Titan from Cassini-Huygens (eBook)

Robert Brown, of the Lunar and Planetary Laboratory of the University of Arizona, Tucson, USA, is the Team Leader for Cassini's Visible and Infrared Mapping Spectrometer (VIMS). Dr. Brown is  Professor of Planetary Sciences. His research interests center on observational, theoretical, and laboratory studies of planetary surfaces and surface processes. Of particular interest in his research are Titan and the rest of Saturn's icy moons.Jean-Pierre Lebreton is the ESA Project Scientist and Mission Manager for the Huygens mission. His particular speciality is planetary science, studying plasma physics.Dr. Lebreton is also in the ESA team working on the Rosetta mission and, in particular, he is involved with the Plasma Consortium Experiment. He led the divisional activities on the Tethered Satellite System.Jack Hunter Waite is a planetary scientist specializing in the application of mass spectrometry to the study of solar system biogeochemistry and aeronomy. He is involved in research projects in ion/neutral mass spectrometry, gas chromatography, biogeochemistry, thermospheric modeling, and planetary astronomy. Dr. Waite is the Team Leader for the Cassini Ion and Neutral Mass Spectrometer investigation, co-investigator and lead SwRI hardware manager for the Rosetta/Rosina Reflectron Time-of-Flight, principal investigator for the development of a Jupiter Thermosphere-Ionosphere General Circulation Model, a co-investigator in planetary observing programs with Hubble Space Telescope (HST), Chandra, and the Canada France Hawaii Telescope and leads a major effort for the development of analytical techniques for use in the study of planetary biogeochemistry funded by NASA JPL and the NASA ASTID programs.

Preface 5
Contents 6
Chapter 1 8
Overview 8
1.1 Introduction 8
1.2 Organization 8
1.3 Synopses of the Main Results for Titan 9
1.3.1 Origin, Evolution and Interior (Chapters 3 and 4) 9
1.3.2 Surface (Chapters 5 and 6) 9
1.3.3 Volatiles (Chapters 7–9) 11
1.3.4 Atmosphere (Chapters 10–14) 12
1.3.5 Magnetospheric Interactions (Chapters 15 and 16) 13
1.4 Open Questions 13
1.4.1 Titan’s Interior 13
1.4.2 Titan’s Geology and Surface Composition 13
1.4.3 Titan’s Volatiles 14
1.4.4 Titan’s Atmosphere and Ionosphere 14
1.4.5 Titan’s Magnetospheric Interactions 14
References 14
Chapter 2 15
Earth-Based Perspective and Pre-Cassini–Huygens Knowledge of Titan 15
2.1 Context/Introduction 15
2.2 Early History of Titan Observations and First Interpretations 15
2.3 Pre-Voyager Observations and Predictions 16
2.4 The Voyager Mission to Titan 18
2.4.1 Visual Appearance and Haze Properties 18
2.4.2 Atmospheric Bulk Composition and Mean Thermal Structure 19
2.4.3 Atmospheric Trace Composition and Photochemistry 20
2.4.4 Thermal Balance 22
2.4.5 Circulation and Meteorology 23
2.4.6 Speculations About the Surface and the Interior 24
2.5 Observations of Titan from the Earth and Earth-Orbit in the Post-Voyager Era 25
2.5.1 Radar Observations 25
2.5.2 Near-IR and Visible Spectroscopy and Imaging 25
2.5.2.1 Surface Composition and Morphology 26
2.5.2.2 Atmospheric Phenomena 26
2.5.3 Mid-Infrared, Far-Infrared and Millimeter Spectroscopy 29
2.5.4 Stellar Occultations 31
2.5.4.1 Temperature Profi les 31
2.5.4.2 Gravity-Waves 32
2.5.4.3 Winds 32
2.5.4.4 Haze 33
2.5.5 Earth-Based Observations During the Huygens Mission 34
