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

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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

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n

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Articlehistory:

Availableonline15September2016

Keywords:

Radar Spacemissions

Electromagneticwavepropagation

Mots-clés :

Radar

Missionsspatiales

Propagationdesondesélectromagnetiques

SincetheveryﬁrstobservationsoftheMoonfromtheEarthwithradarin1946,radars aremoreandmorefrequentlyselectedtobepartofthepayloadofexplorationmissionsin theSolarSystem.Theyare,infact,abletocollectinformationonthesurfacestructureof bodiesorplanetshiddenbyopaqueatmospheres,toprobetheplanetsubsurfaceoreven torevealtheinternalstructureofasmallbodycometnucleus.

AbriefreviewofradarsdesignedfortheSolarSystemplanetsandbodies’explorationis presentedinthepaper.Thisreviewdoesnotaimatbeingexhaustivebutwillfocusonthe majorresultsobtained.Thevarietyofradarsthathavebeenorare currentlydesignedin termsoffrequencyoroperationalmodeswillbehighlighted.

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Depuisles premièresobservations radar delaLune depuis laTerre en 1946, lesradars fontdeplusenplusfréquemmentpartiedelachargeutiledesmissionsd’explorationdu systèmesolaire. Ilssont,en effet,capablesde recueillirdesinformationsàlafoissurla 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 résultatsmajeurs sont présentés.L’accent est mis surlavariété des radars quiontété etsontactuellementconçusentermedefréquenceoudemodeopératoireenfonctiondes contraintesdelamissionetdesobjectifsvisés.

E-mailaddress:[email protected].

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

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processing, theradardeterminesforeachdetected echoapropagationdelayanditsassociatedamplitude. Bothquantities arethenprocessedtoretrieveinformationonthegeometricalanddielectricpropertiesofthetargetandofthepropagation environmentbetweenthetarget andtheantennas.Theinstrument’sﬁnalradiometricperformancesdependontheradar’s parameters,onthegeometricalconﬁguration,onthesoundedenvironmentpropertiesandontheoptimalusemadeonthe redundancywithinthemeasurements.

Section2ofthispaperpresentsthemainkindsofradarsthat havebeendesignedtoexploretheSolarSystem’sbodies andplanets.The emphasisisputontheplatformsthoseradarscan beaccommodatedon(i.e.spacecraftinorbitorﬂyby, rovermobileatthesurface,andevenstationarylanderonthesurface),andonthechoiceofacenterfrequencyaccording tothescientiﬁcandtechnicalobjectivestoreach.

Anoverviewofthemainradarexperimentsdesignedforplanetaryexplorationandsomeofthemostsigniﬁcantresults theyobtainedarepresentedinthesecondsection.

2. Radars for planetary exploration and science

Radarswereoriginallydevelopedformilitarypurposes,whichhasrapidlycontributedtothedevelopmentofanumber ofcivilapplicationsforsafetravelandremotesensingoftheEarthenvironment.Fordecadesnow,thefamousradar equa-tionhas beenthebasis todesign radars [1],especially forplanetary explorationwhere distancesto thetargetsare often extremelylargeandwhereverylittleinformationisavailableonthetarget’scharacteristics.

2.1. Choosingtheplatform

Thefirst historicobservationsoftheMoonwere madefromtheEarthin1946at138MHzbythe Corps’EvansSignal Laboratory,NewJersey[2,3].Aftersomeunsuccessfulattempts,in1961,VenusbecamethefirstplanetoftheSolarSystem tobeexploredbyaradarfromtheEarth:the34-mdishoftheGoldstoneObservatoryinCalifornia[4,5]wasusedatfirst, followedlaterbythe300-mdiameterantennaatArecibo[6].

Sincethe1980s,thevastmajorityofradarsusedinplanetarysciencehavebeenoperatedonmissions thatofferedthe opportunitytomap awholeplanetfromorbit(theMarsExpress[7]andMarsReconnaissanceOrbiter[8]missionsaround Mars, the Magellanmission to Mercury[9]) orduring a numberof limitedﬂybys (the Cassini mission to Saturn andits moons[10]).Inaddition,radarsdedicatedtoamoreaccurate butlocalsubsurfacecharacterizationscanbeaccommodated onboardvehiclesmobileatthesurfaceofplanets.TherecentChang’e-3mission[11]landedontheMoonaroverequipped withaGroundPenetratingRadar(GPR)inordertosoundthelunarregolithfromthesurface.ThenextESAandNASArover missions to Mars, ExoMars [12] andMars2020 willalso operateGPRsto soundthe Martiansubsurface along therover’s paths.

