Volume 1: Cassini Final Report, Rings Discipline Working Group · RWG_FinalReport-2.docx 10/22/18...
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RWG_FinalReport-2.docx 10/22/18 9:40 AM 1 Volume 1: Cassini Final Report, Rings Discipline Working Group Figure 1: Main rings of Saturn; the image is registered along its top edge with a radial profile of the ring optical depth from a Cassini UVIS stellar occultation. The optical depth (usually denoted τ ) is the vertical integral through the ring layer of the particle number density times its cross-sectional area, times extinction efficiency (for the plot above, the efficiency is unity). The B ring – especially its central part – has the highest optical depth, but the highest values shown are just noise limits. Unlabeled regions mentioned in the following text include the series of “plateaus” in the outer C ring, centered on the Maxwell gap near 87500km, which contains an elliptical ringlet. The Encke gap is visible in the outer A ring, around 133600km. The narrow, stranded F Ring is just off the figure at right. Figure from Colwell et al 2009. RWG Executive Summary: Cassini has rewritten the textbooks regarding our understanding of Saturn's Rings, a full generation after Voyager first revealed their complex structure. In fact, Cassini studies of Saturn’s rings have already been thoroughly reviewed in two sets of review chapters and articles; In the first major review volume “Saturn after Cassini-Huygens”, very thorough reviews are given by Colwell et al 2009, Charnoz et al 2009, Cuzzi et al 2009, Horanyi et al 2009, and Schmidt et al 2009, which remain largely valid as of this writing even if they predate many important results. A short, nonspecialist review is given by Cuzzi et al (2010). More recent, complete reviews can be found in the Cambridge “Planetary Rings” book, including Cassini’s results prior to the Ring-Grazing and Grand Finale orbits (Charnoz et al 2018, Cuzzi et al 2018, Estrada et al 2018, Hedman et al 2018, Murray and French 2018, Nicholson et al 2018, Spahn et al 2018, and Spilker et al 2018). Finally, all the instrument-based chapters in this volume update the story to the very end of the mission, including results from Cassini’s last moments, and some provide exhaustive bibliographies as well. We will attempt to cover the highlights of ring science from beginning to end of the mission as organized by our own goals and objectives, and, in a readable way, provide a thread of continuity connecting Cassini’s vast accomplishments to
Transcript of Volume 1: Cassini Final Report, Rings Discipline Working Group · RWG_FinalReport-2.docx 10/22/18...
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Volume1:CassiniFinalReport,RingsDisciplineWorkingGroup
Figure1:MainringsofSaturn;theimageisregisteredalongitstopedgewitharadialprofileoftheringopticaldepthfromaCassiniUVISstellaroccultation.Theopticaldepth(usuallydenotedτ )istheverticalintegralthroughtheringlayeroftheparticlenumberdensitytimesitscross-sectionalarea,timesextinctionefficiency(fortheplotabove,theefficiencyisunity).TheBring–especiallyitscentralpart–hasthehighestopticaldepth,butthehighestvaluesshownarejustnoiselimits.Unlabeledregionsmentionedinthefollowingtextincludetheseriesof“plateaus”intheouterCring,centeredontheMaxwellgapnear87500km,whichcontainsanellipticalringlet.TheEnckegapisvisibleintheouterAring,around133600km.Thenarrow,strandedFRingisjustoffthefigureatright.FigurefromColwelletal2009.RWGExecutiveSummary:
CassinihasrewrittenthetextbooksregardingourunderstandingofSaturn'sRings,afullgenerationafterVoyagerfirstrevealedtheircomplexstructure.Infact,CassinistudiesofSaturn’sringshavealreadybeenthoroughlyreviewedintwosetsofreviewchaptersandarticles;Inthefirstmajorreviewvolume“SaturnafterCassini-Huygens”,verythoroughreviewsaregivenbyColwelletal2009,Charnozetal2009,Cuzzietal2009,Horanyietal2009,andSchmidtetal2009,whichremainlargelyvalidasofthiswritingeveniftheypredatemanyimportantresults.Ashort,nonspecialistreviewisgivenbyCuzzietal(2010).Morerecent,completereviewscanbefoundintheCambridge“PlanetaryRings”book,includingCassini’sresultspriortotheRing-GrazingandGrandFinaleorbits(Charnozetal2018,Cuzzietal2018,Estradaetal2018,Hedmanetal2018,MurrayandFrench2018,Nicholsonetal2018,Spahnetal2018,andSpilkeretal2018).Finally,alltheinstrument-basedchaptersinthisvolumeupdatethestorytotheveryendofthemission,includingresultsfromCassini’slastmoments,andsomeprovideexhaustivebibliographiesaswell.Wewillattempttocoverthehighlightsofringsciencefrombeginningtoendofthemissionasorganizedbyourowngoalsandobjectives,and,inareadableway,provideathreadofcontinuityconnectingCassini’svastaccomplishmentsto
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thebigpictureofringoriginandevolution.Insummary-theringsarechangingbeforeoureyesdynamically,andtheyaremuchyoungerthanthesolarsystemcompositionally.
Wavesandcollectivedynamics:SpiraldensityandbendingwaveswerediscoveredbyVoyager.Theyaredrivenatorbitalresonanceswithvarioussatellites,blankettheAring,andaresprinkledthroughouttheBandCRings.Improvedanalysistoolshaveallowedeventheweakestofthesefeaturestobestudiedindetail,andtheyprovidepowerfulconstraintsontheunderlying,localsurfacemassdensityonseveral-kmspatialscales.AboutadozenspiraldensityandbendingwavesintheCringhavebeenshowntobecausedbygravitationalandpressuremodesinsidetheplanet,potentiallyconstrainingtheinteriorstructureofSaturnanditsrotationrate.
Ringmicrostructureonsub-kmscalesisseeneverywherebystellarandradiooccultations,andvariesonshorttimescales.Ubiquitoustransientgravitationalinstabilitiescalledself-gravitywakesarise,andaretornapartbydifferentialrotation.Thesewakesvaryinconfigurationacrosstherings,andtheirpropertiesimplyaringverticalthicknessoftensofmetersorless.Anunrelated,axisymmetrickindofmicrostructure,duemoretoviscousforcesthanself-gravity,canalsobeseenatvariousplacesinthedenserpartsoftherings.Imagestakenfromcloseorbits,withsub-kmresolution,showthatfine-scalestructureiswidespreadinopticallythickregions,someofitgranularandsomeaxisymmetricorstreaky.Denseclumpscalled“straw”,eachthesizeofaconvoyofaircraftcarriers,areseeninbetweenthedensecrestsofstrongspiraldensitywaves,probablycompactedasparticlespassthroughthecrests,andsubsequentlybrokenapartbycollisions.
Embeddedmoonlets:AnalysisofCassiniimageshavenowshownthatskirted,embedded10-kmsizemoonletsareresponsibleforopeningboththeEnckeandKeelergapsintheARing(seealsochapterbytheIcySatellitesWorkingGroup).Ingeneral,thesmallringmoonslyingwithinandneartotheringshaveverylowdensitiesandarelikelyrubblepileswithdensecores.However,moonletshavenotbeenfoundinsimilargapsintheCassiniDivisionorCringwiththe~1kmsizespreviouslythoughtnecessarytoclearthem.
Flocksofsmaller“propeller”objectsof100-200msize,detectableonlyintheirdisturbanceofnearbyringmaterial,occupythreeradialbandsintheARing.Adozenorsoofthelargestoftheseobjects,0.1-1kminsize,wanderinsemimajoraxis,perhapsbecauseofgravitationalscatteringbytheclumpyringmaterialtheyinteractwith.Discretekm-sizeobjectshavealsobeeninferredfromdisturbancesattheouteredgesoftheAandBringsandintheHuygensringletoftheCassiniDivision.OnesuchobjecthasbeenimagedintheBring.Someoftheseobjectsseemtodisaggregateandperhapsreaggregateinplace,ontimescalesofmonthsoryears,suggestingongoingrecyclingofmaterialdrivenbysatelliteforcing,self-gravity,andcollisions.
Ringsin3D:Cassinidiscovereddramaticverticaldistortionsoftherings,inadditiontothewell-knownspiralbendingwaves,nearEquinoxwhenthesun'silluminationwasatagrazingangle.TheedgesoftheKeelergaparenoticeablywarped,consistentwiththesmallinclinationofitscentralringmoonDaphnis,butthoseoftheEnckegaparenot.TheouteredgeoftheBringshowedalpen-like,spikypeaksandshadows.TheouteredgeoftheARingshowedaveryclumpystructure,lackingdistinctpeaksbutwithshadowssuggestingverticalrelief.
RingParticleproperties:Voyagershowedthatthemainringparticlesizedistributionstendtowardspowerlawsrangingbetweencm-mradii.Cassinihasshownthattheslopeandthe
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minimumandmaximumsizesofthesepowerlawsvaryacrosstherings.Enhancedcollisionsinspiraldensitywavescreateblizzardsofsmallparticles,whichseemtoaffectthebrightnessandcolorofthesurroundingregion.AsshownbycomparisonofoccultationsatUV,nearIR,mid-IR,andradiowavelengths,andbytheringspectraandcolor,thiseffectisespeciallysignificantintheoutermostARing,theregionmoststronglystirredbyspiraldensitywaves.
Acombinationof10-400µmthermalemissionandscattering,and2.2cmradiometry,suggeststhatringparticlesprobablyhavealowdensitymantle–ie,areporousaggregates-butmayalsohavemoredensecores.Therings'thermalemissionsuggestsaregolithofnearlypurewatericegrainsof10-100μmsize.Cassini2.2cmradiometrydetectsafractionofapercentofnon-icymaterialthroughtheAandBrings,greatlyimprovingonpre-Cassinimicrowaveobservationswhichcouldconstrainthetotalnon-icymaterialonlytolessthanafewpercentrelativeabundance.However(whencombinedwithspiraldensitywavesurfacedensities/opacities),2.2cmradiometryhasalsorevealedwhatlookslikeaburiedsilicate-richrubblebeltinthemidCRing–possiblytheremainsofthecoreoftheringparent.
VIMSspectrashowthatAandBringcompositionisnearlypurewatericewithoutanyotheridentifiableicessuchasCO,CO2,NH3,CH4,butthereisastrongUVabsorptionmakingtheringsunusuallyreddish,thatvariesinstrengthfromplacetoplace.Theredcoloroftheringsisstrongestwherethewatericeabsorptionbandsaredeepest,suggestingthattheredmaterialresideswithintheicyregolithgrains,andincreasesradiallyinwardsatasteadyrate.Athird,more“neutral”non-icymaterialhasacompletelydifferentradialdistribution.Theverylowmassfractionofnonicymaterial(exceptperhapsintheCRingrubblebelt)providesourstrongestconstraintontheageandoriginoftheRings,asdiscussedbelow.Therings’redcolorintheUV-visibleregionmaybeexplainedbyfragmentsofcarbon-bearingtholins(perhapssimilartomaterialblanketingthesurfaceofcometCGasobservedbyRosetta),or(someargue)bytinyiron-richparticles,embeddedinthedominantwaterice.RecentCassiniin-situresultsandremotesensinganalyses,andHST-STISobservations,favortheorganics.
NewinsitudatafromCDAandINMSontheRing-Grazing(henceforthRG)andGrandFinale(henceforthGF)orbitshavegreatlyenrichedtheringcompositionstory.Abundant,Fe-poorsilicatesandorganicmoleculesareassociatedwiththeinnermostDandperhapsCRings.EvidencefororganicsisalsofoundoutsidetheringsbyINMS.Reconciliationoftheseinsituresultswithremotesensingisonlybeginning.
Ringvariabilitywithtime:TheouteredgesofthedenseAandBringsvaryincomplexwayswithbothlongitudeandtime,indicatingtheinterferenceofmultiplefreeor“normal”modes–possiblytheresultoflarge-scale,nonaxisymmetricviscousoverstabilities--withtheexpectedsatellite-drivenforcedmodes.Theouter100kmoftheBringshowscomplex,fine-scalebrightnessvariationspossiblycausedbythesedeformations.GravitationaleffectsofthemodesmightevenplayaroleinsculptingthenarrowergapsintheCassiniDivision,sofarfoundtobefreeofsmallmoonlets.
ChannelsopenandcloseinSaturn'sFringstrands,inresponsetocloseapproachesbyPrometheus,andtheseeffectsvaryastheorbitsofPrometheusandtheFringmutuallyprecess,leadingtostrongerandweakergravitationalinteractions.Kinksand“mini-jets”comeandgointheFringcore,excitedbysmallunseenobjectsatlowrelativevelocities,andmoredramaticjetsofmateriallastingmonthsaretriggeredbyobjectseccentricenoughtocrashthroughtheringathighrelativevelocities.TheseviolentcollisionscontinuebecausetheentireFringregion
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isdynamicallychaotic,mostlybecauseofPrometheus,andforthisreasonthelong-termstabilityoftheFRingcorehasbeenproblematic.TheFRinghasanarrow``truecore"oflargeparticles,ascharacterizedbestbyRSS,confinedandstabilizedbyanewkindofdynamicaltrappingdueonlytoPrometheus.ThefinedustseeninimagesandstellaroccultationsisatinyfractionoftheFRingtotalmass.ClumpactivityhasvarieddramaticallybetweenVoyagerandCassini.
Verticalripplesorwarpscoveringtheinnerpartoftherings,whichhavechangedevenoverthedurationofthemission,suggestthatSaturn'sDandinnerCringsweretippedrelativetoitsequatorseveraltimesoverthelastmillennium,andasrecentlyasafewdecadesago.Impactsbyrubblestreamsproducedbydisruptedobjectsarethemostlikelycause.Severalimpactsbyindividualm-sizeprojectileshaveactuallybeenseenandcatalogued.
Diffuseringsdominatedbytinydustgrainsareaffectedbysunlightandmagneticfields:ThesourceoftheERingparticleshasbeenconfirmedasthesouthpolarjetsofEnceladus(seesectiononIcySatellites,thisvolume).Severalnewarcsandringletsareassociatedwitherosionofsmallembeddedmoons.TheERingandotherdiffuseringsandringlets(suchasthoseintheEnckegap)areaffectedbysunlightandelectromagneticforcesaswellasbygravity,andthusshowseasonalvariationsintheirstructure.Somediffuserings,notablytheDRingandthefaintmaterialbetweentheAandFRings,aremodulatedbyazimuthalvariationsinSaturn'smagneticfield.
Thedurationandbehaviorofthe“spokes”VoyagerdiscoveredintheBringhavebeenfurtherconstrained,thoughtheirultimatecauseortriggerremainsunknown.Cassinifoundthattheyseasonallyappearanddisappear,probablyduetovariablephotochargingofthemainringlayer,andtheirtemporalperiodicitiesmatchbestthatofthenorthernhemisphereSaturnKilometricRadiation(SKR)duringthe2008/2009timeframe,thoughcontributionsfromthesouthernSKRsourcecouldnotberuledout.AnyassociationwiththeSaturnElectrostaticDischarges(SEDs)wasruledout.
Ringmass:Voyager-eraestimatesoftheringmasswereroughly0.7-0.8Mimasmasses.Itwassincesuggestedthatthebreakdownoftheringlayerintoopaqueclumps(theself-gravitywakes),separatedbynearlyemptygaps,couldallowalotofmasstobe“hidden”intheclumps.ModelingofdozensofspiraldensityandbendingwavesoverCassini’sfirstdecadehadconstrainedthemassoftheA,C,andinnerBrings,soanyhiddenmasshadtoresideinthedensestpartsoftheBring.IntheGrandFinaleOrbits,theCassiniRSSteamtrackedthespacecrafttoconstraintheringmassbyitsperturbationonCassini'sorbit,andconcludedthattheringmassisnotverydifferentfrom,andlikelylowerthan,ourVoyager-eraexpectations.
OriginandAgeoftheRings:Thestrongestconstraintonringageisprovidedbythegradualdarkeningandrestructuringoftheringswithtimebyinfallingmeteoroids.Inferringtheactualringagerequiresknowledgeofthecurrentmassfractioninnonicy“pollutant”,aswellasboththeincomingmeteoroidmassfluxandthetotalmassoftherings.TheCassiniDustAnalyzerexperimenthasdetermined,cumulativelyovertheentiremission,theinfallingmeteoroidmassfluxanddynamicalpopulation.Themassfluxfarfromtheplanetisnottoodifferentfrompre-Cassiniestimates.However,thedynamicalpopulationislikethatofKuiperBeltobjects,sohasalowerencountervelocityandismorestronglyfocussedbySaturn'sgravitythanthoughtpreviously.Thismeansthatthefluxofmeteoriticmaterialactuallyhittingtheringsisprobablyevenlargerthanpreviouslythought.CombiningtheringmasswiththeCDA-
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determinedmeteoroidmassflux,andestimatesofnonicy“pollution”currentlyintherings,leadstoaringexposureageof100-200Myr,perhapsevenalittleyoungerthanVoyager-era“young-ring”scenarios.
