The Cassini- Huygens Mission Orbiter Remote Sensing Investigations Editor: C.T. Russell THE CASSINI-HUYGENS MISSION THECASSINI-HUYGENSMISSION OrbiterRemoteSensingInvestigations VVVolume3 Editedby CHRISTOPHERT.RUSSELL UniversityofCalifornia,California,U.S.A. ReprintedfromSpaceScienceReviews,VVVolume115,Nos.1–4,2004 KLUWER ACADEMIC PUBLISHERS DORDRECHT/BOSTON/LONDON A.C.I.P.CataloguerecordforthisbookisavailablefromtheLibraryofCongress ISBN:1-4020-3147-5 PublishedbyKluwerAcademicPublishers PPP.O.Box17,3300AADordrecht,TheNetherlands SoldanddistributedinNorth,CentralandSouthAmerica byKluwerAcademicPublishers, 101PhilipDrive,Norwell,MA02061,U.S.A. Inallothercountries,soldanddistributed byKluwerAcademicPublishers, PPP.O.Box322,3300AHDordrecht,TheNetherlands Printedonacid-freepaper Coverillustration:CourtesyofESA AllRightsReserved (cid:1)c 2004KluwerAcademicPublishers Nopartofthematerialprotectedbythiscopyrightnoticemaybereproducedor utilisedinanyformorbyanymeans,electronicormechanical, includingphotocopying,recordingorbyanyinformationstorageand retrievalsystem,withoutwrittenpermissionfromthecopyrightowner PrintedintheNetherlands TTTABLEOFCONTENTS Foreword vii A.J. KLIORE, J.D. ANDERSON, J.W. ARMSTRONG, S.W. ASMAR, C.L. HAMILTON, N.J. RAPPAPORT, H.D. WWWAHLQUIST, R. AMBROSINI, F.M. FLASAR, R.G. FRENCH, L. IESS, E.A. MAROUF AND A.F. NAGY / Cassini Radio Science 1–70 C. ELACHI, M.D. ALLISON, L. BORGARELLI, P. ENCRENAZ, E. IM, M.A. JANSSEN, W.T.K. JOHNSON, R.L. KIRK, R.D. LORENZ, J.I. LUNINE, D.O. MUHLEMAN, S.J. OSTRO, G. PICARDI, F. POSA, C.G. RAPLEY, L.E. ROTH, R. SEU, L.A. SODERBLOM,S.VETRELLA,S.D.WALL,C.A.WOODandH.A. ZEBKER/Radar:TheCassiniTitanRadarMapper 71–110 R.H. BROWN, K.H. BAINES, G. BELLUCCI, J.-P. BIBRING, B.J. BURATTI, F. CAPACCIONI, P. CERRONI, R.N. CLARK, A. CORADINI, D.P. CRUIKSHANK, P. DROSSART, V. FORMISANO, R. JAUMANN, Y. LANGEVIN, D.L. MATSON, T.B.MCCORD,V.MENNELLA,E.MILLER,R.M.NELSON,P.D. NICHOLSON, B. SICARDY and C. SOTIN / The Cassini VVVisual andInfraredMappingSpectrometer(VIMS)Investigation 111–168 F.M. FLASAR, V.G. KUNDE, M.M. ABBAS, R.K. ACHTERBERG, P. ADE, A. BARUCCI, B. BE´ZARD, G.L. BJORAKER, J.C. BRASUNAS,S.CALCUTT,R.CARLSON,C.J.CE´SARSKY,B.J. CONRATH, A. CORADINI, R. COURTIN, A. COUSTENIS, S. EDBERG, S. EDGINGTON, C. FERRARI, T. FOUCHET, D. GAUTIER, P.J. GIERASCH, K. GROSSMAN, P. IRWIN, D.E. JENNINGS,E.LELLOUCH,A.A.MAMOUTKINE,A.MARTEN, J.P. MEYER, C.A. NIXON, G.S. ORTON, T.C. OWEN, J.C. PEARL, R. PRANGE´, F. RAULIN, P.L. READ, P.N. ROMANI, R.E. SAMUELSON, M.E. SEGURA, M.R. SHOWALTER, A.A. SIMON-MILLER, M.D. SMITH, J.R. SPENCER, L.J. SPILKER and F.W. TAYLOR / Exploring the Saturn System in the Thermal Infrared:TheCompositeInfraredSpectrometer 169–297 L.W.ESPOSITO,C.A.BARTH,J.E.COLWELL,G.M.LAWRENCE, WWW.E. McCLINTOCK, A.I.F. STEWART, H.U. KELLER, A. KORTH, H. LAUCHE, M.C. FESTOU, A.L. LANE, C.J. HANSEN, J.N. MAKI, R.A. WEST, H. JAHN, R. REULKE, K. WWWARLICH, D.E. SHEMANSKY and Y.L. YUNG / The Cassini UltravioletImagingSpectrographInvestigation 299–361 C.C. PORCO, R.A. WEST, S. SQUYRES, A. McEWEN, P. THOMAS,C.D.MURRAY,A.DELGENIO,A.P.INGERSOLL,T.V. JOHNSON,G.NEUKUM,J.VEVERKA,L.DONES,A.BRAHIC, J.A. BURNS, V. HAEMMERLE, B. KNOWLES, D. DAWSON, T. ROAAATSCH,K.BEURLEandW.OWEN/CassiniImagingScience: Instrument Characteristics and Anticipated Scientific Investigations atSaturn 363–497 Foreword AtthiswritingtheCassinispacecrafthasfireditsengineandsuccessfullyinserted itself and its precious cargo of