Lessconventional experimentshavebeenspeciﬁcallydesignedtoretrieveinformationon thesubsurfacestructure and 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 separatedunits,the ﬁrst one beinglocated onthe surfaceof thenucleus whiletheotheroneison-boardtheorbiter[13];

•

theEISS(ElectromagneticInvestigation oftheSub-Surface) bistaticradarwas selectedon thenowcanceled Humbold payloadoftheExoMarsmission.Theinstrumentisabletotakeadvantageofthesimultaneouspresenceofastationary landeranda mobilerovertoperformdeepsoundingoftheMartiansubsurfacearoundthelanding site.Inmonostatic mode,the 35-m-longelectrical antennaslocated onthe stationaryplatformofthe missionare usedfortransmission andreception.While inbistatic mode,a 10-cm-long magnetic receiveraccommodated onthe mission’srover isable to measure three orthogonal components of the magnetic field too. These measurements allow one to retrieve the directionof arrival of each reflected wave and thus to retrievethe 3D location of the buried reflectors fora better understandingofthegeologicalcontextofthesite[14–16].

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

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Fig. 1. Centerfrequencyofthemainradarsthathavebeendesignedforplanetaryexploration.Foreachinstrument,thetargetisindicatedbythecolor code.

subsurface at grazingincidence angles (for example,the Mars Radio Science experiment[17] on-boardthe MarsExpress spacecraft,theClementinebi-staticradarexperimentattheMoon[18],andtheVikingbistaticradaronMars[19]). 2.2. Designingtherightradar

The specificationsforeach instrumentofamissionpayload arebasedonthescientific objectivesofthemission.Fora radar, thecharacteristicsofthetarget(range,geoelectricalandgeometricalproperties)aswellasthefinalproduct(image, topography, estimatedroughnessanddielectricproperties,subsurfacecharacterization)andresolutionaimedatwillguide theradardesign.

2.2.1. Avarietyofoperatingmodes

Mostofthetime,giventhelimitedresources(ofmass,volumeandpower)availableinspacemissions,radarsdesigned anddeveloped forplanetary missions areadaptable enoughso that they canbe operatedin differentmodesmaking the bestwayofthehardware.Theyveryoftencanbe usedasaltimeters,synthetic-apertureradars,subsurfacesounders, scat-terometersandradiometers,andoftenalsousedtocharacterizetheatmosphereandionospherewhenrelevant.

Aradar altimeterisusually anadir-lookingradar. Itmeasuresthepowerreceived afteraninteractionwiththesurface asafunctionofthepropagationdelayandthusprovidesanestimate ofthedistancebetweentheradarandthereﬂecting surface. Itisusedtoinferthetopographyofthesurface.Theradaraltimeterechoproﬁleiscontrolledbytherangetothe surface andthesurface back-scatteringproperties,whichdependonthesurface roughnessatscaleslarger thanthewave length andonthedielectricpropertiesoftheshallowsubsurface.Highfrequenciesareusuallychosen forradaraltimeters, soastolimitpenetrationintothesubsurface.

On the contrary, soundings radars are low-frequency HF Ground-Penetrating Radars. They are used to perform deep soundings (uptoseveralhundredsofmeters)ofplanetarysub-surfacestructures.The ﬁrstradarsounderﬂown wasALSE (Apollo LunarSounderExperiment)[20]onboardApollo17in1972.TwoothersoundershaveexploredtheMartian sub-surface(§3.1.1):MARSIS(MarsAdvancedRadarforSubSurfaceandIonosphereSounding)[21]onboardtheEuropeanSpace Agency’s MarsExpressprobe,andSHARAD(marsSHAllowRADarsounder)[22] ontheNASAMarsReconnaissanceOrbiter (MRO).AradarsounderhasalsobeenusedontheJapaneseMoonprobeSELENE[23],launchedin2007(§3.1.2).

