Vision and Voyages for Planetary Science in the Decade 2013-2022 (2011) / Chapter Skim
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8 Satellites: Active Worlds and Extreme Environments
8 Satellites: Active Worlds and Extreme Environments
Pages 217-256
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From page 217... ...
In addition, continued analysis from past missions such as Galileo has produced surprises as well as helping to inform the planning for future missions. All three of the crosscutting science themes for the exploration of the solar system motivate further exploration of the outer planet satellites; their study is vital to addressing many of the priority questions in each of the themes.
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From page 218... ...
The surfaces and interiors of the icy satellites display a rich variety of organic molecules -- some believed to be primordial, some likely being generated even today; Titan presents perhaps the richest planetary laboratory for studying organic synthesis ongoing on a global scale. Europa, Enceladus, and Titan are central to another key question in this theme: Beyond Earth, are there modern habitats elsewhere in the solar system with necessary conditions, organic matter, water, energy, and nutrients to sustain life, and do organisms live there now?
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From page 219... ...
Composition Notes Jupiter Io 422,000 1,822 3.53 0.6 S, SO2, 10–9 SO2 Intense silicates tidally driven volcanism, plumes, high mountains Europa 671,000 1,561 3.01 0.7 H2O, hydrates 10–12 O2 Recent complex resurfacing, probable subsurface ocean Ganymede 1,070,000 2,631 1.94 0.4 H2O, hydrates 10–12 O2 Magnetic field, ancient tectonism, probable subsurface ocean Callisto 1,883,000 2,410 1.83 0.2 H 2O Partially Phyllosilicates? undifferentiated, heavily cratered, probable subsurface ocean Saturn Mimas 186,000 198 1.15 0.6 H 2O Heavily cratered Enceladus 238,000 252 1.61 1.0 H 2O Intense recent tectonism, active water vapor/ice jets Tethys 295,000 533 0.97 0.8 H 2O Heavily cratered, fractures Dione 377,000 562 1.48 0.6 H 2O Limited resurfacing, fractures Rhea 527,000 764 1.23 0.6 H 2O Heavily cratered, fractures Titan 1,222,000 2,576 1.88 0.2 H2O, 1.5 N2, CH4 Active organics, hydrocarbon liquid CH4 hydrologic cycle, complex organic chemistry Iapetus 3,561,000 736 1.08 0.3 H2O, Heavily organics?
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From page 220... ...
They are as follows: • How did the satellites of the outer solar system form and evolve? • What processes control the present-day behavior of these bodies?
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From page 221... ...
Tidal and other energy sources drive a wide range of geologic processes, whose history is recorded on the satellite surfaces. Objectives associated with the goal of understanding the formation and evolution of the giant-planet satellites include the following: • What were the conditions during satellite formation?
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From page 222... ...
What Determines the Abundance and Composition of Satellite Volatiles? Volatiles on the outer planet satellites are contained mainly in ices, although volatiles can also be retained in the rocky components (e.g., hydrated silicates on Europa or Io)
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From page 223... ...
The evolution of the internal structure of a satellite is also affected by the radiogenic heating of the rocky component, and this alone will guarantee convection in the ice-rich parts of the larger satellites.11 Convection can in turn drive surface tectonics and may cause outgassing or cryovolcanism. Although Enceladus was already recognized at the time of the 2003 planetary science decadal survey as a likely location of tidal heating, it has emerged as an active body of great interest, primarily through Cassini observations.
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These data mainly elucidate formation conditions but might also inform researchers about tidal heating in Ganymede or the role of later impacts. Important Questions Some important questions about the thermal and orbital evolution of satellites and how it relates to their internal structure include the following:
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Future Directions for Investigations and Measurements Many of the future investigations needed to understand satellite formation arise here as well because of the interplay of formation conditions and subsequent thermal evolution. A better understanding of the internal structure and thermal evolution of satellites requires measurements of static gravitational fields and topography to probe interior structure and of tidal variations in gravity and topography, as well as electromagnetic induction in the satellites at multiple frequencies to search for oceans.
