Enceladus Public Report FC Opt

8/2/2019 Enceladus Public Report FC Opt

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ENCELADUS

SaturnsActive

Ice Moon

1.2.2007 (Revision 1, Edited for Public Release)


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PrefaceUpon the recommendation o the NASA Advisory Council Planetary Science Subcommittee and

the Outer Planets Assessment Group (OPAG), NASA Headquarters (HQ) Planetary Science Divi-sion commissioned prePhase A studies o Flagship missions to Europa, Ganymede/Jupiter system,Enceladus, and itan. Te purpose o these studies is to inorm near term strategic decisions or the

next Flagship mission. NASAs Goddard Space Flight Center (GSFC) was directed to conduct theEnceladus study.

NASA HQ appointed an Enceladus Science Denition eam (SD) consisting o members drawnrom the science community. Te Enceladus SD developed the science objectives, prioritized subobjectives, dened an example strawman payload, and worked with the mission design team to createthe mission scenarios, concept o operations and instrument accommodation requirements. Te SDbased science priorities on those recommended in the 2003 Decadal Survey or planetary scienceand on work perormed by previous science teams in support o the JPLled study documented initan and Enceladus $1B Mission Feasibility Report, JPL D37401 (Reh et al. 2007). (Te two SDCoChairs provided a consolidated view o the SDs advice as input or this study.)

Te NASA GSFC assembled a mission design team (listed in Section 5) to develop mission ar-

chitecture concepts to address the science goals identied by the SD. A Champion eam (listed inSection 5), whose members have expertise in the keys areas required or this study, provided advice tothe mission design team at critical decision points, and reviewed and endorsed this report.

Relative to the initial edition o the report released 29 August 2007, this Revision 1 edition containschanges that permit the report to be released to the public. It also includes editorial corrections alongwith a ew minor technical corrections which materially afect neither the results nor the recommenda-tions.


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ENCELADUSTable of Contents

1.0 Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-11.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-11.2 EnceladusScience. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

1.2.1 ScienceGoals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-11.2.2 MeasurementRequirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-21.2.3 InstrumentTypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-21.3 MissionArchitectureAssessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-21.3.1 KeyChallengestoStudyingEnceladus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-21.3.2 TechnicalApproach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-31.3.3 ArchitectureTradeSpace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-31.3.4 TradeSpaceConceptDesigns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-31.3.5 RemainingArchitecturesinTradeSpace. . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-31.4 Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-31.5 ConclusionsandFindings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7

2.0 Enceladus Science Goals and Objectives . . . . . . . . . . . . . . . . . . . 2-12.1 ScienceGoals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-12.1.1 Introduction:TheImportanceoEnceladus . . . . . . . . . . . . . . . . . . . . . . . . . . .2-12.1.2 Priority1Goals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-22.1.2.1 BiologicalPotential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-22.1.3 Priority2Goals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-42.1.3.1 Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-42.1.3.2 Cryovolcanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-62.1.3.3 Tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-72.1.3.4 TidalHeatingandInteriorStructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-8

2.1.4 Priority3Goals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-112.1.4.1 SaturnSystemInteraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-112.1.4.2 SuraceProcesses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-122.1.5 RelationshiptoNASAStrategicGoalsandDecadalSurveyGoals . . . . . . . . . . . . . . . 2-142.1.5.1 TheFirstBillionYearsoSolarSystemHistory. . . . . . . . . . . . . . . . . . . . . . . . . 2-142.1.5.2 VolatilesandOrganics:TheStuoLie . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-142.1.5.3 TheOriginandEvolutionoHabitableWorlds . . . . . . . . . . . . . . . . . . . . . . . . . 2-142.1.5.4 Processes:HowPlanetarySystemsWork . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-152.1.5.5 RelevancetoDecadalSurveyLargeSatellitesSub-PanelThemes . . . . . . . . . . . . . . . 2-152.2 MeasurementRequirementsOverview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-172.2.1 TraceabilityMatrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-17

2.2.2 CassinisAbilitytoMakeTheseMeasurements . . . . . . . . . . . . . . . . . . . . . . . . 2-172.3 InstrumentTypes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-202.3.1 OrbiterRemoteSensingInstruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-212.3.1.1 ThermalMapper(Category1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-212.3.1.2 Near-IRMapper(Category1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-212.3.1.3 VisibleMapper(Category1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-222.3.1.4 FramingCamera(Category2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-222.3.1.5 UVSpectrometer(Category2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-22

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ENCELADUS2.3.2 OrbiterGeophysicsInstruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-232.3.2.1 LaserAltimeter(Category1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-232.3.2.2 RadioScience(Category1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-232.3.2.3 Magnetometer(Category1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-242.3.2.4 RadarSounder(Category1or2). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-24

2.3.3 SaturnOrbiterIn-Situ

Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-252.3.3.1 IonandNeutralGasMassSpectrometer(Category1). . . . . . . . . . . . . . . . . . . . .2-252.3.3.2 DustAnalyzer(Category1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-252.3.3.3 LowEnergyPlasmaAnalyzer(Category2) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-262.3.3.4 EnergeticParticleSpectrometer(Category2) . . . . . . . . . . . . . . . . . . . . . . . . .2-262.3.4 EnceladusOrbiterIn-SituInstruments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-272.3.4.1 GasChromatographMassSpectrometer(Category1) . . . . . . . . . . . . . . . . . . . . . 2-272.3.4.2 DustMicro-Analyzer(Category1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-272.3.5 EnceladusSotLanderInstruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-282.3.5.1 LanderCamera(Category1). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-282.3.5.2 Seismometer(Category1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-28

2.3.5.3 RadioScience(Category1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-292.3.5.4 SuraceChemistryPackageandOxidantDetector(Category1) . . . . . . . . . . . . . . . .2-292.3.5.5 LaserDesorptionMassSpectrometer(Category1) . . . . . . . . . . . . . . . . . . . . . .2-292.3.5.6 Magnetometer(Category2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-302.3.5.7 TunableLaserSpectrometer(Category2) . . . . . . . . . . . . . . . . . . . . . . . . . . .2-302.3.6 HardLanderInstruments(Category2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-312.3.7 MatricesRelatingInstrumentstoScienceGoals. . . . . . . . . . . . . . . . . . . . . . . .2-312.4 ScienceEvaluationoArchitectureTradeSpace . . . . . . . . . . . . . . . . . . . . . . . .2-312.4.1 MissionConfgurationsNotChosenorDetailedStudy . . . . . . . . . . . . . . . . . . . .2-312.4.1.1 SampleReturn. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-312.4.1.2 DumbImpactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-34

2.4.1.3 SaturnOrbiterOnly(nolanders) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-342.4.1.4 SingleFlybyMissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-352.4.1.5 Lander-OnlyMissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-352.4.6 MissionsChosenorDetailedStudy..............................2-352.5 PlumeParticleSizesandAbundances:PotentialHazardsandSamplingOpportunities . . . .2-352.6 Reerences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-38

3.0 Mission Architecture Assessment. . . . . . . . . . . . . . . . . . . . . . . 3-13.1 TechnicalApproach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-13.1.1 RiskReduction/FactFindingActivities . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-13.1.1.1 KeyChallengestoStudyingEnceladus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-1

3.1.1.2 TrajectoryWork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-23.1.1.2.1 GravityAssiststoSaturn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-23.1.1.2.1.1 SEPTrajectories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-23.1.1.2.1.2 ChemicalPropulsionTrajectories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-23.1.1.2.2 SaturnOrbitInsertionandGravityAssistswithintheSaturnSystem. . . . . . . . . . . . . . .3-33.1.1.2.3 FreeReturnTrajectories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-53.1.1.3 Aerocapture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-63.1.1.4 ParticleShielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-8

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ENCELADUS3.2 ArchitectureTradeSpace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-113.3 EnceladusOrbiterwithSotLander(Enceladus-OL). . . . . . . . . . . . . . . . . . . . . . 3-123.3.1 Enceladus-OLArchitectureOverview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-123.3.2 Enceladus-OLScienceInvestigation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-133.3.3 Enceladus-OLMissionDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16

3.3.3.1 Enceladus-OLFlightDynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-163.3.3.1.1 Enceladus-OLLaunchWindow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-163.3.3.1.2 Enceladus-OLCaptureatSaturnandRhea/DioneWalkdown . . . . . . . . . . . . . . . . . 3-173.3.3.1.3 Enceladus-OLOrbitDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-183.3.3.1.4 Enceladus-OLLandingApproach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-203.3.3.1.5 Enceladus-OLTimelineoKeyEvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-213.3.3.1.6 Enceladus-OLMissionDVBudget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-213.3.3.2 Enceladus-OLFlightSegmentDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-213.3.3.2.1 Enceladus-OLConfguration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-223.3.3.2.2 Enceladus-OLMassProperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-223.3.3.2.3 Enceladus-OLBoosterandOrbiter(B&O)Description . . . . . . . . . . . . . . . . . . . .3-27

