorarecurrentlydesignedin termsoffrequencyoroperationalmodeswillbehighlighted.

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Depuis lespremières observationsradar de laLune depuisla Terreen 1946, lesradars fontdeplusenplusfréquemmentpartiedelachargeutiledesmissionsd’explorationdu systèmesolaire.Ils sont,eneffet,capablesderecueillirdesinformationsàlafoissurla structuresuperﬁcielled’uncorpsoud’uneplanèteàtraversuneatmosphèreoptiquement opaque,desonderlesous-sold’uneplanète,ouencorederévélerlastructureinterned’un petitcorps.

Une revue non exhaustive des radars scientiﬁques développés pour l’exploration des planètes et autres corps du système solaire est présentée dans cet article. Quelques résultats majeurssont présentés. L’accentest mis sur lavariété des radarsqui ontété etsontactuellementconçusentermedefréquenceoudemodeopératoireenfonctiondes contraintesdelamissionetdesobjectifsvisés.

E-mailaddress:Valerie.Ciarletti@latmos.ipsl.fr.

http://dx.doi.org/10.1016/j.crhy.2016.07.022

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1. Introduction

Radarshaveauniquecapacitytoprovideremoteinformationabouttargetsthatcannotbereachedbyotherkindsof in-struments.Inplanetaryscience,theyhavebeensuccessfullyusedtorevealthetopographyofthesurfaceofasolarsystem’s bodies hiddenbythick atmospheres(suchasimpactcraters andvolcanoesonVenus,lakesandpossiblecryovolcanoeson Titan), thestructuresandlayers buriedunderthesurface ofplanetsgivingthusaccesstotheirgeologicalhistory(internal layering ofthe polar caps of Mars, thickness oflava ﬂows on the Moon) or even the internal structure of smallbodies (Churyumov–Gerasimenko).

The basicprincipleofradars issimple:it isbasedonthetransmission andeventually receptionofan electromagnetic wave that has propagated through andinteracted withthe sounded environment. Aftermore orless complicatedsignal processing, theradardetermines foreach detectedechoapropagationdelay andits associatedamplitude.Both quantities arethenprocessedtoretrieveinformationonthegeometricalanddielectricpropertiesofthetargetandofthepropagation environment betweenthetargetandtheantennas.Theinstrument’sﬁnalradiometric performancesdependontheradar’s parameters,onthegeometricalconﬁguration,onthesoundedenvironmentpropertiesandontheoptimalusemadeonthe redundancywithinthemeasurements.

Section 2ofthispaperpresentsthemainkindsofradarsthathavebeendesignedtoexplore theSolarSystem’sbodies andplanets.Theemphasisisputontheplatformsthoseradars canbeaccommodated on(i.e.spacecraftinorbitorﬂyby, rovermobileatthesurface,andevenstationarylander onthesurface),andonthechoiceofacenterfrequencyaccording tothescientiﬁcandtechnicalobjectivestoreach.

Anoverviewofthemainradarexperimentsdesignedforplanetaryexplorationandsomeofthemostsigniﬁcantresults theyobtainedarepresentedinthesecondsection.

2. Radarsforplanetaryexplorationandscience

Radarswere originallydevelopedformilitarypurposes,whichhasrapidlycontributedtothedevelopmentofanumber ofcivilapplicationsforsafetravelandremotesensingoftheEarthenvironment.Fordecadesnow,thefamousradar equa-tion hasbeenthe basisto designradars [1],especiallyforplanetary exploration wheredistances tothetargetsare often extremelylargeandwhereverylittleinformationisavailableonthetarget’scharacteristics.

2.1. Choosingtheplatform

The firsthistoricobservations ofthe MoonweremadefromtheEarthin1946at138MHzby theCorps’EvansSignal Laboratory,NewJersey[2,3].Aftersome unsuccessfulattempts,in1961,VenusbecamethefirstplanetoftheSolarSystem tobeexplored byaradarfromtheEarth:the34-mdishoftheGoldstoneObservatoryinCalifornia[4,5] wasusedatfirst, followedlaterbythe300-mdiameterantennaatArecibo[6].

