Saturn from Cassini-Huygens (eBook)

Michele Dougherty, Cassini Saturn Orbiter (NASA/ESA) - Acting Principal Investigator (PI) for magnetometer instrument.   Dr. Esposito the principal investigator of the Ultraviolet Imaging Spectrograph (UVIS) experiment on the Cassini space mission to Saturn. He was chair of the Voyager Rings Working Group. As a member of the Pioneer Saturn imaging team, he discovered Saturn’s F ring. He has been a participant in numerous US, Russian and European space missions and used the Hubble Space Telescope for its first observations of the planet Venus. He was awarded the Harold C. Urey Prize from the American Astronomical Society, The NASA Medal for Exceptional Scientific Achievement, and the Richtmyer Lecture Award from the American Association of Physics Teachers and the American Physical Society. Dr. Esposito has written his Ph.D dissertation, numerous scientific publications, scholarly reviews on the topic of planetary rings as well as the Cambridge University Press book Planetary Rings. Along with his students and colleagues he continues to actively research the nature and history of planetary rings at the University of Colorado, where he has been since 1977. He is now Professor of Astrophysical and Planetary Sciences and a member of the Laboratory for Atmospheric and Space Physics (LASP). He has been an officer of the Division for Planetary Sciences of the American Astronomical Society and of the Committee for Space Research (COSPAR) of the International Council of Scientific Unions. He was chair of the National Academy of Sciences Committee on Planetary and Lunar Exploration (COMPLEX). He is a member of American Astronomical Society, American Geophysical Union and International Astronomical Union. Dr. Stamatios (Tom) Krimigis has been at APL since 1968, after earning his B. Physics from the University of Minnesota (1961), and his M.S. (1963) and Ph.D. (1965) in Physics from the University of Iowa and serving as Assistant Professor of Physics and Astronomy there. He became Supervisor of Space Physics and Instrumentation in the Space Department, Chief Scientist in 1980, Department Head in 1991, and Emeritus Head in 2004. He is Principal Investigator on several NASA spacecraft, including Voyagers 1 and 2 to the Outer Planets and the Voyager Interstellar Mission, and the Cassini mission to Saturn and Titan. He has designed and built instruments that have flown to seven of the nine planets, and hopes to complete the set with his participation in the MESSENGER mission to Mercury and New Horizons mission to Pluto. He has published more than 370 papers in journals and books on the physics of the sun, interplanetary medium, planetary magnetospheres, and the heliosphere. He is recipient of NASA's Exceptional Scientific Achievement Medal twice, is a Fellow of the American Physical Society, American Geophysical Union, and American Association for the Advancement of Science, recipient of COSPAR's Space Science Award in 2002, a recipient of the Basic Sciences Award of the International Academy of Astronautics where he serves on the Board of Trustees, and was elected recently to the newly established chair of "Science of Space" of the Academy of Athens.

Preface 5
Contents 6
Chapter 1 Overview 8
1.1 Introduction 8
1.2 Organization of This Volume 8
1.3 Synopsis of theMain Results for the Saturn System 8
1.3.1 Origin and Interior 9
1.3.2 Saturn’s Atmosphere 9
1.3.3 Magnetosphere 10
1.3.4 Saturn’s Rings 10
1.3.5 Icy Satellites 11
1.4 Open Questions and FutureWork 12
1.4.1 Origin and Interior 12
1.4.2 Saturn’s Atmosphere 12
1.4.3 Saturn’s Magnetosphere 13
1.4.4 Saturn’s Rings 13
1.4.5 The Satellite System 14
1.5 Continued Saturn System Exploration 15
References 15
Chapter 2 Review of Knowledge Prior to the Cassini-Huygens Mission and Concurrent Research 16
2.1 Introduction 16
2.2 Saturn’s Composition 17
2.2.1 Brief Historical Overview 17
2.2.2 Bulk Composition: H2, He/CH4 17
2.2.3 Tropospheric Composition 18
2.2.3.1 Ammonia,Water, and Hydrogen Sulfide 18
