Comets at radio wavelengths

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C. R. Physique 17 (2016) 985–994

Contents lists available atScienceDirect

Comptes

Rendus

Physique

www.sciencedirect.com

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

Comets

at

radio

wavelengths

L’étude des comètes en ondes radio

Jacques Crovisier

∗

,

Dominique Bockelée-Morvan,

Pierre Colom,

Nicolas Biver

LÉSIA,ObservatoiredeParis,CNRS,UPMC,UniversitéParis-Diderot,5,placeJules-Janssen,92195Meudon,France

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Articlehistory:

Availableonline10August2016 Keywords:

Comets

Chemicalcomposition Radioastronomy SolarSystemformation Mots-clés :

Comètes

Compositionchimique Formationdusystèmesolaire Radioastronomie

CometsareconsideredasthemostprimitiveobjectsintheSolarSystem.Theircomposition providesinformationonthecompositionoftheprimitivesolarnebula,4.6Gyrago.The radiodomainisaprivilegedtooltostudythecompositionofcometaryices.Observations ofthe OHradicalat18 cm wavelengthallow usto measurethe water productionrate. A wealth ofmolecules (and some oftheir isotopologues)coming fromthe sublimation of ices in the nucleus have been identified by observations in the millimetre and submillimetredomains.Wepresentanhistoricalreviewonradioobservationsofcomets, focusingontheresultsfromourgroup,andincludingrecentobservationswiththeNançay radio telescope, the IRAM antennas, the Odin satellite, the Herschel space observatory, ALMA,andtheMIROinstrumentaboardtheRosetta spaceprobe.

©2016Académiedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Lescomètessontconsidéréescommelesvestigeslesmieuxpréservésdusystèmesolaire primitif.Leurcompositionnousrenseignesurlacompositiondelanébuleuseprimitiveil ya4,6milliardsd’années,fournissantdescontraintessurlaformationdusystèmesolaire. Laradioastronomieestunoutilprivilégiépourl’étudedesglacescométaires.Ledomaine décimétrique permetde mesurer la productionen eau,par l’observationdu radical OH à 18 cm de longueur d’onde. Le domaine millimétrique et submillimétrique permet d’observer de nombreuses molécules provenantde lasublimation desglaces du noyau, ainsiqueleursisotopologues.Nousprésentonsunpanoramahistoriquedesdécouvertessur lescomètesfaitesenradioastronomie,mettantl’accentsurlesrésultatsdenotregroupe, etincluantdesobservationsrécentesfaitesavecleradiotélescopedeNançay,lesantennes del’IRAM,lesatelliteOdin,l’observatoirespatialHerschel,ALMAetl’instrumentMIROdela sondespatialeRosetta.

©2016Académiedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

*

Correspondingauthor.

E-mailaddresses:jacques.crovisier@obspm.fr(J. Crovisier),dominique.bockelee@obspm.fr(D. Bockelée-Morvan),pierre.colom@obspm.fr(P. Colom),

nicolas.biver@obspm.fr(N. Biver).

http://dx.doi.org/10.1016/j.crhy.2016.07.020

1631-0705/©2016Académiedessciences.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Fig. 1. Cometaryimages.Left:CometC/1995O1(Hale–Bopp)observedfromtheEarthon6April1997(photo:N.Biver).Theionanddusttailsarespread hereoverseveral107_{km (tocomparewiththeSun–Earthdistance1 AU=1}

.5×108_{km).Right:Thenucleusofcomet67P/Churyumov–Gerasimenko}

observedatadistanceof198 kmbytheRosetta spaceprobeon18February2015(©ESA/Rosetta/NAVCAM).Thesizeofthenucleusisabout4 km.This imagewasprocessedtoenhancethedustjetsthatescapethenucleus.

1. Introduction

Witha sizeofafewkilometres, cometarynucleiarepractically unobservableata distance.Theirdirectstudyrequires explorationbyspacecraft.However,theyareicybodies.WhentheyapproachtheSun,icessublimate,releasinggasanddust, which forman atmosphereandtails that maybecome very spectacular(Fig. 1).As they contain matter whichremained practically intactsincethebeginningoftheformationoftheSolarSystem, thesebodiesareprecioustestimoniesfromthe pastthatjustifydedicatedstudies.