A synthetic-apertureradar (SAR) isused to obtainimages ofan area atwavelengths that are signiﬁcantlylower than thoseusedbyopticalcameras.Itusesthemotionoftheantennaoveranareatoimprovethespatialresolution;therefore, SARsaretypicallymountedonmovingspacecraft,buttheymightalsobeusedonmoving/rotatingtargets.Syntheticaperture radarsarethemostsuitableinstrumentstoprovideimagesofplanetarysurfaceshiddenbyopaqueatmosphereslikeVenus and Titan. The two Soviet spacecrafts Venera 15 andVenera 16 [24] provided images of the planet in 1983using SAR and Radar altimeters.The SAR of the Magellanmission [25] later in 1990achieved an almost complete coverage of the surface of Venus(§2.1.1).The SAR ofthe Cassini[26] spacecraft,inorbitaround Saturn,has providedimpressive images ofTitansurfaceduringﬂybys(§2.1.2).Whenpossible,enhancedprocessingofSARdataallowustoprovidetheSAR-based topographyofasurfacewithamuchbetterresolutionthantheoneobtainedbysimpleradaraltimeters.

2.2.2. Abroadfrequencyrange

Thechoiceofthefrequencyisoftheuppermostimportancesinceitwilldrivethepenetrationdepth.Moreover,because thebandwidthislimitedbythecentralfrequency,thischoicewillalsosettherangeresolution(i.e.itsabilitytodistinguish between two targets at different 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].

extremelydifferentfrequencieshavebeenused(see Fig. 1).ThelargestwavelengthhasbeenusedbytheMARSISsounder onboard the MarsExpress ESAmission: 230m in the vacuum fora radar designedfor kilometricpenetration depths to searchforpotentialwaterreservoirsintheMartiansubsurface.Theshortestis2.18cmfortheradaroftheCassini mission, whoseprimaryobjectiveistocharacterizeTitan’ssurfacethroughitsthickatmosphere.

Asmentionedpreviously,thechoiceofthecenterfrequencyhasanimpactontherangeresolutionsincetheresolution improveswhenthebandwidthincreases.Asa consequence,radars designedtoperformdeepsoundingsofthesubsurface mustoperateatlow centerfrequencies andconsequently haveapoorvertical resolution.Comparisonofthe MARSISand ShaRadsoundersoperatedonMarsisagoodillustrationofthematter(Fig. 3).Anotherexampleofthistrade-offisprovided bytheWISDOMradarofthefutureExoMarsmission’srover.Ithasbeendesignedtoprovidehighresolutionimagesofthe shallowMartiansub-surfaceinordertosupportdrillingoperations;asaconsequence,itwilloperateonanespeciallylarge frequencybandwidthfrom0.5to3GHz,whichwilllimititspenetrationtoafewmeters.

Inadditiontotheseconsiderations, inthespeciﬁc contextofspacemissions,wherethesizeandmassofthepayload’s instrumentsarealwaysstronglylimited, thechoiceofthefrequencyisalsoconstrainedbythesizeoftheantennausedto transmitandreceivetheelectromagneticwaves.

3. A variety of targets in the Solar System

The radar investigations ofthe Solar Systemstartedlogically withEarth-based observations andaimed at the closest target to Earth, namelythe Moon.Nextcame Venus andMercury,followed by Marsandthe moonsof Saturn.Soon the moonsofJupiterwillalsobeexploredby radars.Thissection illustratestheimpressiveknowledgethat hasbeenacquired ontheSolarSystemthankstoradarinstruments.

3.1. Peeringthroughadenseandopaqueatmosphere

Radarshavebeensuccessfullyusedto revealthesurface topographyandgeologyofplanetary bodiescoveredby thick andopticallyopaqueatmospheres.VenusandTitan’sexplorationbyradarsprovidethebestillustrationsofthiscapacity. 3.1.1. Venus

ThesurfaceofVenusishiddenbythickcloudsofsulfuricacidthatpreventopticalremotesensingtechnicstoaccessthe surface[27](seeFig. 2/right).Venerawasaseriesof16ﬂyby,orbital,andlandedmissionstoVenusconductedbytheSoviet Unionfrom1961to 1983.Mostofthesemissions failedbecauseoftheharshVenusian environment.Butsince Venera7, successfullandingsofmodules ableto survivefor30to 120minat thesurface providedthe ﬁrstoptical pictures ofthe Venusianground.

