Passage to a Ringed World - NASA's History Office

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tan was accreting, it is possible that methane and ammonia would have been mixed with the water ice. The formation of Titan by accretion was undoubtedly at temperatures warm enough for Titan to “differentiate,” that is, for the rocky material to separate to form a dense core, with a water–ammonia–methane ice mantle. The mixture of ammonia with water could ensure that Titan’s interior is still partially unsolidified, as the ammonia will effectively act like antifreeze. Radioactive decay in the rocky material in the silicate-rich core would heat the core and mantle for approximately three billion years, further strengthening the case that a liquid layer in the mantle could exist to the present. Titan’s solid water-ice crust may be laced with methane clathrate — methane trapped in the structure of water ice. This could provide a possible long-term source for the methane in Titan’s atmosphere — if the methane were freed by ongoing volcanism. An alternative model for providing the methane is a porous crust where the methane–water clathrate ratio is approximately 0.1. Cassini–Huygens Experiments On the first orbit after Cassini has executed its Saturn orbit insertion maneuver, the Huygens Probe will be targeted for Titan. Probe data are obtained during the descent through Titan’s atmosphere and relayed to the Orbiter; then the Orbiter mission continues with over 40 more Titan flybys. Magnetospheric Investigations. As the Orbiter encounters Titan, the science instruments will work together to investigate the fields and particles science associated with Titan and help determine if the satellite has an intrinsic magnetic field. The Cassini Plasma Spectrometer will map out the plasma flow. The Magnetospheric Imaging Instrument will measure highenergy particles and image the distribution of neutral particles surrounding Titan during the approach and departure of the Orbiter. Through the measurement of plasma waves, the Radio and Plasma Wave Science instrument will investigate the interaction between the plasmas and Depth, kilometers 0 250 Mostly Solid Surface 450 700 high-energy particles, and will also search for radio emission associated with Titan. The Cosmic Dust Analyzer will map out the dust distribution. The Ion and Neutral Mass Spectrometer and the Cassini Plasma Spectrometer will measure the composition of Titan’s atmosphere and exosphere directly during the flyby, to determine composition and study the interaction with Saturn’s magnetosphere. Atmospheric Investigations. The main purpose of the Huygens Probe is to return in situ measurements as it descends through Titan’s atmosphere and lands on the surface. The Probe’s Methane Clathrate Hydrate and Water Ice Ammonia–Water Ocean Ammonia–Water Ice Rock Core Subsurface Liquid Methane This model of Titan’s interior envisions an underground ocean of liquid ammonia and water, 250 kilometers beneath the solid surface. THE MYSTERIOUS TITAN 37
Titan’s subsurface structure may be key to understanding the properties of its surface. Photochemical models of Titan’s atmosphere predict the existence of a global ocean of liquid ethane and methane, acetylene and other hydrocarbon precipitates — while 38 PASSAGE TO A RINGED WORLD suite of instruments has been optimized for the return of information on critical atmospheric parameters. The atmospheric structure instrument will measure temperature, density and pressure over a range of altitudes, and also measure the electrical properties of the atmosphere and detect lightning. The Aerosol Collector and Pyrolyser will vaporize aerosols, then direct the resulting gas to the Gas Chromatograph and Mass Spectrometer, which will provide a detailed inventory of the major and minor atmospheric species, including organic molecules and noble gases — in particular argon. The instrument for the Doppler Wind Experiment will track the course of the descending Probe, giving wind direction and speed at various altitudes in Titan’s atmosphere. The Descent Earth-based radar data and near-infrared images suggest a solid surface. If Titan’s crust is sufficiently porous, storage space could exist for large quantities of liquid hydrocarbons, which would not show up in the radar data. The porous regolith produced by impacts could be over one kilometer thick, providing sufficient volume to hide the expected hydrocarbons. Cosmic Impacts Liquid Percolation Imager and Spectral Radiometer will take pictures and record spectra of the clouds and surface. Measurements of the Sun’s aureole collected by this instrument will help scientists deduce the physical properties of the aerosols (haze and cloud particles) in Titan’s atmosphere. The instrument will also acquire spectra of the atmosphere at visible and near-infrared wavelengths and measure the solar energy deposited at each altitude level of the atmosphere. The data acquired by the Probe will be compared with the continuing coverage provided by the Orbiter’s subsequent flybys. The Orbiter’s three spectrometers (the Ultraviolet Imaging Spectrograph, the Visible and Infrared Mapping Spectrometer and the Composite Infrared Spectrometer) will detect chemical species at a variety of levels in the atmosphere. By “Regolith” Porous Surface Layer Broken Up by Impacts Methane Gas Exchange mapping the distribution of hydrocarbons as a function of time, the information from the spectrometers will address the sources, sinks and efficiency of the photochemical processes making up the hydrocarbon cycle. The Ultraviolet Imaging Spectrograph can also detect argon and establish Titan’s deuterium–hydrogen ratio. The Orbiter will study the circulation of Titan’s atmosphere using the Visible and Infrared Mapping Spectrometer and the Imaging Science Subsystem’s cameras to track clouds. For the study of Titan’s weather, scientists will combine data from the Orbiter’s Composite Infrared Spectrometer and the Radio Science Instrument experiments to develop temperature profiles and thermal maps. Explosive Methane Release Methane Reservoirs Ammonia–Water Magma Ammonia–Water Volcanic Activity
Page 1: Passage to a Ringed World The Cassi
Page 4 and 5: On the Cover: A collage of images s
Page 6 and 7: FOREWORD This book is for you. You
Page 8 and 9: ACKNOWLEDGMENTS This book would not
Page 10 and 11: Three separate images, taken by Voy
Page 12 and 13: 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 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 64 and 65: Before the advent of spacecraft exp
Page 66 and 67: The bright surface of Saturn’s mo
Page 68 and 69: Galilean satellites. Most of what i
Page 70 and 71: duced that one hemisphere is compos
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
Page 80 and 81: field reverses direction across the
Page 82 and 83: SATURN’S MAGNETIC FIELD Saturn’
Page 84 and 85: Solar Wind Magnetosheath Titan neut
Page 86 and 87: per atmosphere. Energetic particles
Page 88 and 89: and plasma wave emissions from narr
Page 90 and 91: CHAPTER 7 Space mission design aims
Page 92 and 93: (SOI) propulsive maneuver to initia
Page 94 and 95: the tour cannot continue. Of course
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crease inclination to this value. T
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This illustration shows the main de
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discussed earlier) and data handlin
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of the spacecraft! Below the oxidiz
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• Opening or shutting thermal lou
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management subsystem and the Probe
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falls below its storage temperature
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The Cassini-Huygens spacecraft stan
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The Imaging Science Subsystem (ISS)
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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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