Early in thehistory ofradio astronomy,the first attempts toobserve greatcomets C/1956 R1 (Arend–Roland), C/1965 S1 (Ikeya–Seki), C/1969Y1 (Bennett)and a few othersdid not yield probing results.Indeed, it isnot easy to observea cometatradiowavelengths.Thesignalisweaksothatlargeradiotelescopes,equippedwithsensitivereceivers,areneeded. The observationsshould onlybeattempted onbrightcomets,whichare rare.Forthesemovingobjects,one mustblindly rely on ephemeridesandthe telescopetrackingsystem. Last butnot least, comets are variable,unpredictable objects, so that an observationcandifficultly berepeatedforconfirmation. Itisthus not asurprisethat thebeginnings oftheradio investigation of comets were a succession offailures,missed opportunities,contradictory results anddoubtful detections thatcouldnotbeconfirmed.

2. 1973:cometKohoutek

Attheendof1973,theNASAorganizedaworldwidecampaigntoobservecometC/1973E1(Kohoutek),insupporttoits observationaboardtheSkylaborbitalstation.Followingthisopportunity, theobservationoftheOH radicallinesat18-cm wavelengthwasattemptedatNançay.Theirdetectionwasthefirstdetectionofacometatradiowavelengths(Fig. 2,[1,2]). The OH radical isa photodissociationproduct ofwater,the majorconstituentof cometaryices.Its observation allows one to determinetheproductionrateofwater, andthusto quantifytheactivityofthe comet.Thisobservationof comet Kohoutekwasthebeginningofaprogrammeofsystematicobservations,whichisstillongoingattheNançayradiotelescope. Morethan130comets werethusobserved.Theevolutionoftheiractivitywasfollowed,insomecasesovermanymonths, toprepareortocomplementtheirobservationswithotherinstruments.

3. 1986:Halley’scomet

The historicandmythicalHalley’scomet(1P/Halley)wasin1986thetarget ofaspaceexplorationbynolessthanfive spacecraft. Its observationfrom the Earthwas also the topicof an internationalsupporting campaign advocated by the InternationalHalleyWatch.Theradioaspectsofthiscampaign[4]werecoordinatedbyÉricGérard(France),WilliamIrvine, andF.PeterSchloerb(UnitedStates).

The OH 18-cmlines inHalley’s comethavebeen monitoredwiththe Nançay radio telescopefora yearanda halfin 1985–1986 andwithseveralothertelescopes[3,4].ThesamelineswerealsomappedwiththeVeryLargeArray(VLA) in-terferometer[5].Hydrogencyanide(HCN)wasdetectedbythenewlycommissioned30-mradiotelescopeofIRAM(“Institut deradioastronomiemillimétrique”)atPicoVeleta(Spain)(Fig. 3,[6]).

Thismarkedtheentryofcometaryradioastronomyintheworldofmolecularradiospectroscopy,alreadysosuccessful in thestudyofinterstellar molecules.Ashorttime aftercame theidentification, still atIRAM,ofmethanol(CH3OH)and

J. Crovisier et al. / C. R. Physique 17 (2016) 985–994 987

Fig. 2. OHspectrafromNançay.Left:SpectrashowingthedetectionoftheOHlinesat18 cmforthefirsttimeinacomet,C/1973E1(Kohoutek),in December1973withtheNançayradiotelescope[1].TheupperpanelsshowforcomparisontheOHlinesobservedinabsorptioninthegalacticsource W12.Thelowerpanelsshowthesamelinesobservedfrom1to12Decemberinthecomet.Theobservationswereperformedwithfrequencyswitching sothatapositiveandnegativesignalpatternappears.Right:AselectionofOHspectraobtainedatNançayincometC/2001A2(LINEAR)fromAprilto July2001[3].Dependingonthedate,theexcitationconditionschangeandthelineappearsinemissionorinabsorption.Inthisfigureasinmostofthe spectrashowninthisarticle,thefrequencyscaleofthex-axishasbeenconvertedintovelocitiesintheframeofthecometfollowingtheDopplerlaw.The intensityscales,asisusualinradioastronomy,aregivenasantennatemperatureorbrightnesstemperatureinunitsofkelvins,whichistheequivalent temperatureofablackbodyintheRayleigh–Jeanapproximation,orfluxdensityinunitsofjanskys(1 Jy=10−26_W·m−2_·Hz−1_).