2.6 Concluding Remarks and Open Questions Before Cassini–Huygens 34
References 35
Chapter 3 41
The Origin and Evolution of Titan 41
3.1 Introduction 41
3.2 General Constraints on the Environment Around Early Saturn 41
3.3 Compositional and Physical Constraints on Titan Formation 44
3.3.1 Satellite Formation and the End of the Saturn Subnebula 44
3.3.2 Trapping of Volatiles 45
3.3.3 Resulting Composition of Titan and Confrontation with Cassini–Huygens Data 47
3.4 Accretion of Titan 48
3.4.1 Range of Accretional Heating Values 48
3.4.2 Properties of a Primitive Atmosphere Immediately After Accretion 49
3.5 Core Formation, Crustal Freezing and Initial Outgassing 50
3.5.1 Internal Differentiation and Evolution 50
3.5.2 Primitive Crust Formation 51
3.5.3 Internal Reservoir of Volatiles 53
3.6 Particular Problems 55
3.6.1 Origin of Nitrogen 55
3.6.2 Origin of Methane 56
3.6.2.1 Accretion of CH 4 or of CO/CO 2 56
3.6.2.2 Methane Replenishment via Cryovolcanism 58
3.7 Presence of an Atmosphere on Titan But Not Ganymede and Callisto 60
3.8 Questions for Future Missions 61
References 62
Chapter 4 66
Titan’s Interior Structure 66
4.1 Introduction 66
4.2 Observations that Constrain the Interior Structure 67
4.2.1 Orbital Data (Eccentricity, Obliquity and Rotation Rate) 68
4.2.2 Gravity Field 69
4.2.3 Titan’s Shape 71
4.2.4 Titan’s Electric and Magnetic Field 72
4.2.5 Geological Data - Apparent Misregistration of Geological Features 73
4.3 Modeling the Interior Structure 74
4.3.1 Description of the Nominal Model 74
4.3.2 Heat Sources (Radiogenic, Tidal Heating, Latent Heat) at Present 74
4.3.3 Ice Crust: Convection or Conduction in the Ice Crust 75
4.4 Discussion and Remaining Questions to be Addressed by Future Mission 76
4.5 Conclusions 77
References 77
Chapter 5 79
Geology and Surface Processes on Titan 79
5.1 Introduction 79
5.2 Cassini’s Exploration of Titan’s Surface 83
5.3 Titan’s Morphology and Topography 85
5.3.1 Morphology 85
5.3.2 Topography 90
5.4 Geology at the Huygens Landing Site 102
5.4.1 The Huygens Landing Site Terrain 103
5.4.2 Topography and Geomorphology at the Huygens Landing Site 106
5.4.3 Regional Context, Connection with Orbiter Observations 111
5.5 Tectonic and Volcanic Surface Features and Processes 112
5.5.1 Tectonic Features 112
5.5.2 Volcanic Features 113
5.5.3 Theoretical Considerations for Endogenic Processes 115
5.6 Erosional/Depositional Surface Features, Processes and Redistribution of Material 116
5.6.1 Aeolian Features and Processes 116
5.6.2 Fluvial Features and Processes 120
5.6.3 Depositional Features 126
5.6.4 Lakes 128
5.7 Cratering and Surface Ages 131
5.8 Geological Evolution 133
5.9 Summary and Conclusion 135
References 138
Chapter 6 145
Composition of Titan’s Surface 145
6.1 Introduction: Sources of Titan’s Surface Composition 146
6.1.1 Atmospheric Organics and Nitrile Compounds 146
6.1.2 Titan Aerosols: Tholins 147
6.1.3 Hydrocarbon Oceans and Seas 147
6.1.4 Subsurface Oceans, H 2 O Ice, Clathrates, Hydrates - Sources of Cryovolcanism 148
6.2 In Situ Observations of Surface Composition from the Huygens Probe 148
6.2.1 Gas Chromatograph Mass Spectrometer/Aerosol Collector Pyrolyzer 148
6.2.2 Descent Imager/Spectral Radiometer 150
6.3 Surface Composition from Short-Wavelength Infrared Spectroscopy 153
6.3.1 The Case for H 2 O Ice 154
6.3.2 Mapping Global Compositional Units-Spectral and Geomorphological Correlations 157
6.3.3 The Search for CO 2 Ice 160