2.2.4 Stratospheric Composition 21
2.2.4.1 Hydrocarbons and Photochemistry 21
2.2.4.2 External Supply of Oxygen 22
2.2.4.3 Isotopic Ratios 22
2.3 Saturn’s Interior 23
2.4 Saturn’s Clouds and Aerosols 23
2.4.1 Pioneer/Voyager Era Measurements of Aerosol Structure 23
2.4.2 From Pioneer/Voyager to Cassini: Earth-Based Observations of Aerosol Structure 24
2.5 Saturn’s Temperatures 26
2.6 Saturn’s Atmospheric Dynamics 30
2.7 Saturn’s Upper Atmosphere and Auroral Emission 33
2.7.1 Ultraviolet Auroral Observations 33
2.7.2 Infrared Auroral Observations 34
2.8 Planetary Magnetic Field andMagnetosphere 36
2.8.1 Magnetic Field 36
2.8.2 Magnetospheric Plasma Environment 38
2.8.3 Energetic Particles 39
2.8.4 PlasmaWaves 42
2.8.5 Summary of Magnetosphere 43
2.9 Saturn’s Ring System 43
2.9.1 Particle Size Distribution 45
2.9.2 Ring Particle Composition 46
2.9.3 Origin and Evolution 47
2.10 Icy Satellites 48
2.10.1 Introduction 48
2.10.2 Sources of Information 48
2.10.2.1 Dimensions, Densities, and Rotational Properties 48
2.10.2.2 Surface Compositions and Surface Optical Properties 48
2.10.2.3 Geology 49
2.10.3 The Eight Large, Airless Satellites 49
2.10.3.1 Mimas 49
2.10.3.2 Enceladus 49
2.10.3.3 Tethys 50
2.10.3.4 Dione 51
2.10.3.5 Rhea 51
2.10.3.6 Hyperion 51
2.10.3.7 Iapetus 52
2.10.3.8 Phoebe 52
2.10.4 The Small Satellites 52
2.10.4.1 Satellites Near the Rings and Associated with Larger Satellites 52
2.10.4.2 Irregular Satellites 52
References 53
Chapter 3 Origin of the Saturn System 62
3.1 Introduction 62
3.2 Planet and Satellite Formation 63
3.2.1 Big Bang to the Solar Nebula 63
3.2.2 The Solar Nebula to Planets 63
3.2.2.1 The Inner Solar System 63
3.2.2.2 The Outer Solar System 63
3.2.2.3 Giant Planet Formation 64
3.2.2.4 Planetary Migration and the NiceModel 64
3.2.3 Circumplanetary Disks to Satellites 66
3.2.3.1 Formation of the Subnebula 67
3.2.3.2 Conditions for Satellite Accretion 67
3.3 Cassini Results and Discussion 69
3.3.1 Saturn and Rings 69
3.3.2 Satellite Composition 69
3.3.2.1 Satellite Bulk Densities 69
3.3.2.2 EquilibriumCondensation and Solar Composition 71
3.3.3 Satellite Structures 75
3.3.3.1 Background 75
3.3.3.2 Observations 75
3.3.4 Satellite Geological History 77
3.3.4.1 Crater ages 77
3.3.4.2 Iapetus’ Formation Time 77
References 78
Chapter 4 The Interior of Saturn 82
4.1 Diagnostics of Interior Structure and Dynamics 82
4.1.1 Gravity Field and Shape 82
4.1.2 Differential Rotation and Equations of State 83
4.2 Evolution of Saturn 85
4.3 Coupling of Detailed Evolutionary Models for Saturn to the Helium Partitioning Problem, and Comparison with Jupiter 86
4.4 Summary 87
References 87
Chapter 5 Saturn: Composition and Chemistry 89
5.1 Introduction 89
5.2 Observed Composition 90
5.2.1 Major Gases or Bulk Composition 90
5.2.1.1 Helium Abundance 90
5.2.1.2 The Para Fraction of Molecular Hydrogen 92
5.2.1.3 Carbon Elemental Composition 92
5.2.1.4 Nitrogen, Sulfur and Oxygen 94
5.2.1.5 Isotopic Composition 94
5.2.2 Thermochemical Products 95
5.2.3 Photochemical Products 96
5.2.3.1 Evidence for Photochemistry in the Upper Troposphere 96
5.2.3.2 Stratospheric Products 98
5.2.4 External Oxygen Flux 100
5.3 Chemistry 101
5.3.1 Tropospheric Chemistry 101
5.3.1.1 Thermochemistry 101
5.3.1.2 Photochemistry in the Troposphere 102
5.3.2 Stratospheric Hydrocarbon Chemistry 104
5.3.2.1 Chemical Reactions 104
5.3.2.2 One-Dimensional Models 105
5.3.2.3 Seasonal Variation in 1-D Models 107
5.3.2.4 Two-Dimensional Models 109
5.3.3 Oxygen Chemistry 110
5.3.4 Auroral Chemistry 112
5.4 Summary and Conclusions 112
References 113
Chapter 6 Saturn Atmospheric Structure and Dynamics 119
6.1 Introduction 119
6.1.1 Saturn’s Place Among Planetary Atmospheres 119
6.1.2 Saturn Science Between the Voyager and Cassini Epochs 121
6.1.3 Questions About Saturn Entering the Cassini Era 122
6.1.4 Scope and Organization of the Chapter 122
6.2 Observational Inferences About the Deep Atmosphere 123
6.2.1 Saturn’s Rotation Period 123
6.2.2 Convective Heat Flux and Condensation Levels 124