Sincethe1980s,thevastmajorityofradars usedinplanetary sciencehavebeenoperatedonmissionsthatofferedthe opportunity tomapawholeplanetfromorbit(the MarsExpress[7]andMarsReconnaissanceOrbiter [8]missions around Mars, theMagellan missionto Mercury [9]) or during a number oflimited ﬂybys (the Cassini missionto Saturn andits moons[10]).Inaddition,radars dedicatedtoamoreaccuratebutlocalsubsurfacecharacterizationscanbeaccommodated onboardvehiclesmobileatthesurfaceofplanets.TherecentChang’e-3mission[11]landedontheMoonaroverequipped withaGroundPenetratingRadar(GPR)inordertosoundthelunarregolithfromthesurface.ThenextESAandNASArover missions to Mars, ExoMars [12] andMars2020 will alsooperateGPRs to soundtheMartian subsurfacealong the rover’s paths.

Less conventionalexperimentshavebeenspeciﬁcally designedtoretrieveinformationonthe subsurfacestructureand itsdielectricpropertiesinveryspeciﬁccontexts.Amongthose,wecanmentiontwobi-staticradars:

•

the CONSERT experiment of the Rosetta mission, designed to perform the tomography of comet 67P/Churyumov– Gerasimenko nucleus, is composed oftwo separated units,the ﬁrstone beinglocated on the surfaceofthe nucleus whiletheotheroneison-boardtheorbiter[13];

•

the EISS(ElectromagneticInvestigationofthe Sub-Surface)bistaticradar wasselected onthenow canceledHumbold payloadoftheExoMarsmission.Theinstrumentisabletotakeadvantageofthesimultaneouspresenceofastationary lander andamobilerovertoperformdeepsoundingoftheMartiansubsurfacearoundthelandingsite.Inmonostatic mode, the35-m-longelectrical antennas locatedon thestationaryplatformof themissionare usedfor transmission andreception. Whilein bistaticmode, a 10-cm-long magneticreceiveraccommodated on themission’s roveris able to measure three orthogonal components of the magnetic field too. These measurements allow one to retrieve the direction of arrival of each reflected wave andthus to retrieve the 3D location ofthe buried reflectors for a better understandingofthegeologicalcontextofthesite[14–16].

Finally,onanumberofspacemissions,opportunitybi-staticexperiments(oftenreferredtoasradioscienceexperiments) takeadvantageofexistingcommunicationchannelsofthespacecraftwithEarthtoperformlimitedsoundingsoftheplanet’s

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JID:COMREN AID:3334 /SSU [m3G; v1.186; Prn:15/09/2016; 15:29] P.3 (1-10)

V. Ciarletti / C. R. Physique•••(••••)•••–••• 3

Fig. 1. Centerfrequencyofthemainradarsthathavebeendesignedforplanetaryexploration.Foreachinstrument,thetargetisindicatedbythecolor code.

subsurface atgrazing incidence angles (forexample, the Mars Radio Science experiment [17] on-board the MarsExpress spacecraft,theClementinebi-staticradarexperimentattheMoon[18],andtheVikingbistaticradaronMars[19]).

2.2. Designingtherightradar

Thespecificationsforeachinstrumentofamissionpayloadare basedonthescientificobjectivesofthemission.Fora radar,thecharacteristicsofthetarget (range,geoelectricalandgeometricalproperties)aswellasthefinalproduct(image, topography,estimatedroughnessanddielectric properties,subsurfacecharacterization) andresolutionaimed atwillguide theradardesign.

2.2.1. Avarietyofoperatingmodes

Mostofthetime,giventhelimitedresources(ofmass,volumeandpower) availableinspacemissions,radars designed anddevelopedfor planetarymissions are adaptableenough sothat they can be operatedindifferentmodesmaking the bestwayofthehardware.Theyveryoftencanbeusedasaltimeters,synthetic-apertureradars,subsurfacesounders, scat-terometersandradiometers,andoftenalsousedtocharacterizetheatmosphereandionospherewhenrelevant.

Aradaraltimeterisusuallyanadir-looking radar.Itmeasures thepowerreceivedafteran interactionwiththe surface asafunction ofthepropagationdelayandthusprovides anestimateofthedistancebetweentheradarandthereﬂecting surface.Itisusedtoinferthetopographyofthesurface.Theradaraltimeterechoproﬁleiscontrolledbytherangetothe surface andthe surfaceback-scatteringproperties,whichdepend onthesurface roughnessatscales largerthanthe wave lengthandonthedielectricpropertiesoftheshallowsubsurface.Highfrequenciesare usuallychosenforradaraltimeters, soastolimitpenetrationintothesubsurface.