3.3.3.2.3.1 Enceladus-OL(B&O)MechanicalSubsystem. . . . . . . . . . . . . . . . . . . . . . . . . 3-273.3.3.2.3.2 Enceladus-OL(B&O)PowerSubsystem . . . . . . . . . . . . . . . . . . . . . . . . . . .3-273.3.3.2.3.3 Enceladus-OL(B&O)ThermalControlSubsystem . . . . . . . . . . . . . . . . . . . . . .3-283.3.3.2.3.4 Enceladus-OL(B&O)PropulsionSubsystem. . . . . . . . . . . . . . . . . . . . . . . . .3-293.3.3.2.3.5 Enceladus-OL(B&O)AttitudeControlSubsystem . . . . . . . . . . . . . . . . . . . . . .3-313.3.3.2.3.6 Enceladus-OL(B&O)AvionicsandFlightSotwareSubsystem. . . . . . . . . . . . . . . . 3-313.3.3.2.3.7 Enceladus-OL(B&O)CommunicationsSubsystem. . . . . . . . . . . . . . . . . . . . . . 3-323.3.3.2.4 Enceladus-OLLanderDescription . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-333.3.3.2.4.1 Enceladus-OLLanderDescent,Landing,andSuraceOperations. . . . . . . . . . . . . . .3-333.3.3.2.4.2 Enceladus-OLLanderMechanicalSubsystem . . . . . . . . . . . . . . . . . . . . . . . . . 3-343.3.3.2.4.3 Enceladus-OLLanderPowerSubsystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-34

3.3.3.2.4.4 Enceladus-OLLanderAvionicsandFlightSotwareSubsystems . . . . . . . . . . . . . . .3-343.3.3.2.4.5 Enceladus-OLLanderPropulsionSubsystem . . . . . . . . . . . . . . . . . . . . . . . . .3-353.3.3.2.4.6 Enceladus-OLLanderAttitudeControlSubsystem. . . . . . . . . . . . . . . . . . . . . . . 3-353.3.3.2.4.7 Enceladus-OLLanderThermalControlSubsystem . . . . . . . . . . . . . . . . . . . . . .3-353.3.3.2.4.8 Enceladus-OLLanderCommunicationsSubsystem. . . . . . . . . . . . . . . . . . . . . .3-353.3.3.2.4.9 Enceladus-OLLanderIntegrationandTest(I&T) . . . . . . . . . . . . . . . . . . . . . . . . 3-353.3.3.2.5 Enceladus-OLMissionReliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-353.3.3.2.6 Enceladus-OLOrbitalDebrisProtection . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-363.3.3.2.7 Enceladus-OLMissionLevelI&T. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-363.3.4 Enceladus-OLOperationalScenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-363.3.5 Enceladus-OLPlanetaryProtectionApproach . . . . . . . . . . . . . . . . . . . . . . . . .3-38

3.3.6 Enceladus-OLMajorOpenIssuesandTrades . . . . . . . . . . . . . . . . . . . . . . . . .3-383.3.6.1 FlightDynamicsandDVReserve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-383.3.6.2 RadiationEnvironment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-383.3.6.3 Reliability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-393.3.6.3.1 MissionReliabilityEstimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-393.3.6.3.2 HibernationMode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-393.3.6.4 LandingOperations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-393.3.6.5 FaultDetectionandCorrection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-39

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ENCELADUS3.3.6.6 MappingDutyCycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-393.3.6.7 Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-393.3.6.7.1 ScienceDownlink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-393.3.6.7.2 MediumGainAntenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-393.3.6.8 StagingandBoosterDisposalStrategy . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-39

3.3.7 Enceladus-OLTechnologyNeeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-393.3.8 Enceladus-OLTechnicalRiskAssessment . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-403.3.8.1 MissionLietime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-403.3.8.2 EnceladusGravityModel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-403.3.9 Schedule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-403.4 EnceladusOrbiter(Enceladus-O). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-413.4.1 Enceladus-OArchitectureOverview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-413.4.2 Enceladus-OScienceInvestigation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-413.4.3 Enceladus-OMissionDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-413.4.3.1 Enceladus-OFlightDynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-413.4.3.1.1 Enceladus-OLaunchWindow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-41

3.4.3.1.2 Enceladus-OCaptureatSaturnandRheaWalkdown. . . . . . . . . . . . . . . . . . . . . . 3-413.4.3.1.3 Enceladus-OOrbitDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-423.4.3.1.4 Enceladus-OTimelineoKeyEvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-423.4.3.1.5 Enceladus-OMissionDVBudget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-423.4.3.2 Enceladus-OFlightSegmentDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-423.4.3.2.1 Enceladus-OConfguration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-443.4.3.2.2 Enceladus-OMassProperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-443.4.3.2.3 Enceladus-OBoosterandOrbiter(B&O)Description . . . . . . . . . . . . . . . . . . . . . 3-443.4.3.2.3.1 Enceladus-O(B&O)PowerSubsystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-443.4.3.2.3.2 Enceladus-O(B&O)PropulsionSubsystem . . . . . . . . . . . . . . . . . . . . . . . . . . 3-443.4.3.2.3.3 Enceladus-O(B&O)ThermalControlSubsystem . . . . . . . . . . . . . . . . . . . . . . . 3-48

3.4.3.2.3.4 Enceladus-O(B&O)CommunicationsSubsystem. . . . . . . . . . . . . . . . . . . . . . . 3-483.4.3.2.4 Enceladus-OMissionReliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-483.4.3.2.5 Enceladus-OMissionLevelI&T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-483.4.4 Enceladus-OOperationalScenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-493.4.5 Enceladus-OPlanetaryProtection(&Disposal) . . . . . . . . . . . . . . . . . . . . . . . .3-493.4.6 Enceladus-OMajorOpenIssuesandTrades . . . . . . . . . . . . . . . . . . . . . . . . . . 3-493.4.6.1 Singlevs.TwoStageVehicle/BoosterDisposalStrategy . . . . . . . . . . . . . . . . . . .3-493.4.6.3 AdditionalASRG. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-493.4.6.4 UseoACSThrustersPriortoSOI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-493.4.6.5 RadiatorConfguration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-493.4.7 Enceladus-OTechnologyNeeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-49

3.4.8 Enceladus-OTechnicalRisks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-503.4.9 Enceladus-OSchedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-503.5 SaturnOrbiterwithSotLander(Saturn-OL) . . . . . . . . . . . . . . . . . . . . . . . . . . 3-513.5.1 Saturn-OLArchitectureOverview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-513.5.2 Saturn-OLScienceInvestigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-513.5.3 Saturn-OLMissionDesign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-513.5.3.1 Saturn-OLFlightDynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-513.5.3.1.1 LaunchWindow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-51

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ENCELADUS3.5.3.1.2 MultiplePassesoEnceladus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-543.5.3.1.3 LandingonEnceladus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-543.5.3.1.4 Saturn-OLTimelineoKeyEvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-543.5.3.2 Saturn-OLFlightSegmentDesign. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-543.5.3.2.1 Saturn-OLConfguration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-54

3.5.3.2.2 Saturn-OLMassProperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-543.5.3.2.3 Saturn-OLSEPModuleDescription . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-573.5.3.2.3.1 Saturn-OLSEPModuleMechanicalSubsystem. . . . . . . . . . . . . . . . . . . . . . . .3-573.5.3.2.3.2 Saturn-OLSEPModulePowerSubsystem. . . . . . . . . . . . . . . . . . . . . . . . . . . 3-593.5.3.2.3.3 Saturn-OLSEPModulePropulsionandAttitudeControlSubsystem . . . . . . . . . . . . .3-593.5.3.2.3.4 Saturn-OLSEPModuleThermalControlSubsystem . . . . . . . . . . . . . . . . . . . . .3-593.5.3.2.3.5 Saturn-OLSEPModuleAvionics,FlightSotware,andCommunicationsSubsystems . . . . 3-603.5.3.2.3.6 Saturn-OLSEPModuleRequirementsonOrbiter . . . . . . . . . . . . . . . . . . . . . . . 3-603.5.3.2.4 Saturn-OLOrbiterDescription. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-603.5.3.2.4.1 Saturn-OLOrbiterMechanicalSubsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-603.5.3.2.4.2 Saturn-OLOrbiterPowerSubsystem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-60

3.5.3.2.4.3 Saturn-OLOrbiterThermalControlSubsystem . . . . . . . . . . . . . . . . . . . . . . . . 3-623.5.3.2.4.4 Saturn-OLOrbiterPropulsionSubsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-623.5.3.2.4.5 Saturn-OLOrbiterAttitudeControlSubsystem . . . . . . . . . . . . . . . . . . . . . . . . 3-633.5.3.2.4.6 Saturn-OLOrbiterAvionicsandSotwareSubsystem . . . . . . . . . . . . . . . . . . . . . 3-633.5.3.2.4.7 Saturn-OLOrbiterCommunicationsSubsystem. . . . . . . . . . . . . . . . . . . . . . . . 3-633.5.3.2.5 Saturn-OLLanderDescription. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-633.5.3.2.5.1 Saturn-OLLanderBraking,Descent,Landing,andSuraceOperations . . . . . . . . . . . . 3-643.5.3.2.5.2 Saturn-OLLanderMechanicalSubsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-643.5.3.2.5.3 Saturn-OLLanderPowerSubsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-643.5.3.2.5.4 Saturn-OLLanderAvionicsandFlightSotwareSubsystems . . . . . . . . . . . . . . . . . 3-653.5.3.2.5.5 Saturn-OLLanderPropulsionSubsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-65