Fig. 3. Thedetectionofthethreehyperfinecomponentsofthe J 1–0rotationallineat88.6GHzofhydrogencyanideHCNinHalley’scometwiththe30-m antennaofIRAMinNovember1985[6].

4. 1997:cometHale–Bopp

Another exceptional comet was giant comet C/1995 O1 (Hale–Bopp). Its early discovery,one yearand 8 months be-foreitsperihelionpassageon1stApril1997, favouredtheorganization ofitsobservingcampaign. Itcould bedetected at Nançayattherecorddistanceof4.6 AUfromtheSun.Itswaterproductionratereachedatperihelion300tonspersecond, 10 times morethan Halley’s comet. The monitoringof theOH production served asa referenceforthe other molecular radioobservations(Fig. 4).

Itisremarkablethatamongthetwodozensofmolecularspeciesreleasedbycometaryiceswhichwerethenidentified inthiscomet,two-third wereidentifiedbyradiospectroscopy(Figs. 4,5,[8–10]).Inadditiontocarbonmonoxide,onecan note:

– CHOmolecules:H2CO,CH3OH,HCOOH,HCOOCH3,(CH2OH)2;

– nitrogenousmolecules:NH3,HCN,HNC,HC3N,CH3CN,NH2CHO;

– sulphurettedmolecules:H2S,CS,SO,SO2,OCS,H2CS.

Inaddition, isotopicratios D/H, 12_C/13_C,14_N/15_N,32_S/34_{Swere alsomeasured. Mostofthesedetections were madeat}

interferom-Fig. 4. EvolutionoftheproductionratesofvariousmoleculesincometC/1995O1(Hale–Bopp)asafunctionofitsdistancetotheSun[8].Theleftpart referstothepre-perihelionperiod,withthecometapproachingtheSun,andtherightpart,tothepostperihelionperiod,withthecometrecedingfromthe Sun.ThedatafortheOHradical,whichtraceswaterproduction,arefromNançay(squares),withadditionalpost-perihelionvaluesfromUVobservations (circles).TheothermoleculeswereobservedwiththeIRAM30-mantenna(Spain)ortheSEST15-mtelescope(Chile).Downward-pointingtrianglesare upperlimits.Watersublimationappearstobethedominantprocessatlessthan3 AUfromtheSun,whereascarbonmonoxidecouldberesponsiblefor cometaryactivityatlargerdistances.Theasymmetryofthecurvesmaybeduetoseasoneffects,ortothermalevolutionofthecometarynucleus.

Fig. 5. AspectrumofcometC/1995 O1(Hale–Bopp)observedwiththeIRAM30-mantenna,showingthelinesofseveralcomplexorganic molecules

[10].Thelinesofethyleneglycol(CH2OH)2wereidentifiedsevenyearsaftertheobservation,followingthepublicationofthelaboratoryspectrumofthis

J. Crovisier et al. / C. R. Physique 17 (2016) 985–994 989

eteratPlateau-de-Bure),theCSO(CaltechSubmillimeterObservatory10.5-mantenna)andtheJCMT(JamesClerk Maxwell Telescope15-mantenna)atHawaii.Ammoniawasdetected atcentimetricwavelengthswiththe100-mradio telescopeat Effelsberg(Germany) [11].Othermolecules,whicharenon-polar,andthusdevoidofrotationallinesatradiowavelengths, weredetectedbyinfraredspectroscopy:carbondioxide,andthehydrocarbonsCH4,C2H2,C2H6.Anotherwealthof

identifi-cationsofcometarymoleculesisexpectedfromtheinsituinvestigationsofthemassspectrometersaboardRosetta. Radioobservationsalsoallowustoprobekeyphysicalparametersofthecometaryatmosphere:

– itsexpansionvelocity, fromtheshapesofthelines,takingadvantageoftheverygoodspectralresolutionoftheradio technique;

– its temperature, fromthe simultaneous observationofthe intensityof severalrotationallines of amolecule such as methanol.