On the contrary, soundings radars are low-frequency HF Ground-Penetrating Radars. They are used to perform deep soundings(upto severalhundreds ofmeters)ofplanetary sub-surfacestructures.Theﬁrst radarsounderﬂownwas ALSE (ApolloLunar SounderExperiment)[20] onboardApollo17in1972. Twoother soundershaveexploredtheMartian sub-surface(§3.1.1):MARSIS(MarsAdvancedRadarforSubSurfaceandIonosphereSounding)[21]onboardtheEuropeanSpace Agency’sMarsExpressprobe,andSHARAD(marsSHAllowRADarsounder)[22]ontheNASAMarsReconnaissanceOrbiter (MRO).AradarsounderhasalsobeenusedontheJapaneseMoonprobeSELENE[23],launchedin2007(§3.1.2).

Asynthetic-aperture radar (SAR)is used toobtain imagesof an area atwavelengthsthat are signiﬁcantly lower than thoseusedbyopticalcameras.Itusesthemotionoftheantennaoveranarea toimprovethespatialresolution;therefore, SARsaretypicallymountedonmovingspacecraft,buttheymightalsobeusedonmoving/rotatingtargets.Syntheticaperture radarsarethemostsuitableinstrumentstoprovideimagesofplanetarysurfaceshiddenbyopaqueatmosphereslikeVenus and Titan. The two Soviet spacecrafts Venera 15 and Venera 16[24] provided images of the planet in 1983 using SAR andRadar altimeters. The SAR ofthe Magellan mission[25] later in1990 achievedan almost complete coverage ofthe surface ofVenus (§2.1.1).The SARof theCassini[26] spacecraft,in orbitaroundSaturn, hasprovided impressive images ofTitansurfaceduring ﬂybys(§2.1.2).Whenpossible,enhancedprocessingofSARdataallowustoprovidetheSAR-based topographyofasurfacewithamuchbetterresolutionthantheoneobtainedbysimpleradaraltimeters.

2.2.2. Abroadfrequencyrange

Thechoiceofthefrequencyisoftheuppermostimportancesinceitwilldrivethepenetrationdepth.Moreover,because thebandwidthislimitedbythecentralfrequency,thischoicewillalsosettherangeresolution(i.e.itsabilitytodistinguish between two targets atdifferent distances of the instrument). In the context of planetary missions, radars operating at

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Fig. 2. ImagesofVenusCredit:NASA/JPL/USGS.Left:UltravioletimagebythePioneerVenusOrbiterspacecraft.Thisimagerevealsstructuresoftheatmosphere thatarenotdetectedinthevisiblelightimagebutthesurfaceremainshidden[27].Center:TopographyofsurfacebasedontheMagellanradar’sdatawith additionalinformationfromVeneraandPioneerVenusmissionsandEarth-basedradarmeasurements.Thelowestelevationsareinblue(PIA00159).Right: imageofa1.5-km-highvolcanowitharesolutionof∼675 mobtainedbytheMagellanSAR[33].

extremelydifferentfrequencies havebeenused(seeFig. 1).ThelargestwavelengthhasbeenusedbytheMARSISsounder onboard the MarsExpress ESA mission: 230 m in the vacuumfor a radar designed forkilometric penetration depths to searchforpotentialwaterreservoirsintheMartiansubsurface.Theshortestis2.18cmfortheradaroftheCassini mission,

whoseprimaryobjectiveistocharacterizeTitan’ssurfacethroughitsthickatmosphere.

Asmentionedpreviously,thechoiceofthecenterfrequencyhasanimpactontherangeresolutionsincetheresolution improves whenthebandwidthincreases.Asaconsequence,radarsdesignedtoperformdeepsoundings ofthe subsurface must operateatlow centerfrequencies andconsequently havea poorverticalresolution.Comparison oftheMARSISand ShaRadsoundersoperatedonMarsisagoodillustrationofthematter(Fig. 3).Anotherexampleofthistrade-offisprovided bytheWISDOMradarofthefutureExoMarsmission’srover.Ithasbeendesignedtoprovidehighresolutionimagesofthe shallowMartiansub-surfaceinordertosupportdrillingoperations;asaconsequence,itwilloperateonanespeciallylarge frequencybandwidthfrom0.5to3GHz,whichwilllimititspenetrationtoafewmeters.

Inadditiontotheseconsiderations,inthespeciﬁccontextofspacemissions,wherethesizeandmassofthepayload’s instrumentsarealwaysstronglylimited,thechoiceofthefrequencyisalsoconstrainedbythesizeoftheantennausedto transmitandreceivetheelectromagneticwaves.