scientific instruments into orbit, the first step of its exploration of the Saturnian system. The suspense is not over, however. While excitingimagesoftheringshavebeencaptured,anexoticcompositionofPhoebe sensed by the mapping spectrometer and unexpected panoply of magnetic waves andplasmadynamicsencounteredontheincomingtrajectoryandinitialorbit,the HuygensprobeisstillonboardandthefirstcloseflybyofTitanhasnottakenplace. Not until Christmas Day will the probe be released. Navigators are still checking theircalculations,worryingaboutknownunknownslikethemassofSaturn’smoons thatcouldcauseeversosmalladeviationfromtheplannedtrajectoryoftheprobe. TheorbiterinvestigatorsarealsoanxiousbuttheygettheirtasteofTitanearlier,on October 26. How well will they detect the surface? How thick is the atmosphere? Does Titan have a magnetic field? Is there lightning in the atmosphere of Titan? WhileterrestrialandHubbleSpaceTelescopepictureshaveimprovedgreatlyover theyears,theycannotmatchtheresolutionobtainablefromorbitabouttheplanet, andmuchofthedataissimplyunobtainablewithoutdirectinsitusensing. VVVolume 1 of this three volume set described the Cassini/Huygens mission, its scientific objectives and the Huygens probe that will soon enter the Titan atmo- sphere. Volume 2 described the in situ investigations on the orbiter. In this, the thirdandfinalvolumeofthecompendium,wedescribetheremotesensinginves- tigations: radio science, radar, visible and infrared spectroscopy, thermal infrared studies,ultravioletspectroscopyandvisibleimagery. Thisvolumecompletesourdescriptionofthismostambitiousmission.Forthe editor,thishasbeenaveryambitioustask,extendingoveraneight-yearperiod.We trustthatthereaderwillfindthesepagesbeneficial,gaininginsightintothehowand whyoftheCassiniinvestigationsandallowingthebroaderscientificcommunityto shareintheadvanceinourunderstandingthatthemissionbrings.AswithVolumes 1and2,thisvolumeisduetotheeffortsofmanyindividualsespeciallythereferees and authors who have helped produce a very readable and complete descriptions oftheinvestigations.WeespeciallywishtothankAnneMcGlynnwhoassistedin theinitialstageoftheassemblyofthiscollectionandMarjorieSowmendranwho completedtheeffortuponAnne’sretirement.Lastly,noneofthiswouldhavebeen possiblewithouttheyearsoflaborbythewomenandmenoftheCassini/Huygens projectwhobuiltthespacecraft,testedit,programmedthesoftware,andnavigated andoperatedthespacecraftsoflawlessly. C.T.Russell UniversityofCalifornia LosAngeles,CA September2004 SpaceScienceReviews 115:vii,2004. (cid:1)C 2004KluwerAcademicPublishers. PrintedintheNetherlands. CASSINIRADIOSCIENCE A.J.KLIORE1,∗,J.D.ANDERSON1,J.WWW.ARMSTRONG1,S.WWW.ASMAR1, C.L.HAMILTON1,N.J.RAPPAPORT1,H.D.WWWAHLQUIST1,R.AMBROSINI2, F.M.FLASAR3,R.G.FRENCH4,L.IESS5,E.A.MAROUF6andA.F.NAGY7 1JetPrrropulsionLaboratory,CaliforniaInstituteofTechnology,4800OakGroveDrive, PPPasadena,CA91109,USA 2IstitutodiRadioastronomiaCNR,ViaGobetti101,I-40129,Bologna,Italy 3NNNASA-GoddardSpaceFlightCenter,Greenbelt,MD20771,USA 4WWWellesleyCollege,Wellesley,MA02481,USA 5Universita`diRoma‘LaSapienza’,ViaEudossiana18,I-00184Roma,Italy 6SanJoseStateUniversity,OneWashingtonSquare,SanJose,CA95192,USA 7UniversityofMichigan,2455HaywardAvenue,AnnArbor,MI48109,USA ∗ ( authorforcorrespondence,e-mail:[email protected]) (Received29December1999;Acceptedinfinalform4April2002) Abstract. Cassiniradioscienceinvestigationswillbeconductedbothduringthecruise(gravitational