6.2.3 Cassini Probing of the Atmosphere Below Cloud Top 125
6.3 Observations at and Above the Visible Cloud Top 127
6.3.1 Levels Sensed by Cassini Instruments from Cloud Top to the Stratosphere 127
6.3.2 Albedo Patterns vs. Jets on Saturn vs. Jupiter 128
6.3.3 Cloud LevelWinds and Dynamical Fluxes 129
6.3.4 Thermal Structure and Circulation Above Cloud Level 131
6.3.5 Temporal Variation of the Equatorial Jet 133
6.3.6 Upper Troposphere Temperature Knee: Structure and Seasonality of Solar Heating 135
6.3.7 Stratospheric Circulation 136
6.3.7.1 Meridional Circulation 136
6.3.7.2 Equatorial Oscillations 136
6.3.8 Potential Vorticity Diagnosis 139
6.4 Discrete Features as Constraints on Processes and Structure 141
6.4.1 Anti-Cyclonic and Cyclonic Vortices 141
6.4.2 Convective Clouds and Lightning 142
6.4.3 Upper Troposphere Thermal Features and Rossby Waves 146
6.4.4 North Polar Hexagon 147
6.4.5 South Polar Vortex 149
6.4.6 OtherWavelike Features 150
6.5 Theories and Models of the General Circulation 152
6.5.1 Deep Cylinders vs. Shallow Weather Layers 152
6.5.2 DistinguishingDeep-or-Shallow Structure from Deep-or-Shallow Forcing 153
6.5.3 Models for Jet Pumping 153
6.5.4 Do the Observations Constrain Our Models? 157
6.6 Discussion 158
6.6.1 Future Modeling Directions 159
6.6.2 Long-Term Observational Needs 160
References 160
Chapter 7 Clouds and Aerosols in Saturn’s Atmosphere 166
7.1 Introduction 166
7.2 Expectations fromThermochemical Equilibrium Theory and Photochemistry 167
7.3 Observational Constraints on Particle Composition and Chromophores 168
7.4 Aerosol Optical and Physical Properties 170
7.5 Aerosol Vertical Structure 173
7.5.1 Radiative Transfer Models and Haze Structure 173
7.5.2 Mean Vertical Structure 174
7.5.3 Latitudinal Structure 176
7.5.4 Short-term Changes 176
7.5.5 Seasonal Changes 177
7.5.6 Regional Structure: The Equatorial Jet 177
7.5.7 Regional Structure: The GreatWhite Spot (GWS) 179
7.5.8 Regional Structure: The South Polar Vortex 180
7.5.9 Convective Clouds 180
7.6 Solar Radiation Penetration and Deposition 182
7.7 Summary and FutureWork 182
References 183
Chapter 8 Upper Atmosphere and Ionosphere of Saturn 185
8.1 Introduction 185
8.2 Structure and Composition of the Neutral Upper Atmosphere 185
8.2.1 Determination of Atmospheric Properties from Ultraviolet Occultations 186
8.2.2 Determination of Atmospheric Properties from UVIS Spectra and Emission Maps 189
8.3 Theoretical and Empirical Models of the Neutral Upper Atmosphere: Chemistry and Atmospheric Transport in the Homopause Region 189
8.4 Theoretical and Empirical Models of the Upper Atmosphere: Temperature Structure, Energy Balance, and Dynamics 192
8.4.1 Thermal Structure 192
8.4.2 Energy Balance and Dynamics 193
8.5 Observations of the Ionosphere 195
8.5.1 Radio Occultation Observations of Electron Densities 195
8.5.2 Electron Density Variations Inferred from SEDs 195
8.5.3 Ground Based Observations of H3+ Emission 196
8.6 Models of Ionospheric Structure, Composition and Temperatures 197
8.6.1 Background Theory and EarlyModels 197
8.6.2 Modern Theory and Time Dependent Models 198
8.6.3 Plasma Temperatures in Saturn’s Ionosphere 201
8.7 Summary 201
References 203
Chapter 9 Saturn’s Magnetospheric Configuration 206
9.1 Introduction 206
9.1.1 Pre-Cassini Understanding 206
9.1.2 Major Cassini Discoveries 207
9.1.3 Earth, Jupiter and Saturn 209
9.2 Magnetic Field 212
9.2.1 Intrinsic Magnetic Field 212
9.2.2 The Magnetodisk 214
9.2.3 Empirical Magnetic FieldModels 215
9.3 Plasma Sources and Sinks 217
9.3.1 Rings (< 3RS)
9.3.2 Icy Satellites (3RS to 6RS) 219
9.3.2.1 Enceladus 219
9.3.2.2 HC andWC 220
9.3.2.3 NC 222
9.3.3 Minor Sources 222
9.3.4 Loss Processes 223
9.3.5 Plasma Density Models 224
9.4 Magnetospheric Regions 224
9.4.1 Trapped Radiation 224
9.4.2 Ring Current 228
9.4.3 Plasma Sheet 234
9.4.4 Magnetotail 237
9.4.5 GlobalMHD Models 240
9.5 Ionosphere-Magnetosphere Coupling 240
9.5.1 Radial Transport 240
9.5.2 Corotation, Subcorotation and Corotation Breakdown 243