TheNASAPioneerVenusOrbiterwaslaunchedin1978[28].Oneofthemainexperimentsonitspayloadwastheradar (ORAD)operatingat1.757GHz.Thisradaraltimeterwasusedtogetinformationonthesurfacetopographywhichhasbeen reconstructedwithaverticalaccuracybetterthan200manda150-kmresolution.Thesurface electricalconductivityand roughnessat1-mscalehavebeenestimatedtoo.ThePioneerVenusOrbiterwas theﬁrstspacecrafttouseradartoobtain aglobalimageoftheVenussurface[29].

In1983, the twinVenera15and16 spacecraftscarried aradar altimeterandaSAR operating at3.75GHz that were usedtomapthetopographyofthesurfacewitha1-kmresolution(from30NlatitudetotheNorthPole)[30].Theachieved verticalaccuracywasaround230m.

Finally, the Magellanspacecraft [31] was launched by NASA in 1989. The radar on-board [32] collected SAR images, combinedwithaltimetryandradiometrymeasurementsatafrequencyof2.3GHz,overthemajorityofVenus’surface.The resolutionoftheSARimageswasbetterthan150movermorethanhalftheplanet.Theverticalaccuracyofthealtimeter wasbetterthan50m.Combiningalltheinformationavailable,amapoftheelevationwasbuiltforthewholeplanet(see Fig. 2, center). TheSAR ofthe Magellanspacecraftalso provided verydetailed images[33] (see Fig. 2,right) ofvolcanic

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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

The Cassini NASAmissionwaslaunchedin2005toexploreSaturn,itsringsandsatellites.Aspecialattentionwas paid toTitan,whichistheonlymoonintheSolarSystemwithathickatmosphere(10timesthickerthantheEarth’sone)that preventsobservationofitssurfaceatopticalwavelengths(seeFig. 3,left).TheHuygensESAprobesuccessfullylandedonits surfacewhilethemainspacecraftremainedinorbitaroundSaturn.Impressivesetsofradardatahavebeencollectedduring morethanonehundredﬂybysofTitan.Theradaron-boardtheorbiter[25]isapowerfulmultimodesystemthatcanoperate asa SAR,radaraltimeter,scatterometerandradiometer. Itsprincipal objectiveistocharacterizethesurface ofTitanon a regional scale.TheobtainedSAR images,whose resolutionis

>

350 m,haveshowntheexistence, mostlyinthe northern hemisphere, ofﬂat andsmoothareas,which havebeeninterpreted aslakesandseas ofethaneandmethane(see Fig. 3, right)[34].OverlappingSARimageshavebeenusedlocallytobuildSARdigitalelevationmodelsbyradarstereogrammetry [35].Theachievedresolutionisofseveralkilometershorizontallyandaround100mvertically,whichissigniﬁcantlybetter thanthealtimeter’scapacity.

The radar observations alsorevealedthe presence ofnumerousimpact craters, ofdunes whose sizeand patternvary withaltitudeandlatitude[36],ofriverchannels[37],ofmountains[38],ofsedimentarybasins[39],andkarsticlandscapes [40].

3.2. Soundingbeneaththesurface

UnlikeTitan andVenus’s surface, thesurface ofMars, Mercuryand theMoon arereadily accessibleto optical remote sensingfromorbit,butradaroperatedatlow-enoughfrequencyhavetheuniquecapacitytosoundthroughthesurfaceand accesstheburiedstructures.

3.2.1. ThesubsurfaceofMars

So far Mars hasbeen the object ofa numberof missions that are more and moreinterested in discovering what is beneathitssurface.TherarecloudspresentintheMartianatmospheredonotpreventopticalremotesensingofthesurface fromorbitandlarge datasetsobtainedby spectro-imagersandcontextandhigh-resolution stereoscopiccamerasare now available. Tillnow, two orbitalradars havebeendesignedandused toretrieveinformationabouttheMartiansubsurface. The MARSIS orbital radarsounder, onboardESA’s MarsExpress spacecraft launched in2003, 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 idealsounding 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 the latter,themostnotableistheMedusaeFossaeFormation,whoseradarpropagationcharacteristicsare consistentwith a compositionranging froma dry,highlyporous pyroclasticdepositatoneextremetoan ice-richsedimentarydepositat the other,potentially formedby theredistributionofpolarvolatilesattimesofhighobliquity[45].Hintsofseveralother moderatelydeepreﬂectorshavealsobeenfoundinthevicinity ofMa’adimVallis,wherethereisevidencethat theymay originatefromlithologicalinterfaces[46].