3.5.3.2.5.6 Saturn-OLLanderAttitudeControlSubsystem . . . . . . . . . . . . . . . . . . . . . . . . 3-653.5.3.2.5.7 Saturn-OLLanderThermalControlSubsystem . . . . . . . . . . . . . . . . . . . . . . . . 3-653.5.3.2.5.8 Saturn-OLLanderCommunicationsSubsystem........................ 3-653.5.3.2.5.9 Saturn-OLLanderIntegrationandTest. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-653.5.3.2.6 Saturn-OLMissionReliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-653.5.3.2.7 Saturn-OLMissionOrbitalDebrisProtection . . . . . . . . . . . . . . . . . . . . . . . . . 3-663.5.3.2.8 Saturn-OLI&T. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-663.5.4 Saturn-OLOperationalScenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-663.5.5 Saturn-OLPlanetaryProtection(&Disposal) . . . . . . . . . . . . . . . . . . . . . . . . . . 3-673.5.6 Saturn-OLMajorOpenIssuesandTrades. . . . . . . . . . . . . . . . . . . . . . . . . . .3-673.5.6.1 SEPSize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-67

3.5.6.2 IMUCalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-673.5.6.3 LanderHeaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-673.5.6.4 RadiationModel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.6.5 DebrisShielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.6.6 LanderSeparationTime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.6.7 SEPModuleRequirementsonOrbiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.6.8 OrbiterPowerSystemSizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.6.9 ThrusterLocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-68

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ENCELADUS3.5.6.10 LanderFaultTolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.6.11 SEPTrajectoryAnalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.6.12 LanderDVReserve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.7 Saturn-OLTechnologyNeeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.7.1 SEP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-68

3.5.7.2 Landing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.8 Saturn-OLTechnicalRiskAssessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-683.5.9 Saturn-OLSchedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-693.6 OtherIdentifedArchitecturesintheTradeSpace . . . . . . . . . . . . . . . . . . . . . . .3-703.6.1 EnceladusOrbiterthatLandsw/ChemicalPropulsion . . . . . . . . . . . . . . . . . . . . .3-703.6.2 EnceladusOrbiterUsingSEP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-703.6.3 EnceladusOrbiterwithHardImpactor(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-703.6.4 SaturnOrbiterwithSotLanderUsingChemicalPropulsionandGravityAssists . . . . . . .3-723.6.5 SaturnOrbiterwithSotLanderUsingSEPandMoreSaturnianMoonFlybys. . . . . . . . . 3-723.6.6 SaturnOrbiterwithHardImpactor(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-733.6.7 SampleReturnwithorwithoutanOrbiter . . . . . . . . . . . . . . . . . . . . . . . . . . .3-73

3.6.8 DualLaunchVehicleScenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-733.7 Reerences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-75

4.0 Conclusions and Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

5.0 Team Members and Roles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

Appendix A: PlanetaryProtectionDefnitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A-1: Reserved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .B-1Appendix B

Appendix C:BasicPlanetaryData . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .C-1:Reserved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .D-1Appendix D

Appendix E:ComparisonoTradeSpaceConceptDesigns . . . . . . . . . . . . . . . . . . . . . . . . .E-1Appendix F:Acronyms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F-1

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1.1 Oeew

Based on existing knowledge o Enceladus and2003 Decadal Survey goals, the Science Denition

eam (SD) developed science goals or studyingEnceladus and identied the possible mission con-gurations that could meet those goals. Orbiters,as well as a single yby spacecrat, were consideredwith the added possibility or sample return andvarious landertype options. Table 1.1-1 shows themission conguration trade space and provides abrie assessment o science value. Section 2 o thisreport outlines why low science value, high risk, orother reasons removed some congurations romurther study.

Within this trade space the mission design team

identied 12 possible architectures that could bedeveloped into mission concepts to meet the stat-ed science goals. O those 12, three were selectedor concept development because o their high sci-ence value and their ability to provide insight intothe remainder o the trade space; an Enceladusorbiter with a sot lander (EnceladusOL), anEnceladus orbiter (EnceladusO) and a Saturn or-biter with a sot lander (SaturnOL). Tese threecases were purposely selected to enable evaluationo diferent points in the architecture trade spaceand to expedite developing an understanding obasic system sizing, perormance, and cost over

the broad range o potential implementations.Sections 3.3 through 3.5 o this report presentthese three concepts. Section 3.6 uses these re-sults, along with the trajectory and technologytrade study work perormed, to provide insightinto the easibility, advantages, and disadvantageso the other possible architectures identied.

tble 1.1-1: Full Confguration Trade Space

1.2 Enelds Sene

Enceladus, a 500km diameter moon o Saturn,is one o the most remarkable celestial bodies inthe solar system, as revealed by recent discoveriesrom the Cassini mission. It is the only icy world in

the solar system proven to have current geologicalactivity, ofer the possibly o biological potential,and provides a way to sample resh material romits interior via active plumes. Te plume sourceregion on Enceladus provides a plausible site orcomplex organic chemistry and even biologicalprocesses, and resh samples rom this environ-ment can be obtained by ying past Enceladusand sampling its plume.

Enceladus provides dynamic examples o phe-nomena that have been important at some timethroughout the outer solar system. Also, because

Enceladus is the source o the Saturnian Ering,as well as the extensive neutral O and OH cloudsthat ll the middle Saturnian magnetosphere, themoon plays a pivotal role in the Saturnian system,similar in some ways to Ios role in the Jovian sys-tem. For all these reasons, a mission to Enceladuswould produce valuable science that is highlyrelevant to NASA goals as laid out in the 2003Decadal Survey or planetary science.

1.2.1 Sene Gols

Te overarching science goal or a uture

Enceladus mission is the investigation o itsbiological potential, as that ties together manyinterrelated disciplines and has high priority inthe Decadal Survey. Secondlevel goals, which areessential to addressing the primary goal, are theunderstanding o Enceladus tidal heating and in-terior structure, its composition, its cryovolcanism,and its tectonism. O tertiary importance is the

confgon Onl+ SoLnde

+ HdLnde(s)

+ Dbipo

+ Ple Spleren

Sn Obe Incrementalscience return High sciencereturn Seismic networkadds value Modest sciencereturn High potentialscience return

Enelds ObeHigh sciencereturn

Highestscience return

Seismic networkadds value

Modest sciencereturn

High potentialscience return

Sngle Flb Low science returnLow sciencereturn

No way to returndata



Lnde Onl Low science return N/ANo way to returndata



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understanding o surace processes, and the in-teraction o Enceladus with the rest o the Saturnsystem.

1.2.2 meseen reqeens

Te key measurements needed to address thesescience goals would:

Characterize the surace o Enceladus withglobal imaging, topographic, compositional,and thermal maps

Probe the interior structure seismically and/orwith sounding radar

Probe the interior through tidal response,electromagnetic induction signature, andhighorder gravity and shape mapping

Investigate the chemical, prebiotic, and poten-tial biotic evolution o Enceladus with in-situchemical analysis o plume gases and solids,and surace analysis.

Cassini will continue to add to our knowledge oEnceladus through the small number o close y-bys planned or the remainder o its mission. How-ever, Cassini has limited ability to make the abovemeasurements due to its brie time near Enceladus,the high speed o its encounters, instrumenta-tion that is not optimized or these measurements,

and its inability to perorm in-situ surace science.Tus, the science goals dened here or a Flag-ship mission are not expected to be undamentallychanged by new knowledge rom Cassini, unlesstruly unexpected discoveries are made.

1.2.3 insen tpes

A broad suite o instruments were considered inthis study to make these measurements. Remotesensing instruments include pushbroom visible,nearinrared, and thermal inrared mappingcameras, with a raming camera and an ultravi-

olet spectrometer as lower priority instruments.Geophysics instruments include a laser altim-eter and radio science or measurement o tidalexing and static topography and gravity, and aradar sounder when possible. In-situ instrumentsinclude plume dust and gas analyzers, includingmass spectrometers, and or analysis o samplescollected at low speed, a scanning electron micro-scope. Lander instruments include a comprehen-sive surace chemistry package and a seismometer.

It is recognized that many diferent science in-struments can be used to address the same sciencegoals. While the instruments presented are not anexhaustive list, they provide examples o balancedscience payloads and allowed or operations andmass scoping o detailed mission designs.

1.3 msson ahee assessen

1.3.1 Ke chllenges o Sdng Enelds

Missions to study Enceladus will present somekey challenges. Tose common to any missionto Saturn include designing a trajectory that willdeliver the spacecrat to the Saturn system in a rea-sonable amount o time, with a reasonable amounto payload. Tose unique to Enceladus include thelarge DV required once in the Saturn system toeither orbit or land on Enceladus. Alternatively,

they include methods to mitigate that large DVat the expense o adding to mission duration andlie cycle cost. Tey also include methods to pro-tect the spacecrat while it samples the plume nearthe Enceladus south pole. Additionally, planetaryprotection considerations become important notonly or disposal o landers let on Enceladus, butalso or orbiters which may impact Enceladus. Tesame is potentially true or boosters which sepa-rate between itan and Enceladus and which mayimpact other icy moons within the Saturn systemover the same time interval.

Risk reduction analyses were conducted tohelp address some o these challenges includ-ing: a) evaluation o inner planet gravity assistsenroute to Saturn, b) the use o Solar ElectricPropulsion (SEP) as well as chemical propulsiontrajectories, c) the use o either Saturn moons be-tween itan and Enceladus and aerocapture toreduce the DV required to either orbit or land onEnceladus, d) the viability o a reereturn trajec-tory or a sample return mission, and e) require-ments or debris shielding.