9.6 Upstream and SolarWind Boundaries 245
9.6.1 Upstream Conditions 245
9.6.2 Foreshock Region 246
9.6.3 Bow Shock and Magnetosheath 249
9.6.4 Magnetopause 250
9.6.4.1 Standoff Distance 250
9.6.4.2 Compressibility 250
9.6.4.3 Shape 251
9.7 Some Open Questions 251
References 252
Chapter 10 The Dynamics of Saturn’s Magnetosphere 259
10.1 Introduction 259
10.2 Transport of Mass and Energy, and Plasma Flow 259
10.3 Rotational Modulation 263
10.4 Magnetic Field Structure and Dynamics 269
10.5 Magnetotail Dynamics 271
10.6 Magnetospheric Compression 275
10.7 Conclusion 277
References 277
Chapter 11 Fundamental Plasma Processes in Saturn’sMagnetosphere 282
11.1 Introduction 282
11.2 Plasma–Material Interaction Processes 283
11.2.1 Signatures of Gas and Plasma Sources 283
11.2.1.1 Neutral Gas Components 283
11.2.2 Charged Particle and Plasma Interactions with Surfaces 285
11.2.2.1 Introduction and Background 285
11.2.2.2 Application to Saturn 286
Surface Radiolysis 286
Surface Irradiation 286
Surface Sputtering in Saturn’s Magnetosphere 286
Radiolytic Production of Molecular Oxygen 286
11.2.3 Charged Particle and Plasma Interactions with Neutral Gas 287
11.2.3.1 Introduction and Background 287
11.2.3.2 Application to Saturn 287
Neutral Sources and Predictions 287
Neutral Loading at Saturn and Jupiter 288
Charge Exchange 288
Neutral Scattering 289
Electron-Induced Processes 289
Comet-Like Interactions 289
11.2.4 Moon–Magnetosphere Interactions 290
11.2.4.1 Introduction and Background 290
Internal Magnetic Fields 290
Interaction Environments and Features 291
Physical Processes 291
11.2.4.2 Plasma Absorbing Interactions at Saturn 291
Cold Plasma Response 291
Magnetic Field Response 292
Energetic Particle Response 293
Special Cases 294
11.2.4.3 Mass Loading Interactions at Saturn 294
Thermal Plasma and Magnetic Field Response 294
Energetic Particle Response 296
11.3 Transport 296
11.3.1 Rotational versus Solar-WindDrivers 297
11.3.2 Magnetosphere–Ionosphere Coupling 298
11.3.3 Corotation Lag 299
11.3.4 Centrifugal Interchange Instability 301
11.3.5 The Radial Diffusion Formalism 304
11.4 Energy Conversion 305
11.4.1 Reconnection 306
11.4.1.1 Magnetopause Reconnection 306
Proxy Studies at Earth 307
In Situ Studies at Earth 307
In Situ Studies at Saturn 307
Discussion/Future Directions 308
11.4.1.2 Tail Reconnection 308
Tail Stretching and the Growth Phase 309
Dipolarization 309
Near the Neutral Point 309
Plasmoid Formation 309
Discussion/Future Directions 311
11.4.2 Particle Acceleration 311
11.4.2.1 Adiabatic Acceleration and Related Processes 311
Background 311
Adiabatic Acceleration at Saturn 312
11.4.2.2 Pickup Acceleration and Related Processes at Saturn 314
Background 314
Pickup Energization at Saturn 315
Pickup-Associated Electron Acceleration 315
11.4.2.3 Other Acceleration Processes 316
Injection Energization 316
Miscellaneous Acceleration Mechanisms 317
11.4.3 Current Generation 317
11.4.3.1 Background 317
11.4.3.2 Force Balance at Jupiter and Saturn 318
11.4.4 Wave Particle Interactions 319
11.4.4.1 Background onWave Particle interactions 319
11.4.4.2 Background onWaveModes 320
11.4.4.3 Application to Cassini 320
ECH, UHR, and Narrowband Radio Emissions 320
Whistler-Mode Emissions 323
Miscellaneous Plasma Waves 324
11.5 Closing Remarks 325
References 325
Chapter 12 Auroral Processes 333
12.1 Introduction 333
12.2 Observations of Auroral Emissions 334
12.2.1 Pre-Cassini Summary 334
12.2.2 Earth-Based Observations Concurrent with Cassini 334
12.2.2.1 HST-Cassini Campaigns 334
12.2.2.2 Saturn’s Ultraviolet Auroral Morphology 335
12.2.2.3 H2 Auroral Spectroscopy 336
12.2.2.4 Effects of Auroral Energy Input to the Atmosphere 337
12.2.2.5 Ground-Based Observations of Saturn’s IR Aurora 338
12.2.3 Cassini Remote Sensing Observations of Auroral Emissions 339
12.2.3.1 UVIS Results 339
12.2.3.2 VIMS Results 340
12.2.3.3 ISS Results 340
12.2.4 Auroral Radio Emissions 341
12.3 Magnetospheric Dynamics and the Aurora 347
12.3.1 Response of the Aurora to Solar Wind Input 348