For comet Hale–Bopp, it was possible to follow the evolution of theseparameters over a large range of heliocentric distances(upto14 AU;Fig. 4).Thecomaexpansionvelocitywasobservedtoincreasefrom0.5to1.3km/s,andthe temper-aturefromabout20 Kto130 K,asthecometapproachedtheSun.Thesevariationsareinagreementwiththermodynamical modelsofthecoma.

5. RecentobservationsatIRAMandwithALMA

TheversatilityofthespectrometersnowequippingtheIRAM30-mradiotelescopeispreciousforcometaryobservations, whicharetime-criticalandwhichcannotbeeasilyrepeated.Withaninstantaneousfrequencycoverageof2

×

8 GHz, spec-tralsurveysarenoweasilyfeasible,allowingsimultaneousobservationsofmanymoleculesandserendipitousdetections.In recentcometsC/2012F6(Lemmon)andC/2013R1(Lovejoy),thispossibilityallowedustoretrievesomemoleculeswhich upto nowwereonlyspotted incometHale–Bopp[12].AndincometC/2014Q2 (Lovejoy),twonewmolecules–ethanol (C2H5OH)andglycolaldehyde(CH2OHCHO)–weredetectedinJanuary2015[13].

Continuumemissionaswellasmolecularlineemissionscanbemappedbyradiointerferometers.Thecometary contin-uumisdominatedbythethermalemissionofdustinthecoma,thesignalfromthenucleusbeingverydifficulttodetect with Earth-based observations, except forcomets comingclose to the Earth. Continuumemission fromdust particles is onlysignificantatwavelengthssmallerthantheparticlesize.Thusitsobservationatmillimetricwavelengthscharacterizes large-sizeparticles.Thisiscomplementarytovisibleandinfraredobservations,whicharesensitivetoparticlesofverysmall sizes.

Mapping molecular lines allows usto know thedistribution of thesemolecules within the comaand to probe coma chemistry.Theoriginofthegasmoleculesmaythusbetraced:theyareeithercomingdirectlyfromthenucleus,or progres-sivelyinjectedinthecomafollowingchemicalreactionsorthesublimationoficygrains.Asymmetriesintheirdistribution maybeduetojets originatingfromactiveregions ofthenucleus’surface.

The IRAMinterferometer atPlateau-de-Bure consistspresently ofsix15-m antennas,which are tobe extended to 12 antennas withtheNOEMA(NOrthernExtendedMillimetreArray)project.Itsfirstsignificantresultswereobtainedoncomet Hale–Boppin1997[14,15],thenattheoccasionoftheoutburstofcomet17P/Holmes[16]andofthepassageclosetothe Earthofcomet107P/Hartley 2(Fig. 6,[17]).TheAtacamaLargeMillimeter/submillimeterArray(ALMA)inChile,withits50 12-mantennas,ismoresensitive.Itsfirstcometaryobservationswereperformedin2013oncometsC/2012F6(Lemmon) andC/2012S1(ISON).(Fig. 7,[18].)

6. Spaceradiotelescopes:SWAS,Odin,Herschel

Althoughwateristheprimeconstituentofcometaryices,itsobservationisdifficult.Itsrotationallines,whichareinthe submillimetrerange,arenotobservable fromthegrounddueto theopacityoftheEarth’satmosphere. Theirdetectionin cometsisrelativelyrecent.

