The Cassini-Huygens Mission: Orbiter Remote Sensing Investigations

502 Pages·2004·31.023 MB·English

by  A. J. Kliore, J. D. Anderson, J. W. Armstrong, S. W. Asmar (auth.), Christopher T. Russell (eds.)

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
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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.