Passage to a Ringed World - NASA's History Office

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Before the advent of spacecraft exploration, planetary scientists expected the satellites of the outer planets to be geologically dead. They assumed that heat sources were not sufficient to have melted the satellites’ mantles enough to provide a source of liquid or even semiliquid ice or ice silicate slurries. The Voyager and Galileo spacecraft have radically altered this view by revealing a wide range of geological processes on the moons of the outer planets. Enceladus may be currently active. Several of Saturn’s medium-sized satellites are large enough to have undergone internal melting with subsequent differentiation and resurfacing. Recent work on the importance of tidal interactions and subsequent heating has provided the theoretical foundation for explaining the existence of widespread activity in the outer solar system. Another factor is the presence of non-ice components, such as ammonia hydrate or methane clathrate, which lower the melting points of near-surface materials. Partial melts of water ice and various contaminants — each with their own THE KNOWN MOONS OF SATURN Diameter, Distance, Orbital Year Discovered: Moon kilometers kilometers Period, days Discoverer Pan 20 133,580 0.56 1990: Showalter Atlas 30 137,670 0.60 1980: Terrile Prometheus 100 139,350 0.61 1980: Collins Pandora 90 141,700 0.63 1980: Collins Epimetheus 120 151,450 0.69 1966: Walker Janus 190 151,450 0.69 1966: Dolfus Mimas 392 185,520 0.94 1789: Herschel Enceladus 500 238,020 1.37 1789: Herschel Tethys 1060 294,660 1.89 1684: Cassini Telesto 30 294,660 1.89 1980: Smith Calypso 26 294,660 1.89 1980: Smith Dione 1120 377,400 2.74 1684: Cassini Helene 32 377,400 2.74 1980: Laques and Lecacheux Rhea 1530 527,040 4.52 1672: Cassini Titan 5150 1,221,830 15.94 1655: Huygens Hyperion 290 1,481,100 21.28 1848: Bond Iapetus 1460 3,561,300 79.33 1671: Cassini Phoebe 220 12,952,000 550.40 1898: Pickering melting point and viscosity — provide material for a wide range of geological activity. Because the surfaces of so many outer planet satellites exhibit evidence of geological activity, planetary scientists have begun to think in terms of unified geological processes occurring on the planets — including Earth — and their satellites. For example, partial melts of water ice with various contaminants could provide flows of liquid or partially molten slurries that in many ways mimic the terrestrial or lunar lava flows formed by the partial melting of silicate rock mixtures. The ridged and grooved terrains on satellites such as Enceladus and Tethys may have resulted from tectonic activities occurring through- Hyperion is an irregular, pockmarked satellite in an unusual, nonsynchronous rotational state. THE ICY SATELLITES 55
Mimas 56 PASSAGE TO A RINGED WORLD out the solar system. Finally, the explosive volcanic eruptions possibly occurring on Enceladus may be similar to those occurring on Earth, which result from the escape of volatiles released as the pressure decreases in upward-moving liquids. Formation and Bulk Composition The solar system — the Sun, the planets and their families of moons — condensed from a cloud of gas and dust about 4.6 billion years ago. This age is derived primarily from radiometric dating of meteorites, which are believed to consist of primordial, unaltered matter. Because there existed a temperature gradient in the protosolar cloud, or nebula, volatile materials (those with low condensation temperatures) are the major components of bodies in the outer solar system. These materials include water SMALL ONES, BIG ONES… The moons, or satellites, of Saturn comprise a diverse set of objects. Of the 18 currently known satellites, nine are illustrated below to show their relative sizes. The Enceladus Tethys Dione planet-like Titan is easily the largest of Saturn’s moons. It has a dense atmosphere and possibly Iapetus liquid oceans on its surface. Hyperion, by contrast, is irregularly shaped and seems to be covered with ice and Titan ice, ice silicate mixtures, methane, ammonia and the hydrated forms of the latter two materials. Two types of dark contaminants exist on the surfaces of the satellites: C-type or carbon-rich material, and D-type material, which is believed to be rich in hydrocarbons. The planets and their satellite systems formed from the accretion of successively larger blocks of material, or planetesimals. One important concept of planetary satellite formation is that a satellite cannot accrete within Roche’s limit, the distance at which the tidal forces of the primary become greater than the internal cohesive forces of the satellite. Except for Titan, Saturn’s satellites are too small to possess gravity sufficiently strong to retain an appreciable atmosphere against thermal escape. Hyperion dark, rocky material. The Cassini– Huygens mission, through its detailed observations, will provide us with much more information about Saturn’s many moons. Phoebe Rhea Evolution Soon after the satellites accreted, they began to heat up from the release of gravitational potential energy. An additional heat source was provided by the release of mechanical energy during the heavy bombardment of their surfaces by remaining debris. Mimas and Tethys have impact craters (named Herschel and Odysseus, respectively) caused by bodies that were nearly large enough to break them apart; probably such catastrophes did occur. The decay of radioactive elements found in silicate materials provided another major source of heat. The heat produced in the larger satellites may have been sufficient to cause melting and chemical fractionation; the dense material, such as silicates and iron, went to the center of the satellite to form a core, while ice and other volatiles remained in the crust. A fourth source of heat is provided by the release of frictional energy as heat during tidal and resonant interactions among the satellites and Saturn. Several of Saturn’s satellites underwent periods of melting and active geology within a billion years of their formation and then became quiescent. Enceladus may be currently geologically active. For nearly a billion years after their formation, the satellites all underwent intense bombardment and cratering. The bombardment tapered off to a slower rate and presently continues. By counting the number of craters on a satellite’s surface and making certain assumptions about the flux of impacting ma