Te evaluation o these risks identied urther

considerations. Te use o gravity assists at Venusdrives the spacecrat thermal system, and the use ogravity assists at Earth with a spacecrat that usesa radioisotope power supply imposes special saetyconstraints. Also, the extended duration betweenlaunch and the start o science operations result-ing rom the use o multiple gravity assists drivesmission reliability. Additionally, or architecturesthat include orbiting Enceladus (a small moon),the characterization o the gravitational eld o

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Enceladus is a challenge as many o the orbitsabout Enceladus are signicantly perturbed by thesize and proximity o Saturn. Furthermore, a land-ing on Enceladus must be conducted in a ully au-tonomous manner, which means the lander mustbe able to identiy and react to surace hazards as it

approaches the surace.1.3.2 tehnl appoh

Beore commencing development o conceptdesigns, the mission design team perormed riskreduction and act nding studies. Tis phase o-cused primarily on identiying trajectories or or-biting Saturn, or orbiting Enceladus and or reereturn or sample collection. Te team also con-sulted with experts rom other NASA centers andthe Department o Energy (DOE) to examine:

solar electric and chemical propulsion aerocapture to reduce DVrequirements debris shielding radioisotope power systems

Following this initial study phase, the teamperormed studies in the GSFC IntegratedMission Design Center (IMDC)1 to initiate thedevelopment o the EnceladusOL, EnceladusOand SaturnOL concepts.

1.3.3 ahee tde Spe

Te architecture trade space or this study isshown in Table 1.1-1. Some o these options wereconsidered to be o low science priority, such assingle ybys, and were not considered any urtheras explained in Section 2. In addition, there aremany ways to implement each mission concept.For example, both solar electric and chemicalpropulsion systems were considered. Te launchvehicles considered were limited to Atlas V 551and Delta IV Heavy due to perormance needs.Within this trade space, the ollowing technicalarchitectures were identied:

1. Enceladus orbiter with sot lander withchemical propulsion (EnceladusOL)

2. Enceladus orbiter with chemical propulsion(EnceladusO)

3. Saturn orbiter with sot lander with SEP(SaturnOL)

4. Enceladus orbiter that lands with chemicalpropulsion

5. Enceladus orbiter using SEP6. Enceladus orbiter with hard impactor(s)

7. Saturn orbiter with sot lander using chemi-cal propulsion and gravity assists

8. Saturn orbiter with sot lander using SEPand more Saturnian moon ybys

9. Saturn orbiter with hard impactor(s)

10. Sample return with or without orbiter

11. Dual launch vehicle loosely coupled orbit-er/lander

12. Dual launch vehicle Low Earth Orbit(LEO) assembly

1.3.4 tde Spe conep Desgns

Tree promising mission concepts were devel-oped: EnceladusOL, EnceladusO and SaturnOL. Sections 3.3 through 3.5 o this reportpresent the details o these designs. Table 1.3-1

summarizes their salient eatures. No requirementsor missionspecic technology development wereidentied or any o these concepts.

1.3.5 renng ahees n tde Spe

Te implications o the remaining architec-ture concepts in the trade space are discussed inSection 3.6 o this report and summarized inTable 1.3-2.

1.4 cos

Tables 1.4-1 to 1.4-3 show the cost estimatesor the three concepts developed during thisstudy, broken out by WBS element or scal year(FY) 2007 dollars.

The NASA/GSFC IMDC provides engineering analyses, endtoendmission design products and grassroots and parametric cost estimatesduring concept development studies, which nominally last one to one-andahal weeks per concept

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-

ENCELADUStble 1.3-1: Summary o Trade Space Concept Designs

Mission Description

Enelds -OL

Enceladus orbiter w/sot lander

Enelds O

Enceladus orbiter

Sn OL

Saturn orbiter w/sot lander

Instruments

Orbiter: imagers and in-situLander: imager, seismometer,sample analysis

Orbiter: imagers,radar, and in-situOrbiter: imagers and in-situLander: imager, seismometer,sample analysis

Trajectory (all use Saturn &Titan gravity assists)

VVEES + Rhea & Dione gravityassists

VVEES + Rhea gravityassists

Earth gravity assist

C3 (km2/s2) 9.05 9.05 9.2

Launch Date 29 Sep 208 29 Sep 208 Mar 208

Nominal Mission Duration(years)

8.3 7.3 9.5

Orbiter Science Ops (years) 2.4 2.4 .3

Lander Science Ops (days) 58 N/A 58

Plume passages 2 @ 0.43 km/s 2@ 0.43 km/s 2@ 3.8 km/s

Number o Stages 3 2 3

Propulsion TypeDualmode chemicalbooster & orbiterMonoprop lander

Dualmode chemicalbooster & orbiter

25 kW SEP moduleDualmode chemical orbiterBi prop lander

DV rom ChemicalPropellant (m/s)

Booster and orbiter: 4497Lander: 45

Booster and orbiter:4977

Orbiter: 2797Lander: 435

Launch Mass (kg) 6320 580 696

Launch Vehicle Type Delta IV Heavy Delta IV Heavy Delta IV Heavy

Cost (FY07 $B) 2.8 to 3.3 2. to 2.4 2.6 to 3.0

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ENCELADUS

tble1.3-2:Summaryo

RemainingTradeSpaceArchitectures

rem

iningachiecuesintdeSpce

achiecue

Deivion

Benefs

Dwb

cks

Enceladusorbiterthatlands

withchemical

propulsion

Enceladus-

OLorEnceladu

s-

O

Longerlanderlietime

Complexityrequiredorinstruments

andsubsystemstoworkintwodierent

environments,

missio

nlie

EnceladusorbiterusingSEP

OrbiterromE

nceladus-O

andSEProm

Saturn-

OL

Shortermissionlietime,

~5.5

years,

than

Enceladus-

O

SEPaddscomplexity

andexpense

Enceladusorbiterwithhardimpactor(s)

Enceladus-

OLorEnceladu

s-

O

Lowcostmultipointsuraceobservation

orenhancedgeophysics

Conceptsandtechnologyorhard

landersareimmature

Saturnorbiterwithsotlanderusing

chemicalpropulsionandgra

vityassists

Saturn-

OLwithgravityass

istsromt

he

Enceladus-

OL

Smallerlaunchvehicle

Longerrequiredmiss

ionlietimethan

Saturn-

OL

SaturnorbiterwithsotlanderusingSEP

andmoreSaturnianmoonfy

bys

Saturn-

OLwithlessermoongravity

assistsoEnceladus-

OL

LessDVorthelander,slowerfybys,

and

increasedplumedwelltime

Longerrequiredmiss

ionlietime;2.5

to5

yearslonger

Saturnorbiterwithhardimpactor(s)

Saturn-

OL

Lowcostmultipointsuraceobservation

orenhancedgeophysics

Conceptsandtechnologyorhard

landersareimmature

Samplereturnwithorwithou

torbiter

Notrelated

Muchlesspropellantreq

uiredthanTrade

SpaceConceptDesigns

Longmission(~26ye

ars),singlesample

opportunity;highEar

threentryvelocity

Duallaunchvehicle

looselycoupled

orbiter/lander

Enceladus-

OL

Morerobustlanderando

rbiter

Cost

Duallaunchvehicle

LEOassembly

Enceladus-

OL

Morerobustlanderando

rbiter

NotFeasible

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ENCELADUStble 1.4-1: EnceladusOL Cost Estimate tble 1.4-2: EnceladusO Cost Estimate

Fy07 ($m)

cos Eleen

Project Elements:

.0 Project Management 61

2.0 Mission Sys Engr 52

3.0 Mission Assurance 38

4.0 Science 124

5.0 Payload 225

6.0 Spacecrat 417

7.0 Mission Ops 270

9.0 Ground System 52

0.0 System I&T 40

Sbol 1,279

unen rnge (40% o 70%) 511 o 895

Sbol w/ unen 1790 o 2173

resees 531 o 645

Sb ol w/esees (30%) 2320 o 2818

Eleens w/o on:

8.0 Launch Vehicle 486

.0 E/PO 3

Fy07 ($B)

msson tol rnge 2.8 o 3.3

Fy07 ($m)

cos Eleen

Project Elements:

.0 Project Management 53

2.0 Mission Systems Engineering 43


4.0 Science 54

5.0 Payload 128

6.0 Spacecrat 251

7.0 Mission Operations 240


0.0 System I&T 24

Sbol 876

unen rnge (40-70%) 350 o 613

Sbol w/unen 1225 o 1488

resees 363 o 441

Sbol w/esees (30%) 1589 o 1929

Eleens w/o on:


.0 E/PO 2

Fy07 ($B)

msson tol rnge: 2.1 o 2.4

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ENCELADUStble 1.4-3: SaturnOL Cost Estimate and the interaction o Enceladus with the rest o

Fy07 ($m)

cos Eleen

Project Elements:

.0 Project Management 612.0 Mission Sys Engr 52


4.0 Science 78

5.0 Payload 200

6.0 Spacecrat 495

7.0 Mission Ops 137


0.0 System I&T 35

Sub total 1,151unen rnge (40-70%) 460 o 805

Sbol w/ unen 1611 o 1956

resees 476 o 579

Sbol w/esees (30%)

Eleens w/o on:

2087 o 2534


.0 E/PO 3

Fy07 ($B)

msson tol rnge: 2.6 o 3

1.5 conlsons nd Fndngs

A mission to Enceladus would produce highvalue science that is highly relevant to NASAgoals as laid out in the 2003 Decadal Survey anddescribed in this report. Te accessibility o sub-surace water enable sampling through conven-tional means and without complicated drillingscenarios. Te SD dened a comprehensive set

o science goals that can be met, to varying de-grees, by a wide range o mission congurations.