Thefundamentalsubmillimetriclineofwaterat557 GHz(the110–101 transitionat0.5 mmwavelength)wasfinally

ob-servedfromtwosatellitesdedicatedtothestudyofthisline,theSubmillimeterWaveAstronomySatellite(SWAS,launched bytheUnitedStatesin1998),andOdin (launchedbytheSwedishspaceagencyin2001, witha Frenchparticipation con-sistinginthedelivery ofan acousto-opticspectrometer).Odin observed waterina dozenofcomets.It alsoobserved NH3

andH18₂ O,measuringforthefirsttimebyremotesensingthe18O/16Oisotopicratioinacomet.Odin isstilloperationaland observedcometC/2014Q2(Lovejoy)inFebruary2015.(Fig. 8,[19,20].)

The next step was the Herschel space observatory, operating inthe submillimeter andfar-infrared domains, launched bytheEuropeanSpaceAgency.Withits3-mdiametermirroranditsliquid-helium-cooledinstruments(includingHIFI,the HeterodyneInstrumentfortheFarInfrared),itwasmuchmoresensitivethanitspredecessors.From2009to2013,itcould studyandmapseverallinesofwaterinadozenofcomets,someofthembeingweak,distantcomets.Inthreeofthem,it measuredtheD/Hisotopicratio.(Figs. 9,12 [21–23].)

Fig. 6. Mapsofcontinuumemissionat1,1.5and3 mmwavelengthofcomet103P/Hartley2observedwiththeIRAMinterferometeratPlateau-de-Burein October–November2010[17],whenthiscometpassedatonly0.12AU(1.8×107_{km)fromtheEarth.Thesemapsshowthedistributionoflarge-sizedust}

particlesinthecomaandthecontinuumemissionofthenucleus.ThearrowsshowthedirectionoftheSun.

Fig. 7. Fromlefttoright,mapsoftheemissionlinesofhydrogencyanideHCN,itsisomerHNCandformaldehydeH2COobservedwithALMAincomets

C/2012F6(Lemmon)(top)andC/2012S1(ISON)(bottom)[18].Thespectraoftheindividuallinesareshownintheupperleftinserts.Thesemapsshow thatmoleculessuchasHNCandH2COareprogressivelyreleasedinthecomafromstillill-knownsources,whereasHCNseemstocomedirectlyfromthe

J. Crovisier et al. / C. R. Physique 17 (2016) 985–994 991

Fig. 8. The water lines of the water isotopologues H162O and H 18

2O observed by the Odin satellite in comet C/2001 Q4 (NEAT)[20].

Fig. 9. MapsandprofilesofwaterlinesincometC/2009P1(Garradd)observedwiththeHerschel spaceobservatory[23].The110–101 lineat557GHz

isnotsymmetric:thisverystronglineissaturated,showinganindentationatnegativevelocitiescausedbyself-absorptionfromtheforegroundexternal layersofthecometaryatmosphere.

7. CloseobservationofacometwithMIRO,theradiotelescopeaboardRosetta

TheRosetta spaceprobe,launchedbytheEuropeanSpaceAgencyin2004,exploredcomet67P/Churyumov–Gerasimenko, orbitingitsnucleusatdistanceswhich,atsomemoments,wereascloseas8 km.Itwasequippedwithadozenof instru-ments,oneofthembeingMIRO(MicrowaveInstrumentfortheRosetta Orbiter),aradiotelescopewithanantennaofonly 30 cmdiameter.Inthevicinityofthecomet,thismodestsizeisenoughtostudyaselectionoflinesofwater(several iso-topicspecies),methanol,ammoniaandcarbonmonoxidewithaspectrometeroperatingaround0.5 mmwavelength.MIRO isalsoequippedwithtwocontinuumchannelsat0.5and1.6 mmwavelength,dedicatedtotheobservationofthethermal emissionofthenucleus(Figs. 10,11,[24,25]).

Rosetta anditsinstrumentsweretomonitortheevolutionofthecometuntiltheendofSeptember 2016andtofollow itsactivity,whichclimaxedatitsperihelioninAugust2015.