Theassociatedpuzzlesof“WhyisSaturntheonlygiantplanetwithrings?”and“Istheringsystemyoung?”havebeen,ifnotyetexplained,atleastilluminatedbyanemerginghypothesisinvolvingaclosely-coupledtidalevolutionofthemid-sizedicymoonsleadingtoaSaturn-specificdynamicalinstabilityoftheinnersatellitesystem,withpossibledisruptionsandreaccretion,ontheorderof100Myrago.Constraintsfromsatellitesurfacegeologywillneedtobefoldedintoevaluatetheplausibilityofthescenario.(1)OverallAssessmentofRingandDustScience
NotonlywereallkeyscienceobjectiveslistedintheAOandSolsticeTraceabilitymatrixsuccessfullyaccomplishedoverthecourseoftheCassinimission,butanumberofnewscienceobjectiveswereformulatedandaddressedalongtheway,eitherinresponsetounexpectedresultsorcapitalizingonobservationalopportunities(bothsomenotoriginallyenvisioned,andsomeingreaternumbersthanenvisioned).Whenthemissionwasapproved,theAOObjectivesreflecteda4-yearbaselinetourwithonly59orbits,allowingatmost16Radiooccultations,andofcoursedidnotincorporatetheRingGrazing(henceforthRG)orGrandFinale(henceforthGF)observinggeometries.Bytheendofthemission,eventhefour-yearbaselinetourhadbeenoptimizedtoinclude74(generallyshorter)orbits,wastinglesstimefarfromtheplanet,andhadalsobeenextendedbynineyears,triplingthetotaltimebaselineforobservationsrelativetotheAO.Thefinaltourlasted13years,samplingthreedifferentseasonsandreachingNorthernSummerSolstice(themaximumopeningangleoftheringsasseenfromEarthandthesun)whichallowedtheradiooccultationstobetterpenetratetheopticallythickestpartsoftheBRing.Themissionincluded282orbitscapturingmanynewcombinationsofilluminationandviewingangles–importantforimaging,VIMSandUVISspectralreflectancemapping,andCIRSthermalmapping.Datasetsreturnedincluded135ringradiooccultations(eachat3wavelengths),170,000ringimages(manyat<1km/pxl),over200UVstellaroccultations,170near-IRstellaroccultations,30solaroccultations,andevenafewoccultationsatthermal,mid-IRwavelengths.Thestellaroccultationsmadeuseofdozensofstarswithdifferentelevationanglerelativetotherings;low-opticaldepthringsarebeststudiedbylow-elevationstars,andhighopticaldepthringsbyhigh-elevationstars.Inaddition,severalcomplete2.2cmmicrowaveemissionmapsorradialscancombinationswereobtainedindifferentgeometriesandresolutions,and3activeradarbackscatteringscanswereobtainedintheRGandGForbitswhenthespacecraftwascloseenoughtoovercomethefourth-powerdependenceofradarsensitivityondistance.Moreover,theRGandGForbitsprovidedIn-situringparticlecompositionsamplingopportunitiesforCDA,INMS,andMIMIthatwerenotpartoftheAOplan,andenableddirectdeterminationoftheringmassfromspacecrafttrackingoftheRSSsignal.VIMSstellaroccultationswerenotevenenvisionedintheAO,butwereaddedafterselectionoftheinstruments.ThisnewstellaroccultationcapabilityprovidedaveryusefulwavelengthdifferencecomparedtoUVIS,andanentirelydifferentangulardistributionofsourcesonthesky.UsingtheRadarasapassive,pencil-beammicrowaveringbrightnessmapper,ortoobtainactiveradarbackscatteringradialprofilesfromtherings,werealsonotpartoftheoriginalplan.
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TheoverallsciencedatavolumereturnofCassiniwasmorethan100timesthatofVoyagerintheinstrumentstheybothcarried,nottomentiontheroughly200timeslongertimebaselineatinterestingresolution,andthenumerousuniquenewremoteandin-situobservationsprovidedbyCassini.ThereisnoquestionthatthesciencereturnofCassinivastlyexceededtheexpectationsasdescribedintheAO.(2)SummaryofKeyOpenQuestions
Overall,CassinihasansweredmanyofthequestionsraisedbyVoyager,raisednewones,andleavesuswithalargelyuntappedreservoirofdatawithwhichtoanswerthemall.Theseandothersarediscussedinmoredetailinsection6.
Theoverallringmassisknownnow,butwithlowprecisionandnoradialresolution.IstheBringmassuniformlydistributedaccordingtoopticaldepth,orisitevenmorestronglyconcentratedinthestillnearlyopaquecentralregions?DoestheCRingcontainarockyrubblebelt?Futuremissionswithtrackingcapabilityandmultipleclosepasseswillbeneeded,andshouldbeplannedtoalsoprovideabetterunderstandingofwhySaturn’sowninteriorissodifferentfromJupiter’s.
Mostringstructureisstillapuzzle!TheirregularstructureblanketingtheentireBRing,withlengthscalesnarrowlyconcentratednear80kmintheinnerBRingbutcoveringawiderrangeofscalesfromtheresolutionlimittoseveralhundredkminthemid-andouterBRing,remainsamystery.Intheinner-midBRingthereareverysharpoptical-depthjumps(butnottoemptyspace)thatarebarelyexploredandremainunexplained.
Theentireensembleofso-called“plateaus”intheouterCRing,symmetricallyplacedaroundtheMaxwellgapintheCRing(whichisknowntobecausedbystructureinSaturn’sinterior)andincorporatingsomeotheremptygaps,Isnotunderstood.Theplateausalsocontainauniquekindof“streaky”microstructurenotseenelsewhere,thatwasonlyseeninalimitedwayduringthefinalorbits.
Thereareseveralsignificantfeatures(Aringinneredge,CRingplateaus)whichhavesharp,well-definedboundariesthataredefinedonlybychangesintheparticlesizedistribution,acrosswhichthesurfacemassdensitydoesnotchange.Thisstructuralcontrolofparticlesizedistribution,orviceversa,isnotunderstood.
ThemerepresenceoftheCassiniDivisionisanunsolvedproblem,althoughitisplausiblethatastrongdensitywavedrivenbythe2:1resonancewithMimashadaroleincreatingitwhentheringsweremoremassiveandmaterialfilledtheregion.Theapparentlackofmoonletscapableofclearingthe13gapsintheCassiniDivisionandCring,especiallythe5-6thathavenoembeddedringlets1,isapuzzle.Acluemaybefoundintheapparentlynon-randomspacingsoftheedgesoftheCassiniDivisionbandsandgaps,perhapssomehowmanifestinggravitationalinfluenceoftheBRingouteredge.
Theunusually“red”coloroftheAandBrings,whicharemorethan95%waterice,seemstobeduetoorganicmaterial,basedonacombinationofremoteandinsitumeasurements.However,compositionvariesfromplacetoplaceandonarangeofscales.1CassiniDivisiongapswithringlets:Huygens,Herschel,Laplace;andwithoutringlets:Russell,Jeffreys,Kuiper,Bessel,Barnard;Cringgapswithringlets:G1,Colombo,Maxwell,Bond;andwithoutringlets:Dawes
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Modelscapableofinterpretingcolorandspectraldataquantitativelyintermsofunderlyingcompositionareinadequate,andneedimprovement.Thehintofarubblebeltofnonicymaterialburiedinthemid-CRingisintriguing,butremoteobservationsareambiguousandtheanalysisofinsituobservationsisonlygettingunderway.Whetherornotorganicsexplainthesimilar,butrelativelyweak,UVabsorptionseeninmostoftheicymoonsisnotyetknown(thedarksideofIapetushasbeenfitbycarbon,nano-iron,andmetaloxides).WhyistheGRingparentbody,soclosetotheicymainrings,sodark?
AretheA(andB)ringpropellerobjectstheshardsleftoverfromtheringparent?Whydothedozenorso“giantpropellers”foundonlyoutsideoftheEnckegapwanderinsemimajoraxis,andwhat(ifany)patternsarepresentintheevolution?Aretherecyclic,self-limitingprocessesofgrowthanddisruptionofsub-kmsizedobjectsintheouterA(andB)rings?TheFRingcoreisstabilizedbyPrometheus,butwhatcausesitsuniformlyprecessingeccentricity?DorelativelylargeobjectsforminregionscompressedbyPrometheusandgoontocrashthroughthecore,creatingclumpsandstrands?
TheringsseemtobemuchyoungerthantheSolarSystem,basedonnominalresultstodate,andthereisoneencouraginghypothesis(a200Myr-agosystem-wideinstability)providingthepropercontext,butquestionsremainincludingconsistency(orlackofit)oftheobservedicymooncrateringdistributionswithreaccretionofmostoftheinnericymoonsonthatsametimescale.
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(3)KeyObjectivesoftheRingsWorkingGroup TheoriginalringobjectivesfromtheAO,andthoseaddedfortheextendedCassiniSolsticeMission,aregivenbelow.AscanbeseenintheTable,WeassessCassiniasfullysuccessfulatleast,inallregards.Itcannotbeemphasizedenoughthatinalmostallcases,therehasbeenfarmoredataobtained,reduced,andarchivedthantheteamshavehadtimetoanalyzeandinterpret.
RWGScienceAssessment,matrixformat
RWG Science Objectives
AO and TM
Science Objectives
RWG Science Assessment
Comments if yellow (partially fulfilled)
Ring Structure and Dynamics R_AO1 Ring Particle Composition and Size R_AO2 Ring-Satellite Interactions R_AO3 Dust and Meteoroid Distribution R_AO4 Ring-Magnetosphere-Ionosphere Interaction R_AO5 Changing Rings RC1a Ring Temporal Variability RC1b F Ring RC2a Ring Age and Origin RN1a Ring Composition RN1b Ring Structure RN1c Ring Microstructure RN2a New Ring Structures RN2b Theseobjectivesarediscussedbelowinmoredetail.
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(4)RWGScienceResults4.1PrimeMission(AO)OriginalObjectives(Highest,top-levelresults)4.1.1RingStructureandDynamics(R_AO1)-Studyconfigurationoftheringsanddynamicalprocesses(gravitational,viscous,erosional,andelectromagnetic)responsibleforringstructure.
Thisobjective–perhapsthebroadestanddeepest-wassatisfiedbeyondourinitialexpectations.Stellarandradiooccultationsarethemostpowerfultoolforexploringstructureathighresolution.Duringinitialplanningsessions,theUVISteamwasconsideringdozensofstellaroccultationsasanoptimisticgoal,andendedupwithover200,includingthenovel“turnaround”occultationsinwhichthelineofsighttothestarmovesparalleltotheparticleorbits,atverylowvelocity,allowingazimuthalstructuretobedeterminedathighresolutioninperhapsadozenradiiandstructureonthescaleofaringthicknesstobeaddressedstatistically.Moreover,VIMSdevelopedastellaroccultationcapabilityoftheirownthatwasnotpartoftheoriginalplan,andwentontoproducealmost200occultationsoftheirowningeometriesthatwereindependentofandcomplementarytothoseavailabletoUVIS,becausetheyobservedadifferentsetofstars.AllthesestellaroccultationsresideontheRingsPDSnodein1and10kmresolutionformat.Inaddition,RSSconducted135occultationsatthreeradiowavelengths(0.94,3.6,and13cm),whichallresideontheRingsPDSnodein1and10kmresolutionformat,andforwhichfurtherprocessing(bydiffractioncorrection)cangenerallyprovide100morevenbetterradialresolution.Moreanalysisofthesedatawillprovideadditionalsurprisesandconstraints.ReviewchaptersbyColwelletal(2009)andCuzzietal(2018)coverthemajoradvancesregardingobservedringstructure;Schmidtetal(2009)reviewadvancesinthetheoryofdenseringstructure,andHoranyietal(2009)reviewadvancesinthetheoryofelectromagneticinfluences(mostsignificantontinydustgrains).
(a)Configurationoftherings:The``GrandStructureoftherings”(Figure1)waselucidatedforthefirsttimebyVoyager(reviewedbyOrtonetal2009andColwelletal2009).Inthissectionwewilldiscusstheso-called“Mainrings”–theA,B,andCringandtheCassiniDivision,allofwhichhaveopticaldepthgreaterthanabout0.1(Figure1).TheAringisthebestunderstoodbyVoyager-eratheory-blanketedbyspiraldensityandbendingwavesfromnearbymoons,withembeddedmoonletsthatcarveemptygapsbytheprocessofgravitationalshepherding,andaubiquitousnonaxisymmetric“microstructure”havingscalescomparabletotheringthicknessduetolocalgravitationalinstabilities.AllofthesestructuralpropertieshavebeencloselyobservedbyCassini,ingreaternumbers,withgreaterradialcoverage,inmoregeometries,infinerdetail,andatbettersensitivitythaneverbefore.TheBandCringscontainafewofthesewell-understoodfeaturestoo,butareoverwhelminglydominatedbystructureonallscalesthatissimplynotunderstoodatpresent(seesubsectione,below).Theso-called“Diffuse”D,E,andGrings,anddustyringsassociatedwithvariousmoonlets,haveopticaldepthsmuchlessthanunityandareaseparateclassofstructurediscussedinsection4.1.4-4.1.5.Thenarrow,kinkyFRinglyingjustoutsidetheARinghascapturedanicheofitsownandisdiscussedmostlyinsection4.2.3.
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Thelocal“ringthickness”wasaperennialtopicofdiscussionbeforeVoyager,andmeasurementsofkilometerswerereported(nowknowntobebetterexplainedbyverticalcorrugationsand/ortheinclinedFRing).ThesingleVoyagerstellarandRadiooccultationsshowedthatringedgesweresosharpthatthelocalthickness-atleastatedges-couldbenomorethantensofmeters.Cassini’shundredsofstellarandradiooccultationshaveextendedthisknowledgetonearlyalllocationsexcepttheopaquecentralBring(Colwelletal2006,2009;Hedmanetal2007;seealsoTiscarenoetal2007).Thissmallverticalthicknessmeansthattheringsarenotthe“manyparticlethick”classicallayerenvisionedevenintheVoyagerera,andhaveamoderatevolumedensityof0.01-0.1(SaloandFrench2010)orevenlarger.Thereareimplicationsofhighvolumedensityforringviscosity(see(c)below).Anotherindicationofvolumedensity(andthuslocaldynamics)comesfrommutualshadowing.Visualbrightnessobservationshavelongshownasharp“oppositionpeak”thatwastraditionallyinterpretedintermsofshadowinginamany-particle-thicklayer;howeverdynamicalconstraints,andtheincreasedrecognitionoftheimportanceofcoherentbackscatteringingrainyparticleregoliths(seeCuzzietal2009forareview)complicatethisinterpretation(SaloandFrench2010).CIRSobservationsshowamuchbroaderphaseeffectinparticletemperatures,however,thatisplausiblyshadowing-related,unlikelytobecontaminatedbycoherentbackscattering,andconsistentwithnonclassicalrings(Altobellietal2007,2009;Reffetetal2015,Morishimaetal2017).Thesethermalstudiesleadtomidplaneparticlevolumedensitiesaslargeas0.3-0.4intheBring.Finally,analysisof2.2cmradiometryisbestfitbymodelingcloselypacked,cm-msizeparticlesinthenear-fieldscatteringregime(Zhangetal2017b).
Therehasbeenastarttakenonmodelingofimagingobservationsoftheringsoverawiderangeofgeometries,withanemphasisonunravelingoppositioneffectsofdifferentkindsusingclassicalradiativetransfermodels,(Deauetal2013,2018;Deau2015).However,interpretingUV,visual,andnear-IRobservationsusingrealisticmodelsofcloselypackedparticleswithrough,shadowedsurfaces,whichmightprovidemoreinsightintoradialvariationsoflocalparticlevolumedensityandthuslocaldynamics,remainsinitsinfancy(seePorcoetal2008fortheonlyexample).