Te highest priority science goal or a utureEnceladus mission is the investigation o its bio-logical potential. O secondary importance arethe understanding o Enceladus tidal heatingand interior structure, its composition, its cryo-volcanism, and its tectonism. O tertiary impor-tance is the understanding o surace processes,

the Saturn system. Cassini can still make valuablecontributions towards addressing these questions,but is limited by its instrumentation, its orbit andby its inability to land on Enceladus. Tus Cassinicannot adequately address the advanced science

goals dened here.Tese goals can be met most efectively by

both orbiting Enceladus and landing on its sur-ace. Orbiting Enceladus allows comprehensivemapping o its surace morphology, compositionand heat ow, including detailed investigation othe active plume vents. Te interior structure andtidal heating mechanisms, including the presenceor absence o a subsurace ocean, can also be in-vestigated through determination o the moonsgravity and global shape, its potential and shapeLove numbers, its magnetic induction signature,

and crustal structure can be probed using sound-ing radar. Multiple plume passages at the low or-bital speed o ~150 m/s will allow collection ointact plume particles and complex organic mole-cules rom the plume or onboard study. A landerprovides the opportunity or seismometryin-situchemical analysis and unique views o suraceprocess.

Te mission design team developed threepromising concepts using state o the practicetechnology: EnceladusOL, EnceladusO, anda SaturnOL, with cost estimates in the two to

three billion dollar ($FY07) range. All three pres-ent the possibility o providing valuable Flag-shiplevel science, allow the urther evaluationo the ull architecture trade space, and representsingle points in the architecture trade space. Foreach case, trades can be made that aect missionlietime and deliverable mass. In addition, com-mon key challenges, risks and technology liensemerged, including:

trajectory design and resultant DV budget

chemical propulsion (enables more delivered

mass at the cost o time)

SEP (saves time at the expense o mass andcomplexity)

aeroassist to decrease propellant mass (at the ex-pense o complexity and mass or the aeroshell)

deiciency o current gravity models oEnceladus

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ENCELADUS aects orbital stability estimates

paucity o ight data in Saturn environmentradiation model

long required lietimes, regardless o which

trajectory is chosen has implications or overall mission

reliability

technology lien or critical spacecrat com-ponents to undergo additional longlie test-ing (particularly true in the case o samplereturn missions which could have lietimesin excess o 25 years)

planetary protection guidelines

lander concerns: sot landers must maintain anchoring

to the surace during any sample collec-tion and surace coupling or seismometerexperiments

sot lander concepts that operate on batterypower result in short lie

hard impactor package needs urther def-nition in several areas (e.g., battery sizing,thermal design, deployment approach,

landing shock attenuation orienting in pre-erred attitude on surace, coupling to thesurace, etc.)

In summary, based on SDdened goals andmeasurement requirements, the architecture tradestudy presented in this report ound promisingEnceladus mission concepts that would providevaluable, Flagshiplevel, science in the two tothree billion dollar ($FY07) range. Te three

study concepts that were developed use state othe practice technology and could be developedin time to meet the proposed launch dates. Keychallenges, considerations and risks have beenidentied, some o which are common to anymission to Saturn and some o which are uniqueto missions to study Enceladus. Possible missiondesign trades and their efects were discussed,along with insights gleaned about remainingtrade space architectures. Te SD concludedthat a Flagship mission to Enceladus can achievea signicant advance in knowledge and severalmission concepts are identied that merit urther

study.

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ENCELADUS

2.0 EncEladus sciEncE Goals and objEctivEs

2.1 see G

2.1.1 ir: the impre Ee

Enceladus (Figure 2.1.1-1) is one o the mostremarkable moons in the solar system. It hascaptured the attention o planetary scientistssince the early 1980s, when Voyager revealedEnceladus extraordinarily high albedo and itsyouthul and heavily modied surace (Smith etal. 1982). Ground-based observations urtherdemonstrated that Saturns diuse E-ring is con

centrated at the orbit o Enceladus (Baum et al.1980). Te very short estimated lietime o E-ringparticles requires a constant source o replenishment, and speculation about geyser activity onEnceladus supplying resh material to the ring isnot new (Haf et al. 1983). However, it was a series

o Cassini observations in 2005 that provided denitive proo that Enceladus is one o the very ewsolid bodies in the solar system that is currentlygeologically active. Multiple Cassini instrumentsdetected plumes o gas and ice particles emanating rom a series o warm ractures centered onthe south pole, dubbed the tiger stripes.

EN059

Fgre 2.1.11: Global Cassini view o Enceladus (diameter 500 km). The active south polar region is ringed bya scalloped racture zone and includes the our parallel tiger stripe ractures in its central region. The south poleitsel is marked by a red circle. Credit: NASA/JPL/Space Science Institute, PIA06254.

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ENCELADUSTe plume source region on Enceladus provides

a warm, chemically rich, environment, perhapsincluding liquid water, that is a plausible site orcomplex organic chemistry and even biologicalprocesses. Most importantly, resh samples romthis environment can be obtained and studied by

ying past Enceladus and sampling its plume, allowing investigation o Enceladus interior and itsbiological potential. No other icy satellite oersthis opportunity.

As the only proven example o a geologicallyactive ice world (with the possible exception oriton), Enceladus provides active examples o phenomena that have been important at one time oranother throughout the outer solar system. Teseprocesses, including tidal heating, cryovolcanism,and ice tectonism, can be studied as they happenon Enceladus, leading to understanding that can be

applied throughout the outer solar system. Finally,because Enceladus is the source o the E-ring, aswell as the extensive neutral O and OH cloudsthat ll the middle Saturnian magnetosphere, themoon plays a pivotal role in the Saturnian systemsimilar in some ways to Ios role in the Jovian system. For all these reasons, a mission to Enceladuswould produce compelling science that is highlyrelevant to NASA goals (see Section 2.1.5).

Prioritized science goals or a uture Enceladusmission are summarized in Table 2.1.1-1 anddiscussed in detail below. Tere is no prioritiza

tion within the three broad categories, and manyo the goals are inter-related (Figure 2.1.1-2) andrequire similar measurements. Detailed ow-down rom these goals to specic measurementsand mission requirements is given as traceabilitymatrix in Section 2.2.1.

te 2.1.11: Prioritized Science Goals

Prry G se

1 Biological Potential 2.1.2.1

Composition 2.1.3.1

2Cryovolcanism 2.1.3.2

Tectonics 2.1.3.3

Tidal Heating and Interior Structure 2.1.3.4

3Saturn System Interaction 2.1.4.1

Surace Processes 2.1.4.2

EN060

Interior

Exterior

Surface

BiologicalPotential

SurfaceProcesses

TectonicsTidalHeatingInteriorStructure

CompositionSystem

Interactions

Cryovolcanism

Fgre 2.1.12: Illustration o the overlapping andinterdependent nature o the science goals discussedhere.

2.1.2 Prry 1 G

2.1.2.1 bg Pe

Te search or extraterrestrial habitable environments is a driving orce in planetary exploration,as outlined in the 2003 Decadal Survey. BecauseEnceladus is arguably the place in the solar systemwhere space exploration is most likely to nd a

demonstrably habitable environment, evaluatingits biological potential is the overarching goal oEnceladus exploration. Evaluating the habitability o Enceladus involves understanding nearly allother aspects o Enceladus science, so much willbe learned even i the conclusion reached is thatEnceladus cannot support lie as we currently understand it. In addition, though detection o extant lie is perhaps unlikely, the enormous impacto such a discovery makes it worthwhile to carrysome instrumentation (or instance, to measuremolecular chirality) that is specically designedor that task.

Current State of Knowledge

Despite its small relative size, there are manyreasons to suspect that lie might have evolvedand could be supported more easily on Enceladusthan on other icy moons in the outer solar systemsuspected o having liquid-water oceans, such asEuropa or Callisto. Oxidation/reduction reactions(redox chemistry) provide the only known, and

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ENCELADUSmost plausible, energy partitioning and storage disappear. Hand et al. (2006) have since arguedsystems that might drive a biosphere (see discus- that the concentration o oxidants in planetarysion in Gaidos et al. (1999)). All known biochem- ices may be much higher than estimated previistry is certainly dependent on electron-transport, ously, but still alls short by three orders o mag-as evidenced by the electron transport chains that nitude when compared with the most energy-depermeate all o biochemistry. In terms o sup- prived ecosystem known on Earth (Parkinson et

porting redox-based lie, the ice-covered oceans al. 2007). Tis is a serious problem, because or-o the outer planets need to have access to both ganisms on Earth that are able to survive in theseend-members o a signicant redox couple, and energy poor environments are highly adapted tothe urther apart the end-members are chemical- exploit them, and clearly evolved rom more exily, the more plausible are the initial steps in the ble ancestors. Hence, it is unlikely that they wouldevolution o metabolism (Kirschvink and Weiss be conducive to any scenario or the origin o lie2002). Both end-members o the redox scale are, in the rst place (Kirschvink and Weiss 2002).thereore, o equal importance.