waaaveandconjunctionexperiments)andtheSaturniantourofthemission(atmosphericandionospheric occultations,ringoccultations,determinationsofmassesandgravityfields).Newtechnologiesinthe constructionoftheinstrument,whichconsistsofaportionon-boardthespacecraftandanotherportion ontheground,includingtheuseoftheKa-bandsignalinadditiontothatoftheS-andX-bands,open opportunitiesforimportantdiscoveriesineachoftheabovescientificareas,duetoincreasedaccuracy, resolution,sensitivity,anddynamicrange. Keywords: atmospheres,Cassini,gravitationalfields,gravitationalwaves,generalrelativity,iono- spheres,occultations,planetaryrings,radioscience,Saturn,Titan 1. Introduction Thispaper,producedbytheCassiniRadioScienceTeamduringtheearlycruiseof the Cassini spacecraft en route for the Saturnian system, records major reference informationconcerningtheinvestigationstobeconducted,andtheinstrumentthat will be operated to conduct these investigations. Section 2 describes the radio scienceinvestigations.Section3isdevotedtotheradioscienceinstrument.Section4 containsabriefconclusion. 2. RadioScienceInvestigations Foreachoftheradioscienceinvestigations: – Gravitationalwaveexperiments, – Conjunction experiments (a new test of general relativity, study of the solar corona), SpaceScienceReviews 115:1–70,2004. (cid:1)C 2004KluwerAcademicPublishers. PrintedintheNetherlands. 2 A.J.KLIOREETAL. – Gravitationalfieldmeasurementsandcelestialmechanicsexperiments(pertain- ingtoSaturn,Titan,andtheIcySatellites), – Ringoccultationexperiments, – Atmospheric and ionospheric occultation experiments (pertaining to Saturn, Titan,andtheicysatellites), wedescribethescientificobjectiveswithrespecttothepresentstateofknowledge, thetechniques,andthemajorrequirements. 2.1. GRAVITATIONAL WWWAVE EXPERIMENTS 2.1.1. ScientificObjectives ThissectionofthepaperoutlinesthemethodandexperimentalsetupoftheCassini GravitationalWaveExperiment(GWE). Theextensiveandstringenttestsofgravitationtheoriescarriedoutinthesolar system, together with observations of binary pulsars, have dispelled most doubts about Einstein’s theory of general relativity and the existence of the gravitational waaaves it predicts. The direct detection of gravitational waves constitutes an out- standing challenge for experimental physics, however, and – when successful – will open up a new window for observational astronomy (Thorne, 1987). Since gravitational waves are virtually unaffected by intervening matter, their observa- tion will probe the dynamics of cataclysmic events in the deep interiors of, for example, supernovae and active galactic nuclei, regions which are inaccessible to electromagneticobservations. Therearethreemainfrequencybandsofastronomicalinterest: – The“high”frequencyband,around1kHz,wherethesourcesincludesupernovae andstellarcoalescences; – The “low” frequency band, around 1 mHz, where the sources include compact binary systems such as a binary pulsar, the formation of super-massive black holes,andthecoalescenceofsupermassivebinaryblackholes; – The “very low” frequency band, <1 µHz, where one might expect a stochas- tic background of waves from the superposition of stellar binaries, distant past collapse events, and possibly the red-shifted remnant of a primordial cosmic background created by density fluctuations in the Big Bang. (See Armstrong etal.,2003forresultsfromCassini’sGWE). Thevarietyofpossiblesourcesinthelow-frequencybandaccessibletospace- craft Doppler tracking necessitates using a variety of signal detection