(b)Gravitationalprocesses(self-gravitywakes,spiralwaves,andshepherdingtorques):Probablythelongest-knownandmostwidespreadgravitationalprocessleadstothe“self-gravitywakes”,causedbylocal,transient,incipientgravitationalinstabilitieswhicharestretchedintotrailingclumpsofscalecomparabletotheringthickness(indeeddefiningtheringthickness)byKeplerianshear,keepingtheringinaconstantstateoffrustratedsatelliteformation(Ortonetal2009,Schmidtetal2009,Colwelletal2009).ImagingobservationshavecharacterizedtheeffectintheAring,usingtheazimuthalbrightnessasymmetryknownfordecades,butfindingtheparticlecollisionalelasticitytobesmallerthanthought–indicatingeitheraroughsurfaceoraporousregolith(Porcoetal2008).TheeffecthasalsobeenobservedandmodeledinthethermalIR(Leyratetal2008,Ferrarietal2009,Morishimaetal2014;seesection4.2ofFlasaretal,thisvolume,andSpilkeretal2018).A“CAT-scan”techniqueofcharacterizingthehorizontalandverticalscalesofthesestructuresaswellastheiranglerelativetotheorbitdirectionwasdevelopedbasedonstellaroccultationsfromavarietyofslantdirections.Thesemodelsofthedensegravitationalwakesasopticallythick“granolabars”orellipticalcylinders(Colwelletal2006,2007;Hedmanetal2007a,NicholsonandHedman
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2010,seeColwelletal2009forareview)providevaluablemeanvaluesoftheorientation,width,height,andseparationofdenseclumpsforuseinmodelsofringbrightness.Thepitchangleofthewakes,relativetotheorbitdirection,decreasesradiallyinwardsasdifferentialrotationbecomesstronger.
Perhapsthemostobvious,beautiful,andusefulgravitationalringstructuresarespiraldensityandbendingwavesatisolatedsatelliteresonances.Thesestrikingfeaturessharethephysicsofthearmsofspiralgalaxiesbutaretightlywrappedlikewatchsprings(Shu1984;Schmidtetal2009).Theypropagateawayfromtheirdrivingresonancewithadecreasingwavelength–densitywavesmoveoutwardsandbendingwavesinwards.Thewavelengthitselfisdiagnosticoftheunderlyingsurfacemassdensityonscalessmallerthanawave.Thus,spiraldensitywavemeasurementsprovideourbestideaoftheradialprofileofmassdensity,inregionswheretheyarefound.Thispropertyisextremelyimportant,becausethealmostubiquitousclumpingduetoself-gravitywavesbreakstheringsintoasetofopaqueregionsseparatedbynearlyemptygaps,forwhich“theopticaldepth”(seefigure1)isofquestionablebasicvalue.
Forthisreason,Robbinsetal(2010)suggestedthatinferencesofringsurfacemassdensityfromacombinationofobservedopticaldepthandparticlesizedistributionmightgreatlyunderestimatetheringmass.ThisreasoningledtoadebateastowhethertheringmasscouldbemuchlargerthanVoyager-eraestimates,becausemostoftheBRing(whichcontainsmostofthemass)isunsampledbydiagnosticdensityandbendingwaves,withimplicationsforringage(seesection4.2.4).ThedebatetiltedinfavoroflowmasswhenHedmanandNicholson(2016)succeededinobtainingsomeBringdensitywavemeasurementsdespitethenoisybackground,andhasnowbeendefinitivelyresolvedinthatdirectionbytheRSStotalmassmeasurementsbasedongravitationaltracking(section4.2.4a).TheRSSgravitymeasurementshavelittleornoradialresolution,butusedAandCRingmassesasdeterminedfromthesewaves(Colwelletal2009,Tiscarenoetal2013)andassumedtheremainderofthemeasuredmasstobethatoftheBring.Theydosuggest,though,thatthesurfacemassdensitiesobtainedfromspiraldensitywavesareinsensitivetothesuggestedclumpingdifficulty. Oneimportantsurprisefrommeasuringsurfacemassdensityisthatseveralimportantstructuralfeatures,longseeninimagesandoccultations,actuallymanifestasdistinctstructuresnotbecausetheyhavehigherlocalmassdensity,aswasusuallyassumed,butbecausetheirlocalparticlesizedistributionchangesinadramaticway,makingthemmoreopaquethantheirsurroundingswithoutmuchchangeintheirunderlyingsurfacemassdensity.Thishappensbecausethesurface-to-massratioofaparticle,orofadistributionofparticles,increasesasparticlesizedecreases.TheeffectisseenattheinneredgeoftheAringandalloftheCRingplateaus–sharpopticaldepthjumpsareseenwherethereIsatmostagradualchangeinsurfacemassdensity(Tiscarenoetal2007,2013a;TiscarenoandHarris2018.Baillieetal(2011)andHedmanandNicholson(2014a)showsurfacemassdensitiesfortheplateausandbackgroundCring,andColwelletal.(2018)showdropsineffectiveparticlesizeintheplateaus,consistentwithasmallchangeinsurfacemassdensitydespitealargeincreaseinopticaldepth.Thisimportantphenomenonoflocallyabruptchangesintheparticlesizedistributionisnotcurrentlyunderstood,butisclearlyofcentralimportancetounderstandingtheformationofthesestructures.
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Anotherimportantcluefromdensityandbendingwavesisobtainedfromtheir(sparse)samplingoftheCRing,showingthatthesurfacemassdensityradialprofilediffersfromtheopticaldepthprofileinafashionmosteasilyexplainedifthelocalparticles,whosesizesarewellknownfromRSSoccultations,areseveraltimesmoredenseinthecentralCringthanelsewhereinit–possiblyhintingatburiedrocksinunusuallyhighabundance,andperhapstoahiddenrockyrubblebelt(Zhangetal2017a;seesection4.1.2onringorigin).SamplingoftheBRingisevenmoresparse,butgenerallypointstosurfacemassdensitiessomewhatlowerthanpreviouslythought(HedmanandNicholson2016,TiscarenoandHarris2018)
AninterdisciplinarybonuswastheconfirmationofthepredictionbyMarleyetal.(1987,1989),Marley(1990),andMarleyandPorco(1993)thatdensityfluctuationsinSaturn’sinteriorproducedbyplanetaryscaleacousticoscillations(modes)candrivespiraldensityandbendingwavessimilartothosedrivenbyanexternalsatellite.AnumberofunidentifiedwaveshadbeennotedbyRosenetal(1991)inVoyagerRSSoccultationdata.MarleyandPorcosuggestedseveralspecificassociationsbetweenpredictedresonancelocationsandthese“Rosenwaves”.Theyalsosuggestedthatthestrongestpredicted(two-lobed,orm=2)planetaryinteriorresonancemightbeassociatedwiththeunexplainedMaxwellgap,wheretherewasanunexplainedeccentric(m=1)ringlet.
Cassini,alongwithsubsequentimprovementsintheSaturninteriormodel,haveconfirmedMarleyandPorco’spredictions,puttingthestudyof“ringseismology”onfirmgroundandfindingintheprocessotherfeatureslikelyduetovariationsininternalstructurenotaccountedforinthesimple1993interiormodel.ThegreatadvantageofCassinidataismultiplicityandaccuracyofoccultationmeasurements,allowingboththenumberofspiralarmsandthepatternspeedofthewavestobedeterminedunambiguously,andthusfinerassociationstobemadewiththeoreticalpredictions(HedmanandNicholson2013,2014a,Marley2014).Initially,someRosenwaves,andnewlydiscoveredones(Baillieetal2011),werefoundtohavethesamevalueofm,somethingnotpredictedbyMarleyandPorco.However,Fulleretal(2016)showedthatallowingacompositionally-stratifiedlayerdeepinsideSaturncouldexplainthesemultiplicitiesbecausesuchagradualdensitytransitionwouldallowdifferenttypesofmodestomixinsidetheplanet.Next,atwo-armedspiraldensitywavewasdiscoveredwithinthe(m=1)Maxwellgapringlet(Frenchetal2016b),validatingtheassociationofthegapwiththestrongest(m=2)modeaspredictedbyMarleyandPorco(1993).Finally,Mankovichetal(2018)haverecentlyshownthatslightlychangingtheinteriorrotationperiodofSaturnallowsthedetailedpredictionsofMarleyandPorco(1993)tobeverywellmatchedacross16setsofwavesoverawiderangeofrelativestrengths,withthestrongestperturbationopeningtheMaxwellgap.Infact,thisexcellentagreementprovidesthebestconstraintyetonthedeepinteriorrotationperiodofSaturn–afundamentalpropertythathasbeenelusivebecauseofthealmostcompletelackofmagneticfieldazimuthalasymmetry(Doughertyetal2018,andtheMAPSDWGreport,thisvolume).Inaddition,anewclassofdensitywaveshasbeenfoundintheBandArings–wavesthataredrivenbydensitystructuresofunknownorigin,fixedtothebodyofSaturn(El-Moutamidetal2016a,b).Surprisingly,someofthesewavefeaturesareactuallyseentodriftradiallyinwardsintherings,indicatingtemporalchangesinSaturn’sinteriorstructureonadecadetimescale(Hedmanetal2014a,2017,2018b).So,ringpropertiesandphysicshaveledtothreenew,importantinsightsintothedeepinteriorofSaturn.
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Seealso4.1.3formorediscussionofinteractionsbetweensatellitesandmoonletswiththerings,whicharemostlygravitational.
(c)Viscousprocesses:Becauseoftheirinnumerableparticlesinaconstantstateofgentlecollisions,theringsactlikeagasorfluidandobeytheequationsoffluiddynamics,withlocalpressureandviscosity.Theyspreadradiallyduetoviscosity,animportantaspectoftheirevolutionthatcombineswithotherangularmomentumandmasstransportprocesseslikeshepherding,wavetorques,and“ballistictransport”ofmeteoroidejecta.Thetheoryofspiraldensitywavesismatureenoughthattherateatwhichtheiramplitudedampsastheypropagatecanbeusedtoconstrainthelocalringviscosity(Schmidtetal2009,2016;Colwelletal2009).
Asecondkindofso-called“ringmicrostructure”,calledaviscousoverstability,occursonscalesoftheringverticalthicknessandispartlydrivenbyviscosity(Schmidtetal2009).Inthiseffect,astable(non-growing)radialoscillationcandevelopthatcomprisesaxisymmetric,alternatingringletsofhighandlowdensity,thatpulsateontheorbitaltimescale.ThesestructuresarealignedwiththeorbitaldirectionandwerefirstdistinguishedbysubtleDopplereffectsinoff-axisscatteringduringCassiniRSSoccultations(Thomsonetal2007)buthavealsobeendetectedusingstatisticalanalysisofstellaroccultations(Colwelletal2009,Hedmanetal2014b).Theyarewidespreadacrosstheringsandgenerallyfoundwiththeexpectedlengthscales–comparabletoaringthickness-inregionswheretheopticaldepthismoderatetohigh.Self-gravityplaysonlyasmallroleinthesestructures.
Viscousoverstabilitiescantakeonnon-axisymmetricformsaswell.Forexample,inadenselypackedring,collectivebehaviorsakintothoseseeningranularflowcanoccur,andviscousstresscanactuallydecreasethedampingbetweenadjacentorbitalmotions,effectivelybylockingparticles,andthereforetheirorbits,together.Wherethereisalsotheopportunityfordoublereflectionofspiraldensitywaves,aswithinanarrow,isolatedringboundedbysharpedges,oraso-called“resonantcavity”,theycanbecomeamplified(Borderiesetal.1985),generatinglow-order(m=1,2,3,etc)modes.Them=1andm=2structureofthenarrow,sharp-edgedringsofSaturn(theMaxwellringletinparticular,exhibitsanm=2spiraldensitywaveforcedbyanm=2acousticmodewithinSaturn(seeSection4.1.1babove)butattainsanm=1overallstructure)andUranusmaybeexplainedinthisway.However,Cassinidiscoveredlarge-scale,unforcedor“free”modeswithm=1,2and3intheouterportionoftheBRing,inadditiontotheresonantm=2modeforcedbyMimasattheBring’sedge(SpitaleandPorco2010;section4.2.2).Thelarge-amplitudem=1mode,inparticular,woulddampquicklywithoutboththeoverstabilizingviscouseffectsdescribedaboveandtheamplificationaccompanyingmultiplereflectionswithintheresonantcavityformedbytheBringouteredgeandthemode’sownresonantradiussome250kminwards.Thediscoveryoftheseunforcedmodesisthefirstindicationofsubstantialwaveamplificationonlargescales,inbroadrings.Thesemodesmayhaveapplicationstoothercelestialdisks,suchasprotoplanetarynebulae(Laughlinetal.1997)andspiralgalaxies.
Viscosityandpressureduetoringparticlecollisionsdeterminestheabilityofringparticlestomovefromonefaceoftherings(saythelitface)totheother(unlit)face,andmodelshavebeenappliedtoCIRSthermalobservationstoaddressthisconstraint(Flandesetal2010,Pilorzetal2015,Morishimaetal2016).Anothermanifestationofparticlecollisionswouldbetopacksurfaceregolithstovaryingdegrees,dependingonlocaldynamics.CIRStried
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tomeasureradialvariationsinparticlethermalinertia(higherformorepackedparticlesurfaces),butthecomplexitiesofthemodels,thecomplicationsduetosmalland/orrapidlyrotatingparticles,andtheincompletenessoftheobservingphasespacehassofarprecludedmorethanabasicdeterminationthattheringparticleslooklikeotherfrostyoutersolarsystemobjects.ThechapterbyFlasaretalinthisvolumedescribeshowmodelsofringthermalobservationscanaddresstheseandotherissues,inmoredetail.
(d)Erosionalprocesses:“Pollution”ofthemostlyicyringmaterialbymeteoroidbombardment,erosionofringparticlesintoimpactejecta,andrestructuringoftheringsby“BallisticTransport”oftheejecta,werefirstsuggestedanddiscussedinthepre-Cassiniera(seeDurisenetal1989,1992;CuzziandEstrada1998;andsection4.2.4cformorediscussion).Theprocessisalittlesubtleandcounterintuitive(seemorerecentworkinCharnozetal2009andEstradaetal2015).Extrinsicmeteoroidshittheringsattensofkm/s,notonlydepositingtheirsubstantialamountofnonicymaterialbutejectingchipsofthetargetringparticleat1-100m/s;thesechipsgooffonorbitsthatre-impacttherings,wheretheybecomepartofanewringparticle.ExchangesoflossesandgainsofmassandangularmomentumcanresultinsignificantradialrestructuringoftheringsurfacemassdensityoveratimeshortcomparedtotheageoftheSolarSystem(givenwhatwenowunderstandaboutthemeteoroidflux;seesection4.1.4).
Signaturesofthisprocesscanbefoundbothinringstructureandringcomposition.Todate,intriguingpossiblecorrespondencestoobservedstructurehavebeenseen(amongstthemtheabruptinneredgesoftheAandBringsandtheoddlinearrampsinwardofthem,andthe80-kmscale“irregularstructure”intheinnerBandinnerArings),andagoodmatchisprovidedtothesmoothcompositionalprofilethataccompaniestheabruptBRing-CRingboundary.Theseindependentstructuralandcompositionalpropertiessuggesta“young”ringageofafewhundredmillionyears,extendingthesimpler“pollution”argumentrelyingondepositionofnonicymaterialalone(sections4.1.2),butthemodelshavemanyparametersandfacenewchallengesfromnewdiscoveriesofseveralkinds(seesection6).
(e)Electromagneticandradiativeprocesses:Particlesinthespaceenvironmenttendtoattainamaximumchargethatisonlylargeenoughtoaffectthedynamicsoftiny(atmost10micronsize)grains.Anentirebranchofringdynamicshastodowithresonancesinvolvingforcingoftinychargedgrainsbytheperiodicallyfluctuatingplanetarymagneticfield–so-called“LorentzResonances”(Burnsetal1985,SchafferandBurns1987,seeHoranyietal2009andHedmanetal2018aforreviewsoftheseandotherelectrodynamicaleffects).Themostobviousplaceswheretheseprocesseswillbeimportantisinthediffuserings,suchastheD,E,andGrings,whichareknowntobemosteasilydelineatedbytheirtinyparticles(Hedmanetal2009,2018a).TheERingshowsthemostclearinfluences,includingitspreferenceforanear-monodispersionofparticlesizeataroundonemicronradius,aradialprofilewithalocalminimumatthelocationofEnceladus,and3DvariationsthatcorrelatewiththeorientationrelativetotheSun(Hedmanetal.,2012,Yeetal2016).Itisnowknown,ofcourse,thattheERingisthefrostybreathofEnceladus,andanalysesofISSimageshaveclarifiedhowitisfedinsomedetail(Mitchelletal2015).