6.3.4 Spectral Evidence of Organics and Nitriles in the 5 m m Window 162
6.3.5 The Case for Methane and Ethane Surface Ice 165
6.3.6 The Case for Ammonia Ice 165
6.4 Compositional Constraints from Microwave Observations 166
6.4.1 Compositional Constraints from the Cassini RADAR Scatterometer 166
6.4.2 Constraints from the Cassini RADAR Radiometer 168
6.5 Compositional Constraints on Polar Lakes from RADAR Radiometry and VIMS 170
6.6 Summary 174
References 175
Chapter 7 180
Volatile Origin and Cycles: Nitrogen and Methane 180
7.1 Historical Perspective: From Christiaan Huygens to Cassini–Huygens 180
7.2 Origin and Evolution of Titan ’ s Nitrogen Atmosphere 182
7.2.1 Direct Capture of N 2 182
7.2.2 N 2 as a Secondary Atmosphere from Primordial NH 3 183
7.2.2.1 N 2 from NH 3 Photolysis 183
7.2.2.2 N 2 from Impacts 184
7.2.2.3 Endogenic N 2 184
7.2.2.4 Origin and Evolution of Titan’s Nitrogen Atmosphere: The Cassini – Huygens Perspective 185
7.3 The Cycle of Methane on Titan 188
7.3.1 The Meteorology of Methane 188
7.3.1.1 Clouds 190
7.3.2 Methane and the Climate on Titan 191
7.3.3 Photochemical Destruction of Methane in the Stratosphere: The Ethane Ocean Dilemma 194
7.3.4 Methane Replenishment – The Source of Methane 196
7.3.5 Origin of Methane 197
7.4 Summary and Future Observations 198
References 199
Chapter 8 203
High-Altitude Production of Titan’s Aerosols 203
8.1 Cassini Observations of Heavy Hydrocarbons in Titan’s Upper Atmosphere 204
8.2 New Chemical Models Based on the Cassini Results 211
8.3 Conclusions: Laboratory Simulations and the Future of Titan Exploration 212
References 215
Chapter 9 217
Titan’s Astrobiology 217
9.1 From Astrobiology to Titan 217
9.2 Titan: A Fiercely Frozen Echo of the Early Earth? 218
9.2.1 Introduction 218
9.2.2 The Origin of the Earth’s Atmosphere 218
9.2.3 General Remarks 219
9.2.4 Lessons for Earth 220
9.2.5 Original Sources of Volatiles: Titan and Earth 220
9.2.6 Summary 221
9.3 Prebiotic-like Organic Chemistry 222
9.3.1 Laboratory Simulations, Modeling and Observation 222
9.3.2 Organic Chemistry in the Atmosphere 222
9.3.2.1 Photochemical Modeling 222
9.3.2.2 Laboratory Simulation Experiments 223
Gas Phase Products 224
CH 3 C * H(C 2 H 5 )CH=CH 2 and CH 3 C * H(CN)CH=CH 2 224
Titan’s Tholins 225
9.3.2.3 Observational Data 226
9.3.3 Organic Chemistry on the Surface and Sub-surface 227
9.3.3.1 Surface Chemistry 227
9.3.3.2 Sub-surface 228
9.3.4 Summary 230
9.4 Habitability and Life 230
9.5 Titan and the Destiny of Life on Earth 231
References 232
Chapter 10 236
Atmospheric Structure and Composition 236
10.1 Historical Introduction 236
10.2 Vertical Structure of the Atmosphere: Mass Density, Pressure, and Temperature 237
10.3 The Height and Latitude Structure of the Atmosphere 239
10.3.1 Radio Occultations 239
10.3.2 Remote Sensing 240
10.3.3 Solar and Stellar Occultations 241
10.3.4 In Situ Measurements 241
10.4 Interpretation of Atmospheric Temperature Structure 242
10.4.1 Radiative Budget of Troposphere and Stratosphere 242
10.4.2 Radiative Processes in the Upper Atmosphere 243
10.5 Composition 244
10.5.1 Major Constituents and Inert, Noble Gases 244
10.5.2 Minor Constituents - Hydrocarbons Other than Methane 245
10.5.3 Minor Constituents - N-Bearing Species and Nitriles 248
10.5.4 Minor Constituents - Oxygen Compounds 249
10.5.5 Isotope Ratios 251
10.6 Sources, Sinks, and Photochemistry of Atmospheric Composition 252