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226 VISION AND VOYAGES FOR PLANETARY SCIENCE The science return from the Cassini mission has been phenomenal. Multiple flybys of Titan have confirmed the presence of numerous methane lakes on the surface -- the only bodies of surface liquid on any known world other than Earth -- along with fluvial channels (Figure 8.3)
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• What geologic processes operate on Triton's unique surface, how old is that activity, and what do its surface features reveal about whether Triton is captured? Future Directions for Investigations and Measurements Advancing understanding of the full range of surface processes operative on outer planet satellites requires global reconnaissance with 100-meter scale imaging of key objects, particularly Europa, Titan, and Enceladus as well as topographic data and high-resolution mapping (~10 meters/pixel)
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• What processes control the chemistry and dynamics of satellite atmospheres? • How do exogenic processes modify these bodies?
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An isolated active region on Europa comparable in size to Enceladus's south polar province could easily have been missed by previous missions. Evidence for ongoing endogenic activity on Titan has been suggested, and Triton's plumes may be driven by ongoing endogenic processes.23 Cassini measurements have revealed active cryovolcanism on Enceladus, which provides a window into its interior structure and composition and provides a case study for tidal heating of icy satellites; associated tectonic and other resurfacing activity is seen along and near the tiger stripes (the active geologic features near Enceladus's south pole)
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SOURCE: NASA/JPL/GSFC/Southwest Research Institute/Space Science Institute. 1.5 bar (~1.5 times Earth's surface pressure)
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SOURCE: NASA/ JPL/Space Science Institute. FIGURE 8.7 Three frames from a movie of Io's Tvashtar plume, showing plume motion on minute timescales, taken by New Horizons during its 2007 Jupiter flyby.
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From page 232... ...
• What is Triton's surface distribution of molecular nitrogen and methane, and how does it interact with the atmospheric composition and dynamics? Future Directions for Investigations and Measurements Improved understanding of the chemistry and dynamics of Io's atmosphere will require improved mapping of the spatial distribution and temporal variability of its atmosphere and associated correlations with local time and volcanic activity, as well as measurement of the diurnal variation in frost temperatures, and direct sampling of the atmosphere to determine composition.
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Reh, Titan Saturn System Mission Study Final Report.
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, and radiolytic processing generates reactive species such as molecular oxygen and hydrogen peroxide in surface ices, which might, in the case of Europa or Enceladus, deliver chemical energy to underlying bodies of liquid water in quantities sufficient to power biological activity. 34 Micrometeoroids play a crucial role in regolith generation and in redistributing radiolytic products to the subsurface layers through impact gardening.
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Future Directions for Investigations and Measurements Important investigations and measurements into exogenic processes include improved mapping of satellite surface composition to understand and separate the distributions of endogenic and exogenic materials. Because most of the exogenic materials are carried between the moons by plasma processes, in situ measurements of the field and plasma environments are required to understand the relative roles of exogenic and endogenic processes in defining the surface chemistries of the moons.
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• What is the nature of Triton's inferred dense neutral torus? Future Directions for Investigations and Measurements Investigations and measurements important to advancing understanding of how satellites influence their own magnetospheres and those of their parent planets include (1)
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• Which satellites elsewhere in the solar system possess long-lived subsurface bodies of liquid water? Titan and Enceladus are obvious candidates, but other mid-size icy satellites, including those of Uranus and Neptune, could in theory have retained internal oceans to the present day.45 Triton in particular, with its geologically young surface and current geysering, is another interesting candidate.
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would provide valuable insights into the presence of liquid water at the plume source. What Are the Sources, Sinks, and Evolution of Organic Material?
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Important Questions Some important questions about the available energy sources for sustaining life include the following: • What is the nature of any biologically relevant energy sources on Europa? • What are the energy sources that drive the plume on Enceladus?
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INTERCONNECTIONS Connections with Other Parts of the Solar System The satellites of the outer planets embody processes that operate throughout the solar system. Io's hyperactive silicate volcanism provides living examples of volcanic processes that have been important now or in the past on all the terrestrial planets and the Moon.