3. AvarietyoftargetsintheSolarSystem

The radar investigations of the SolarSystemstarted logicallywith Earth-based observationsand aimed atthe closest target to Earth, namelythe Moon.Next came VenusandMercury, followedby Marsandthe moons ofSaturn. Soonthe moons ofJupiterwillalsobeexplored byradars.Thissectionillustrates theimpressiveknowledgethathasbeenacquired ontheSolarSystemthankstoradarinstruments.

3.1. Peeringthroughadenseandopaqueatmosphere

Radars havebeensuccessfullyusedtoreveal thesurfacetopography andgeologyofplanetary bodiescovered bythick andopticallyopaqueatmospheres.VenusandTitan’sexplorationbyradarsprovidethebestillustrationsofthiscapacity.

3.1.1. Venus

ThesurfaceofVenusishiddenbythickcloudsofsulfuricacidthatpreventopticalremotesensingtechnicstoaccessthe surface[27](seeFig. 2/right).Venerawasaseriesof16ﬂyby,orbital,andlandedmissionstoVenusconductedbytheSoviet Union from1961to1983. Mostofthesemissionsfailedbecauseofthe harshVenusianenvironment.But sinceVenera7, successful landingsofmodules abletosurvive for30to120 minatthesurface provided theﬁrst opticalpictures ofthe Venusianground.

TheNASAPioneerVenusOrbiterwas launchedin1978[28].Oneofthemainexperimentsonitspayloadwastheradar (ORAD)operatingat1.757GHz.Thisradaraltimeterwasusedtogetinformationonthesurfacetopographywhichhasbeen reconstructed withaverticalaccuracybetterthan200manda150-km resolution.The surfaceelectricalconductivityand roughnessat1-mscalehavebeenestimatedtoo.ThePioneerVenusOrbiter wastheﬁrstspacecrafttouseradartoobtain aglobalimageoftheVenussurface[29].

In 1983, thetwinVenera15 and16spacecrafts carrieda radaraltimeteranda SAR operatingat3.75GHz that were usedtomapthetopographyofthesurfacewitha1-kmresolution(from30NlatitudetotheNorthPole)[30].Theachieved verticalaccuracywasaround230m.

Finally, the Magellan spacecraft [31] was launched by NASA in1989. The radar on-board[32] collected SAR images, combinedwithaltimetryandradiometrymeasurementsatafrequencyof2.3GHz,overthemajorityofVenus’surface.The resolutionoftheSAR imageswasbetterthan150movermorethanhalftheplanet.Theverticalaccuracyofthealtimeter was betterthan50m.Combiningalltheinformationavailable,amapoftheelevationwasbuiltforthewholeplanet(see Fig. 2,center). The SAR ofthe Magellanspacecraft alsoprovided very detailedimages [33] (see Fig. 2, right)of volcanic

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Fig. 3. ImagesofTitan:Left:.ColorcompositeimageofTitantakenduringtheCassini spacecraft’sﬂybyin2005[PIA06230].Athickorangehazepreventsany viewofthesurface.Right:ColorizedmosaicofTitan’snorthernlandoflakesandseasobtainedbythemultimoderadar[34].Thedryareasaredisplayed inbrown,andtheareasinterpretedasliquidhydrocarbonbodiesinblue(PIA17655).

andtectonicstructuresthatrevealedthatVenus’geologyisdominatedbyvolcanism.Theimpactcratersthatweredetected tooappearedtobeevenlyspreadovertheVenusiansurface,whichimpliesthattheplanet’sentiresurfaceisthesameage. Cratercountingsuggestsanageof1billionyears.

3.1.2. Titan

TheCassini NASAmissionwas launchedin2005toexplore Saturn,itsringsandsatellites.Aspecialattentionwaspaid toTitan,whichistheonlymoonintheSolarSystemwithathickatmosphere(10timesthickerthantheEarth’sone)that preventsobservationofitssurfaceatopticalwavelengths(seeFig. 3,left).TheHuygensESAprobesuccessfullylandedonits surfacewhilethemainspacecraftremainedinorbitaroundSaturn.Impressivesetsofradardatahavebeencollectedduring morethanonehundredﬂybysofTitan.Theradaron-boardtheorbiter[25]isapowerfulmultimodesystemthatcanoperate asaSAR, radaraltimeter,scatterometerandradiometer.Its principalobjectiveisto characterizethesurface ofTitanona regionalscale.The obtainedSARimages,whoseresolutionis

>

350 m, haveshowntheexistence,mostly inthenorthern hemisphere, ofﬂatandsmooth areas,whichhavebeen interpretedaslakes andseasofethaneandmethane(see Fig. 3, right)[34].OverlappingSARimageshavebeenusedlocallytobuildSARdigitalelevationmodelsbyradarstereogrammetry [35].Theachievedresolutionisofseveralkilometershorizontallyandaround100mvertically,whichissigniﬁcantlybetter thanthealtimeter’scapacity.