MARSIS wascloselyfollowed bythe ﬂightof anotherradarsounder calledtheSHallow RADar(SHARAD)on theMars Reconnaissance Orbiter (MRO) [47]. SHARAD was designed to complement the capabilities and performance of MARSIS

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mapoftheareabasedondatafromtheMOLAlaseraltimeteronboardNASA’sMarsGlobalSurveyor.

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

by operating at higherfrequencies around 20 MHz to better resolve variations innear-surface composition, stratigraphy andstructure–particularlywithregard toresolving theinternal layersofthe polarcaps (see Fig. 5[48]).The horizontal resolution of the instrumentis between 0.3 and3 km andthe vertical resolution is about 15 m (10 times better than MARSIS).Comparison ofFigs. 4 and5givesa clearunderstandingofthebetter resolutionthat canbe obtainedathigher frequenciesbecauseoftheassociatedincreaseinthefrequencybandwidth.

Outside the polarcaps, ShaRad soundings performedin theeastern Hellas region revealedelectromagnetic properties thatareconsistentwithmassivewaterice,supportingthehypothesisthatgroundiceispresentinthesubsurfaceatlower latitudes[49].

ThefutureESA/RoscomosExoMars roverMission[12] toMars,whichis currentlyplannedfor2018, willhaveonboard a ground penetrating radar designedto sound the very shallow subsurface (toa depth of

∼

3 m)along the roverpath. ThisradarcalledWISDOM[50](WaterIceSubsurfaceDepositsObservationonMars)hasbeendesignedtoprovidearange resolutionbetter than 10cminthe subsurfaceandis thusoperating overa large-frequencybandwidthbetween0.5 and 3GHz.Itwillprovideinformationaboutthegeologicalcontextandhelp totheidentiﬁcationofpotentially habitablesites. 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 frequencyrather thana single pulse. Acompleteset ofmeasurements isperformedforthe rangeoffrequencies covered by theinstrument’sbandwidth. Performingan InverseFourierTransformonthe wholeset offrequencyresponses allows the computation ofthe impulse response,which wouldbe readily obtainedby a pulseGPR. This step frequencytechnic allowsabetteruseofthelimitedpoweravailableontherover.Inaddition,theantennasystemhasbeendesignedtoallow polarimetric measurements, which will provide additional information aboutthe shape and orientation of the reflecting structures.Giventhemission’sconstrains,WISDOMwillbeaccommodatedontheroveratadistanceofabout30cmfrom thesurface,whichisunusualforaGPRwhoseantennasareusuallyincontactwiththeground.Thisspecificconfiguration is not the most favorable, but provides the opportunity to make use ofthe surface echo to get an estimate of the top layerdielectricconstant.Aprototypeoftheinstrumentverysimilartothefinal flightmodel(withatotalmassof1.7kg) iscurrently testedinvarious environments. Fig. 6 illustratesthe abilityof theinstrumentto resolve finelayering thanks to its broad frequencybandwidth [51]. A radargram obtainedwith only 1GHz of frequencybandwidth is presented for comparison.

ThenextMars2020NASAmissionhasalsoselectedaGPRoperatingatslightlylowerfrequenciestobepartofitspayload [52].ThisGPRcalledRIMFAX(RadarImager forMars’Subsurface Exploration)iscurrentlyunderdevelopment.RIMFAXis a gated FrequencyModulatedContinuous Wave(FMCW) radaroperating overthe frequencyband 150–1200 MHz,which shouldallowittoprobetheMartiansubsurfacetodepths

>

10 m.

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

3.2.2. ThesubsurfaceoftheMoon

EveniftheMartianpolarcapsprovidedsofarthebestexamplesofplanetaryfeaturesexploredbyradar,theMoonwas also sounded by radars.The NASA Explorer 35bistatic experiment[53] was launched in 1967;its purpose was tostudy the electromagneticreﬂectivepropertiesofthelunarsurface.The experimentusedthe136.10-Hz communicationchannel betweenthespacecraftandtheEarth,theelectromagneticwaveswerescatteredfromthelunarsurface andthenrecorded atStanfordonEarth.Statisticsofthelunarsurface’sslopewereobtainedintheequatorialregion.