Asecondringphenomenonwidelythoughttobeconnectedtoelectromagneticforcesistheflickeringofshadowy“spokes”acrosstheAandBrings,discoveredbyVoyager(seereviewsbyOrtonetal2009andHoranyietal2009).VariousperiodicitiesintheVoyagerspokeoccurrencerates,rotationratesatspokeboundaries,andtheangularandspectralscattering
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propertiesofspokeregionshaveimplicatedelectromagneticeffectsactingontinygrains(withsizesmostrecentlyshownbyD’Aversaetal2010).Thetinygrainsareprobablyreleasedsporadicallyfromringparticlesurfacessomehow,perhapsbym-sizeparticleimpacts,energybeamedfromtheplanet,ormagneticfield/plasmainstabilitiesnearthering(reviewedbyHoranyietal2009).Earth-basedobservationsalsocontributedtothisunderstanding(McGheeetal2005).SurprisinglytomostoftheCassiniteam,spokeswereinvisibleonapproachandforthefirstyearormoreofthemission,firstseenonlyweaklyinlate2005(Mitchelletal2006).ThisobservationsupportedanideafirstsuggestedbyNitteretal(1998)thatseasonalchangesinphotochargingbytheSunwereresponsible.Inthistheory,dustisconstantlybeingloftedoutoftheringplanebyimpacts,butwhenthesolarelevationishigh,electronsarephotosputteredoutofthedensemainringlayer,causingthemainringstobepositivelychargedwhileembeddedinaverticallyextended,negativelychargedelectronplasma.Grainsejectedfromtheringsbecomenegativelychargedinthisplasmaandaresweptimmediatelybackintotheringsbythestrongverticalelectricfield.Theprocessislikeanelectrostaticdustprecipitatorinanindustrialsmokestack.Thetheorypredictedthatatacertainlowelevationangle,theelectrostaticprecipitationwouldceaseandspokeswouldreappear(Voyager1and2flybyswerebothatatimeoflowsolarelevation),andindeedtheirdisappearanceagainayearorsoafterequinoxprovedthistobethecase.Duringthebrieftimespokeswereabundant(roughly2009-2011),detailedstudiesoftheirmorphology(Mitchelletal2013)showedthatthespokeshadextendedactivegrowthtimes,bothradiallyandazimuthally,andthattheiractivitylevelwasmostlikelyassociatedwithoneoftheSaturnKilometricRadiation(SKR)periods,butmaximizesatadifferentlongitudeinthatsystemthanfoundbyVoyager.
Finally,certaindustyringletsinvariousotherwiseemptygapsintheringssuggestevidenceforelectromagneticorevensolarradiativecontrolontheirpredominantlymicron-sizedgrains.Onedusty,eccentricso-called“charming”ringletintheoutermostCassiniDivisiongap,thatwasnotverynoticeableduringeitherVoyagerencounter,alwayspointsitsapoapsetowardsthesunduetoradiationpressure(Hedmanetal2010a).Severaldusty,clumpyringletsintheEnckegaphavesimilarproperties(Hedmanetal2013c).Ingeneral,onewouldexpectthatthesetinygrainsareconstantlyresuppliedbygenerallyunseenstrandsorclumpsofmacroscopicparticleswithcomparablylowopticaldepths,butsuchabeltofmoremassiveparticleswouldnotbeinfluencedbyelectromagneticorradiativeforces.4.1.2RingParticleCompositionandSize(R_AO2)-Mapcompositionandsizedistributionofringmaterial. Asdescribedinmoredetailinsection4.2.4c,understandingthenonicymassfractionintheringsiscritical,asitprovidesperhapsthestrongestconstraintontheageandoriginoftherings. (a)Composition(remotesensing):Thegoalwastodeterminethecompositionoftheringmaterialwiththebestpossibleradialresolutionacrossthemainrings.SpectralobservationsatUV,visual,andNIRwavelengthsaregenerallyinsensitivetoringparticlesizebecausetheringparticleswereknowntobemuchlargerthanthesewavelengthsfromgroundbasedandVoyagerobservations.Knownringstructureisonscaleson100-300km,soobservationsneededtobeplannedtoenableVIMS,UVISandCIRS–allhavingmuchlowerresolutionthanISS–toresolvethesescales.Longerobservationtimesallowingdeeper
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integrationswerepossibleatgreaterdistances,andhighSNRspectraofbroadsegmentsoftheA,B,andCRingswerealsoobtained.Astimewenton,welearnedmoreabouttheobservationsandabouttherings.Forinstance,Saturnshinewasfoundtocontributeorganic-lookingspectralfeaturesincertaingeometries(ringlongitudesbetween120-240oSolarhouranglesasobservedfromhighphaseangles,forinstance).TheVIMSandUVIScalibrationpipelinesevolvedandimprovedasthemissionwenton,anddataanalyzedandpublishedearlyinthemissionmightstillprofitfromreanalysis.ISS,whilehavingmuchhigherresolutionandtypicallyobtaining3-5km/pxlinextendedcolorsequences,hasverylimitedspectralresolution.In-depthISSsequenceswereplanned,butonlyonelitfaceobservationinall15filterswasobtainedinPrimeMission,whichwascorrupted,soextensiveandredundantfollowupwasplannedforthe“extended”orCassiniSolsticeMission(CSM),overarangeofilluminationandviewinggeometries(see4.2.6).RegionalandlocalradialvariationsincolorandspectrawereseenbyUVIS,ISS,andVIMS,greatlyexpandinggeometricalandwavelengthcoveragetoadepthofdatathathasyettobeplumbed.However,goingfromobservedreflectivity(orI/F,theratioofobservedintensitytothatofacolocatedLambertsurface)toquantitativeconstraintsonringparticlecompositionisasomewhatimperfectmodelingstep,involvingthepoorlyunderstoodwaycloselyspacedparticleswithrough,grainyregolithsscatterlight(seesection6).
Allowingformostofthesecomplexities,VIMSspectraldatahasbeenespeciallypowerfulinconnectingtheabundanceandregolithgrainsizeofwatericewiththerelativeabundanceofothermaterials.Earlyinthemission,Nicholsonetal(2008)found,andsubsequentlyHedmanetal(2013)showedinmoredetail,thattherednessofthemainringscorrelatedverywellwiththewatericebanddepth,suggestingthattheUVabsorberwasspatiallycolocatedwithwaterice,mostlikelyastiny,submicronorsmallersizeinclusionswithinthewatericeregolithgrains(newinsituobservationsmayhaveevendetectedthesetinyfew-nm-sizeinclusionsdirectly–seesection4.2.4b).DetailedstudiesofradialvariationofcolorandspectraarereviewedbyCuzzietal(2009,2018,andreferencestherein).RegardingtheactualnatureoftheUVabsorber,alivelydebatehascontinuedforadecadeastowhethertherings’rednesscanbebetterexplainedbygoodold-fashionedrustorothermetaloxides,asonMars,orbylarge,organicmoleculeslikePolycyclicAromaticHydrocarbons(PAHs)thatgivefruitsandvegetablestheirorange-redcolor(seeCuzzietal2009foradiscussion).
Broadlyspeaking,theopticallythinnerCRingandCassinidivisionaremore“polluted”bysomespectrallyneutralnonicymaterialthantheAorBrings,makingthemdarkeratvisualwavelengthsanddecreasingthestrengthsofthenear-IRwatericebands;thisisthenaturaloutcomeifthedarkeningmaterialisdepositedfromextrinsicmeteoroidbombardment(CuzziandEstrada1998;Bradleyetal2018;seeCuzzietal2009forareview).MorerecentresultsfromCIRSthermalmodelsgivetheringparticlebolometricalbedo–anothermeasurementoftheirnonicypollutants;itisinterestingthat,likeinferencesfromvisualwavelengthreflectivities,thebolometricalbedosarenotonlylowerintheCringandCassinidivisionbutalsovarysmoothlyacrossthesharpAandBringboundaries(Morishimaetal2010),asgenerallypredictedbyballistictransportmodels.
Cassini’slate-orbitin-situobservationsfindplentyoforganicmaterialandsomesilicates,butnodiscerniblefreemetalormetaloxides(seesection4.2.4bfordetails).Moreover,themostrecentCassiniandnon-CassinistudiesusingbothclassicalandMonte-Carloringradiativetransfermodelsagreethatthespectraaremoreconsistentwithreddishorganicsthanwith
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othersuggestedmaterials(Ciarnielloetal2018,Cuzzietal2018b).However,eventhebestcurrentregolithscatteringmodelssimplifythephysicsandinvolveseveralparameters.Becausedifferentflavorsof“Hapke-like”regolithscatteringmodelsleadtosystematic,model-baseduncertaintiesinabsoluteabundancesatthefactor-or-severallevel,orevenworse,quantitativeinferencesaboutabundancesmustbetreatedwithcaution.Forinstance,Hedmanetal(2013)interpretedfine-scaleradialcolorvariationsintermsofregolithgrainsize,butthismayhavebeenbecauseofthelimitationsoftheir(classical)model,whereasothersecondaryoptical-depth-relatedeffectsmayaslikelybethecause.Ontheotherhand,Cassini’sUVspectrometerUVISobservedthestrong170nmwatericeabsorptionedgeandhowitvariesacrosstherings(Bradleyetal2010,2013);theseanalysesprovideasensitivemeasurementoftheregolithgrainsize,whichcanremoveambiguitiesinregolithmodelsatcomparablewavelengthsandleadtoabsoluteabundancesofnonicymaterial(seesection4.1.2formorediscussionofgrainsizeeffects).So,whilethecompositionaldebatehasclarifiedregardingoverallcomposition,thereisaneedforconsiderablymoreworkregardingradialvariationandinferenceofquantitativeabundancesofnonicymaterial(seesection6).
OthernewcontributorstotheringcompositionweretheCIRSfar-IRcapability2andtheCassini2.2cmRadar,usedinradiometermode3.Theseobservationshaveconsiderablehistoricalmotivation,asmicrowaveobservationswerethefirsttoconstraintheringcompositionasnearlypurewatericeparticlesofcm-msize(seeEspositoetal1984forareview).ThebasicobservationisthatthebrightnesstemperatureoftheringsdropsfromthephysicaltemperatureinthethermalIR,toasmallfractionofitinthemicrowave(thusrequiringalowparticleemissivity).CIRSwasabletogetringspectracoveringthetransitionspectralrangebetweenthermalIRandseveralhundredmicronwavelength,resolvinginconsistentVoyager-eraobservations(Spilkeretal2005,2018,andFlasaretalthisvolume),and2.2cmradiometrywasusedtocreatesensitive,high-resolutionmapsoftheverylowmicrowavebrightnesstemperature(Zhangetal2017a,b).Microwaveradiometryisuniquebecausethelongwavelengthspenetratetodepthsofseveralmetersincoldwaterice(thathasaverylowmicrowaveabsorptioncoefficient),thusmicrowavessamplethebulkcompositionoftheringparticlesinawaythatmicronandsubmicronwavelengthscannot.TheCassini2.2cmradiometryhadsufficientlyhighresolutiontoseparatetheA,B,andCRingsintodozensofradialbins,andgreatlyimprovedongroundbasedinterferometrybysettingupperlimitsofafractionofapercentonnonicymaterialintheAandBRings. Analysisofthe2.2cmRADARradiometryobservationsalsofoundthattheCRing(withupto6%nonicymaterialuniformlymixed)ismuch“dirtier”thantheAandBrings(Zhangetal2IthadbeenhopedthatthelongestwavelengthchannelofCIRS–advertisedas0.5mmwavelengthbutmorerealistically1mm,couldbeusedasaradiometerinthisway,butcomplicatedcalibrationproblemswiththeMichelsoninterferometrytechniqueprecludedthisevenafterconsiderableeffortbytheteam.3LittlewasproposedorexpectedalongthelinesofringsciencefromRADARintheAOorearlydiscussions,astheteamwasexclusivelyfocusedonTitan.However,asthemissionwentonandtheantennabeamwascarefullycalibrated,theverylowringbrightnesstemperaturewasmappedeveninthepresenceofthe10-times-brighter,hugeglobeofSaturnloominginthecomplexsidelobes.
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2017a,b).TheassumptionofuniformmixingmayseriouslyunderestimatetheCRingnonicymaterialabundancethough;constraintsonsurfacemassdensityfromahandfulofspiraldensityandbendingwavesstronglysuggestthattheparticlesinthesamebeltofthecentralCRingwhereunusuallyhighabundancesofsilicatesarefound,haveaninternaldensityseveraltimeshigherthanwaterice–far“heavier”thanexplainedbyamere6%inuniformlymixedsilicatedust.Thesituationsuggestsburiedsolidchunksofnonicymaterial–silicateorcarbonaceous–inlargevolumefraction.This“rubblebelt”iswhatwemightexpecttheremainsofadisruptedcoreofadifferentiatedobjecttolooklike.However,modelingthemicrowaveemission/scatteringprobleminthemainringsisverydifficult,sincetheparticlesizesandseparationsarenotmuchlargerthanthewavelength,soapproximationsareneededevenintheverybestcurrentmodels,leadingtosomedegreeofcompositionaluncertainty;moreovertheseanalysesareinsensitivetothenatureofthenonicymaterial. RecentcomparisonsofspectralpropertiesofthemainringswiththoseoftheringmoonsandclassicalicymoonsareprovidedbyFilacchioneetal(2012,2013,2014),andotherrecentreviewsarebyCuzzietal(2009,2018a).Forcomplementarydiscussionoftheringcompositionbasedoninsituobservations(notpartoftheAOobjectives),whichareespeciallyrelevanttotheDRing,seesection4.2.4.Theso-called“diffuse”(D,E,F,G)ringsandsmallerrubblebelts)arediscussedinsection4.1.4,bothregardingstructureandcomposition. (b)SizeDistribution:EvenbeforeVoyager,itwasknownthatbyfarmostofthemainringparticleswereinthecm-fewmeterssizerange,andforthisreasonmicrowavewavelengthswereexpectedtobethemostpowerfulatspecifyingtheirdetails.Tothisend,Cassini’sobservationplanningincorporated135radiooccultationsofthemainandFrings,atthreewavelengths:0.94,3.6,and13cm.Theseoccultationsprovidethedirect-pathopticaldepthatallthreewavelengths,clearlyshowinglocalandregionaldifferencesintheabundanceofthesmallerparticles(afewcminradius).Inaddition,theoff-axisscattering(so-calledbistaticscattering)oftheradiobeamcanbeusedtoconstrainvariationsintheparticlesatthelarge-sizeendofa(usually)powerlawsizedistribution,aswellastheslopeofthepowerlaw(egZebkeretal1985;Cuzzietal2009).TheRSSoccultationprofilingobservationshavebeenreduced,correctedfordiffraction,andprovidedtotheRingsNodeofthePDS;someexamplesareshowninCuzzietal2009(figures16.1a-16.2).Distinctionsassubtleaschangesintheslopeofthepowerlawsizedistributionandtheminimumandmaximumparticlesizebetweentypicallyafewmmandafewmeterscanbediscerned.Inaddition,stellaroccultationscontributeinformationontherelativeabundanceofsmallerparticles. Somegeneralitiescanbeextractedevenatthispreliminarystageoftheinvestigation.ThefractionofsmallparticlesincreasesoutwardsthroughtheAringbasedontheRSSdata,consistentwithmorevigorouscollisionsassociatedwiththeincreasingradialdensityofresonancesandspiraldensitywaves(Cuzzietal2009;Beckeretal2016,Jerouseketal2016).IntheRSSdata,CRing“plateaus”showlocaldropsinthedifferentialopticaldepth,characteristicoflocallyfewerwavelength-sizeparticles,andasimilareffectisseencrossingfromtheouterCRingtotheinnerBring,andfromtheouterCassiniDivisiontotheinnerARing–withtheopticallythickerringshavingrelativelyfewerwavelength-sizeparticleslyingnearthesmallendofthesizedistribution(Cuzzietal2009).