10.7 Chemistry and Transport of Atmospheric Constituents 254
10.8 Concluding Remarks 254
References 256
Chapter 11 259
Composition and Structure of the Ionosphere and Thermosphere 259
11.1 Introduction 259
11.1.1 Brief Overview of How Titan’s Upper Atmosphere and Ionosphere Fit into a Comparative Picture of Atmospheres and Ionospheres in the Solar System 259
11.1.2 Brief Review of Our Knowledge of Titan’s Thermosphere and Ionosphere Prior to the Cassini Mission 260
11.1.3 A Brief Overview of the Current Chapter 261
11.2 Structure and Composition of the Upper Neutral Atmosphere 261
11.2.1 Brief Review of Basic Processes Relevant to the Neutral Upper Atmosphere 261
11.2.2 Observed Variations of the Total Neutral Density and the Major Neutral Species and the Thermospheric Temperature 262
11.2.2.1 Total Neutral Density 262
11.2.2.2 Dynamical Implications of Measured Neutral Density Structure 265
11.2.2.3 Diffusive Separation - Methane Structure in the Thermosphere 265
11.2.3 Structure of the Exosphere and Atmospheric Escape 266
11.2.4 Interpretation of the Structure of the Neutral Atmosphere, Energy Balance, and Small-Scale Structure 268
11.2.5 Observed Variations of Minor Neutral Composition with Altitude, Latitude, Longitude, and Local Time 269
11.2.6 Interpretation and Theoretical Considerations for the Minor Neutral Composition - Chemistry 272
11.3 The Structure and Composition of the Ionosphere 275
11.3.1 Review of Basic Processes Relevantto the Ionosphere 275
11.3.2 Sources of Titan’s Ionosphere 276
11.3.3 Observed Variations of the Total Ion Density, the Electron Density and Electron Temperature 277
11.3.4 Observed Composition of Titan’s Ionosphere 282
11.3.5 Ionospheric Dynamics 284
11.3.6 Ionospheric Chemistry 285
11.3.7 Ionospheric Energetics 289
11.4 The Role of the Upper Atmosphere and Ionosphere for Titan Overall 289
11.4.1 Titan’s Upper Atmosphere as an Interface Between the Magnetosphere and the Lower Atmosphere - Tranport of Energy and Momentum and Atmospheric Loss 289
11.4.2 Questions and Issues That Remain Concerning the Upper Atmosphere and Ionosphere of Titan - the Extended Cassini Mission 291
References 292
Chapter 13 321
Atmospheric Dynamics and Meteorology 321
13.1 Introduction 321
13.2 Radiative and Dynamical Time Constants 322
13.2.1 Radiative 323
13.2.2 Dynamical 323
13.3 Temperatures and Zonal Winds 323
13.3.1 Temperatures 324
13.3.2 Zonal Winds 327
13.3.2.1 Indirect Methods 327
13.3.2.2 Direct Methods 328
Doppler Line Shifts 328
Cloud Tracking 329
Huygens Doppler Wind Experiment (DWE) 330
Descent Imager/Spectral Radiometer (DISR) 333
13.4 Meridional Circulations 333
13.4.1 Temperatures 334
13.4.2 Gas Composition 335
13.4.3 Aerosols and Condensates 336
13.4.4 In-situ Measurements 337
13.5 Surface-Atmosphere Coupling 338
13.5.1 Structure of PBL 338
13.5.2 Energy Exchange 339
13.5.3 Momentum Exchange 340
13.6 Waves and Their Effect on the General Circulation 341
13.6.1 Gravitational Tides 341
13.7 Titan’s General Circulation 342
13.7.1 Thermally Direct and Indirect Circulations 342
13.7.2 Zonal Circulation and Superrotation 344
13.8 Key Questions and Future Prospects 345
References 346
Chapter 14 351
Seasonal Change on Titan 351
14.1 Introduction 351
14.2 Titan’s Haze 352
14.2.1 Observations of Titan’s Main Haze 352
14.2.2 The Detached Haze 356
14.2.3 Polar Hood 356
14.3 Temperatures and Zonal Winds 357