12.3.2 Response of the Aurora to Rotational Dynamics 350
12.4 In-Situ Measurements 351
12.4.1 Energetic Particles 351
12.4.2 Auroral Currents 358
12.5 SolarWind-Magnetosphere–Ionosphere Coupling Currents and Their Relation to Saturn’s Aurora 361
12.5.1 Proposed Steady-State Theoretical Framework 361
12.5.2 Time-Dependent Auroral Processes 365
12.6 Summary 369
References 370
Chapter 13 The Structure of Saturn’s Rings 375
13.1 Grand Structure of the Rings 375
13.2 A Ring 377
13.2.1 Satellite Resonance Features 378
13.2.1.1 Density and BendingWaves 378
13.2.1.2 Outer Edge of the A Ring 381
13.2.2 Satellite Impulse-Driven Features 381
13.2.2.1 Wavy Edges of the Encke and Keeler Gaps 381
13.2.2.2 SatelliteWakes 382
13.2.3 Self-GravityWakes and Propellers 382
13.2.4 The Inner A Ring 386
13.2.5 Small-Scale Periodic Structures 387
13.3 B Ring 387
13.3.1 Overview 387
13.3.2 The Inner B Ring (Region B1) 389
13.3.3 The Central B Ring (Regions B2 and B3) 390
13.3.4 The Outer B Ring (Regions B4 and B5) 391
13.3.5 Density and BendingWaves 392
13.3.6 Self-GravityWakes 392
13.3.7 Correlations of Structure with Particle Properties 394
13.4 The Cassini Division and C Ring 394
13.4.1 Overview 394
13.4.2 Gaps and Ringlets in the Cassini Division 394
13.4.3 DensityWaves in the Cassini Division 397
13.4.4 C Ring Structure 398
13.4.5 Waves and Other Resonant Features in the C Ring 400
13.5 F Ring 401
13.5.1 Overview 401
13.5.2 Perturbations by Prometheus 402
13.5.3 Jets and Kinematic Spirals 403
13.5.4 Embedded Objects 403
13.5.5 A Narrow Component 405
13.6 Summary 406
13.6.1 DensityWaves 406
13.6.2 Microstructure of the Rings 407
13.6.3 Vertical Structure 407
13.6.4 Large-Scale Structure 407
13.6.5 Future Prospects 408
References 408
Chapter 14 Dynamics of Saturn’s Dense Rings 413
14.1 General Theory and Recent Advances 413
14.1.1 Steady-State of a Dense Non-Gravitating Particle Disk 414
14.1.1.1 Dissipative Collisions 414
14.1.1.2 Shear Stress 414
Local Shear Viscosity 414
Non-Local Shear Viscosity and Pressure 415
Total Shear Viscosity and Pressure 415
14.1.1.3 Steady State and Thermal Stability 415
14.1.1.4 Mechanical Properties of Particles 416
14.1.1.5 Steady State Dynamical Properties 417
14.1.2 Balance Equations for Dense Rings 418
14.1.2.1 Kinetic Theory 418
14.1.2.2 Hydrodynamics 419
14.1.3 Self-Gravity of the Ring 421
14.1.3.1 Gravitational Encounters 422
14.1.3.2 Vertical Self-Gravity 422
14.1.3.3 GravitationalWakes 422
14.1.3.4 Survey of Self-GravityWake Structures 425
14.1.3.5 Gravitational Viscosity 425
14.1.3.6 Observational Signatures of Self-Gravity Wakes 426
14.2 Instabilities 429
14.2.1 ViscousOverstability 430
14.2.1.1 A Linear Model 430
14.2.1.2 Overstability and Self-Gravity 432
14.2.2 Viscous Instability 433
14.2.3 Instabilities due to Ballistic and Electromagnetic Transport 435
14.2.4 Shear Rate Instability 435
14.3 RingMoon Interactions and Narrow Rings 436
14.3.1 SpiralWaves 436
14.3.1.1 Background 436
14.3.1.2 Elements of Theory 437
14.3.1.3 Advances in Modeling 438
The Waves Associated with the Co-Orbital Satellites 438
Power Spectrum Density Methods 438
Application of the Nonlinear Theory 438
14.3.2 Moonlet Induced Gaps 439
14.3.3 Propellers – the Action of Tiny Moons 440
14.3.4 Dense NarrowRings 443
14.3.4.1 Confinement of Narrow Rings 443
14.3.4.2 Rigid Precession 444
14.3.4.3 Excitation of Eccentricities and Inclinations 444
14.3.4.4 Viscous Overstability 444
14.3.4.5 Ring Edges 444
14.4 Size Distribution and Spins of Ring Particles 445
14.4.1 Particle Size Distribution and Its Evolution 445
14.4.1.1 Particle Size Distribution Derived From Observations 445
14.4.1.2 Accretion of Particles in the Roche Zone 446
14.4.1.3 Processes and Models for Particle Size Evolution 448
14.4.2 Particle Spins 449
14.4.2.1 Dynamical Studies 449
14.4.2.2 Relation to Observations of the Rings’ Thermal Emission 451
14.5 Open Problems 451
References 452
Chapter 15 Ring Particle Composition and Size Distribution 459
15.1 Introduction 459
15.2 Ring Particle Size Distribution 460