8. Otherstudiesandperspective

Thisrapidsurvey,mainlyfocusedontheresultsfromourgroup,isfarfromexhaustingallaspectsofthestudyofcomets atradiowavelengths.Oneshouldalsomention:

– Continuum observationswithbolometersonsingledishtelescopes,sensitivetodustgrainsinthecoma[26].

– Radar studiesof cometary nuclei, whichdetermine the size andshape of theseobjects whenthey comewithin the rangeoflargeEarth-basedantennas[27];

– The studyoftheplasmaenvironmentofcomets,wherethesolarwindandthecometarycomainteract[28];

– The velocimetryofcometaryspaceprobes,still theonlywaytoevaluatepreciselythemassofcometarynuclei.

Com-pared with the nucleus dimensions obtained by imaging, the nucleus density can then be obtained. Rosetta thus

determinedadensityof533 kg

·

m−3for67P/Churyumov–Gerasimenko[29];

– The tomography of the nucleusof 67P/Churyumov–Gerasimenko performed by the CONSERTinstrument, a bi-static

radaroperatingbetweentheRosetta orbiteranditslanderPhilae[30];

– Laboratory measurementsofcometarymatteranalogues,fortheinterpretationofradarandradiometricobservationsof cometarynuclei[31].

Fig. 10. Anexampleofthewaterlinesobservedat0.5 mmwavelengthincomet67P/Churyumov–GerasimenkowithMIROonRosetta[25].Top:observation on23June2014atadistanceof128 000 km.Bottom:observationon19August2014atadistanceof81 km;thelinesarethenseeninabsorptionagainst thecontinuumofthenucleus.

Fig. 11. Apartialmapofthetemperatureofthenucleusofcomet67P/Churyumov–Gerasimenko,fromthesubmillimetriccontinuumobservationsofMIRO onRosetta,superimposedonthemodelofitsshapededucedfromimaginginthevisible[25].ItcanbeseenontheleftsidethatMIROobservesthenight side,forwhichvisibleimagingprovidesnoinformation.Thetemperaturesvary,followingsolarinsolation,fromabout30 K(nightside)to130 K.Thecomet wasthenat3,5AUfromtheSun.

The spaceexploration of comets, dueto its complexity and its high cost, will be limited fora long time to a small number of targets, which precludes statistical investigations. Theselatter can only be done from Earth-based systematic observations.The observationsofseveraltensofcomets,andespeciallyspectroscopicradioobservations,haveshownthat the relative productionsof water,carbonmonoxide, methanol, andmanyother molecularspecies mayhighly differfrom oneobjecttotheother.Thissuggeststhatcometshavedifferentchemicalcompositions,butwedonotknowyethowthese differencesarerelatedtothehistoryoftheformationandevolutionofthesebodies.

The D/Hisotopic ratioofcometarywater,comparedtothat ofEarthoceans, iscustomarilyusedtotestthehypothesis of a cometary origin forterrestrial water.This ratio ispresently only known fora small numberof comets (from radio observationsforseveralofthem).TheseD/Hvaluesrangefromonetothreetimestheterrestrialvalue,whichseemstorule outanoriginfromonlycomets(Fig. 12,[22,23,32]).Inthefuture,abetterknowledgeofthisstatisticwillhelpustobetter knowtowhichextentthefallofcomets,asteroidsandothersmallbodiesonEarthcontributedtoterrestrialwater.Further constraintsontheoriginofcometsandcometarymaterialarealsoprovidedfromtheD/Handotherisotopicratiosthatare measuredinmoleculesotherthanwater.

J. Crovisier et al. / C. R. Physique 17 (2016) 985–994 993

Fig. 12. TheD/HratioobservedincometarywaterandinotherSolarSystembodies.Mostofthecometarydataarefromradioobservations.Thecometary D/Hratiorangesfromone(103P/Hartley 2observedwithHerschel)tothreetimes(67P/Churyumov–GerasimenkoobservedwiththeROSINAmass spec-trometerofRosetta)itsvalueinEarthoceans[22,23,32].

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