Parkinson et al. (2007) note that the oxidantAt the reducing end o this scale it is not di- supply on Europa may pale in comparison with

cult to nd suitable materials or driving a bio- that o Enceladus. Although the production osphere. Hot H2 gas in the inner portion o the oxidants per unit area on the suraces o the twoancient solar nebula led to the widespread chemi- bodies should be roughly the same, the presence

cal reduction o Fe and Ni-bearing dust particles, o the E-ring o Saturn may tip the balance inwhich were later accreted into progressively larger avor o Enceladus. Ice particles that are ejectedobjects, and processed in the core o proto-plan- in the plumes to orm Saturns E-ring act as aetary bodies. Hence, all solid bodies in the solar chemical processor when exposed to energeticsystem are intrinsically capable o providing the particles and UV radiation in the space environreducing couple or a biosphere, assuming that ment. As it sweeps through its orbit, Enceladusa suitable geological process (like silicate volca- will sweep up many o these particles again, addnism) is present to mix these materials with more ing them to the oxidants ormed in situ. Coupledoxidizing counterparts. On Enceladus, the chem- with the presence o an active ice cycle as indiical composition o materials in the plume and on cated by the plumes themselves (Hurord et al.the surace suggests the presence o a heat source 2007b; Nimmo et al. 2007) this also argues orhot enough to decompose ammonia into N2 and an enhanced biological potential or Enceladus indrive reactions with hydrocarbons, implying in- comparison with Europa. As discussed urther in

ternal temperatures on the order o 500-800 K. Section 2.1.4.1, particles in the E-ring are alsoIn turn, this suggests some orm o silicate vol- responsible or reducing the background ux ocanism presumably driven by tidal interactions lethal radiation in its portion o Saturns ring sys(Matson et al. 2007). A volcanic source near tem, which could well expand the habitable zoneEnceladus south pole, and a substantial body o o surace-based lie in extrasolar planetary syswater in an ice-covered ocean, is also consistent tems with similar stressed moonlets.with the moons shape and inerred true PolarWander events which would have moved this e- Finally, and most importantly, the plumes oective negative mass anomaly to the bodys spin Enceladus simpliy the problem o collectingaxis (Collins and Goodman 2007; Nimmo and samples rom the ice-covered ocean. AlthoughPappalardo 2006). Similar tidal processes could the plumes themselves may or may not be direct-plausibly supply a source o reductants on Europa ly sampling the liquid water reservoir (Nimmo et(Squyres et al. 1983). al. 2007), the warm ice needed to support these

plumes when tidal orces open cracks would mostGaidos et al. (1999) noted that or Europa, the likely have risen rom a deeper, liquid body nearavailability o oxidants is the primary actor which the heat source at depth. Under these circummakes lie in ice-covered oceans difcult, particu- stances it is quite plausible that bits o an oceaniclarly in situations where hydrothermal circulation biosphere (even intact microorganisms) could getin response to continuous volcanic activity will act trapped in these ice plumes as they rise, and beto cycle and recycle the same uid over billions o expelled into orbit around Saturn. Rather thanyears on a time scale much aster than the ice dy- searching the Jovian system or reeze-dried shnamics; chemical equilibrium is reached quickly ejected rom the occasional massive impact onand virtually all signicant chemical gradients Europa, as suggested byDyson (1997), detecting

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ENCELADUSreeze-dried microbes around Enceladus might in Table 2.1.3-1. Water ice occurs in bothactually be possible. crystalline and amorphous orms over much o

Enceladus surace. CO2 has been unambiguouslyMajor Questions detected by Cassini VIMS both as ree ice and

complexed with another material (Brown et al.Specic questions relevant to this science goal 2007). Tis host may be water ice, a mineral, or

include: Is liquid water present on Enceladus, another volatile ice. Clathrates have been suggest-either in a subsurace ocean, in the plume vent re- ed by several authors, e.g., Kiefer et al. (2006);gions, or elsewhere? How extensive and long-lived Brown et al. (2007); Kargel et al. (2007). Evidenceis the water, i present, and what is its chemistry? or low concentrations o short-chain organicWhat energy sources are available or lie? And on molecules in the ice is strong, especially near theEnceladus, one may even be able to answer the tiger stripes (Figure 2.1.3-1, Brown et al. 2006),most important question o all: is lie present there and other absorption eatures have been observednow? that have yet to be identied. Curiously, carbon

monoxide and ammonia have not yet been seen,Measurement Requirements though they are predicted by several lines o rea

soning, including their possible role in produc-Almost all measurement requirements con- ing the plume through melting point depression.

sidered here are important to this overarching Silicates also have not been identied, though

science goal. Most critical are measurements that Enceladus high density o 1608 kg/m3

suggests aprobe interior conditions, particularly measure- high silicate/ice ratio.ments o the plume, its source region, and itsallout on Enceladus surace, but also geophysi- te 2.1.31: Molecules known or predicted to be oncal measurements that can establish the presence Enceladus. Entries in lower two rows o the table haveo a subsurace ocean and constrain the nature been predicted on the basis o theoretical argumentso the tidal heat engine. Some measurements in but have not been observed.particular, however, hold the potential or directdetection o extant lie. In-situ microscopic analysis o plume particles might be able to directlyimage biological structures rozen within theparticles, i any existed, and a surace chemistrypackage with the ability to measure the chirality

o organic compounds would be able to measureany enantiomeric excess (i.e., a preerence or onechirality over the other), which would be a strongindicator o biotic origin.

2.1.3 Prry 2 G

2.1.3.1 cmp

elescopic and recent spacecrat observationshave provided most o what is known about thecomposition o Enceladus surace. Compositionis important to understanding the answers to sev

eral major questions regarding chemistry, suraceprocesses, interactions with the rings, the orma- Plume material is likely to all back to the surtion and subsequent evolution o the Saturn sys- ace on ballistic trajectories. A combination otem, and the evaluation o astrobiology potential. mass spectrometry (Figure 2.1.3-2) and inraredStrategies or answering these questions involve a spectroscopy reveals it to be dominated by H2O,combination o remote and in-situ approaches. with rom one to our percent o CO2, CH4 (Waite

sre Pme

ObservedH2O (crystalline, amorph.)CO2 (ree, trapped)

H2OCO2CH4CO or N2

Trace Organics

NH3C2H2C3H8HCN

ExpectedCONH3 or NH3nH2OClathrates

COOHO+

Theorized

SaltsAcidsAmmoniumMethanol

et al. 2007), and a molecule o mass 28. TisCurrent state of knowledge molecule is believed to be either CO or N2; most

likely N2, because o the absence o CO in theTe current state o knowledge is summarized Cassini FUV spectra (Waite et al. 2007; Hansen et

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ENCELADUSal. 2007). Additional materials observed in tracequantities include ammonia, acetylene, propaneand HCN (Waite et al. 2007). Also expected,but not yet detected, are OH and O+ (Hansenet al. 2007), which were previously observed tooccupy a torus linked to Saturns magnetosphere

(Shemansky et al. 1993).

EN061

Fgre 2.1.31: Near-inrared composite image oEnceladus showing the concentration o the 3.44 mC-H stretch band (red) along the south polar tigerstripes. Brown et al. (2006).

Major Questions

Te presence, or absence, o various materials on the surace or in the plume is inextricablylinked with several major issues. What chemicalreactions are occurring in the plume, or in thesubsurace, possibly in liquid H2O? Aqueous

chemistry in a subsurace liquid environment isexpected to produce a number o compounds diagnostic o interior composition and circulationas well as low-temperature chemistry within thecrust. Are biologically relevant materials beingcreated? What are the physical conditions in theactive regions? Is the plume composition the sameeverywhere, or are there variations in space andtime? o what extent are materials transported tothe surace altered through photolytic or radio

H2O

N2

CO2

CH4C2H2

C3H8100.00

10.00

1.00

0.10

0.010 20 40 60 80 100

Mass (Da)

Signal(counts/IP)

EN062

Fgre 2.1.32: Cassini INMS mass spectrum o theEnceladus plume, taken in July 2004, showing masspeaks due to H2O, CO2, N2, CH4, and possibly C2H2and C3H8. (Waite et al. 2006).

lytic processing, or low-temperature chemistry atthe surace? What are the details o the suracechemistry? How chemically heterogeneous is thesurace? What are the timescales or cycling ocrustal materials? Is there a chemical distinctionbetween the materials o the optical surace layerand those below?

Te composition o Enceladus should also bereected in the composition o the plume andconsequently o the E-ring as well. How does thisaect the ring system? What are the rates andquantities o supplied materials, and the relativeabundances o dierent components? What arethe consequences or the rest o the ring system,or or the Saturn system?