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Stellaroccultationvariancedata4fromUVISprovidesanindependentandcomplementaryparticlesizedeterminationtotheRSSdata.AlloftheCringplateausandnarrower"embeddedringlets”haveasmallermeanparticlesizecomparedtothebackgroundCringrevealedbyUVISoccultationvariancedata.ThebackgroundCringhasanundulatingopticaldepthoverscalesofhundredsofkm,andeffectiveparticlesizecorrelateswiththisstructure;thismaysuggestsize-dependentdynamicaltransportwithinthering.Theinnermost700kmoftheBring,andregionsaroundthestrongresonancesintheAring,exhibitdifferenteffectiveparticlesizesbasedonoccultationvariance(eventhoughself-gravitywakesintheseregionsdominatethesignal).ThereisamarkeddecreaseineffectiveparticlesizecoincidingwiththeMimas5:3bendingwaveintheouterAring.Thisandotherobservationssuggestaverticallyextendedhazeofsmallparticlesacrossthewave;oddly,theeffectisnotseeninthenearlydensitywaveswherecollisionsmightbeexpectedtobeevenmorevigorous.ThevarianceintheCassiniDivision"ramp"revealstheparticlesizedistributiontheretobemoresimilartotheAringthantotheCassiniDivision,withadiscreteincreaseineffectiveparticlesizeattheinneredgeoftheramp.Ontheotherhand,thecomparableCring-Bringboundarytransitionismoregradualinvarianceandimpliedparticlesize(Colwelletal.2018). Moreinformationonparticlesizescanbegainedbycomparingthelocalsurfacemassdensity,fromwavelengthsofspiraldensityandbendingwaves(seeColwelletal2009;Schmidtetal2009)withthelocalopticaldepth.Foridenticalparticles,theopacity(ratioofopticaldepthtosurfacemassdensity)isinverselyproportionaltoparticlesize.Basedonanumberofobservationsofspiraldensitywaves,thereseemtobeonlyverysmooth,orno,dramaticchangesinsurfacemassdensityacrosstheplateausoracrosstheinnerAringboundary(Tiscarenoetal2013a,Colwelletal2009;section4.1.1b).Becausetheopticaldepthdoeschangeabruptlybyafactorofseveral,theremustbeacorrespondingabruptchangeintheopacityattheselocations,requiringachangeintheparticlesizedistribution.Thesenseofthisisthattheparticlesareonaveragesmallerintheplateaus,andintheinnerAandBrings,thaninadjacentmaterial,consistentwiththevariancedatanotedabove.Butyet,RSSoccultationsshowthatthesesameregionshavefewercm-sizeparticles,requiringeitherthattheremustalsobefewerlargeparticles,orthatthelargestparticlesaresmaller,intheplateausandsimilarregions.Alltogetherthissuggestsnarrowersizedistributionsfortheplateaus,andinnerAandBRings,thanfortheirsurroundings–relativelyfewerparticlesatbothsizeextremes.Thereiscurrentlynoexplanationforthisstrongstructuralcontrolonparticlesize,withveryabruptboundaries. Animportantkindof“sizedistribution”isthesizedistributionofregolithgrainsonthesurfacesofringparticles.Whilenotwhatadynamicistwouldcalla“ringparticle”,thesegrains,alongwiththeirunderlyingmaterialcomposition,determinethespectralsignatureatanywavelength.Regolithgrainsizesareusuallybestdeterminedinandaroundstrongabsorptionbands.UVIS,forinstance,seesthestrongwaterice“edge”at170nm,andfromthedetailsofitspositiondeterminesaregolithgrainsizeofabout5microns(Bradleyetal2010,2013).However,atlongerwavelengths(say,thenear-IRwaterbands),largergrainsizesinthetensofmicrons4ShowalterandNicholson(1990)introducedthistechniquetorelatetheexcessvarianceintheoccultationsignal(aboutaradiallyvariable,locallydefinedmean)tothesizeofthelargestlocalparticles.
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aregenerallyderived(Cuzzietal2009,Clarketal2012,Filacchioneetal2012,2013).Thisisconsistentwiththeideathatshorterwavelengthssenseandarescatteredbysmallerscalestructures.Indeed,evenbeforeCassini,modelsbyPouletetal(2002,2003)ofEarthbasedspectraoftheringscoveringthe0.3-4.5umspectralrangerequiredverybroadregolithsizedistributions–from10micronstoseveralmillimeters.Analysesofthelong-wavelengthrollofffrommuchlongerwavelengthCIRSdata(Spilkeretal2005),anda30microniceabsorptionfeature(Morishimaetal2012),suggest(notsurprisingly)abroadsizedistributionfromtensofmicronstocentimetersradius,withthelargeronesperhapsstartingtomergefromregolithparticlestofreely-floatingparticles.Muchmoreworkisneededwithimprovedregolithradiativetransfermodelsandbroaderspectralranges. Theso-called“diffuserings”havetheirowndistinctsizedistribution–generallydominatedbymicron-sizedparticlesthatdiffract,ratherthanbackscatter,lightatUV,visual,andNIRwavelengths(seesection4.1.4).4.1.3Ring-SatelliteInteraction(R_AO3)-Investigateinterrelationofringsandsatellites,includingembeddedsatellites.
ThefirstactualsightingofasmallmoonembeddedinacleargapwasPan,intheARingEnckegap(Showalter1991;seeColwelletal2009andOrtonetal2009).Cassiniquicklyfoundasecondexample:Daphnis,intheKeelergap,hasaninclinedorbitandthewavyedgesitimpartstothegapflapvertically(Weissetal2009).Theknowndimensionsandmassesoftheseobjects(Porcoetal2005,2007),combinedwiththeestimatedringviscosity,providesourbestvalidationofthe“shepherdingtorque”theory(GoldreichandTremaine1980,1982;Schmidtetal2009).Cassinithenembarkedonanextensive,systematicmappingoftheothermultipleemptygapsintheCassiniDivisionandCRingoverthedurationofthemission,comingupemptyhanded,and(eventhoughnoteverylongitudeofeverygapwasmapped)concludingthatthesegapsdonotseemtobeclearedbymoonlets(Spitale2017),atleastbasedontheshepherdingtorquetheoryascurrentlyunderstood(Schmidtetal2009).Cluesaboutothergap-clearingprocessesmaybefoundinsomeofthedetailsoftheedgelocationsoftheCassiniDivisiongaps(HedmanandNicholson2010,SpitaleandPorco2010,Frenchetal2016a).However,therearenocorrespondingcluesregardingthefewCRinggapslackingmoonlets,whichmayinsteadbeexplainedinthecontextofwhatevermysteryprocessgeneratestheCRingplateauensemble,withwhichtheunexplainedgapsareassociated.It’sironicthattheVoyagerimagingteamredirectedresourceslateintheplanningphasetotesta(then-new)hypothesisforgap-clearingbysearchingforgap-embeddedmoonlets,butresourceandtimeconstraintsforcedachoiceoftargetingonlyonegap,andasitturnsout,theCassiniDivisionwheretheylookedwasnottherightplace(Smithetal1982).
Ringedgescanalsobemaintainedbysingle,isolatedresonances,asattheouteredgesoftheBring(Mimas2:1resonance;SpitaleandPorco2010andreferencestherein)andtheAringouteredge(Janus7:6resonance;Porcoetal1984).RecentlyElMoutamidetal(2016)andTajeddineetal(2017a)showedindetailhowJanusactuallyisabletocreateanabruptedgefortheouterARingandrestrainitsspreading,evenasitswapsorbitsbackandforthwithEpimetheus,insomewhatofateameffortwithalltheotherringmoonsandresonancesactingondifferentpartsoftheAring,sotheclassical“shepherding”theoryseemstobeingoodshape.
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Thekinky,strandedFRinghasamorecomplicated,oftenchaotic,relationshipwithitsnearbymoons,though(seesection4.2.3).
Aspecialkindofembeddedmoonletisonethatistoosmall(smallerthan1kmradius)toactuallyopenagap;thesewerepredictedbySpahnetal(1994),butmostCassiniscientistsweresurprisedtoactuallyseesome,initiallyintheSOIimages(Tiscarenoetal2006).Thesehavebeendubbed“propellers”byvirtueofthedisturbancestheycreate,whicharetooweaktopreventviscosityfrombackfillingthedisturbancebeforethenextencounterofthemoonletwiththesamematerial.ItseemsthereareontheorderofamillionoftheseobjectsintheAring,andtheyareconfinedmostlytothreedistinctbandsintheARing(Sremcevicetal2007,Tiscarenoetal2008).TherehavebeenreportsofpropellersintheBRingaswell(Sremcevicetal2012,2013,2014a,2014b).Itisnotcurrentlyunderstoodwhethertheseobjectsareshardsofaringparent,oraresomehowspontaneouslyformingandperhapsdispersinginplace(Espositoetal2012).ThethreeAringpropellerbandsareanticorrelatedwiththe“halos”oflargespiraldensitywaves,andcorrelatedwiththestrengthofself-gravitywakes,whichmightfavorthelocalformationideaormightsimplyindicatechangesinthephotometricbalanceorlocalviscositythatcausespropellerstobevisible.Asubsetofmuchlargerobjects,dubbed“GiantPropellers”wasdiscovered,whichcouldbetrackedandfoundtobeevolvinginsemimajoraxis(Tiscarenoetal2010).Thisdiscoverymotivatedsignificantdedicatedobservingduringtheextendedmission(seesection4.2.8).Meanwhile,thetheoryofthesedisturbanceshasbeenactivelydiscussedandcontinuestoadvance(Schmidtetal2009,Cridaetal.2010,PanandChiang2010,2012,ReinandPapaloizou2010,Panetal2012,BromleyandKenyon2013,Tiscareno2013,Seileretal.2017).
ApossiblephysicalextensionoftheseobjectstosmallersizeshasbeenseenintheopticallythickerbandsoftheCRing(includingtheplateaus)andCassiniDivision(includingitsouterramp);so-called“ghosts”orpartialclearingsappearinUVISoccultationswithverysmalllengthscales(Baillieetal2013).Thesearethoughttobecausedbyrelativelylargeringparticles–maybemeterstotensofmetersinsize–smallerthantheAandBringpropellers(objectssosmallwouldnotcleargapsinthegenerallyhigheropticaldepth,andmorestronglystirred,AandBrings).Theseobjectswouldneedtobemorethan3timeslargerthanthelargestringparticleinapowerlawsizedistributioninordertocreatethe“ghost”clearings.
FleetingglimpsesofwhatappeartobeembeddedKeplerianobjectshavealsobeenobtainedneartheedgesofseveralringsandringlets.AttheouteredgeoftheARing,theso-called“Peggy”objectwasseenbriefly(Murrayetal2014),seeminglygettingreadytobreakfreeofthemainrings.Thisexcitingmomentmight,itwasthought,allowtheobjecttobespunawayfromtheringsduetoringtorquessuchasmodeledbyCharnozetal(2010)–aneffectbywhichtheageoftheringsmightbeconstrained(seealsosection4.2.4c).However,Peggyactuallywasreabsorbedbackintotheringsandapparentlybrokeintoseveralpieces,provingthatleavinghomecanbedifficult.ModeratelylargeKeplerianobjects,butstilltoosmalltobeseendirectly,werealsoseenneartheedgesoftheAring’sKeelergap(Tajeddineetal2017b),intheHuygensringletoftheCassiniDivision(SpitaleandHahn2016),andneartheouteredgeoftheBRingitself(SpitaleandPorco2010;seealsoCuzzietal2018a).Ithasbeensuggestedthatmanyorevenalloftheseobjectsaretransient,formingbycompactionduetosatelliteperturbations,andthenbeingdisruptedastheyincreasinglystirtheregionsaroundthem(Espositoetal2012).
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Finally,itcannotbeforgottenthatallthewonderfuldynamicalrelationshipswearestartingtounderstandbetweentheringsandvariousmoonsdependonagoodunderstandingoftheorbitaldynamicsofthemoonsthemselves,andtheso-called“ringmoons”inparticular,thatinteractstronglywiththerings(Spitaleetal2006,Jacobsonetal2008,Tajeddineetal2013).AdditionalresultscanbefoundintheISSteamreport.Ofspecialimportanceistheparadigm-breakingobservationalresultofLaineyetal(2017)thatthemidsizeicymoonsofSaturn,fromMimasthroughatleastRhea,aretidallyevolvingoutwardsmuchfasterthanhadbeenpreviouslyassumedbasedontraditionaltidal“Q”theory.Thiswasextremelysurprising,becauseSaturn’sinteriorhadbeenthoughttobequitenon-dissipative,likeJupiter,withahigh“Q”value(seeSaturnsection).Amorerecenttidaltheorysuggeststhat“Q”ishighlyfrequencydependent,andthatmoonsgettrappedonacomboflow-QfrequenciesthatslowlydecreaseasSaturn’sinteriorstructureevolves,drivingtheentiresetofmoderate-sizedicymoonsoutwardatamoreorlessdistance-independentrateinsteadofthestronglydistance-dependentrateoftraditionaltidaltheory(see,eg,Fuller2016).Thisnewperspectivehasprofoundimplicationsfortheoriginand(nowincreasinglybelievedtobe)geologicallyyouthfulageoftherings(section4.2.4c).4.1.4DustandMeteoroidDistribution(R_AO4)-Determinedustandmeteoroiddistributionbothinthevicinityoftheringsandininterplanetaryspace. (a)Thediffuserings:Theso-called“diffuse”rings(D,E,F,andG)arebeststudiedbytracingtheirmicron-sizeparticles.SuchsmallparticlesscatterUV,visual,andnear-IRlightintosmallsolidanglesintheforwarddirectionsotheymaybemoreeasilydetectedthaninthelowerbrightnessoftheirlargerparticles,whichisspreadovermoreanglesnearbackscatteringandalsomaybesubjecttothelowbrightnessofindividualparticles.Inallthesecaseswhereonlyfinedustbeltsareobserved,itisgenerallyacceptedthatalarger-particle“skeleton”–orsomesmallparentmoonlet-mustbepresenttoresupplythemicron-sizedmaterial,whichisratherquicklylost(Burnsetal1984;Hedmanetal2018a).Basedpartlyonobservationsofchargedparticledepletions(VanAllen1982;seealsoMAPSchapterinthisvolumeforCassini-basedexamples),theGringhaslongbeenknowntocontainanunderlyingrubblebeltorarcofmacroscopicparticles(seereviewsbyOrtonetal2009andHedmanetal2018),andCassinidiscoveredthattheultimatesourceofthisrubblebeltisa0.5kmsize,highlynonsphericalobjectcalledAegaeon,whichhasaverydark,primitive-bodytypesurface(Hedmanetal2011a).Moreover,Aegaeonistrappedina7:6resonancewithMimas(Hedmanetal2007,2010). Severalotherdustyarcsofmaterialwerediscovered,suppliedbythesmallmoonletsAnthe,Pallene,andMethone–ofthese,MethoneandAnthearetrappedinMimasresonances(Hedmanetal2009b).Acomposite“dustyring”phasefunctionandsizedistributionwasobtainedfromwidephaseanglecoverageoftheDandGrings,forapplicationtoso-called“DebrisDisks”aroundyoungstars(HedmanandStark2015);acertainsimilarityinthetworingphasefunctionsmaypointtosomefamilyand/orprocesssimilarityintheparticles–suchasanaggregate,fractalstructure.TheDringshowsseveralkindsoftimevariability,andisdiscussedmoreinsection4.2.2.Also,thegiant,diffuse“Phoebering”(discoveredbySpitzer;Verbisceretal2009)wasfurtherconstrainedinextent,dynamics,andparticlesizeusingshadow-edgeimagingobservations(Tamayoetal2014,2016).
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Adisadvantageofnecessaryrelianceonforwardscatteringforthediffuserings,isthatitisessentiallydiffraction,andonlyweaklydependentoncomposition,sothecompositionoftheseringsisnotquiteaswellknown.Evenwhenthereissomesignalinbackscatteredintensityorreflectivity,itremainsproblematictoderiveevenalbedosforthelargeFandGringparticles(whosescatteringwouldbecompositionallydiagnostic)becausetheiropticaldepthsarevariableand/orsmall,andsouncertain.TheFRingisknowntobedominatedbycrystallinewaterice(Vahidiniaetal2011,Hedmanetal2011b)butmaybesignificantlypollutedbydriftingGringmaterial(Clarketal2012,seefigure3.22ofCuzzietal2018a,andsection4.2.3formoreFRingdiscussion).Bycomparison,thecompositionoftheEringisknownverywellfromCDAin-situsampling(mostlywaterice,withembeddedorganicsandsalt,allcharacteristicoftheundergroundEnceladusoceanfromwhichitcomes);itsstructureisdiscussedinsection4.2.1.In-situsamplingofnanoparticlesintheDRingarediscussedbelowandin4.1.5and4.2.4b.
(b)Theextrinsic(interplanetary)massfluxwasaprimarygoalofCassini.TheCDAInstrumentpaper(Sramaetal2005)estimateddetecting100extrinsicparticlesofmicron-sizeandlarger,andthisis,intheend,justaboutwhattheyfound.However,earlyinthemissionitstartedtolookdifficultorimpossibletomeasureitdirectly(it’saverydustysysteminreality),soanindirectapproachwasexplored,lookingfordustejectahaloesaroundRheaandTethys,byanalogytoGalileoresultsatJupiter(Sremcevicetal2005);thisledtoonlyupperlimitsandaninitialconclusionthattheextrinsicmeteoroidfluxwaslowerthanpreviouslyassumed.However,thebasic,directapproach,simplycountingupthedetectionsandvelocitiesofsmallparticleswithidentifiablyextrinsicorbits,wasultimatelysuccessful,partlyduetothemuchlongertimebaselineoftheCSM(13years)relativetothebaselinemission(seebelow).Thelackofindirectdetection(satellitedusthaloes)wasreconciledafterthefactbyuncertaintiesintheejectayieldmodel.Forexample,theongoing"snowfall"causedbyEnceladus'geysersmighthaveinfluenced/changedthemechanicalsurfacepropertiesofRheaandTethyscomparedtotheJovianicymoons.Futureanalysesofthermalinertiasmightprovideusefulconstraintsonsatellitesurfaceproperties,inthisregard.
