8

Satellites: Active Worlds and Extreme Environments

This chapter is devoted to the major satellites of the giant planets: those large enough to have acquired a roughly spherical shape through self-gravity. There are 17 of these worlds (four at Jupiter, seven at Saturn, five at Uranus, and one at Neptune), ranging in diameter from 5,260 kilometers (Ganymede) to 400 kilometers (Mimas) (Figure 8.1, Table 8.1). They are astonishingly diverse, with surface ages spanning more than four orders of magnitude, and surface materials ranging from molten silicate lava to nitrogen frost. This diversity makes the satellites exceptionally interesting scientifically, illuminating the many evolutionary paths that planetary bodies can follow as a function of their size, composition, and available energy sources, and allowing researchers to investigate and understand an exceptional variety of planetary processes. However, this diversity also presents a challenge for any attempt to prioritize exploration of these worlds, as we move from initial reconnaissance to focused in-depth studies.

The sizes, masses, and orbits of all the large satellites are now well known and are key constraints on the origin of the planetary systems to which they belong. Additional constraints come from their detailed compositions, which scientists are just beginning to investigate. Several worlds have unique stories to tell us about the evolution of habitable worlds, by illuminating tidal heating mechanisms, providing planetary-scale laboratories for the evolution of organic compounds, and harboring potentially habitable subsurface environments. Many of these worlds feature active planetary processes that are important for understanding these bodies themselves as well as worlds throughout the solar system. These processes include silicate volcanism, ice tectonics, impacts, atmospheric escape, chemistry, dynamics, and magnetospheric processes.

While much can still be learned from ground-based and near-Earth telescopic observations, particularly in the temporal domain, and from analysis of existing data, missions to these worlds are required to produce new breakthroughs in understanding. During the past decade, understanding of these worlds has been substantially expanded by the Cassini spacecraft and its Huygens probe that descended to Titan in 2005. Data from Cassini continue to revise and expand what is known about Saturn’s moons. 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. For example, in the building new worlds theme, the satellites retain chemical and geological records of the processes of formation and evolution in the outer solar system—records no longer accessible in the giant planets themselves. As such the satellites are key to attacking the question, How did the giant planets and their satellite systems accrete, and is there evidence that they migrated to new orbital positions? The planetary habitats



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8 Satellites: Active Worlds and Extreme Environments This chapter is devoted to the major satellites of the giant planets: those large enough to have acquired a roughly spherical shape through self-gravity. There are 17 of these worlds (four at Jupiter, seven at Saturn, five at Uranus, and one at Neptune), ranging in diameter from 5,260 kilometers (Ganymede) to 400 kilometers (Mimas) (Figure 8.1, Table 8.1). They are astonishingly diverse, with surface ages spanning more than four orders of mag- nitude, and surface materials ranging from molten silicate lava to nitrogen frost. This diversity makes the satellites exceptionally interesting scientifically, illuminating the many evolutionary paths that planetary bodies can follow as a function of their size, composition, and available energy sources, and allowing researchers to investigate and understand an exceptional variety of planetary processes. However, this diversity also presents a challenge for any attempt to prioritize exploration of these worlds, as we move from initial reconnaissance to focused in-depth studies. The sizes, masses, and orbits of all the large satellites are now well known and are key constraints on the origin of the planetary systems to which they belong. Additional constraints come from their detailed composi- tions, which scientists are just beginning to investigate. Several worlds have unique stories to tell us about the evolution of habitable worlds, by illuminating tidal heating mechanisms, providing planetary-scale laboratories for the evolution of organic compounds, and harboring potentially habitable subsurface environments. Many of these worlds feature active planetary processes that are important for understanding these bodies themselves as well as worlds throughout the solar system. These processes include silicate volcanism, ice tectonics, impacts, atmospheric escape, chemistry, dynamics, and magnetospheric processes. While much can still be learned from ground-based and near-Earth telescopic observations, particularly in the temporal domain, and from analysis of existing data, missions to these worlds are required to produce new break- throughs in understanding. During the past decade, understanding of these worlds has been substantially expanded by the Cassini spacecraft and its Huygens probe that descended to Titan in 2005. Data from Cassini continue to revise and expand what is known about Saturn’s moons. 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 explora- tion of the outer planet satellites; their study is vital to addressing many of the priority questions in each of the themes. For example, in the building new worlds theme, the satellites retain chemical and geological records of the processes of formation and evolution in the outer solar system—records no longer accessible in the giant planets themselves. As such the satellites are key to attacking the question, How did the giant planets and their satellite systems accrete, and is there evidence that they migrated to new orbital positions? The planetary habitats 217

