National Aeronautics and Space Administration

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Moons of Saturn

National Aeronautics and Space Administration

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LG-2009-09-576-HQ — JPL 400-1344O 09/09

Saturn, the sixth planet from the Sun, is home to a vast array ofintriguinganduniquesatellites—53plus9awaitingofficialconfirmation.ChristiaanHuygensdiscoveredthefirstknownmoonofSaturn.Theyearwas1655andthemoonisTitan.Jean-DominiqueCassinimadethenextfourdiscoveries:Iapetus(1671),Rhea(1672),Dione(1684),andTethys(1684).MimasandEnceladuswerebothdiscoveredbyWilliamHerschelin1789.The next two discoveries came at intervals of 50 or more years —Hyperion(1848)andPhoebe(1898).

As telescopic resolving power improved, Saturn’s family of knownmoonsgrew.EpimetheusandJanuswerediscovered in1966.BythetimeCassini–Huygenswaslaunchedin1997,high-resolutionimagingtechniquesusedonEarth-basedtele-scopes had added to the moon count. Cassini has discovered sixmoonsandmayfindmoreduringitsmission.CassinifocusesitscamerasmainlyonobjectsrelativelyclosetoSaturn;thebrightringscomplicatemoon-huntingefforts.Earth-basedtele-scopes focus on the outer part of the Saturn system, and have foundanumberofmoonsintheouterregions.

EachofSaturn’smoonsbearsauniquestory.Twoofthemoonsorbitwithingapsinthemainrings.Some,suchasPrometheusandPandora,interactwithringmaterial,shepherdingtheringinitsorbit.SomesmallmoonsaretrappedinthesameorbitsasTethysorDione.JanusandEpimetheusoccasionallypassclosetoeachother,causingthemtoperiodicallyexchangeorbits.In2006,Cassinifoundevidenceforanewclassof“moonlets”re-siding within Saturn’s rings, sweeping out small gaps in the ring particles.Theremaybeasmanyas10millionmoonletswithinjust one of the rings.

Here’sasamplingofsomeoftheuniqueaspectsofthemoons:

•Titan—At5,150kilometers(3,200miles)across,Titanisthesolar system’s second-largest moon. Titan hides its surface beneathathick,nitrogen-richatmosphere,butCassini’sinstru-ments have revealed that Titan possesses many parallels to Earth—clouds,dunes,mountains,lakes,andrivers.Titan’satmosphereisapproximately95 percent nitrogen with traces of methane.WhileEarth’satmosphereextendsabout60kilome-ters(37miles)intospace,Titan’sextendsnearly600kilometers(10timesthatofEarth’satmosphere)intospace.

•Iapetushasonesideasbrightassnowandonesideasdarkasblackvelvet,withahugeridgerunningaroundmostofitsdark-sideequator.

•PhoebeorbitstheplanetinadirectionoppositethatofSaturn’slarger moons, as do several of the recently discovered moons.

•Mimashasanenormouscraterononeside,theresultofanimpact that nearly split the moon apart.

•Enceladusdisplaysevidenceofactiveicevolcanism:Cassiniobservedwarmfractureswhereevaporatingiceevidentlyes-capes and forms a huge cloud of water vapor over the south pole.

•Hyperionhasanoddflattenedshapeandrotateschaotically,probablyduetoarecentcollision.

•PanorbitswithinthemainringsandhelpssweepmaterialsoutofanarrowspaceknownastheEnckeGap.

•TethyshasahugeriftzonecalledIthacaChasmathatrunsnearly three-quarters of the way around the moon.

•FourmoonsorbitinstableplacesaroundSaturncalledLagran-gianpoints.Theseplaceslie60degreesaheadoforbehindalargermoonandinthesameorbit.TelestoandCalypsooccupythetwoLagrangianpointsofTethysinitsorbit;HeleneandPoly-deucesoccupythecorrespondingLagrangianpointsofDione.

•SixteenofSaturn’smoonskeepthesamefacetowardtheplanetastheyorbit.Called“tidallocking,”thisisthesamephe-nomenonthatkeepsourMoonalwaysfacingtowardEarth.

In addition to studies of Titan, Cassini continues to gather data aboutmanyoftheothersatellitesinanefforttofullyunderstandthe nature, formation, and dynamics of Saturn’s many intriguing moons.

FAST FACTS

•LargestMoonofSaturn Titan Titan’sDiameter 5,150km(3,200mi)

•ClosestMoontoSaturn Pan Pan’sDistancefromSaturn 133,583km(83,022mi)

•FastestOrbit Pan Pan’sOrbitAroundSaturn 13.8hours

•NumberofMoonsDiscoveredbyVoyager 3 (Atlas,Prometheus,andPandora)

•NumberofMoonsDiscoveredbyCassini 6 (Methone,Pallene,Polydeuces,Daphnis,Anthe,andAegaeon)

ABOUT THE IMAGES

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Cassini’1 s visual and infrared mapping spectrometer can view different layers of Titan’s atmosphere simultaneously.

2 Falsecolor(blue)emphasizesicywalls offracturesonEnceladus.

3 TheHerschelcrateronMimasisarelicofalargeimpactthat nearly destroyed this moon.

4 ThisisthelandscapeseenbytheHuygensprobeonitsdescent to the surface of Titan.

A false-color view pr5 ocessed to enhance the individual jets spurting ice particles on Enceladus.

6 ThisimageisamosaicofimagesofPhoebetakenbyCassini during its historic close encounter in June 2004.

This image of Iapetus, the two-toned moon, shows the 7

brighttrailinghemisphere.

Cassini’8 s false-color image of Rhea enhances the slight dif-ferences in natural color across the moon’s face.

FOR MORE INFORMATION

solarsystem.nasa.gov/saturn

ForthemostrecentSaturnmooncount,visit: solarsystem.nasa.gov/planets/profile.cfm?Object= Saturn&Display=Moons

Some of Saturn’s moons are shown at relative distances to the planet.