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From page 241... ...
Specific instrumentation requirements for the next generation of missions to the satellites of the outer planets include the following: • In the immediate future, continued support for Europa orbiter instrument development. Europa instruments face unique challenges: they must survive not only unprecedented radiation doses, but also prelaunch reduction of microbial bioburden to meet planetary protection requirements.
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. The challenges posed by the physical scale of the outer solar system and resulting long flight times, and the relative immaturity of current understanding of outer planet satellites, are best met with the economies of mission scale: large missions are the most cost-effective.
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66 That report states, in words that remain true today, "The first step in understanding the potential for icy satellites as abodes for life is a Europa mission with the goal of confirming the presence of an interior ocean, characterizing the satellite's ice shell, and understanding its geological history. Europa is important for addressing the issue of how far organic chemistry goes toward life in extreme environments and the question of how tidal heating can affect the evolution of worlds.
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and will have two components, to be launched separately: a Jupiter Europa Orbiter (JEO) , which will be built and flown by NASA, and a Jupiter Ganymede Orbiter (JGO)
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SATELLITES: ACTIVE WORLDS AND EXTREME ENVIRONMENTS 245 FIGURE 8.12 After a comprehensive tour of the Jupiter system, the Jupiter Europa Orbiter (foreground) is proposed to investigate Europa's ice shell and ocean from Europa orbit, while the ESA Jupiter Ganymede Orbiter (background)
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found this mission to be a plausible candidate for the New Frontiers program. The science goals of the Io Observer mission include the following: • Study Io's active volcanic processes; • Determine the melt fraction of Io's mantle; • Constrain tidal heating mechanisms; • Study tectonic processes; • Investigate interrelated volcanic, atmospheric, plasma-torus, and magnetospheric mass- and energyexchange processes; • Constrain the state of Io's core via improved constraints on whether Io generates a magnetic field; and • Investigate endogenic and exogenic processes controlling surface composition.
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From page 247... ...
, address some of the science goals of the Io Electrodynamics mission considered by the 2003 solar and space physics decadal survey.75 New Missions: 2013-2022 Flagship Missions Further exploration of Titan is a very high priority for satellite science. White papers from the community provide strong support for Titan science,76,77,78,79 and OPAG endorsed a Titan flagship mission as its second-highest priority flagship mission as part of an outer planets program.
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From page 248... ...
82 Technology development priorities for this mission are those needed to address the mission design risks identified by the outer planet flagship review panel.83 Specific components highlighted as requiring development include the following: • In situ elements enabling extensive areal coverage. The Montgolfière (hot-air)
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From page 249... ...
could potentially address many science goals for Triton. Rationale for Prioritization of Missions and Mission Studies The committee's decision to give higher priority to the Jupiter Europa Orbiter than to the Titan Saturn System Mission was made as follows.
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From page 250... ...
high and comparable in value, but the Jupiter Europa Orbiter mission was judged to have greater technical readiness. The technical readiness of the Europa mission results from a decade of detailed study dating back to the original Europa Orbiter concept, for which an Announcement of Opportunity was issued in 1999.
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From page 251... ...
Other stand-alone Titan mission concepts that could achieve a subset of the goals of the TSSM mission are also possible. However, implementation of such stand-alone missions is challenging, as evidenced by the fact that only one additional mission that could replace an element of TSSM was proposed in any of the community white papers submitted to the decadal survey: a stand-alone Titan airplane.95 This concept is intriguing, and is noted above as a possible alternative to a balloon as an element of a flagship mission.
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Jupiter Europa Orbiter component of EJSM as described in the Jupiter Europa Orbiter Mission Study 2008: Final Report97 and refined subsequently (including several Io science flybys)
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the outer solar system: Exchange processes involving the interiors. Space Science Reviews 153(1-4)
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Strategic Exploration White Paper. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.
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for the Outer Solar System. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.
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White paper submitted to the Planetary Science Decadal Survey, . National Research Council, Washington, D.C.
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