14.4 Stratospheric Gases 359
14.5 Methane Meteorology 359
14.6 Models of Cloud Microphysics and Dynamics 363
14.7 Models of Seasonal Change 364
14.7.1 Stratospheric Modeling 364
14.7.1.1 Haze Models 364
14.7.1.2 Models of Seasonal Composition Changes 365
14.7.1.3 Temperature/Dynamics 365
14.7.1.4 Tropospheric Modeling – Global 365
14.8 Conclusions 367
References 368
Chapter 15 371
Mass Loss Processes in Titan’s Upper Atmosphere 371
15.1 Introduction 371
15.2 Atmospheric Escape 373
15.2.1 Thermal Escape 373
15.2.2 Hydrodynamic Escape 374
15.2.3 Photochemical-Induced Escape 374
15.2.4 Plasma-Induced Escape 374
15.3 Simulations of the Transition Region and Escape 375
15.3.1 Boltzmann Equation 376
15.3.2 Monte Carlo Simulations 376
15.4 Estimates of Escape Flux: Pre-Cassini 377
15.5 Estimates of the Escape Flux: Cassini Data 377
15.5.1 H 2 Escape 377
15.5.2 Carbon Mass Loss by Precipitation 378
15.5.3 Escape of Nitrogen and Carbon: Hot Recoil Models 378
15.5.4 Escape of Nitrogen and Carbon: Continuum Models 381
15.5.5 Summary of Mass Loss: Cassini Data 382
15.5.6 Monte Carlo Simulations: Tests of Continuum Models 382
15.6 Atmospheric Loss as Plasma and Plasma Heating: Cassini 384
15.7 Titan Mass Loss: Magnetospheric Implications 385
15.8 Summary 386
References 387
Chapter 16 390
Energy Deposition Processes in Titan’s Upper Atmosphere and Its Induced Magnetosphere 390
16.1 Introduction 390
16.1.1 Summary of Energy Input Sources to Titan’s Upper Atmosphere 393
16.1.2 Outline of Chapter and Relation to Other Chapters 394
16.2 Pertinent Instrument Characteristics, Observational Constraints and Observational Limitations 395
16.2.1 CAPS IMS, IBS and ELS 395
16.2.2 Cassini INMS 395
16.2.3 Cassini Magnetometer 397
16.2.4 Cassini RPWS and LP 397
16.2.5 Cassini MIMI Instrument 397
16.2.6 Cassini UVIS 398
16.2.7 Titan Observational Constraints and Limitations 398
16.3 Global Properties of Saturn’s Magnetosphere, and Importance of Solar Incidence-Ram-Angle (SRA) 399
16.4 Models of Titan’s Interaction with Saturn’s Magnetosphere: Channeling of Energy Input to Upper Atmosphere: MHD Versus Hybrid Codes 403
16.4.1 MHD Simulations 404
16.4.2 Hybrid Simulations 406
16.5 Radio Science Observations of Titan’s Ionosphere and Its Height Dependence 409
16.5.1 Titan’s Ionospheric Layer as Observed by Voyager 1 409
16.5.2 Cassini Radio Science Observations of Titan’s Ionosphere 410
16.6 Solar Input to Titan’s Upper Atmosphere: Ionosphere Formation, Atmospheric Heating, Haze Layers and Non-Thermal Atmospheric Escape 410
16.6.1 Voyager 1 and Cassini UVIS Airglow Observations 410
16.7 Magnetospheric Interaction and Charged Particle Bombardment: Electrons, Ions and Pickup Ions 415
16.7.1 Titan’s Exosphere as Source of PickupIons, Mass Loading of Incoming Flow, Energy Input to Upper Atmosphere and Atmospheric Loss 415
16.7.1.1 Voyager 1 Observations and Modeling of Titan’s Exosphere 415
16.7.1.2 CAPS Observations of Pickup Ions and Titan’s Exosphere 416
16.7.1.3 Cassini ENA Imaging of Titan’s Exosphere 417
16.7.1.4 Cassini INMS Observations of Titan’s Lower Exosphere 420
16.7.1.5 Hydrodynamic Model and Non-Thermal Model of Titan Exospheric Escape Particles 420
16.7.2 Magnetospheric Electron Energy Input Versus Solar Input to Titan’s Ionosphere 421
16.7.2.1 Initial Cassini Observations of Titan’s Ionosphere 421
16.7.2.1.1 Cassini TA Flyby 421
16.7.2.1.2 Cassini T5 Flyby 423