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Passage to a Ringed World The Cassi
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On the Cover: A collage of images s
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FOREWORD This book is for you. You
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ACKNOWLEDGMENTS This book would not
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Three separate images, taken by Voy
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descent to the surface. The Huygens
Page 14 and 15: address the challenges of the missi
Page 16 and 17: Participant Nations* * First flag i
Page 18 and 19: PROBING A MOON OF MYSTERY Huygens
Page 20 and 21: TOUR T18-3 TARGETED SATELLITE FLYBY
Page 22 and 23: SATURN CASSINI-HUYGENS MISSION SCIE
Page 24 and 25: Cassini-Huygens designed to determi
Page 26 and 27: In this diagram from his book, Syst
Page 28 and 29: This Voyager 1 image shows Saturn
Page 30 and 31: An Extended Atmosphere From Liquid
Page 32 and 33: Haze layers are also observed above
Page 34 and 35: IN THE EYE OF THE BEHOLDER Taken fr
Page 36 and 37: This spectacular view of the edge o
Page 38 and 39: the interaction was described as a
Page 40 and 41: have a signature of a tone decreasi
Page 42 and 43: CONSTITUENTS OF THE TITAN ATMOSPHER
Page 44 and 45: The processes of plate tectonics an
Page 46 and 47: tan was accreting, it is possible t
Page 48 and 49: Surface Science. The highest resolu
Page 50 and 51: This enhanced Voyager 2 image shows
Page 52 and 53: Voyager 1 captured this striking im
Page 54 and 55: Prometheus and Pandora, two tiny sa
Page 56 and 57: In the stellar occultation experime
Page 58 and 59: scattering than are larger particle
Page 60 and 61: In fact, Saturn’s rings — as we
Page 62 and 63: The satellites of Saturn comprise a
Page 66 and 67: The bright surface of Saturn’s mo
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Page 72 and 73: patches on the surface. Although it
Page 74 and 75: Artist’s conception of Ithaca Cha
Page 76 and 77: An artist’s rendition of Saturn
Page 78 and 79: The MAPS Instruments Coordinated ob
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Page 82 and 83: SATURN’S MAGNETIC FIELD Saturn’
Page 84 and 85: Solar Wind Magnetosheath Titan neut
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Page 90 and 91: CHAPTER 7 Space mission design aims
Page 92 and 93: (SOI) propulsive maneuver to initia
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Page 96 and 97: crease inclination to this value. T
Page 98 and 99: This illustration shows the main de
Page 100 and 101: discussed earlier) and data handlin
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Page 104 and 105: • Opening or shutting thermal lou
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• Map the composition and thermal
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cise distance between the surface f
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• Study the interaction of the ma
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• Study the erosional and electro
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midway out on the magnetometer boom
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The RPWS instrument will be used to
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altitudes down to 40 kilometers abo
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dex. A group of platinum resistance
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CHAPTER 10 Mission operations are t
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PLANETARY MISSION OPERATIONS Functi
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two low-gain antennas 1 and will pr
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During much of Cassini’s orbital
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articulation control subsystem’s
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AFTERWORD It has been said that in
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Developments tied to the Cassini-Hu
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APPENDIX A Command. A data transact
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APPENDIX A 138 PASSAGE TO A RINGED
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APPENDIX A Plasma wave. A wave char
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APPENDIX A 142 PASSAGE TO A RINGED
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APPENDIX B H O 2 + Water molecule w
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APPENDIX C 146 PASSAGE TO A RINGED
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APPENDIX D 148 PASSAGE TO A RINGED
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APPENDIX D 150 PASSAGE TO A RINGED
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APPENDIX F 152 PASSAGE TO A RINGED
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APPENDIX F 154 PASSAGE TO A RINGED
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APPENDIX F Peralta, F. and S. Flana
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National Aeronautics and Space Admi
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