Theﬁrstsuccessfulattempttosoundthelunarsubsurfacefromorbitwasperformedin1972duringtheApollo17 mis-sion(ALSEexperiment[54]).Themeasurementsperformedat5and150MHzallowedthedetectionofalayeratkilometric depths intwosoundedlunarmariaandvalidatedthefeasibilityofsuchmeasurements[55].In2007,theLRS(LunarRadar Sounder) instrumenton-boardthe Kaguyaspacecraftofthe JAXA SELENEmission completeda globalcoverage ofthe lu-nar surface andsubsurfaceat 5MHz [56]. Subsurfacefeatures observed by both ALSE andLRSradars are interpreted as interfacesbetweenlavaﬂowsofdifferentages.

Twomini-SAR(ormini-RF)havebeendesignedandusedtoexploretheMoon[57,58].Theﬁrstinstrumentwaslaunched on the Indian SpaceResearch Organization’s (ISRO’s) Chandrayaan-1 spacecraft, while the second one was on-board the NASA’sLunarReconnaissanceOrbiter(LRO).Theyhavemappedbothpolarregionsandanumberofdifferentgeologicunits on the lunar surface. Mini SAR instruments have been designed to map the permanentlydark areas of the lunar polar regions tosearchforpotentialwatericedeposits.Thesearchforwatericeistheirprimaryscienceobjective,buttheyalso characterized the surface roughness andthickness of the regolith.The MiniSAR transmits Right Circular Polarization at 2.5GHz andreceivesbothLeftandRight,whichgivesaccesstothe circularpolarization ratio.Theelevated CPR(Circular PolarizationRatio)valuesobservedwithinpermanentlyshadowedcratersinthepolarareasareconsistentwiththepresence ofwatericeinthesecraters[59].

3.2.3. ThenucleusofcometChuryumov–Gerasimenko

Despite numerous observations of comets either from Earth orduring dedicated space missions among which ESA’s Giotto, NASA’s Deep Space, Stardust andDeepImpact [60],there hasbeen,prior to theRosetta mission,no directaccess to the structure of cometary nuclei. The successful Rosetta mission [61] with the recent descent and landing of Philae on the nucleus of 67P/Churyumov–Gerasimenko at the end of 2014 has provided for the first time in-situ data of the upper most importance. Withreceivers and transmitters onboardboth Rosetta and Philae, the objective ofthe CONSERT (Comet NucleusSounding Experiment by Radiowave Transmission) radar is the characterization of the nucleusin terms of internal structure anddielectric properties. CONSERT[62] is a bi-static radar that consistsoftwo separate units: The LanderCONSERTunit(LCN)locatedonPhilae andtheOrbiterCONSERTunit (OCN)locatedontheorbiter.Aradiosignalat 90 MHzistransmittedbytheOCNandpropagatesthroughthecometnucleustotheLCN.TheLCNworksasatransponder: the signal receivedby the landeris analyzedinreal time todetect thestrongestecho (whichis expectedtobe the first one) forsynchronizationpurposes, andimmediatelyretransmittedback to the orbiter,where thefinal data iseventually recorded. Thepropagationdelay andamplitudeofthe receivedechoesmeasured arereadilyobtainedfromthemeasured impulseresponse.Foreachsoundingconfiguration,themeasuredpropagationdelay correspondingtothefirstechoallows onetoestimatetheaveragedielectricconstantofthenucleusalongthepropagationpath,andtoconstrainthecomposition and porosity of thenucleus. The width ofthe received echoes isan indirectmeasurement of theinternal heterogeneity at a scale larger than the experimentwave length (

∼

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

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andinterpretationofthedatacollectedby CONSERTthat takesintoaccounttheamplitude ofthereceivedsignalswillbe possible.

An updated version of thistomographic radar concept is currentlyconsidered forthe future AsteroidImpact Mission (AIM)aimingatasteroidDydimoscharacterization[66].