Te relative abundances o other materials within the ice can dramatically aect the appearance osurace eatures. What are the global distributionso chemical species? How do they aect the land

scape? Are sublimation-degradation processes concentrating thesematerials?Are ammonia,methanol,chloride salts, or some other materials depressingthe melting point and enabling cryovolcanism, oris it somehow occurring in their absence? Perhapsammonia was present in the past but has been sequestered as ammonium minerals (Kargel 2006).Even small amounts o contaminants can changerheological properties by orders o magnitude,and would, or instance, aect our understanding

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ENCELADUS

o apparent viscous relaxation o craters seen onEnceladus. Tese eects o composition can be exploited to probe the ormation history o Enceladus,the initial complements o planet-orming materials, and subsequent meteoritic and cometary inall,weathering and other processes.

And what is the impact o these processes onthe astrobiology potential? Knowledge o the surace composition will enhance understanding othese processes and the ways in which they inuence the creation and maintenance o viablehabitats, production and transport o biogenic elements and compounds, and energy sources thatcould support biological processes.

Measurement Requirements

Tese questions are best addressed through a

combination o remote and in-situ measurementsthat can detect both predicted and unknowncompounds. From orbit, a multicolor visible imager can quantiy changes in albedo, texture,coloration, and geomorphology, enabling interpretation o surace structures and inerence ocomposition. Te use o discrete lters can allowmapping o specic materials, such as CH4 andH2O. Te real power o the remote sensing systemor compositional analysis though is provided bynear-IR spectroscopy, which will detect diagnosticabsorption eatures or a number o compoundsand map their abundances both locally and glob

ally. Scientic return would be urther enhancedby the addition o UV spectroscopy or suraceand remote plume analysis (see Priority 2 goals).

Inormation about plume composition determined, or instance, by in-situ gas chromatography and mass spectroscopy (GCMS) can provideinvaluable inormation on the nature o the plumesource region. It can also provide valuable inormation or constraining interpretations o theremote sensing data, by providing detailed andbroad (wide mass range) organic and molecularanalysis o plume particles. Analysis o dust com

position, density, and particle sizes will urther enhance the scientic return. Knowledge o plumecomposition, knowledge o surace compositionrom landed instruments, which can provide precise quantitative inormation on major and minorconstituents at a single location, and knowledgeo surace composition rom orbiting instrumentscan all serve to provide checks on results achievedby other methods, and ultimately eed into theother scientic objectives as well.

2.1.3.2 crym

Te most remarkable known aspect oEnceladus is its south polar plume activity(Figure 2.1.3-3, Hansen et al. 2006; Waite et al.2006; Porco et al. 2006; Dougherty et al. 2006;

Srama et al. 2006). Tis is the only known example o active cryovolcanism in the solar system (the origin o ritons very dierent plumesis unknown, but they are plausibly driven byseasonal N2 rost sublimation rather than internal heat (Brown et al. 1990)). Understandingthis remarkable phenomenon should thus be amajor goal o uture missions to Enceladus.

EN063

Fgre 2.1.33: Cassini high phase angle alse-colorimage o the Enceladus plume, showing orward-scattering by micron-sized plume particles. FromPorco et al. (2006).


Te plumes arise rom warm surace ractures(Spencer et al. 2006), the tiger stripes (Porcoet al. 2006). Sublimation o warm surace ice(Spencer et al. 2006), boiling o near-surace

liquid water (Porco et al. 2006), decompositiono clathrates at depth (Kiefer et al. 2006), andsublimation at depth due to rictional heating(Nimmo et al. 2007) have all been proposed asplume generation mechanisms. Te surace ractures radiate 6 GW (Spencer et al. 2006) and theplume latent heat carries away another ~1 GW,and this energy must be continually resuppliedrom the heat source at depth, by movement o

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ENCELADUSgas or liquid water, or (less plausibly) by conduc- is also possible that extrusive cryovolcanism oction through the ice. curs on Enceladus, and might explain some o the

more exotic landorms seen in the Cassini imagesTe mass production rate o plume gas, cru- (Kargel et al. 2006).

cial to understanding the plume source andEnceladus eect on broader the Saturn system, is Measurement Requirements

estimated to be ~150 kg/s rom stellar occultationdata (ian et al. 2006). Tis value is surprisingly Understanding Enceladus plumes requires ahigh, sufcient to remove a signicant raction o combination o techniques. Remote sensing isEnceladus mass over the age o the solar system necessary: high-resolution imaging o the plume(Kargel et al. 2006). Plume ice particle produc- vent morphology and the plumes themselves, detion and escape rates are much more poorly con- tailed near-inrared mapping o the compositionstrained than the gas (Porco et al. 2006; Spahn et o the plume allout and its spatial distribution,al. 2006), because o limited knowledge o plume and thermal mapping o near-vent temperatures.particle sizes. A globally-distributed source o dust idal exing and radar sounding measurementsand gas is necessary to explain the Cassini in-situ will help to understand local heating and crustaldata (Waite et al. 2006; Spahn et al. 2006) but structure near the vents. Measurements o plumewhether this results entirely rom sputtering and chemistry and plume particle morphology willimpacts or requires low-level non-polar plume ac- reveal much about the source region and will

tivity is not yet established. constrain resuracing rates.Major Questions 2.1.3.3 te

Te plume generation mechanism, and how en- Understanding the complex tectonic evoluergy is delivered to the near-surace o Enceladus tion o Enceladus would be a primary goal o anto supply the plumes, is not understood. Under- Enceladus mission. ectonic eatures dominatestanding this mechanism, and thus understand- the surace, and have many intriguing similaritiesing the physical and chemical conditions in the to, and dierences rom, tectonic eatures oundplume sources, is o great importance. on other icy satellites.

Many uncertainties remain in understand- Current State of Knowledgeing the plume gas and particle production, es

cape, and resuracing rates. Particle masses and Cassini images have revolutionized the Voyagersize distributions are an important constraint on view o this satellite (c.,Morrison et al. 1986; Kargelplume mechanisms (Porco et al. 2006), and are and Pozio, 1996), demonstrating, or instance,crucial to understanding mass loss, and supply that smooth terrain is pervasively tectonizedo material to the E-ring. Much o the dust, and (Helenstein et al. 2005; Rathbun et al. 2005).probably some o the gas, alls back to the surace Figure 2.1.3-4 shows that diverse eatures cross-and is probably a major resuracing mechanism, cut the surace, revealing an intricate history. Sevbut rates and spatial distribution o this resurac- eral dierent tectonic processes seem to have beening are unknown. Te detailed chemistry o the at work. Te sinuous chain o scarps that boundplumes is also not yet known. the south polar terrain at a latitude o ~55 S ap

pear to have ormed in response to compressionalTe temporal variability o the plumes is un- orces, while north-trending racture zones that

known. Tere is a suggestion that they might be radiate rom peculiar Y-shaped cusps and inter-

controlled by daily tidal changes (Hurord et al. rupt the chain o scarps appear to be extensional2007b), and longer-term variability is also likely. It (Helenstein et al. 2006a; Porco et al. 2006). Shearis also unknown whether low-level plume or oth- osets along pre-existing rits are also observeder cryovolcanic activity occurs at locations other near the transition between these contractional andthan the south polar terrain. Other orms o cryo- extensional eatures. Te origin o tiger stripes, avolcanism may occur on Enceladus, but details system o parallel rits through which cryovolcanicare unknown. For instance, the presence o large plumes erupt, is currently unclear.boulders near the tiger stripes (Figure 2.1.4-3)may imply occasional episodes o much more vio- While the south polar terrain is a ocus o per-lent activity than have been seen by Cassini. It vasive active tectonism, other regions are less so

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ENCELADUS(e.g., the cratered north polar region). Analysiso the relationship between impact craters andtectonic eatures (Barnash et al. 2006; Bray et al.2007) indicates that the tectonism has persistedthrough time. Furthermore, ossil terrains elsewhere on Enceladus reminiscent o the south po

lar terrain suggest multiple resuracing episodesthroughout the satellites history (Helenstein et al.2006b; Schenk and Seddio, 2006).

EN064Fgre 2.1.34: Mosaic o Cassini ISS images,displaying several dierent tectonic styles that indicatea complex tectonic history. South is towards the right.The radius o Enceladus is 251 km. Courtesy NASA/JPL-Caltech (image # PIA06191).

Te chain o scarps bounding the south polarterrain, as well as the northward-radiating racture zones, may be the product o a change inEnceladuss global gure, possibly associated witha wholesale reorientation o the satellite (i.e., polarwander). Te correspondence o the south polarterrain with a rotational pole is not likely coincidental, and models that seek to explain the southpolar activity (Nimmo and Pappalardo, 2006;Collins and Goodman, 2007) oten incorporatea long-wavelength low in the equipotential surace o Enceladus that can drive polar wander.

Last, the shape o the satellite is changed on adaily basis because o tidal working. Te apparentmorphology and orientation o at least one tigerstripe could be a result (Hurord et al. 2007a),and tidal exing may play an important role increating the vapor plumes issuing rom the tigerstripes, in general (Nimmo et al. 2007; Hurord etal. 2007b).

Major Questions

Several questions motivate this science goal. Arst question concerns the nature o the tectoniceatures, since whether they ormed rom horizontal extension, contraction, or shearing o the sur

ace bears directly on the evolution o Enceladus.An Enceladus mission would also seek to resolvewhy tectonic patterns vary so widely across thesurace and how tectonism has changed over time.It is also important to understand the stressesthat have given rise to tectonic eatures, or instance convection within the icy mantle, possiblyinvolving a regime similar to plate tectonics onEarth (e.g., Helenstein et al. 2006a), can inducetractions on the surace; thus, unraveling the tectonics may illuminate these convective motions.