TheCDAmeteoroidmassfluxmeasurementworksasfollows(Kempfetal2018;seeCDAchapterofthisreport).Foreachparticledetectedenteringtheinstrumentthereisainstrumentalambiguitythatallowstwoequallyvalidpredetectionvelocityvectors.Alowerlimittotheincidentmassfluxcomesfromonlythe25particlesforwhichbothvelocityvectorstracebacktoextrinsicprojectiles,frombeyondSaturn.Anupperlimitofasortalsoincludesthe75orsoparticlesforwhichonlyoneofthevelocityvectorsrequiresanextrinsicorigin.Becausethesizedistributionofthedetectedparticlesisnotwellknown,thelargestparticlesmightcarrymostofthemass,andtheirraritysuggeststhatthenominaldetectionvaluescouldbealowerlimit.Forinstance,asolitary250umparticlewasnotincludedinthemassfluxcalculations(includingitwouldincreasethemassfluxbyafactoroffive)andanotheryearofmissiontimemighthavecapturedmorelargeparticles.Future,moredetailed(perhapsBayesian)analysesmightrefinetheuncertaintiesmore,butatpresentitseemsthatthenominalCDA“minimumflux”isconservative.Thevaluesfortheminimumflux“atinfinity”–beforegravitationalfocusingbySaturn–arewithinafactoroforderunityofpre-Cassiniassumptions(CuzziandEstrada1998).However,theCDAdiscoverythatthepre-Saturnorbitsweredynamically“KBO-like”(loweccentricityandinclination)ratherthan“comet-like”(andthushavelowerencounter
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velocities)thanpreviouslyassumed,leadstoanorderofmagnitudeincreaseinSaturn’sgravitationalfocusingfactorandthusinthefluxattherings.Theimplicationsofthisnewresultformodelsofringstructuralandcompositionalevolution,whichhavemultipleparameters(Estradaetal2015)arenotyetwellunderstoodbuttheyareunlikelytoallowaringageolderthanpreviousestimatesofsomehundredsofmillionsofyears.
Alongthelinesofmassfluxintotherings,weshouldmentionfluxatlargersizesasperhapsprovidingadditionalconstraintsonringevolution;asyet,noefforthasgoneintomergingthesedatasetsandresults.AnobservationbyRPWSonlyreportedatmeetings(Gurnettetal2004)maysuggestimpactsbydozensofparticlesduringthe30minutesCassiniwasskimmingacrossthefaceoftheringsatSOI;significanttheoreticaldevelopmentisneededtotranslatetheseobservationsintoresponsibleprojectilesizes(sand-grains?)butifthiscanbedone,therateswouldsurelybeofinterest.Impactejectatrailsfromahandfulofmeter-sizedparticleimpactswereimagedbyISS(Tiscarenoetal2013,SchmidtandTiscareno2013);UVISmountedanearlyattempttodetectactualimpactflashesthemselvesbutsubsequentthoughtandmodelingconcludedthattheflashcouldonlybeseeninthenear-IR(Chambersetal2008)sotheUVISsearcheswerediscontinued.Perhapsasimple,largeFOV,NIRphotometerexperimenttomonitorimpactsmightbeconsideredforafuturemission.Finally,asdiscussedinsection4.2.2,thereisevidenceformultipleimpactsontheringsby1-10kmsizeobjectsoverdecadesandcenturies.
4.1.5RingMagnetosphere-IonosphereInteractions(R_AO5)–StudyinteractionsbetweentheringsandSaturn’smagnetosphere,ionosphere,andatmosphere.
Ithaslongbeenknownthattheringshavean“atmosphere”oftheirownduetophotosputteringandmeteoroidbombardment,andinteractwithboththemagnetosphereandtheplanet,evenreducingtheionosphericelectrondensity(Shimizu1982,ConnerneyandWaite1984).DuringSaturnOrbitInsertion(SOI)Cassiniobtainedthefirst-everinsitumeasurementoftheringatmosphere,anditwasabigsurprise.Ithadbeenexpectedtobedominatedby“waterproducts”(WP;moleculesandionsofH2Oanditsfragments)butinsteadCAPSfounditwasdominatedbyneutralandionizedOandO2(Tsengetal2010).TheobservationsandtheoryarereviewedinCuzzietal2009;essentially,WPrecombineonringparticlesurfacesbutOandO2donot,buildingupintheringatmospherewithoutfreezingoutoradsorbingontoringparticlesurfaces,andultimatelyfilteringouttothemagnetosphereandintotheplanet(Tsengetal2010).Sincethatreview,theeffecthasbeenmodeledinmoredetail,andtheobservedseasonalvariationofmagnetosphericoxygenionstransportedfromtherings(seeMAPSDWGreport)suggeststhatphotosputteringofringicesdominatesovermaterialfromEnceladusinproducingtheringatmosphere(Tsengetal2010,2013;Elrodetal2014).ThisdiscoveryoftheringatmospherealonewouldsatisfytheAOobjective,butindeedduringtheRGandGForbits,Cassinidiscoveredfarmoreabouttheparticulatecomponentofthering“atmosphere”anditsfluxintotheplanet–theso-called“ringrain”longspeculatedupon(eg,ConnerneyandWaite1984).Wediscussthisindetailinsection4.2.4.
ThediffuseEandGRingsinteractinmoreintimatewayswiththemagnetosphere;theirtinygrainsgetchargedandundergoelectromagneticforces(section4.1.1e).4.2ExtendedMission(CSM)Objectives:
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Theobjectivesbelowweredevelopedin2010,toprioritizeupcomingplanningbyfocussingonspecificproblems,eitherunresolvedAOgoalsorunexpectednewphenomena,andtotakeadvantageoftheuniquegeometriesoftheCassiniSolsticeMission(CSM).4.2.1ChangingRings(RC1a)–Determinetheseasonalvariationofkeyringpropertiesandthemicroscalepropertiesofringstructure,byobservingattheseasonallymaximumopeningangleoftheringsnearSolstice. Theemphasishereison“observingattheseasonallymaximumopeningangleoftheringsnearSolstice”.ThedominantoperativeintentionwastotakeadvantageofthemaximumringopeningangleasseenfromthesunandEarth,to(a)allowCIRStodeterminetheenergeticsofthermalheatingandcoolingoftheringsastheyopenedtotheirmaximumopeningangletothesun,and(b)allowRSSoccultationstopenetratethemostopaquepartsoftherings–thecentralBring–whichtheyhadnotyetbeenabletodoevenduringthemaximumopeningangleallowedduringPrimeMission(21o).Thedifferencerelativetotruemaximumopeningangleof23.7ocanbecriticalforRSS,asitappearsintheexponentialoftheopticaldepthterm.Theobjectivewasverywellsatisfied,inthat36moreradiooccultationswereobtained,withvariouscombinationsofresolutionandsensitivity,atopeningangleslargerthanavailableduringprimemissionandmoresuitableforprobingthedenseBRIng.ThedatahavebeenreducedanddepositedintheRingsPDSnodeat1and10kmspatialresolution.CIRSalsoobtaineddataoverawiderangeofopeningangles;oneimportantconclusionhastodowiththeseasonalcoolingoftheringsthroughequinox;theirminimumtemperatureisconsiderablylargerthaninstantaneousthermalequilibriumwithSaturn’sreflectedandemittedenergy,constrainingtheinternaldensityoftheparticles(atleastintheAringwherethemodelwasapplied)tobeclosertosolidiceinthecentralAringthaninitsinnerorouterregions(Morishimaetal2016).Perhapsthiscanbecorrelatedwiththeradialvariationofself-gravitywakesand/ortheradialdistributionof“propellerobjects”. Whenthisgoalwaswritten,itwasstillnotknownastowhetherthespokeswouldvanishagain,andifso,exactlywhenandunderwhatgeometricalconditions.Indeedthatsituationisnowwellunderstoodintermsofphotochargingofthemainrings(section4.1.1e). ThediffuseERing,composedlargelyofmicron-sizedgrainsjettedoutofEnceladus,deformsandchangesshapewiththeseasonsinresponsetotheelevationangleofthesun,assolarradiationpressureinfluencestheorbitsofthesetinygrains(Hedmanetal2012). Thewaxingandwaningofthemagnetosphericoxygenioncontentisdominantlyseasonal–implicatingphotosputteringoficeinthemainringsratherthanimpactsbygrainsfromEnceladus,onepreviouscandidate(Tsengetal2013).4.2.2:RingTemporalVariability(RC1b)-DeterminethetemporalvariabilityofringstructureonalltimescalesuptodecadalforregionsincludingEnckegap,DRing,FRing,andringedgesbysubstantiallyincreasingthecadenceandtimebaselineofobservations.
Theringsarechangingbeforeoureyes.TheFRingmaybethemostextremeexampleoftimevariability,andmeriteditsownstrategicobjective(section4.2.3).Herewediscussotheraspectsofringtemporalvariability.
ThesharpedgesoftheAandBRingsfloparoundloosely,revealingthefluidnatureoftherings.TheAringouteredgeseven-lobedstructurederivesfroma7:6resonancewith
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(mostly)Janus,anditreorganizesitself,diminishinginamplitude,whenJanusswapstoitsouterorbitandtheresonancemovesoffoftheARingedge(SpitaleandPorco2009,ElMoutamidetal2016c).TheBRingouteredgeismorecomplicated,showingthepredictedtwo-lobedforcingoftheMimas2:1resonance,butinaddition,showinginterferencewithm=1,2,and3patternsthatareprobablyfreemodesdrivenbyringpressureandviscosity.Sometimesthepatternalmostvanishesbecauseofthisinterference(Frenchetal2010,Hedmanetal2010,Espositoetal2012,Nicholsonetal2014,SpitaleandPorco2010;seesection4.1.1c)
Saturn’sinnermostring–theDRing–isonlyvisiblewitheffortandinfavorablegeometry.However,aftertheFRing’sfireworks,ithasprovidedsomeofthemostinterestingevidencefortimevariations(usuallyobservedbyvirtueofitsfinedustcomponent,inforwardscatteringgeometries,asdiscussedinsection4.1.4a).Hedmanetal(2007b)firstnoticedthatitsstructureofirregularlyspacedbandsorbeltshadchangeddramaticallysinceVoyager.Theyalsopointedoutapatternthattheyinterpretedasaverticalspiralripple,andsuggestedthattheripplewastheresultofanongoingwrappingupbydifferentialnoderegressionofatiltedmeanringplane.Fromthewavelengthofthewrap,whichshortenswithtime,theydatedtheeventtotheearly1980s.Hedmanetal(2011c)foundtherippletoextendthroughtheCRing,andsuggestedthatthetiltwasimposedbyanimpactingstreamofrubble,perhapsfromadisrupted,1-10kmsize,Shoemaker-Levy-9typeobject,withanextendednodecrossingtheDandCrings.Hedmanetal(2015b)determined,fromfurtheranalysis,thattheeventmayhavehadtwoparts,separatedbymonths.HedmanandShowalter(2016)subsequentlyfoundpatternevidencefortwootherdisturbancesthatoccurredin1979(inVoyagerdata)and2011!Maroufetal(2011)discoveredasimilarpatternintheCring,fromhigher-resolutionradiooccultationdata,thattheyinterpretedastwoevents,separatedby50years,thatoccurredinthelate1300's.Itisfascinatingthattheringsstillbearsilentwitnesstotheselong-lastingscarsofcometaryimpacts.Arelatedexampleoftimevariabilityisongoingimpactsontotheringbydiscreteparticleslargeenoughtocreatedetectabledisturbances–thelittlecousinsoftheseripple-forming,comet-sizeobjects.Impactsbymeter-sizeparticleshavebeenobserveddirectly;oncealeadingcandidatefortriggeringspokes(section4.1.1e),theconnectionbetweentheobservedmeter-sizeimpactratesandspokeformationrateshasnotbeenpursuedindepth.Theseimpacteffectswerediscussedinsection4.1.4b.
TheDandCringsshowtwoother,stillunexplained,kindsoftimevariation.Hedmanetal(2014c)foundthatoneofthemostprominentbrightringletsintheDRing,alsoseenbyVoyager,ismovinginwardsatmorethan2km/yr,andapparentlymovedoutwardstoitscurrentpositionbetweenVoyagerandCassini.Theringletalsohasstrongazimuthalclumpinessthatseemsstable,butcannotbeassociatedwithanyplausibleresonance-witheithersomeknownmoon,orsomeknownplanetaryinteriormodesorrotatingstructures;allinallitisapuzzlingdynamicalfeature.Also,therearetime-variableaspectstoseveralCringspiraldensitywavesthatappeartobedrivenbydensityfluctuationsfixedtoSaturn’sinternalstructureandrotatingwithitswinds–so-called“tesseral”resonances(seesection4.1.1b).Hedmanetal(2014a,2017,2018b)havefoundthatsomeofthesewavetrainsseemtobedriftinginwardsatabout1km/year,asifthedensityfluctuationscausingthemweremovinginlatitudeordepthinsidetheplanet.
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4.2.3FRing(RC2a)–FocusonFRingstructure,anddistributionofassociatedmoonletsorclumps,assparseobservationsshowclumps,arcs,andpossiblytransientobjectsappearinganddisappearing.
Becauseofthestrongtimevariationsseenduringprimemission,theFRingbecameacentralfocusforCassiniimagingobservationsduringCSM.TheFRingliesabout3000kmoutsidetheARingedge,andisuniqueinthefamilyofplanetaryrings.FaintlyglimpsedbyPioneer11,itskinky,multi-strandedstructurewasrevealedinVoyagerimagestobedominatedbysmalldustgrains,andVoyagerradioandstellaroccultationsshowedittohaveaverynarrowcoreoflargerobjects..EarthbasedobservationsbetweenVoyagerandCassinishowedittobehighlyvariable,withlargeclumpscomingandgoing,havingorbitalperiodsthatdifferedslightlyfromthatofitscentralstrand(McGheeetal2001).Asitwasstraddledbythe100-kmdiameterringmoonsPrometheusandPandora,classicalshepherdingtheorywasquicklyproposedtoexplainitssurvival.However,almostimmediately,problemsarosewiththatinterpretation,growingovertheyearswhenitwasrealizedthatnotonlyweretheso-calledshepherdsonchaoticorbitsthemselves,butobjectsorbitingbetweenthemwereexcitedtoevenmorechaoticorbits.Infact,Cassiniveryquicklyobservedsomeoftheseobjects(2004S6,etc;Porcoetal2005)onorbitsthatcrossedtheFRing,butcouldnotbetrackedformorethanafewmonthsbeforetheyvanished(Cooperetal2017).Meanwhile,largejetsofmaterialwereseensplashedoutfromthecentralcoreduetocollisionsbythesecareeningobjects(Charnozetal2005,Murrayetal2008,Beckeretal2018),andevidencewasfoundforsmaller,unseenobjectsembeddedwithintheFRingstrandsthemselves.DynamicalmodelsshowedhowtheperturbationsofPrometheusalonecouldexplainthealternatingstreamer-channelappearanceoftheFRing(Murrayetal2005),Buerleetal2010).
Severalnewclassesofembeddedobjectswereinferredandmodeled,basedonoftenfaintandinitiallyobscurepatternsseentocomeandgo.“Mini-jets”–smallspikespokingoutofthecentralstrand–werecataloguedandunderstoodascausedbylargeembeddedobjects(metersortensofmetersinsizeperhaps)collidingatlow(fewm/sec)speedswithinthedensecentralringstrand(Charnoz2009,Murrayetal2008,Espositoetal2008,Buerleetal2010,Albersetal2012,Meinkeetal2012,Attreeetal2012,2014).“Fans”orlocalized,regularbrightnessfluctuationswereshowntobeduetosmall,embeddedobjectsonslightlyeccentricorbitsrelativetotheFRing(Murrayetal2008).SomeofthesefanswerelocalizedtopartsoftheringwherePrometheuscompressedringmaterial,perhapsactuallyformingnew,smallmoonletsthenandthere(Buerleetal2010).