The radarobservations also revealedthe presenceof numerousimpact craters, ofdunes whosesize andpatternvary withaltitudeandlatitude[36],ofriverchannels[37],ofmountains[38],ofsedimentarybasins[39],andkarsticlandscapes [40].

3.2. Soundingbeneaththesurface

UnlikeTitan andVenus’ssurface, the surface ofMars, Mercuryandthe Moonare readilyaccessible tooptical remote sensingfromorbit,butradaroperatedatlow-enoughfrequencyhavetheuniquecapacitytosoundthroughthesurfaceand accesstheburiedstructures.

3.2.1. ThesubsurfaceofMars

So far Marshas been the objectof a number ofmissions that are more andmore interested in discovering what is beneathitssurface.TherarecloudspresentintheMartianatmospheredonotpreventopticalremotesensingofthesurface fromorbitandlargedatasets obtainedbyspectro-imagers andcontext andhigh-resolutionstereoscopic camerasarenow available.Till now, twoorbital radarshavebeendesignedandusedtoretrieveinformationabouttheMartiansubsurface. The MARSISorbital radar sounder, onboardESA’s MarsExpress spacecraftlaunched in 2003, was designedto investigate theMartianionosphereandthegeologicalandhydrologicalstructureofthesubsurface,withaparticularemphasisonthe detectionofdeepbodiesofsolidorliquidwater[41].MARSISoperatesatfrequenciesof

∼

2–5 MHz;itshorizontalresolution isabout10kmwhiletherangeaccuracyis150minvacuum[42].Theparticularlylowfrequenciesusedbytheinstrument give it a theoretical ability to detect the presence of liquid water, under ideal sounding conditions, at depths of up to

∼

3–5 km beneaththeMartiansurface[43].Inpractice,MARSIShasachievedthislevelofsoundingperformanceonlyina numberofspeciﬁcenvironments,whichincludetheice-rich(and,thus,lowdielectricloss)NorthandSouthPolarCaps(see Fig. 4[44]),whereitprovidedestimatesofthethicknessoficedeposits,aswellasseveralothersitesatlowerlatitudes.Of thelatter,themostnotableistheMedusaeFossaeFormation,whoseradarpropagationcharacteristicsareconsistent with acomposition rangingfromadry,highly porouspyroclastic depositatone extremetoanice-rich sedimentarydepositat theother,potentiallyformed bytheredistribution ofpolarvolatilesattimesofhighobliquity [45].Hints ofseveralother moderatelydeepreﬂectorshavealsobeenfoundinthevicinityofMa’adimVallis, wherethere isevidencethatthey may originatefromlithologicalinterfaces[46].

MARSISwas closelyfollowedby theﬂight ofanotherradar soundercalledtheSHallow RADar (SHARAD)ontheMars Reconnaissance Orbiter (MRO) [47]. SHARAD was designed to complementthe capabilities andperformance of MARSIS

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Fig. 4. Top:MARSISradargramoftheSouthPolearea.Bottom:Thewhitelineontheimageshowsthecorrespondinggroundtrackshowsonatopographic mapoftheareabasedondatafromtheMOLAlaseraltimeteronboardNASA’sMarsGlobalSurveyor.

Fig. 5. Top:SHARADradargramoftheNorthPolarLayeredDeposits(NPLD)overtheBasalUnit(BU).Bottom:MOLAlasertopographyalongtheground track)[47].

by operating athigher frequencies around 20 MHz to better resolve variations in near-surface composition, stratigraphy andstructure –particularlywithregardto resolvingthe internallayers ofthepolarcaps (seeFig. 5 [48]). Thehorizontal resolution of the instrument is between0.3 and 3km and the vertical resolution is about15 m (10 times better than MARSIS). ComparisonofFigs. 4 and 5givesaclearunderstanding ofthebetterresolution thatcan beobtainedathigher frequenciesbecauseoftheassociatedincreaseinthefrequencybandwidth.

Outside thepolarcaps, ShaRad soundings performed inthe eastern Hellasregion revealed electromagneticproperties thatareconsistentwithmassivewaterice,supportingthehypothesisthatgroundiceispresentinthesubsurfaceatlower latitudes[49].