Finally, Enceladus may hold the key to under

standing tectonic processes on other icy satellites.Indeed, many tectonic eatures on Enceladus maybe analogous to eatures observed on other icysatellites such as Europa (Figure 2.1.3-5), Ganymede, and perhaps itan. Tus, study o the tectonics o Enceladus, which is probably currentlyactive, can be used as a natural laboratory to investigate the response to stresses o the other icysuraces o the outer solar system.


High-resolution images are vital or interpret

ing the individual eatures and the orces that ledto their ormation. Near-complete coverage ohigh-resolution (~10 m/pixel) imaging will provide a detailed tectonic ramework, and morphology o individual eatures, that is essential orunderstanding Enceladus tectonism. Stereo imaging o selected eatures will allow quantitativemodeling o their evolution. Radar sounding caninvestigate the subsurace expressions o tectoniceatures, providing important constraints on theirnature. Measurements o tidal exing, and localgravity and topography, will aid understanding othe stresses driving tectonic activity. It is also pos

sible that motions along active tectonic eaturescould be detected by repeated very precise topographic measurements, or instance using laseraltimetry, or by seismometry rom the surace.

2.1.3.4 t Heg ierr srre

Enceladus ranks alongside Io and possiblyriton as one o the ew geodynamically active

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EN065

Enceladus

5 km

Europa

5 km

EN065

ENCELADUS

Fgre 2.1.35: Comparison o tectonic eatures on Enceladus (let) and Europa (right) at similar scales. Credit:

NASA/JPL/Space Science Institute/Arizona State University (PIA06251, PIA00849).satellites in the solar system. Understanding thetidal heating engine that almost certainly drivesthis activity, and the interior structure that bothcontrols and is controlled by the tidal heating, isvital to understanding Enceladus as a whole.


Enceladus mean density is 1608.3 kg/m3 andits mean radius 252.1 km (Tomas et al. 2007).Te interior structure o Enceladus is not known,

however, calculations suggest that it is likely dierentiated with an icy shell ~90 km thick thatsurrounds a silicate core that could either be hydrated or dehydrated (Figure 2.1.3-6) (Barr andMcKinnon 2007; Schubert et al. 2007).

It is not known whether Enceladus has a subsurace ocean. I a global ocean exists, it woulddecouple the ice shell rom the underlying rockycore, permitting tidal dissipation and tidally driven tectonics similar to Europa. An ocean, or evenisolated pockets o subsurace liquid water couldconceivably provide a habitat or primitive lie.

Te heat ux rom Enceladus south polar region is between 3 to 7 GW based on CIRS observations (Figure 2.1.3-7, Spencer et al. 2006); theglobal heat ux could be ~10 times as high. Radiogenic heating rom Enceladus rocky component supplies only ~0.3 GW at present (Schubertet al. 2007), so tidal dissipation is likely supplying the rest o the heat. Te exact mechanism bywhich tidal deormation in Enceladus results in

160 km11 MPa

60 MPa 50 MPaEN066

Ice Shell

Ocean?

Ice Shell

Ocean?170 km

10 MPa

250 km

HydratedRock

Anhydrous

Rock

Fgre 2.1.36: Interior structure o a dierentiatedEnceladus (Barr and McKinnon 2007), assuming solar-composition rock (Mueller and McKinnon 1988), apure ice shell, and an updated iron abundance (Lodders2003). An internal liquid layer may exist at the base othe ice shell.

internal heating is not known; it is possible thatdissipation occurs within the deep interior o awarm convecting satellite (as envisioned byRossand Schubert(1989)) but also plausible that dissipation occurs close to its surace on shallow ault

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ENCELADUS

2-10

zones (Nimmo et al. 2007). Regardless o themethod o dissipation, Enceladus must be warmand/or partially molten to experience signicanttidal exing and dissipation the mode o ini-tial warming to kick start tidal heating is notknown (though 26Al heating has been suggested

as one possible mechanism (Castillo et al. 2006)).I tidal dissipation is localized to the south pole,it could provide enough heat to maintain a localsubsurace sea, topographic low, and to drivecryovolcanism (e.g., Collins and Goodman 2007).

Temperature, Kelvin

60 65 70 75 80 85

EN067

Figure 2.1.3-7: False-color image of 12-16 microncolor temperatures on Enceladus, from the CassiniComposite Infrared Spectrometer (CIRS) showing theheat radiation from the warm tiger stripes in the southpolar region. Peak temperatures are much warmer, atleast 145 K, than the low-resolution averages shownhere. The dashed line is the terminator. (Spencer et al.

2006).

Evidence that high heat ux has been presentor long periods comes rom the observations thatmany ancient impact craters on Enceladus showclear evidence o viscous relaxation due to locallyelevated near-surace temperatures at some timein the bodys past (Passey 1983; Smith et al. 2007;Schenk and Moore 2007).

o date, measurements o Enceladus tidal heatux are the only quantitative constraints on theamount o tidal dissipation and its spatial local-ization in any icy satellite. idal dissipation oc-curs in Enceladus because the satellite does me-chanical work against its own internal rigidity.

Te amount o tidal dissipation thereore dependson the amount o deormation occurring withinEnceladus over its daily orbital cycle.

Major Questions

Understanding the interior structure oEnceladus, and the heat engine that drives its ac-tivity, is a key goal o a agship mission. It is im-portant to constrain the extent o dierentiation,the presence o an ocean, and the modes o heattransport and generation in the interior. For thisit is necessary to understand the moons heat ow,

the internal density and thermal structure and iceshell thickness, and the location and distributiono liquid water.


Determination o Enceladus static gravity eldto sufcient degree and order to look or subsur-ace density anomalies, and with proper geom-etry to independently measure J2 and C22 (seeMcKinnon 1997or discussion); measurement oits magnetic eld (both intrinsic and inductive);and seismic sounding, will provide essential con-

straints on interior structure. In addition, mea-surements o Enceladus time-variable potentialand surace deormation can provide estimates oits Love numbers, h2 and k2, which can be usedto constrain its interior structure (or example tohelp determine the presence o an ocean), and,serve as rst steps toward modeling tidal dissipa-tion because they relate tidal deormation to theapplied tidal potential. Te determination o thesubsurace thermal structure using some sort osounding technique (such as seismology or ice-penetrating radar) would provide valuable inor-mation about the modes o tidal dissipation and

hold the key to understanding tidal dissipationin other satellites, and thermal inrared measure-ments o surace temperature, coupled with bolo-metric albedo measurements to understand andremove the absorbed sunlight contribution, willconstrain global and local heat ow. Such mea-surements would also provide valuable constraintson the thickness o Enceladus lithosphere, whichaects its modes o resuracing and surace/sub-surace material exchange. Measurements o the


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ENCELADUStopography o viscously relaxed impact craters will exchange, and degrades the energy o the enerprovide important constraints on the time history getic electron population throughout the sameand spatial distribution o heat ow throughout region through coulomb collisions. Tis resultsEnceladus history. in a radiation environment three orders o mag

nitude less intense than that o Earth, and even2.1.4 Prry 3 G weaker relative to Jupiter. And that relatively be

nign environment extends to the moons Mimas,2.1.4.1 sr syem ier ethys, and Dione, and to a lesser degree, Rhea.Tus, Enceladus, at 500-km diameter, com-

In addition to Enceladus being an interesting pletely dominates the radiation environmentbody in its own right, it has a major inuence on throughout the inner Saturnian magnetosphere.the rest o the Saturn system. In turn, the larg- I a process like this is at all common througher Saturn system inuences Enceladus in many out the universe, it may be a signicant actor inways. Te processes involved are particularly in- the probability o encountering habitable zonesteresting because they may aect both Enceladus about giant planets such as Saturn where radiaability to support lie, and the habitability o hy- tion might otherwise preclude lie (at least at thepothetical extrasolar planetary systems that may surace o moon, as, or example, at Jupiter), orcontain Enceladus-like worlds. alternately, might provide energy or subsurace

lie (Chyba 2000). Investigation o Enceladus in-

Current State of Knowledge teraction with the magnetosphere is, thereore, ointerest to the entire question o the evolution oSince Enceladus orbits deep in Saturns magne- lie throughout the universe.

tosphere, the impact o energetic trapped particleson Enceladus surace is an important process. In addition to gas, Enceladus plumes also con-Particle precipitation contributes to the aging and tain very ne dust particles. Many o these par-chemistry o Enceladus surace layer through ticles are ejected with sufcient velocity that they,sputtering and radiolysis. While such processes too, escape Enceladus weak gravity eld andare not unique to Enceladus, the interpretation spread to orm the tenuous E-ring about Saturno surace materials to determine the age o vari- (Figure 2.1.4-1). Te E-ring is dominated byous surace regions on Enceladus depends on an ~1 micron ice particles, with some larger ones,understanding o such surace processes. For this though the size/requency distribution is not wellreason, it is important to characterize the radia- known. Te E-ring is typically less than a Saturn

tion environment o Enceladus in any mission radius in thickness, and extends rom about threethat seeks to understand the resuracing o the to ten Rs, with peak density near its source at themoon by its plume material. Micrometeorite gar- orbit o Enceladus. Particles in the E-ring aredening, either rom interplanetary dust particles composed o ejecta rom the plumes, either di-or returni

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