RSSoccultationsonlydetectedtheFring’sverynarrow(<1km)“truecore”ofcm-and-larger-sizeparticlesabout1/3ofthetime,showingithadtobeazimuthallybrokenanddiscontinuous.Usingtheseobservations,anewtheorywasdevelopedinwhichthestabilityofthe“truecore”,inthepresenceofstrongorbitalchaosdrivenlocallybyPrometheus,wasduetoauniqueoverlapoftwodifferentkindsof“resonances”(Cuzzietal2014,2018c;seealsoAlbersetal2012andCooperetal2013).ThisarrangementoftheFRing’s“truecore”,whichconsistsofincomplete“arcs”ofmacroscopicparticles,mustoccasionallymakesmalladjustments,giventheoccasionalchaoticchangesinPrometheus'orbittowhichtheclumpsmustreadjust.Hedmanetal(2011b)observedvariationsincrystallineicebanddepthduetovariationsintherelativeabundanceofmacroscopicbodiesanddust,whichhaveyettobecorrelatedwiththeRSSclumping.Largeobjectsformingin“fans”,andusuallyoutsidethevery
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narrowlydefinedstabletruecoreorbit(Cuzzietal2014,2018c),canbecomestronglyperturbedbyPrometheusandevolveinto2004S6-likeobjects,ultimatelyrecollidingwiththeFRingathighspeeds.Indeed,while2004S6wastrackedin2006-2008asitcollidedwiththeFringseveraltimesandthenlost,similarobjectshavesinceappeared.DifferentfrequenciesandintensitiesofsuchbigeruptionsasseenbyCassiniandbyVoyagerobservationsunderlinethesporadicnatureofthesevariations(Frenchetal2012,2014).4.2.4RingAgeandOrigin(RN1a)–Constraintheoriginandageoftheringsby(a)directdeterminationoftheringmass,and(b)ofthecompositionofringejectatrappedonfieldlines.
Overtheyearstheringcompositionhasemergedasperhapsthebestgaugeofringageandcluetoringorigin.Whileringcompositionispowerfullyaddressedusingtraditionalremotesensingtechniques(section4.1.2),thisobjectivefocusedoninsitumeasurementsthatwereonlyachievableduringCassini’sRGandGForbits.
(a)Determinationofringmass:Itwasnotknown,whenCassiniwasinitiallyplanned,thatwewouldbeabletoobtainadirectmeasurementoftheringmass;thisonlybecamepossibleintheCassiniSolsticeMission(CSM)GForbits,whenthespacecraftwasabletoflybetweentheringsandtheplanet22times.TheoriginalAOmissionplanwastoconstraintheringmassbymeasuringtheringopticaldepthandparticlesizedistributionstogether,andsampleallavailablespiraldensitywaves,usingstellarandradiooccultations.However,becauseofthepossibleambiguitiesassociatedwith“hiddenmass”intheBringthataroseduringtheprimemission(section4.1.1b),sixcompleteclosepasseswereallocatedintheGForbits,duringwhichthespacecraftremainedquietwhileRSStransmittedacarriersignaltoEarth,toallowthesmallgravitationaleffectsoftheringstobeseparatedfromthoseofSaturn–amini-Junomission.Thedetailsofthetechnique,anddifficultiesencountered,aredescribedbyIessetal(2018)andinthisvolume.Unexpectedandprobablyazimuthallyvariableinternalpropertiesoftheplanet(veryunlikethoseofJupiter)contaminatethegravitationalsignatureoftherings,increasingthemeasurementuncertaintytoperhaps50%ofthesignal.Itispuzzlingthat,iftheRSSvarianceisinterpretedasinternalmassirregularities,theamplitudeoftheseseemsconsiderablylargerthanwouldbeinferredfromthedozenorsospiraldensityandbendingwavescurrentlyseenintherings,thathavebeenidentifiedasbeingdrivenbymassfluctuationsinsidetheplanet(section4.1.1b).Nevertheless,itcanbeconcludedwithconfidencethattheringmassisprobablylessthantheVoyager-eraestimatetowithintheuncertainty(seetheRSSteamreportinthisvolumeandIessetal2018).Atotalringmassevenaslargeas0.8Mimasmassesisregardedasunlikely.WhileRSShasbeenasyetunabletoobtainaradialprofileofsurfacemassdensity,thereisacluefromCIRSthermalmodelsthatthecentralBringregioncalledB3(105000-110000km)isactuallyaquitesharplyboundedannuluscontaining2-3timesthesurfacemassdensityoftherestoftheBRing(Reffetetal2015).
(b)Compositionofringsfromringejecta:TheAOmissionprovidedfornodirect,in-situmeasurementofmainringparticlecomposition;itwasplannedtorelyonremoteobservationsbyVIMSandCIRS,andeventuallyRADAR2.2cmradiometry,sensingthenonicycomponentofringmaterial(seesection4.1.2).Unfortunately,nounambiguouscompositionalsignaturesofanymaterialsbesideswatericewereseenbyVIMS,UVIS,orISS,andotherinstrumentssuchas
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CIRSandRADARcouldonlyconstraintheamountofnonicymaterialinanonspecificway.Fortunately,intheGForbits,whenCassinicrossedtheringplaneinsideoftheDRing(andindeedskimmedthroughtheplanet’stenuousupperatmosphere)synergistic,dedicatedin-situobservationsbyCDA,INMS,MIMI,andRPWShitpaydirt.
CDAdetectedgrainsatavarietyofverticaldistancesabovetherings,withvaryingsilicate/icefraction(Hsuetal2018).TheFe/Siratiosuggestedlow-Fesilicates.Theorder-unitysilicate/IceratioinmaterialdetectedbyCDA(especiallyathighlatitudes)isfarhigherthanseeninthemainringsoreventhemorepollutedCRing,andsuggeststhatdetectedgrainshavebeenstrippedofmostoftheiricebeforedetection,perhapsbyphotosputtering(Hsuetal2018),butthisremainsanactiveareaofresearch.Noevidencewasfoundfor“free”ironorFexOycompounds(candidatesforreddeningtherings;seesection4.1.2aandCuzzietal2009forareviewanddiscussion),eventhoughtheirpresencehadbeensuggestedintrueinterstellargrainsdetectedbyCDA(Altobellietal2016).CDAobservationsoforganicsareexpectedtobelimitedtolargegrains,whichwererarebecausethedustabundanceattheradiiofringplanecrossingswasmuchlowerthanhadbeen(conservatively)estimatedfromspacecrafthazardstudies,andmostofthedetectedgrainsweretootinytogetgoodmassspectrainthepresenceofknowncarboncontaminationofCDA.Forthelargergrains,CDAcansaythattheconcentrationoforganicsislowinanysingledetectedgrain,comparedtotheamountoficeorsilicate.Nevertheless,thedataarestillbeinganalyzedandnewresultsalongtheselinesareanticipated(see,forinstance,Hsuetal2018andthechapterbytheCDAteam,thisvolume).
MIMIdetectedanequatoriallayerof4-5nmradiusgrainswithverticalthicknessofafewthousandkm(Mitchelletal2018)blendingintotheveryrareupperatmosphereofSaturn.ThesetinygrainsplausiblyderivefromtheevenflatterDRinganddriftinwardsundergasdragwhilespreadingvertically.Theyaretoosmalltopickupmuchcharge,soorbitatnearlyKeplerianspeeds,obtainingtheirfew-thousandkmverticalthicknessbyscatteringcollisionswithdifferentiallyrotatingexosphericHatoms.MIMIcouldnotdeterminethecompositionofthesegrainsdirectly,butestimatestheirmassfluxintotheatmosphereat5kg/s(Mitchelletal2018andthisvolume).
Meanwhile,asCassiniskimmedthroughtheveryrarifiedupperatmosphereontheGForbitsclosesttoSaturn,INMSdetectedstrongsignalsduetoaverywiderangeofmasses,uptotheirlimitat100amu(Waiteetal2018,Perryetal2018).TheabundanceofthesemoleculesactuallydecreaseddownwardsintoSaturn,sotheyareenteringfromabove,notdiffusingupfromdepth.Theoverallpatternobservedisroughlydominatedbymultiplesofcarbonatoms,uptoC6,butgiventhelikelihoodthatlargemoleculesfragmentintosmalleroneswithintheINMSchamberpriortoactuallybeingsampled,detailedinferencesrequireacomplexdeconvolutionprocess(Milleretal2018).INMSisunabletodetectnonvolatilemateriallikeironorsilicates,andisevensubjecttounderestimationofH2O.ThegoodagreementbetweentheINMSlatitudinalsignalstrengthandthethicknessoftheMIMI4-5nmparticlelayersuggeststhattheINMScompositionsrefertoimpactfragmentsoftheMIMIparticles.AlsoseenintheINMSspectraareNandO;however,thecomplexityofthedeconvolutionleavessomeambiguityaboutwhethertheO,N(andevensomeoftheC)werecarriedaspartoftheorganicsorinsmallermoleculeslikeNH3orCH4(Waiteetal2018,Milleretal2018,Perryetal2018).Forcomparison,CassiniVIMSobservationsseenoCH4orCO2(Cuzzietal2009)andSTISspectraseenoNH3(Cuzzietal2018b).INMSestimatedatotalfluxofthesematerialsintoSaturn’s
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upperatmosphere,overalatitudinalbandof5-10owide,thatisabout1000timeslargerthantheMIMIestimate(Perryetal2018),andproposethatitmustbecarrieddominantlybyparticlesof1-2nmradius,toosmallforMIMItodetectbutfollowingroughlythesameverticaldistribution.ThisfluxissohighthatINMSsuggestsitisprobablyduetoaflareupintheDring,perhapsduetorecentactivityintheD68ringlet(Hedmanetal2014c,HedmanandShowalter2016). (c)RingOriginandAge:TheageoftheringshasbeendebatedsincetheVoyagerera,whentwogenerallinesofargumentarosesuggestingtheymaybemuchyoungerthanthesolarsystem.Thesearedescribedinmoredetailinseveralrecentreviewchapters(Cuzzietal2009,2018a;Charnozetal2009).First,angularmomentumtransferbyspiraldensitywavesbetweentheringsandtheinnermoonsshouldquicklycollapsetheAringandspinthecloseringmoonsoutwards,atidaleffectlikeourownEarth-moontidalinteraction(GoldreichandTremaine1982;butseePouletandSicardy2001orCharnozetal2000forloopholes).Second,pollutionofthedominantlyicyAandBrings–morethan90%ice,asknownsincepre-Voyager(Espositoetal1984;Ortonetal2009)–bydominantlynonicymeteoroidinfallshoulddarkentheminmuchlessthantheageofthesolarsystem(Doyleetal1989,CuzziandEstrada1998,ElliottandEsposito2011).Ithadbeenargued,however,thattheringmasscouldbemuchlargerthangenerallyacceptedduringtheVoyagerera,becausegravitationalclumpingandlackofsamplingbyspiraldensitywavesofthemostmassive(B)ringmightallowmostofthemasstoescapedetection(section4.1.1b).Also,themeteoroidmassfluxwasverypoorlyknown,andifithadbeenoverestimatedbyanorderofmagnitudeorso,theringscouldremainunpollutedfortheageofthesolarsystem.
ThecombinationofringmassdeterminedbyRSS(Iessetal2018),andtheextrinsicmeteoroidmassfluxmeasuredbyCDA(Kempfetal2018;seesection4.1.4b)atlastallowsustoconstraintheringagetobelessthan250Myrorso(observationalandmodel-baseduncertaintiesremain)–thatis,noolderthanthedinosaurs.Ifwemustreluctantlyrelinquishourotherwisephysicallyplausible“primordialringorigin”scenarios,atleastthereisoneplausible“recentorigin”scenario,relatedtoadynamicalinstabilityincurred100MyragobyastablyevolvingSaturninnermoonsystem(Cuketal2018).Therearemanycurrentissuesassociatedwithconnectingsuchaninstability(nearRhea’sorbit)andactualformationofamassiveringinsidetheRochezone.However,itoffersbothanindependentlyderived,recentageandalsoanindicationwhySaturnistheonlygiantplanettosportsuchayoung,massive,icyring.Constraintsfromcrateringoficymoonsurfacesandinternalevolutionwillplayanimportantroleintestingtheidea.Thecurrentimplicationsthattheringsarecoloredbyorganics,thattheremaybeasilicate-richrubblebeltburiedwithintheCRing(section4.1.2a),andeventhedark,enigmaticGRingparentAegaeon(Hedmanetal2011a)provideimportantconstraintsaswell.4.2.5Ringmoons(RN1b)-Determinethecompositionoftheclose-in“ringmoons”astargetsofopportunity.ThisisdiscussedindetailintheFinalReportoftheIcySatellitesWorkingGroup.4.2.6High-resolutionRingStructureandcomposition(RN1c)-Determinestructuralandcompositionalvariationsathighresolutionacrossselectedringfeaturesofgreatestinterest,usingremoteandin-situobservations.
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Asubstantialnumberofnew,dedicatedobservationswereprioritized,integrated,andimplementedtomakeuseoftheunique,up-close-and-personalgeometryofCassini’sRGandGForbitstoaddress“radialstructure”,meaningonscaleslargerthantheverticalthickness.Also,alargenumberofISS8-15filtercolorscanswith3-5kmradialresolutionwereintegratedandsuccessfullyimplementedwellbeforetheRGandGForbits,toaddresscompositionalvariations,overalargerangeofilluminationandviewingangles,aswellasdifferentsolarhouranglestoassesstheroleofSaturnshine.
Muchhigherspatialresolutionisneededtoassessfinescalestructuralvariations.RecallthattheuniqueSOIobservations(400mresolution)weresparselysampled,wereoftheunlitface,andweretakenatalargerelativevelocityrequiringshortexposures.Ontheotherhand,theRGandGForbitsprovidedsimilar100-300mresolutiononthelitfaceaswellasoftheunlitface,andatlowphaseangles,withtheopportunitytousesmearremovaltracking.Moreover,onenewkindofobservationwasenabledduringtheRGandGForbits–activeradarbackscattering.Radarobservations(fromtheground)openedthemoderneraofSaturn’sRingsresearch40yearsago,buttheirsensitivitydecreasesasdistancetothefourthpower,sotheyonlybecamefeasibleforCassiniintheseveryclose-ingeometries.TheRadarteamdidcarryoutthreeactiveradarbackscatteringradialprofileswiththepotentialtoreachseveralkmradialresolution,butreductionofthissortofdatahasnotbeenpreviouslydonebytheteamandisstillinprogress(seethechapterbytheRadarTeam).Indeed,asofthiswriting,anumberofobservationstakenontheRGandGForbitsremainintheearlystagesofanalysis.However,anoverviewofhighresolutionobservationsbyISS,VIMS,UVIS,andCIRSmaybefoundinTiscarenoandHarris(2018)andTiscarenoetal(2018).4.2.7RingMicrostructure(RN2a)-ConductIn-depthstudiesofringmicrostructuresuchasself-gravitywakes,whichpermeatetherings.Microstructurehastheconnotationofstructurecomparableinscaletotheringverticalthickness(section4.1.1b).DuringtheCSM,anumberofexceptionallyhighradialresolutionscanswereimplementedofthelitandunlitfaces,usingsmearremovaltracking(theso-called“SUPERHRESscans”)andtargetedobservationsof,forinstance,theKeelergapinthevicinityofDaphnis,theCRingplateaus,andotherspecificregionsofinterest.Spatialclumping–ofseveraldifferenttypesandonscalesstilllargerthantheexpectedself-gravitywakes–iswidelyseeninimageswiththesesub-kmresolutions(Tiscarenoetal2018).Anumberofspeciallydesigned“Turnaround”stellaroccultationswereprioritizedandincluded;thesearechordoccultationsinwhichthepathofthestarbecomestangenttotheorbitaldirectionatsomeradius.Theimplicationisthatradialstellarvelocityvanishesandultra-highradialresolutioncanbeobtained,alongwithverygoodtangentialresolution.Finally,thetechniqueofvarianceanalysis4hasbeenwidelyappliedandavarietyofinterestingdifferencesseenfromplacetoplace.Thissortofanalysisisonlygettingunderway(Colwelletal2018).4.2.8NewRingStructures(RN2b)-Performfocusedstudiesoftheevolutionofnewlydiscovered“propeller”objects.Extendtostudiesofstreakyplateaustructure.Adedicatedcampaigntoimageselected“giant”propellers(>1kmsize)asoftenaspossiblewasconductedoverthelastfiveyearsofthemission.Thiscampaignyieldedseveralhundredimagesthatsuccessfullytargetedpropellers,withrichsamplingintime.Ongoinganalysisoftheseimages
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(e.g.,section4.1.3)findsexcursionsfromkeplerianorbitofdiversekinds,includingbothgradualtrendsandsharpchangesofdirection.IntheRGorbits,threegiantpropellerswerespecificallytargetedforclose-flybyimaging,whichsuccessfullyrevealedtheirdetailedstructure.AlsointheRGorbits,high-resolutionimagesofthePropellerBeltsyieldedthefirstfullviewofasizedistributioninwhichthelargestpropellersare4xto5xlargerthanthesmallest(Tiscarenoetal.2018).IntheGForbits,imagesoftheplateausspectacularlyconfirmedprevioussuspicionsofstreakytexture,andfurtherrevealedsharp-edgedbeltsoftextures(somestreaky,someclumpy)atotherlocationsinthemainrings(Tiscarenoetal.2018).Mostofthesedataencompassboththelitandunlitfacesoftherings,andarestillunderreductionandanalysis.(5)Saturn“System”ScienceResults
Thepossibilityof“youngrings”nowhasbroadenedtothepossibilityofayounginnericymoon(andring)system.TheevectionresonancetheoryofCuketal(2016)notonlyindependentlyarrivesatasimilartimescaleasringages,butalsopointstoSaturnasthemostlikelyplanettoincursuchafate.Morestudiesoftheicymoonsurfaces(especiallycrateringstatistics)areneededtoconstrainthevalidityofthishypothesis,andthecompositionofthemoonsandringsneedstobetakenintoaccounttogether.