The futureESA/RoscomosExoMarsroverMission[12] toMars, whichiscurrentlyplannedfor2018, willhaveonboard a groundpenetrating radar designed to soundthe very shallowsubsurface (to a depth of

∼

3 m) along the rover path. ThisradarcalledWISDOM[50](WaterIceSubsurfaceDepositsObservationonMars)hasbeendesignedtoprovidearange resolution better than10cm inthesubsurface andisthus operatingovera large-frequencybandwidth between0.5and 3GHz.Itwillprovideinformationaboutthegeological contextandhelptotheidentiﬁcationofpotentiallyhabitablesites. Thedrillofthemissionwillthencollect,atadepth

<

2 m,samplesthatwillbeanalyzedbyasuiteofinstrumentsonboard the rover. WISDOM is a step frequency radar, thus it transmits a number of continuous wave signals, each at a single frequency ratherthan asingle pulse. Acomplete setof measurementsis performedfortherange offrequencies covered by the instrument’sbandwidth.Performingan Inverse FourierTransformonthewhole setoffrequency responses allows the computation of theimpulse response, which wouldbe readilyobtained by a pulseGPR. Thisstep frequency technic allowsabetteruseofthelimitedpoweravailableontherover.Inaddition,theantennasystemhasbeendesignedtoallow polarimetric measurements, which will provide additionalinformation about the shape andorientation ofthe reflecting structures.Giventhemission’sconstrains,WISDOMwillbeaccommodatedontheroveratadistanceofabout30cmfrom thesurface,whichisunusualforaGPRwhoseantennasareusuallyincontactwiththeground.Thisspecificconfiguration is not the mostfavorable, butprovides the opportunity to make use of the surface echo to get an estimate of the top layer dielectricconstant.Aprototypeoftheinstrumentvery similartothefinalflight model(witha totalmassof1.7kg) is currentlytestedin variousenvironments. Fig. 6 illustrates theability ofthe instrumenttoresolve fine layeringthanks to its broad frequency bandwidth[51]. Aradargram obtained withonly 1 GHz offrequency bandwidth ispresented for comparison.

ThenextMars2020NASAmissionhasalsoselectedaGPRoperatingatslightlylowerfrequenciestobepartofitspayload [52].ThisGPRcalledRIMFAX(Radar Imager forMars’ SubsurfaceExploration)is currentlyunderdevelopment. RIMFAXis a gated FrequencyModulated ContinuousWave (FMCW)radar operatingover thefrequency band 150–1200MHz, which shouldallowittoprobetheMartiansubsurfacetodepths

>

10 m.

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Fig. 6. ExamplesofradargramsobtainedbyWISDOMintheDachsteinicecave[51],Austria(withthewholefrequencybandwidthontheleftandwithonly 1GHzofbandwidthontheright).Thelayerediceisvisibleontheupperrightpartoftheﬁgures.Theredlinehighlightsthetransitionwiththebedrock beneaththeice.

3.2.2. ThesubsurfaceoftheMoon

EveniftheMartianpolarcapsprovidedsofarthebestexamplesofplanetaryfeaturesexploredbyradar,theMoonwas alsosounded by radars. The NASA Explorer35 bistaticexperiment [53] was launched in1967; its purposewas to study theelectromagneticreﬂectivepropertiesofthelunar surface.Theexperimentusedthe136.10-Hzcommunicationchannel betweenthespacecraftandtheEarth,theelectromagneticwaveswerescatteredfromthelunarsurfaceandthenrecorded atStanfordonEarth.Statisticsofthelunarsurface’sslopewereobtainedintheequatorialregion.

Theﬁrstsuccessfulattempttosoundthelunarsubsurfacefromorbitwasperformedin1972during theApollo17 mis-sion(ALSEexperiment[54]).Themeasurementsperformedat5and150MHzallowedthedetectionofalayeratkilometric depthsintwosoundedlunarmariaandvalidatedthefeasibilityofsuch measurements[55].In2007,theLRS(LunarRadar Sounder) instrumenton-boardtheKaguya spacecraftoftheJAXA SELENE missioncompleted aglobalcoverage ofthe lu-nar surface andsubsurface at5 MHz [56].Subsurface features observed by both ALSEand LRSradars are interpreted as interfacesbetweenlavaﬂowsofdifferentages.