15.2.1 Models and Theory 460
15.2.2 Cassini RSS Extinction Observations 462
15.2.3 Model Results 464
15.2.4 Near-Forward Scattered Signal Observations 466
15.2.5 Size Distribution from the Voyager RSS Observations 467
15.2.6 Size Distribution from 28 Sgr Stellar Occultations 469
15.2.7 Size Information from the Excess Variance in Stellar Occultations 470
15.2.8 Summary of Current Knowledge and Limitations 470
15.2.9 Comparison of the Four Main Ring Regions 471
15.2.10 Caveats Regarding Modeling “Ring Particles” vs. “Self-GravityWakes” 472
15.3 “Propeller” Objects: Shards of the Ring Parent or Locally Grown? 473
15.4 Ring Particle Composition, Its Radial Variations, and Comparison with Other Icy Objects 474
15.4.1 Observations 474
15.4.2 Global VIMS Ring Spectra and Overall Composition 476
15.4.3 Regional and Phase Angle Variations of VIMS Ring Spectra 476
15.4.4 UVIS Spectra of theMain Ring Regions 479
15.4.5 Radial Profiles of ISS and VIMS Spectral Properties 479
15.4.5.1 Radial Profiles of ISS and VIMS Spectral Properties 479
15.4.5.2 Particle Albedo Variation from CIRS Ring Temperature Profiles 481
15.4.6 Modeling Individual Particle Properties from Observed Ring Reflectance 483
15.4.6.1 Modeling the Layer of Ring Particles as a Whole 483
15.4.6.2 Modeling Ring Particle Regoliths 483
15.4.7 Laboratory and ModelWater Abundance and RegolithGrain Size 484
15.4.7.1 Water Ice Band Depths fromVIMS Data 484
15.4.7.2 Regolith Properties fromCIRS Spectra at Long Thermal InfraredWavelengths 485
15.4.8 GlobalModels of Ring Composition 486
15.4.9 Comparison of Ring Spectral Properties with Other Icy Objects 489
15.5 Ring Atmosphere and Meteoroid Bombardment 492
15.5.1 Introduction 492
15.5.2 Main Rings 492
15.5.3 Modeling of the Ring Atmosphere 493
15.5.4 Atmosphere-Driven Chemistry on Icy Ring Particle Surfaces 494
15.5.5 The Ring Atmosphere as a Magnetospheric and Atmospheric Source 495
15.5.6 Meteoroid Bombardment, Ring Mass, and Ring Composition 495
15.6 Summary, Discussion, and Future Directions 496
15.6.1 Summary of Observational Properties 496
15.6.2 Origin – the Big Picture 497
15.6.3 Candidate “UV Absorbers” 499
15.6.4 FutureWork Needing to Be Done 500
Appendix 15: The Zero-Phase Opposition Effect 501
References 504
Chapter 16 Diffuse Rings 510
16.1 Introduction 510
16.2 Pre-Cassini Observations 511
16.3 Cassini Observations and Current Theories 513
16.3.1 The D Ring 513
16.3.2 The Roche Division 516
16.3.3 Resonant Structures in the D Ring and the Roche Division 516
16.3.4 Faint Ringlets Within Main-Ring Gaps 517
16.3.5 Spokes in the B Ring 518
16.3.6 The G Ring 520
16.3.7 Other Narrow Outer Faint Rings 522
16.3.8 The E Ring 522
16.3.9 Dust Streams 529
16.4 Summary: Dynamical Connections Between Diffuse Rings 531
References 532
Chapter 17 Origin and Evolution of Saturn’s Ring System 536
17.1 Introduction 536
17.1.1 New Results on an Old Question 536
17.1.2 Organization of the Chapter 537
17.2 Basic Observational Constraints and Theoretical Considerations 537
17.2.1 Ring Structure 537
17.2.1.1 Ring Particle Sizes 538
17.2.1.2 Ring Particle Composition 540
17.2.1.3 Mass of Saturn’s Rings 540
17.2.2 Processes in Saturn’s Rings: Simple Considerations 541
17.2.2.1 Tidal Forces: the Roche Limit 541
17.2.2.2 Collisions: Flattening and Viscous Spreading 541
17.2.2.3 Meteoroid Bombardment 542
17.2.2.4 Cosmic Recycling 543
17.2.2.5 Young or Old Rings? 544
17.2.3 An Overview of Possible Scenarios for the Origin of the Main Rings 546
17.2.4 Beyond the Paradox? 547
17.3 Evolution of theMain Rings 547
17.3.1 Meteoritic Bombardment and Ballistic Transport 548
17.3.1.1 Principles 548
17.3.1.2 Dynamical Evolution 549
17.3.1.3 Spectral Evolution 550
17.3.2 Limited Accretion 551
17.3.2.1 Gravitational Instability 553
17.3.2.2 Tidally Modified Accretion 553
17.3.2.3 Surface Sticking 554
17.3.2.4 Accretion of Small Embedded Satellites? 555
17.3.3 Collisional Cascade 556
17.3.4 Ring–Moon Interactions 557
17.3.5 The Long Term Evolution of Saturn’s Main Rings 558