16.7.2.1.3 Cassini T9 and T18 Flybys 427
16.7.3 Magnetospheric Heavy Ion and Suprathermal Ion Energy Input to Titan’s Upper Atmosphere 431
16.7.3.1 Voyager and Cassini Plasma Observations of Ion Precipitation with Titan’s Upper Atmosphere 431
16.7.3.2 T5 Ion Energy Deposition Results 433
16.8 Meteoric Ionization and Cosmic Rays: Energy Deposition at Lower Ionosphere and Thermosphere 435
16.8.1 Dust Particle Produced Ionization Layers 435
16.8.2 Galactic Cosmic Ray Ionization Layer 436
16.9 Heavy Ion Formation and Aerosol Production 439
16.9.1 Observations of Heavy Ions by Cassini 439
16.9.2 The Formation of Fullerenes and PAHs in Titan’s Upper Atmosphere 440
16.9.3 Role of Oxygen Input from Magnetosphere and Micro-Meteorites 441
16.9.4 Trapping of Free Oxygen and Hydroxyl Ions in Seed Particles 442
16.9.5 Transport to Lower Atmosphere and Surface as Aerosols 442
16.10 Conclusion and Future Outlook 444
References 445
Chapter 17 451
Titan in the Cassini–Huygens Extended Mission 451
17.1 Titan in the Cassini–Huygens Extended Mission 451
17.1.1 Overview 451
17.2 Interior Structure 454
17.2.1 Internal Magnetic Field 454
17.2.2 Internal Ocean 455
17.3 Surface Science 456
17.3.1 Coverage and Resolution 456
17.3.2 Targeted Scientifi c Investigations in the EM 459
17.3.2.1 Surface units 459
17.3.2.2 Surface Composition 459
17.3.2.3 Cryovolcanism 460
17.3.2.4 Dunes 460
17.3.2.5 Lakes and river channels 461
17.4 Atmospheric Investigations 461
17.4.1 Primary Mission Observations 462
17.4.2 Equinox Mission Scientifi c Investigations 463
17.4.2.1 Atmospheric Structure and Distribution of Hydrocarbons 463
17.4.2.2 Clouds and Haze 463
17.4.2.3 Atmospheric Dynamics 464
17.5 Titan’s Thermosphere and Ionosphere, and the Interaction of Titan’s Upper Atmosphere with Saturn’s Magnetosphere 464
17.5.1 Primary Mission Achievements 465
17.5.2 EM Major Scientifi c Goals 465
17.5.2.1 Complex Organic Formation in Titan’s Upper Atmosphere 466
17.5.2.2 Energetic Ion Precipitation 466
17.5.2.3 Oxygen Plasma Injection into Atmosphere 468
17.5.2.4 Geometry of the Induced Magnetic Field 468
17.6 The Solstice Mission 469
References 472
Chapter 18 474
Titan Beyond Cassini–Huygens 474
18.1 Introduction 474
18.2 Unresolved Science Questions 474
18.2.1 Our Pre-Cassini–Huygens Understanding 474
18.2.2 Post-Cassini Science Results 475
18.2.2.1 Cassini’s Prime Mission 475
Titan’s Interior Structure 475
Surface Science 475
Atmospheric Science 475
Upper Atmosphere and Its Interaction with Saturn’s Magnetosphere 476
18.2.2.2 Extended Mission and Cassini Solstice Mission science 476
18.2.3 Unresolved Science Issues During and After the Cassini–Huygens Era 476
18.2.3.1 Exploring Titan as an Earth-Like System 477
18.2.3.2 Titan’s Organic Inventory and Astrobiological Potential 477
18.2.3.3 Titan’s Origin and Evolution 478
18.3 Future Missions 478
18.3.1 The Pre-Cassini Era 478
18.3.2 The Present Epoch 479
18.3.2.1 NASA Studies 479
18.3.2.2 ESA Studies 479
18.3.2.3 Joint Studies – The Titan Saturn System Mission 480
18.4 Summary 481
References 482
Chapter 19 484
Mapping Products of Titan’s Surface 484
19.1 Introduction 484
19.2 Huygens Image Data 486
19.3 Global Maps Derived from ISS Images 489
19.4 Global Maps Derived from VIMS Observations 489
19.5 Global Maps Derived from RADAR Measurements 495
19.6 Thematic Maps Derived from Cassini Data 498
19.7 Titan’s Nomenclature 502
References 504
Appendix 506
Index 519