Twomini-SAR(ormini-RF)havebeendesignedandusedtoexploretheMoon[57,58].Theﬁrstinstrumentwaslaunched on the Indian Space Research Organization’s (ISRO’s)Chandrayaan-1 spacecraft,while the second one was on-board the NASA’sLunarReconnaissanceOrbiter(LRO).Theyhavemappedbothpolarregionsandanumberofdifferentgeologicunits on the lunar surface. MiniSAR instruments have been designedto map the permanently dark areas ofthe lunar polar regionstosearchforpotentialwatericedeposits.Thesearchforwatericeistheir primaryscienceobjective,butthey also characterized the surface roughnessand thicknessof the regolith. The Mini SAR transmits Right Circular Polarization at 2.5GHzandreceives bothLeftandRight,whichgivesaccesstothecircularpolarizationratio.The elevatedCPR (Circular PolarizationRatio)valuesobservedwithinpermanentlyshadowedcratersinthepolarareasareconsistentwiththepresence ofwatericeinthesecraters[59].

3.2.3. ThenucleusofcometChuryumov–Gerasimenko

Despite numerous observations of comets either fromEarth or during dedicated space missions among which ESA’s Giotto, NASA’sDeep Space, Stardust andDeep Impact [60], therehasbeen, priorto the Rosetta mission,nodirect access to the structure of cometary nuclei. The successful Rosetta mission [61] with the recent descent and landing of Philae

on the nucleusof 67P/Churyumov–Gerasimenkoat the end of 2014 has provided for the first time in-situ data of the upper mostimportance. With receivers andtransmitters onboard both Rosetta and Philae, the objectiveof the CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission) radar is the characterization of the nucleus interms of internal structure anddielectric properties.CONSERT [62] isa bi-static radar that consists oftwo separate units: The LanderCONSERTunit(LCN)locatedonPhilae andtheOrbiterCONSERTunit(OCN)locatedontheorbiter.Aradio signalat 90 MHzistransmittedbytheOCNandpropagatesthroughthecometnucleustotheLCN.TheLCNworksasatransponder: thesignal received by thelander isanalyzed inrealtime to detectthe strongestecho(which isexpectedto be thefirst one)for synchronizationpurposes,andimmediately retransmittedback to theorbiter, wherethe final datais eventually recorded.The propagationdelay andamplitude ofthereceived echoesmeasuredare readilyobtainedfromthemeasured impulseresponse.Foreachsoundingconfiguration,themeasured propagationdelaycorresponding tothefirstechoallows onetoestimatetheaveragedielectricconstantofthenucleusalongthepropagationpath,andtoconstrainthecomposition andporosity ofthe nucleus. The widthof the received echoesis an indirect measurement ofthe internal heterogeneity at a scalelarger than the experiment wave length (

∼

3 m), while the measured amplitude can be linked to attenuation

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and/or diffusioninside the nucleus. Despitethe signiﬁcant polarization mismatchbetween lander andorbiter’s antennas duetothefactthatPhilae isnotstandingonitsthreefeet,somegood-qualitydatahavebeenobtained.Theyshowthatthe waves didpropagatethroughthe smallerlobeofthenucleus. Themeasured delaysprovidedan estimateofthe averaged dielectric constantvalueofabout1.27,whichsuggestsaporositylargerthan78%[63].Theanalysisofthedataalsoshows that scatteringinsidethenucleusisverylimited[63],whichsuggeststhat thenucleusisratherhomogeneousatscales of theorderofmagnitudeofthewavelengthandthatthepermittivity atdepthisprobablylowerinsidethenucleusthanat the surface[64].CONSERTmeasurements haveevenbeenableto provideanestimate ofthePhilae locationinsideastrip of 350

×

30 m at the surface [65] but, to actually quantitatively characterizethe nucleus internal structure, an accurate knowledge of the experiment geometry (Orbiter and Lander location and attitude with respect to a 3D nucleus shape model) is needed. Whenever the actual location and position ofPhilae can be better constrained, the thorough analysis andinterpretationofthedatacollected byCONSERTthattakesintoaccountthe amplitudeofthereceivedsignalswillbe possible.

An updated version ofthis tomographicradar concept is currentlyconsidered for the futureAsteroid Impact Mission (AIM)aimingatasteroidDydimoscharacterization[66].

3.2.4. IcymoonsofJupiter

ThethreeJupitericymoons(Ganymede,EuropaandCallisto)arebelievedtohideliquid-wateroceansbeneatha several-km-thickcrustofice.Detectionandcharacterizationoftheseoceanswouldbeessentialtosupportthepotentialhabitability ofthesemoons andtomakesigniﬁcantprogressonourunderstandingofthelife.Sinceelectromagneticwavesexperience little attenuation in ice (see § 2.2.1. the deep soundings of the Martian polarcaps) and would be strongly reﬂected by large bodiesofliquidwater,radarsare particularlywellsuitedto thisobjective.Consequently,thefuturemissions aiming toJupiteranditsicymoonswillcarryonboardradarsdesignedtoperformdeepsoundingoftheicecrust.