17.4 Scenarios for Origin of the Main Rings 558
17.4.1 Remnant from Saturn’s Sub-nebula Disk? 558
17.4.1.1 Satellite Formation 559
17.4.1.2 Implanting the Ring System 559
17.4.1.3 Collapse and Cooling of the Envelope 559
17.4.1.4 The Role of Turbulence 560
17.4.1.5 Caveats 560
17.4.2 Debris from a Destroyed Satellite? 561
17.4.2.1 Bringing and Keeping a Satellite in the Roche Zone 561
17.4.2.2 Destruction of the Satellite 562
17.4.3 Debris from Tidally Disrupted Comets 563
17.4.4 A Conclusion? 565
17.5 Saturn’s F Ring: Processes and Origin 565
17.5.1 Characteristics of the F Ring Relevant for Its Origin and Evolution 565
17.5.2 Processes atWork in the F Ring 566
17.5.3 Origin and Evolution of Saturn’s F Ring 567
17.6 Diffuse Rings: Processes and Origins 567
17.7 Conclusions 569
References 570
Chapter 18 The Thermal Evolution and Internal Structure of Saturn’s Mid-SizedIcy Satellites 575
18.1 Introduction 575
18.2 Satellite Properties 577
18.2.1 Size and Shape 577
18.2.2 Density 577
18.2.3 Porosity 579
18.2.4 Initial Composition 580
18.2.4.1 Volatile Composition 580
18.2.4.2 Rock Composition 581
18.2.4.3 Rhea’s Gravitational Field 581
18.3 Sources of Heat 583
18.3.1 Heating by Radioactivity 584
18.3.2 Tidal Heating 584
18.3.2.1 Despinning 585
18.3.2.2 Orbital Eccentricity 586
18.3.3 Heat from the Gravitational Field 586
18.3.3.1 Accretion 586
18.3.3.2 Internal Conversion of Gravitational Potential into Heat 588
18.4 Thermal Transfer 588
18.4.1 Heat Transfer by Conduction 588
18.4.2 Heat Transfer by Convection 588
18.4.3 Onset of Convection 591
18.4.4 Convective Evolution and an Example 592
18.5 Constraints on Thermal Parameters 593
18.5.1 Ice Thermal Conductivity 593
18.5.2 Rock Thermal Conductivity 593
18.5.3 Effect of Porosity 594
18.6 Structural Evolution 594
18.6.1 Porosity Evolution 594
18.6.2 Melting and Differentiation 595
18.6.3 Long-Term Evolution of a Rock Core 595
18.7 Model Studies of Thermal and Dynamical Evolution 596
18.7.1 Effect of Initial Composition 596
18.7.1.1 Rock-Rich Models 597
18.7.1.2 Rock-Poor Models 597
18.7.2 Global Evolution 597
18.7.2.1 Assessing the State of Differentiation of an Icy Satellite 597
18.7.2.2 Evolution of the Lithosphere 598
18.7.2.3 Shape Evolution 599
18.7.2.4 Age of Iapetus 599
18.7.2.5 Iapetus’ Equatorial Ridge 600
18.8 Other Satellites 601
18.8.1 Phoebe 601
18.8.2 Mimas, Tethys and Dione 602
18.9 At the Frontiers: Space, Laboratory, Processes, and Modeling 603
18.9.1 Space 603
18.9.2 Laboratory Data Needed 603
18.9.3 Processes 604
18.9.4 Modeling 604
18.10 Concluding Remarks 604
Appendix: Glossary of Symbols 605
References 605
Chapter 19 Icy Satellites of Saturn: Impact Cratering and Age Determination 611
19.1 Introduction: Understanding of Saturnian Impact Crater Populations Through the Voyager Era 611
19.2 Impactor Populations 613
19.3 Heavy Bombardments 615
19.4 Cratering Chronologies 616
19.5 Impact Physics and Scaling Laws 617
19.6 Predicted Cratering Rates by Comets 619
19.6.1 Implications for Catastrophic Disruption 623
19.7 Observed Crater Statistics and Interpretation 624
19.8 Conclusions 628
References 629
Icy Satellites of Saturn: Impact Cratering and Age Determination 611
Chapter 20 Icy Satellites: Geological Evolution and Surface Processes 634
20.1 Introduction 634
20.2 Cassini’s Exploration of Saturn’s Icy Satellites 636
20.3 Morphology, Geology and Topography 637
20.3.1 Craters 639
20.3.2 Tectonics 639
20.3.3 Cryovolcanism 647
20.4 Composition and Alteration of Surface Materials 648
20.4.1 DarkMaterial 651
20.4.2 Iapetus’ Hemispheric Dichotomy 653
20.4.3 Surface Alterations and Photometry 655
20.5 Constraints on the Top-Meter Structure and Composition by Radar 660
20.5.1 Radar-Optical Correlations 661
20.5.2 Wavelength Dependence 662
20.5.3 Iapetus Radar Image 664
20.6 Geological Evolution 665
20.7 Conclusions 669
References 670
Chapter 21 Enceladus: An Active Cryovolcanic Satellite 679
21.1 Cassini’s Exploration of Enceladus 679
21.2 Interior and Tidal Heating 681
21.2.1 Introduction 681
21.2.2 Bulk Structure 682