3.2.4. IcymoonsofJupiter

ThethreeJupitericymoons(Ganymede,EuropaandCallisto)arebelievedtohideliquid-wateroceansbeneatha several-km-thickcrustofice.Detectionandcharacterizationoftheseoceanswouldbeessentialtosupportthepotentialhabitability ofthesemoonsandtomakesigniﬁcantprogressonourunderstandingofthelife.Sinceelectromagneticwavesexperience little attenuation in ice(see § 2.2.1. the deepsoundings ofthe Martianpolar caps) andwould be strongly reﬂected by largebodies ofliquidwater,radars areparticularlywell suitedtothisobjective.Consequently,thefuturemissionsaiming toJupiteranditsicymoonswillcarryonboardradarsdesignedtoperformdeepsoundingoftheicecrust.

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

The JupiterIcy MoonExplorer (JUICE)ESAmission[68] will be launchedin 2022tostudyJupiter andinvestigatethe potentialhabitabilityofitsicymoons.Amongthepayloadinstruments,theradarsoundercalledRIME(RadarforIcyMoon Exploration)[69]hasbeendesignedtostudythesubsurfacestructureofthemoonsuptodepthsabout9kminice.A center frequencyof9MHzhasbeenchosentoachievethisobjective.Thehigh-resolutionmodewitha3-MHzbandwidthwillallow theinstrumenttoachieveaverticalresolutionof30mintheice. Alowerresolutionmodeisalsoplannedwitha1-MHz bandwidthtoreducethedatavolume.Theuseofthe9-MHzcenterfrequencywillrestraintheradaroperationtotheside ofEuropawhichisawayfromJupiter.

Among Jupiter’s icy moons, Europa iscurrently considered to be the mostpromising location in theSolar Systemto searchforsignsofextantlife.ThefutureNASAEuropamission,formerlyknownasEuropaClipper,[70] shouldbelaunched inthemid-2020sandperform45ﬂybys ofEuropaataltitudesvaryingfrom2700to25kilometersabove thesurface.The Radar forEuropa Assessmentand Sounding: Ocean to Near-surface(REASON) [71] instrument hasbeen selected forthe mission.Asmentionpreviously,thechoiceofthefrequencyiscrucial,REASONwillusetwofrequencies:ashorter60-MHz wave(whichisnotjammedbytheradioemissionofJupiter)andalonger9-MHzwave(whichwillensuredeeppenetration butwillbeoperatedonlyonthe anti-JoviansideofEuropa).Thisdual-frequencyradarisdesignedtolookfora potential liquidwaterreservoir,butalsotocharacterizethestructuresoftheiceshell.Thepossibilitytocomplementtheorbiterwith alanderthatmighthaveastationaryradaron-boardiscurrentlyunderstudy.

4. Conclusion

ThisbriefoverviewdemonstratestheessentialcontributionofradarstotheexplorationoftheSolarSystem.

Radarscanbehighlyadaptedtodifferentscientiﬁcobjectivesandplatforms.Mostofthefuturespacemissionswillhave amongtheir payloadradars that havebeenspeciﬁcallydesignedtoprovideinformationessential tothe understandingof studiedbodies’formationandevolution.

Fromthesurface, eachofthenext rovermissionstoMars(ExoMarsandMars2020)willaccommodatea Ground Pene-tratingRadartodiscoveralongtheroverpaththegeologicfeaturesburiedundertheMartiansubsurface.GPRsmightsoon becomemandatory instrumentsona roverpayloadjustlike camerasare atthe moment.The synergybetweenthesetwo context instruments is obvious [72], andwill give the possibility to havea better understanding ofthe site’s geological historiesand,forexample,toassesstheircapacitytohavebeenhabitableinthepast.

Fromorbit,some ofthemostspectacularfeaturesrevealedbysoundingradarshavebeenobtainedbytheCassini radar onTitanandtheMARSISandShaRadsoundersontheMartianpolarcaps.Jupiter’sicymoons,withtheirpotentialhidden oceans,arefuturechallengesintheexplorationoftheSolarSystem,andradarswillprovidedatathatwillallowustomake signiﬁcantprogressinourunderstandingoftheSolarSystemformationandevolution.

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

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B study,is aiming atthe65803Didymos binary asteroid system. Thescenario isto land onthe asteroid’s moona small lander that wouldallowan updated versionofthe CONSERTradar tobe operatedinorderto characterizetheinteriorof themoon.

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