Thefrequencyoftheelectromagneticwavesusedhastobecarefullychosen:ononeside,itshouldbeaslowaspossible tomaximizethepenetrationcapabilityandtominimizethescatteringatthesurfaceduetoroughness.Ontheotherside,the intensenaturalJovianradioemissionsatfrequenciesbelow40MHz[67]needtobetakenintoaccountwhenplanningthe radaroperationsandthesizeoftheantenna,whichiscommensuratewiththewavelength,mustbekeptwithinreasonable dimensions.

The Jupiter IcyMoon Explorer(JUICE)ESAmission [68] willbe launched in2022to studyJupiter andinvestigatethe potentialhabitabilityofitsicymoons.Amongthepayloadinstruments,theradarsoundercalledRIME(RadarforIcyMoon Exploration)[69]hasbeendesignedtostudythesubsurfacestructureofthemoonsuptodepthsabout9kminice.A center frequencyof9MHzhasbeenchosentoachievethisobjective.Thehigh-resolutionmodewitha3-MHzbandwidthwillallow theinstrumenttoachievea verticalresolutionof30mintheice.Alowerresolutionmode isalsoplannedwitha1-MHz bandwidthtoreducethedatavolume.Theuseofthe9-MHzcenterfrequencywillrestraintheradaroperationtotheside ofEuropawhichisawayfromJupiter.

Among Jupiter’s icy moons,Europa is currentlyconsidered to be the mostpromising location in the SolarSystem to searchforsignsofextantlife.ThefutureNASAEuropamission,formerlyknownasEuropaClipper,[70]shouldbelaunched inthemid-2020sandperform45ﬂybysofEuropaataltitudesvaryingfrom2700to25kilometersabovethesurface.The Radar for EuropaAssessment andSounding: Ocean to Near-surface (REASON)[71] instrument has beenselected for the mission.Asmentionpreviously,thechoiceofthefrequencyiscrucial,REASONwillusetwofrequencies:ashorter60-MHz wave(whichisnotjammedbytheradioemissionofJupiter)andalonger9-MHzwave(whichwillensuredeeppenetration butwillbe operatedonlyontheanti-Jovianside ofEuropa). Thisdual-frequencyradarisdesignedtolookforapotential liquidwaterreservoir,butalsotocharacterizethestructuresoftheiceshell.Thepossibilitytocomplementtheorbiterwith alanderthatmighthaveastationaryradaron-boardiscurrentlyunderstudy.

4. Conclusion

ThisbriefoverviewdemonstratestheessentialcontributionofradarstotheexplorationoftheSolarSystem.

Radarscanbehighlyadaptedtodifferentscientiﬁcobjectivesandplatforms.Mostofthefuturespacemissionswillhave among theirpayload radarsthat havebeenspeciﬁcally designedto provideinformationessentialto theunderstanding of studiedbodies’formationandevolution.

From thesurface,eachofthenextrovermissionstoMars(ExoMarsandMars2020)willaccommodateaGround Pene-tratingRadartodiscoveralongtheroverpaththegeologicfeaturesburiedundertheMartiansubsurface.GPRsmightsoon become mandatoryinstrumentson aroverpayloadjustlikecamerasareatthemoment.Thesynergy betweenthesetwo context instruments isobvious [72],and will give the possibility to have a better understanding of the site’sgeological historiesand,forexample,toassesstheircapacitytohavebeenhabitableinthepast.

From orbit,someofthemostspectacularfeatures revealedbysoundingradarshavebeenobtainedbytheCassini radar

onTitanandtheMARSISandShaRad soundersontheMartianpolarcaps.Jupiter’sicymoons,withtheir potentialhidden oceans,arefuturechallengesintheexplorationoftheSolarSystem,andradarswillprovidedatathatwillallowustomake signiﬁcantprogressinourunderstandingoftheSolarSystemformationandevolution.

Finally, some quite speciﬁc tomographic radars like the CONSERTradar of the Rosetta mission are designedto sound smallbodiesoftheSolarSystem(comets,asteroids).TheAIM(AsteroidImpactMission)ESAmission,currentlyunderphase

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B study,isaiming atthe 65803Didymosbinary asteroid system. The scenariois to landon theasteroid’s moona small lander thatwouldallow anupdated version oftheCONSERTradarto beoperated inordertocharacterizethe interiorof themoon.

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