Alongtheselines,resultsobtainedthroughthestudyofringshaveledustoanewunderstandingofthedeepinteriorofSaturn.Thediscipline,nowknownasKronoseismology,isinitsinfancy,andhasledtothesuggestionofadiffusecoreboundary,andrapidlychanginginternalstructure,atSaturn(section4.1.1b).RecentresultshaveevenshownthattheelusivedeepinteriorrotationperiodoftheplanetcanbedeterminedverypreciselybymatchinginternalstructurepredictionswithspiralwavesseenintheCRing.
CDAalsodetected46grainscomingfromtheInterstellarFluxstream(itsvelocityapexorramdirectionisknownfromthevelocityofthesolarsystemthroughthelocaldiffuseInterstellarmedium)andconstrainedtheircomposition(Altobellietal2016).ThisprofoundlyimportantinsitudeterminationofthecompositionofISMgrainscomplementsandinformsoftenambiguousremotesensingdeterminations(seealsosection4.2.4above).ThegrainspopulatingthediffuseISM–thosedetectedhereandthemosteasilystudiedbyremotesensing–includeMg-richsilicatesand,perhaps,ironmetaloroxides,butaredepletedinsulfur.Bycomparison,thesilicatesdetectedbyCDAduringtheGForbitswerealsoMg-richbutshowednoevidenceofcompanionFemetaloroxides.TheISMgrainswerequitehomogeneous,probablybyrepeatedprocessingintheISMbyshocksandradiation.Asthesegrainsbecomeembeddedinthedark,dense,presolarparentcloud,theyaccumulatemorevolatileorganicsandices(apparentlysomesulfur-bearingices)beforeflowingintotheformingsolarsystem.But,theirveryrefractorymixofsilicatesandmetalwouldnotbeexpectedtochangeatthelowtemperaturesoftheseclouds.(6)Unsolvedproblemssuitableforongoingresearchandfuturemissions
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Figure2:Amontageofstill-puzzlingstructure.AmosaicviewofSaturn’smainringsisshownin
thecentralpanel.Certainselectedregionsareindicatedbydouble-headerarrows,andbycloseupimagesshownasinsets.
WhyistheinteriorofSaturnsodifferentfromthatofJupiter,asseenbytheRSSgravitypasses,andwhyarethemassirregularitiesinferredsodifferentandsomuchlargerthanwhatareinferredfrom“Kronoseismology”spiraldensityandbendingwavesintherings?ArethesedifferencestiedtothefastertidalevolutionratesseenfortheSaturnianmoonsthanforthejovianmoons(theso-called“largerQ”forJupiterthanforSaturn)?Whataretheembeddedmassconcentrationsresponsibleforthe“tesseral”spiraldensitywavesthatrotateatharmonicsofSaturn’sspinrate,andmoreoverchangewithtime?
Theringmassandinparticularitsradialdistributionwerenotaswellconstrainedashoped,becauseofunexpectedinternalstructurewithinSaturn.Onlyatotalringmasscouldbedetermined,andtheBringcomponentinferredbybackingoutthealreadyverywellknownAandCRingmasses(fromspiraldensitywaves).Thisremaininguncertaintyprovidesanopportunityforafuturemission–touserepeatedclosegravitationalflybystoseparateoutandfullyunderstandtheapparentlyunusualandintriguinginternalstructureoftheplanet,fromtheradialmassdistributionwithintherings,hopefullyatafewhundredkmradialresolutioneven,toseewhichpartsoftheopaqueBRingactuallyholdhowmuchofthemass.BetterradialmassresolutionwouldbeusefulintheCRingalso,becausethepossibilitythatthemiddleCRinghidesafossilsilicaterubblebelthasimplicationsforringorigin;itiscurrentlyconstrainedbyasmallhandfulofsurfacemassdensities.Thereissomeevidencethatthevisualwavelength
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spectraofthecentralCringdifferfromtheotherwisesimilarCassiniDivision;mightthisbeanotherclueregardingaburiedsilicatebelt?
Thesmooth,undulating80-kmscaleirregularstructureintheinnerBringmayberelatedtoballistictransport,butthearrayoffiner-to-coarserscalestructureinthemiddleandouterBRingseemstobesomethingdifferent.Coulditbesomeformofviscousoverstability,operatingatlongerwavelengthsthanmodelsshow,andpossiblycouplingtotheevenlargerscaleazimuthalBringedgevariations?Intheregionbetween100,000-104500km,radioandstellaroccultationsbothseesuccessionsofverysharptransitionsbetweenlargeandverylargeopticaldepth,forwhichnoexplanationsexist.Atthetimeofthiswriting,RSSoccultationsoftheverydensestBringregions,takenatthelargestringopeningangleswheretheywouldbemostlikelytobeevenpartiallytransparent,havenotbeenanalyzedorreleased,butareeagerlyanticipated.
TheouterCRinghostsaseriesof5-8“plateau”featuresinopticaldepth,nearlysymmetricallyarrayedabouttheMaxwellgapwhichisnowknowntobecausedbya2:1resonancewiththestrongestplanetaryinternalvibrationmode(MarleyandPorco1993,Frenchetal2016b,Fulleretal2014);thereasonwhyadense,ellipticalm=1ringletdominatesthegap,apparentlycausedbyanm=2resonance,isnotunderstood.AsimilarsituationoccursataTitanresonanceintheCRing,butthatisanm=1resonance.Theplateausdonothaveanyspecialassociationwithresonancesfromknownorpredictedplanetaryinternalmodes(Marley2014).Theyareoddinthattheirappearanceasfeaturesisdueprimarilytolocallyabruptchangesinparticlesizedistributionratherthansurfacemassdensity(sections4.1.2b,andbelow).Moreover,ultra-highresolutionCassiniimagesrevealanovelandunique“streaky”structureintheplateausthatisnotunderstood(Tiscarenoetal2018). WhyisthereaCassiniDivisionatall,andwhyaretherenoembeddedmoonletsclearingitsseveralemptygaps?IthasbeensuggestedthatweakperturbationsfromthefluctuatingBRingedgemightplayarole(Hedmanetal2010b).PresumablytheCassiniDivisionwasoriginallyclearedbyapowerfulMimas2:1spiraldensitywave,atatimewhentheringswerecontinuousthroughtheregion,buthowdidthatearly,vigorousevolutionleadtotoday’squietbandedstructure?And,howarethemoonlet-freeCringgapsformed?
IntheAring,amillion“propeller”objectspopulatethreedistinctradialbelts.Aretheseobjectstransientclumpsthatemergefromthebackgroundmaterialwithlow,butfiniteprobabilityandthendisperseagain?Giantpropellers(foundonlyoutsidetheEnckegap)seemespeciallyhardtocreatebyspontaneous,ortriggeredformation.Gravitationalencounterswithsmallermassesseemtoexplaintheirwanderingbehavior,butthemassclumpsneededarelargerthanthelargestringparticlesandeventypicalself-gravitywakes.Thereareafew“propeller”objectsintheBRing,buttheirlocationsandabundanceshavenotbeenwellcharacterized.WhatelsecouldbehidinginthedensestpartsoftheBRing?Whatcausesthe“ghost”gapsintheCringandouterCassiniDivision?
IntheFRing,telltalesignaturesofembedded,relativelylargeobjectsareseenpreferentiallyinstrandswheretheyaremaximallycompressedbyPrometheus.Ifthesemimajoraxesofthesenewlycreatedobjectslieoutsidethestable“truecore(section4.2.3),theywilldevelopchaoticorbitalperturbations,diffusingawayfromtheirformationlocationastheireccentricityincreases,andultimatelyrecollidingsporadicallywiththeFRingcore.Cyclesofsuchactivitymaybeexpected(andthereissomeevidencefromcomparingVoyagerand
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Cassini)butnosystematicassessmenthasbeenobtained.Meanwhile,eventhemostrecenthypothesisforthestabilityoftheFRing’struecorecannotexplainthestableprecessionofdozensofseparatedarcsasasingle,welldefinedellipse.Couldthe(mostlyunconstrained)massoftheFRingbeplayingarole?
ThesystematicincreaseoftherednessoftheringsinwardsacrosstheBRingremainsapuzzle.In-situobservations(albeitintheDring)seemtofavorlargeorganicmolecules,PAHSorfragmentsoftholins,possiblycontainingNandmaybeevenO,astheUVabsorberresponsiblefortheringredness.Theseredorganicsseemtobeintrinsictothedominanticymaterial.WhetherornotorganicsalsoexplaintheweakerUVabsorptionseeninmostoftheicymoonswillbeanimportantclueregardingringoriginscenarios,suchasrecentonesthatconnecttheirorigintoamoderatelyrecentcollisionalerosion/destructionofpre-existing,mature,differentiatedicymoons.Canorganicmaterialremainassociatedwithliquidwaterwhenamoondifferentiates,andremainwithitwhenitfreezes?
Colorandbrightnessvariationswithintheringsontenstohundredsofkmscalehavebeeninterpretedassmall-scale,optical-depthdependentregolithgrainsizevariations,butmodelscapableoftreatingthedifficultringradiativetransferproblemhaveyettobedeployedincompositionalanalyses.Thatis,secondaryoptical-depthdependenteffects(likecollisionalpackingorsmoothingofparticlesurfaces)maychangetheparticlephasefunctions,orvolumedensityeffectsmaychangethelayerscatteringfunction.Muchmoreanalysisneedstobedonewithmoresophisticated,nonclassicalradiativetransfermodels,fittingobservationsoverwiderangesofilluminationandviewinggeometry,aswellasassessingwiderrangesofwavelengthsimultaneously,tounderstandthesituationbeforecompositionalimplicationscanbederivedwithconfidence.Shadowingonthesurelyveryraggedring“particle”surfacesneedstobestudied(Cuzzietal2017).Alsoonthesubjectofradiativetransfermodels,Cassini’sthreehigh-radial-resolutionactiveradarbackscatteringscansrequirenewmodelsfortheirinterpretation,thatcanadequatelytreatcoherentbackscattering.
Unexplainedjumpsinparticlesizedistributionareseenatanumberofplacesintheringswheresharpedgesappearinopticaldepth,butacrosstheseedgesthereareonlysmall(orno)contrastsinsurfacemassdensity.Basically,thismeansthe“features”arecausedbyabruptchangesinparticlesizedistribution.TheinneredgeoftheAring,whereittransitionstotheCassiniDivision,isonesuchplace.TheseriesofCRing“plateaus”,nearlysymmetricallysurroundingtheMaxwellgap(whichisknowntobeassociatedwitha2:1resonancewithamodeinsideSaturn)arelikethataswell.Whatcausesthiseffect?Whydotheslopesandminimumormaximumsizesingenerallypowerlawsizedistributionsofringparticlesvaryfromplacetoplace?Aremostparticlesporousthroughoutordotheyconcealdense,solidcores?
Unexplainedmicrostructureisseeninmanyplacesacrosstherings,atopticaldepthslargeandsmall,inlate,high-resolutionimageswhichhaveresolutionevenbetterthanthosetakenatSOI,aswellascombinationsoflitandunlitfacegeometries,lowersmear,andhighersignaltonoiseratio(Tiscarenoetal2018).Thetexturecanchangeratherquicklywithradiusandisnotalwaysrelatedtoopticaldepthchanges. Theprocessactuallyresponsiblefortheinitialformationofspokes–whetheritbeburstsofionizingradiationfromplanetarysuperstorms,magneticfieldinstabilities,orimpacts,remainsunclear.
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TheBallistictransportprocessusuallybelievedtoexplainthesharpinneredgesoftheAandBRing,andhypothesizedtoexplaintheCringplateaus,hasbeenmodeledassumingtheboundariesofthesefeaturesmanifestedjumpsinsurfacemassdensitywithoutparticlesizechanges,andwenowknowtheunderlyingsurfacemassdensityisonlyslowlyvaryingwhiletheparticlesizechangesabruptly.Howdothesenewrealizationsaffectthemodels?Couldtheejectionvelocitychangefromplacetoplace,dependingonthenatureofthelocalparticleregoliths?Howwilltheresultschangeundernewassumptionsaboutmeteoroidimpactfluxanddynamicalpopulation?Thetheoryofringpollutionandrestructuringbymeteoroidbombardmentandballistictransportneedstobetestedmorecarefullyagainstotherstructuresandcompositionalprofiles,andrevisedtocapturemorerealisticimpactoutcomesandnewlyconstrainedincomingmeteoroidfluxes.Matchingradialscansofringcolorandspectralreflectancewithmodelsofcompositionalevolutionmustincorporatenewunderstandingaboutringsurfacemassdensity,particlesize,andviscosity.Inclusionofregoliththermalinertiaswouldprovideanotherhandleonhowparticlesurfacesrespondtometeoroidbombardment. Moresophisticatedmodelsofimpactchemistryandradiationchemistryareneeded–bothinmicrometeoroidimpactsonringparticles,possiblyresultinginsomeC-Ochemistry,andbynm-sizegrainimpactsintotheINMSentrychamber.INMSseesavarietyofmolecules–CH4,CO2,andNH3inparticular,thatarenotevidentinVIMSorHST-STISspectraoftherings.Aresomeofthesecreatedinhypervelocityimpacts,transformingabsorbingcarboncompoundsintovolatilesortransparentices?Ifso(orifnot)thiswouldhaveimplicationsforringpollutionandinferredringage.Howdoesthewatericethroughouttheringsremaincrystallinegivenconstantradiationandmeteoroidbombardment?VIMSspectralandoccultationobservationsstronglysupportcrystallineice.
Explanationforthegeologicallyrecentformationoftherings:Wenowhaveanewsetofconstraints,andevenanappealingtheoreticalhypothesistoexplore;however,wearestilllackingacompleteunderstandingastohowtheringsformedintheircurrentlocation,withinSaturn’sRochelimit,fromadifferentiatedicyobjectwithatrace,intimatelymixedorganiccomponent,250Myrago,evengivendisruptivecollisionsnearthecurrentorbitofRhea.Observationsandinterpretationoficymoonsurfacescancontributemeaningfulconstraints. …Coda… Thisshortreviewcaninnowaycapturethedepthandcomplexityoftheanalysisthathasgoneintoeveryoneoftheincompletelistof200publicationscitedhere,eachofthemseveralyearsworkformultiplescientists.EachofthesepapershascontributedonecriticalanduniquestonetotheedificeofknowledgewenowpossessabouttheringsofSaturn–stonesallformedfrombitsreturnedbyCassini’sspectacular13yearmission.Norcanitadequatelyconveytheevenlargeranddeeper,stillunanalyzedanduninterpretedamountofdatasothoughtfullyconceived,passionatelyarguedfor,painstakinglyplanned,obtained,carefullycalibrated,andstoredinthePlanetaryDataSystemthatawaitstheattentionoffuturegenerationsofringscientists.But,wehopeitcanprovideanideaofthescopeofwherewestandtoday,andhintsatfuturedirectionsofresearch.Eventhissummarywouldbedifferentifitwerewrittenayearfromnow.TheCassiniRingsDisciplineWorkingGrouphasnothingbutadmirationanddeepappreciationforthededicationofthehundredsofengineersanddozensofmanagersatJPLwhomadethismissionpossible,someofwhommaybeluckyenoughtoreturntoSaturnonafuturemissionandhelpanswertheseoutstandingquestions.
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