techniques. Thisisespeciallytruebecauseitisasearchconductedlargely“inthedark”;while astronomical observations and theory almost guarantee the existence of gravita- tionalwaves,itisstillbeyondourcapabilitytopredicttheshape,oreventhelikely strength, of waves from these sources. So the best one can do is to do a system- atic search for a variety of plausible waveforms (wide-band pulses, periodic and quasi-periodicwaveforms,stochasticbackgrounds). CASSINIRADIOSCIENCE 3 Fordetectionpurposes,anidealsourcewouldbethebinarypulsar1913+16. Binary pulsars are objects for which clear evidence of gravitational radiation al- ready exists and everything one needs to know about the system is available to predict precisely the observed signal (Wahlquist, 1987). The wave frequency of the strongest harmonic of the periodic signal from 1913+16 falls exactly where the Doppler method is most sensitive. Unfortunately, the predicted amplitude of the signal from 1913+16 is too weak to detect at Cassini’s sensitivity by many ordersofmagnitude. Inadditiontobeingofgreatintrinsicastrophysicalinterest,supermassivebinary blackholesystemsmaybethemostpromisingsourcesfordetectionwithCassini. Clean binary systems, not too close to coalescence, are sufficiently simple that detailed comparison of theory and observation is possible. Of course, since the parametersofthesystemarenotknowninadvance,itisnecessarytoemployalarge ensembleofsignaltemplatestosearchforwavesfromoneofthesesystems.Near coalescence, the amplitude and frequency of the wave increase with time; special methods to extract these “chirp” signals from both random and systematic noises havebeendeveloped(TintoandArmstrong,1991;Andersonetal.,1993;Bertotti etal.,1994;Bertotti,1997;IessandArmstrong,1997;Bertottietal.,1999).Very closetocoalescence,physicalprocessesbecomemorecomplicatedandtemplates mayatbestbearoughapproximationtotheemittedwaveforms. Theexistenceofbinarysourceshasbecomeincreasinglyplausiblewiththere- centobservationsofsupermassiveobjectsingalacticnuclei,togetherwithevidence forthefrequentmergingofgalaxiesintheearlyuniversewhentheirspatialdensity was much higher than at present. Cassini should be able to detect signals of this type,iftheyarepresentintheDopplerfrequencybandwiththeexpectedstrength, well beyond the Virgo cluster (≈17 Mpc), thus including thousands of candidate galaxies. ThegravitationalwavesearchontheCassinimissionhasbeenthemostsensi- tive Doppler experiment ever performed. The experiment has been repeated three times during the cruise period from Jupiter to Saturn; i.e., when the spacecraft was the antisolar direction from earth (November 2001–January 2002; December 2002–January 2003; December 2003–January 2004). At each opposition, Cassini has been Doppler-tracked as continuously as possible for 40 days. Around-the- clock tracking required using all three deep space network (DSN) complexes (Goldstone, Madrid, Canberra), and may be supported additionally by non-DSN radio antennas in Italy. The highest sensitivity was achieved with DSS-25, a beam waveguide antenna located at the Goldstone complex, which has been carefully designed for the utmost in frequency stability and which has Ka-band uplink, precision frequency standards, and advanced tropospheric correction equipment. Cassini at opposition became one of the largest gravitational wave antenna’s ever used (≈8 AAAU in length), attaining by far the highest sensitivity to date for gravitationalwavesatthelowerendofthelow-frequencyband.