21.2.2.1 Observational Constraints 682
21.2.2.2 Indirect Arguments 683
21.2.2.3 Discussion and Summary 683
21.2.3 Interior Composition and Chemistry 684
21.2.4 Heat Production and Tides 684
21.2.5 Thermal Structure 686
21.2.6 Evolution Through Time 687
21.2.7 Summary and FutureWork 689
21.3 Geology 689
21.3.1 Introduction 689
21.3.2 Major Terrains: Nature and Global Distribution 690
21.3.2.1 Cratered Plains: 690
21.3.2.2 Eastern (Trailing) Hemisphere Fractured Plains 690
21.3.2.3 Western (Leading) Hemisphere Fractured Plains 693
21.3.2.4 South Polar Terrain (SPT) 693
21.3.2.5 Tiger Stripes 695
21.3.3 Surface Age Distribution Through Crater Counting 697
21.3.4 Global Tectonics and Possible Rotational Changes 697
21.4 The Surface: Composition and Processes 698
21.4.1 Surface Composition 698
21.4.1.1 Water Ice, and Its Spatial Variability 698
21.4.1.2 Minor Constituents 699
21.4.2 Surface Processes 700
21.5 Plumes and System Interaction 701
21.5.1 Introduction 701
21.5.2 Observations 701
21.5.2.1 Imaging Science Subsystem (ISS) 701
21.5.2.2 Composite Infrared Spectrometer (CIRS) 703
21.5.2.3 Ultraviolet Imaging Spectrometer (UVIS) 703
21.5.2.4 Ion and Neutral Mass Spectrometer (INMS) 706
21.5.2.5 Cosmic Dust Analyzer (CDA) 707
21.5.2.6 Visual and Infrared Mapping Spectrometer (VIMS) 707
21.5.2.7 Magnetometer (MAG) and Cassini Plasma Spectrometer (CAPS) 708
21.5.2.8 Groundbased Observations 708
21.5.3 Plume Models 708
21.5.3.1 Transfer of Heat to the Surface 708
21.5.3.2 Plume Dynamics and Particle Formation 710
21.5.3.3 Plume Composition 711
21.5.4 SystemInteraction 712
21.5.5 Variability of the Activity 712
21.5.6 Conclusions: The Nature of the Plume Source 713
21.6 Biological Potential of Enceladus 713
21.7 Summary and Future Exploration 715
List of Acronyms 716
References 717
Chapter 22 The Cassini Extended Mission 721
22.1 Introduction 721
22.2 EquinoxMission Design Overview 722
22.3 The EquinoxMission Science Objectives 723
22.3.1 Icy Satellite Objectives 724
22.3.2 Magnetospheric Objectives 724
22.3.3 Rings Objectives 724
22.3.4 Saturn Objectives 725
22.3.5 Titan Objectives 725
22.4 Operational and Safety Constraints 725
22.5 Tour Design and Development Process 726
22.6 The EquinoxMission Trajectory 728
22.6.1 DetailedDescription of EM Trajectory Phases 733
22.6.1.1 High Inclination Phase (T45–T51) 733
22.6.1.2 8-Day Pi-Transfer (T51–T52) 734
22.6.1.3 Saturn Equinox (T52–T62) 734
22.6.1.4 Icy Satellites and Ansa-to-Ansa Occultations (T62–T68) 734
22.6.1.5 High Northern Titan Groundtracks (T68–T70) 736
22.7 Beyond the EquinoxMission 736
22.7.1 Cassini End-of-Mission 738
References 739
Chapter 23 Saturn’s Exploration Beyond Cassini-Huygens 741
23.1 Introduction 741
23.2 Saturn’s Interior 742
23.3 Structure and Evolution of Low-Density Giant Planets 743
23.4 Saturn’s Atmospheric Composition 744
23.5 Saturn’s Atmospheric Dynamics 745
23.6 Saturn’sMagnetosphere 748
23.7 Saturn’s Rings 749
23.7.1 Direct Imaging of Particle Size Distribution 750
23.7.2 Microstructuration 750
23.7.3 Rings Thickness 751
23.7.4 Chemical Composition: the Mystery of Silicates 751
23.7.5 Rings’Mass 751
23.8 Saturn and the Formation of the Solar System 751
23.9 TheMeans of Saturn’s Future Exploration 753
23.9.1 Flyby 753
23.9.2 Orbiter 753
23.9.3 Probes 754
23.9.4 Microprobe in Saturn’s Rings 754
23.9.5 Observations from 1AU 754
23.10 Conclusions 755
References 755
Chapter 24 Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data 758
24.1 Introduction 758
24.2 Shapes and Sizes of the Saturnian Satellites 759
24.3 Global Basemaps Derived from Cassini-ISS Images 761
24.3.1 Data Processing 761
24.3.2 Coordinate System 761
24.3.3 Basemaps 761
24.4 High-Resolution Atlases 761
24.5 Compositional Maps Derived from Cassini-VIMS Data 765
24.5.1 Data Processing 765
24.5.2 VIMS Composition Map of Dione 766
24.5.3 VIMS Composition Map of Rhea 771
24.5.4 VIMS Composition Map of Enceladus 771
24.6 FutureWork 771
References 775
Appendix: The Cassini Orbiter, Behind the Scenes 777
Index 789