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218 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 8.1 Montage of the major outer planet satellites, with Earth’s Moon for scale. Ganymede’s diameter is 5,262 km. The Moon’s diameter is 3,476 km. SOURCE: NASA. theme includes the question, What were the primordial sources of organic matter, and where does organic synthesis continue today? 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? Exhibiting a global methane cycle akin to Earth’s hydrologic cycle, Titan’s complex atmosphere is key to understanding the workings of the solar system theme and the question, Can understanding the roles of physics,

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219 SATELLITES: ACTIVE WORLDS AND EXTREME ENVIRONMENTS TABLE 8.1 Characteristics of the Large- and Medium-Size Satellites of the Giant Planets Distance Surface from Bulk Geo- Dominant Atmospheric Dominant Primary Radius Density metric Surface Pressure Atmospheric (g cm-3) Primary Satellite (km) (km) Albedo Composition (bars) Composition Notes 10–9 Jupiter Io 422,000 1,822 3.53 0.6 S, SO2, SO2 Intense silicates tidally driven volcanism, plumes, high mountains 10–12 Europa 671,000 1,561 3.01 0.7 H2O, hydrates O2 Recent complex resurfacing, probable subsurface ocean 10–12 Ganymede 1,070,000 2,631 1.94 0.4 H2O, hydrates 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? cratered, extreme albedo dichotomy continues

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220 VISION AND VOYAGES FOR PLANETARY SCIENCE TABLE 8.1 Continued Distance Surface from Bulk Geo- Dominant Atmospheric Dominant Primary Radius Density metric Surface Pressure Atmospheric (g cm-3) Primary Satellite (km) (km) Albedo Composition (bars) Composition Notes Uranus Miranda 130,000 236 1.21 0.3 H 2O Complex and inhomogeneous resurfacing Ariel 191,000 579 1.59 0.4 H 2O Limited resurfacing, fractures Umbriel 266,000 585 1.46 0.2 H2O, dark Heavily material cratered Titania 436,000 789 1.66 0.3 H 2O Limited resurfacing, fractures Oberon 584,000 761 1.56 0.2 H2O, dark Limited material resurfacing 10-5 Neptune Triton 355,000 1,353 2.06 0.8 N2, CH4, H2O N2, CH4 Captured; recent resurfacing, complex geology, active plumes chemistry, geology, and dynamics in driving planetary atmospheres and climates lead to a better understanding of climate change on Earth? Finally, the giant-planet satellites exhibit an enormous spectrum of planetary conditions, chemistry, and processes—contrasting with those of the inner solar system and stretching the scientific imagination in addressing the question, How have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time? SCIENCE GOALS FOR STUDIES OF PLANETARY SATELLITES The planetary science community has made remarkable progress over the past decade in understanding the major satellites of the giant planets (Table 8.2), but despite this progress, important questions remain unanswered. The committee developed some specific high-level goals and associated objectives to guide the continued advance- ment of the study of planetary satellites. The goals cover the broad areas of origin and evolution, processes, and habitability. 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? • What are the processes that result in habitable environments? Each of these goals is described in more detail in subsequent sections. HOW DID THE SATELLITES OF THE OUTER SOLAR SYSTEM FORM AND EVOLVE? Understanding the origin and evolution of the satellites is a key goal of satellite exploration. Satellite compo- sition and internal structure (particularly the state of differentiation) provide important clues to the formation of

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221 SATELLITES: ACTIVE WORLDS AND EXTREME ENVIRONMENTS TABLE 8.2 Major Accomplishments by Ground- and Space-Based Studies of the Satellites of the Giant Planets in the Past Decade Major Accomplishments Mission and/or Techniques Discovered an active meteorological cycle on Titan involving liquid Cassini and Huygens; ground-based observations hydrocarbons instead of water Discovered endogenic activity on Enceladus and found that the Cassini Enceladus plumes have a major impact on the saturnian environment Greatly improved understanding of the origin and evolution of Titan’s Theory and modeling based on Cassini and Huygens atmosphere and inventory of volatiles and its complex organic chemistry data Major improvement in characterizing the processes, composition, and Theory and modeling based on Cassini data histories for all the saturnian satellites Developed new models improving understanding of Europa, Io, and the Theory and modeling based on Galileo data; ground- other Galilean satellites based observations; and Cassini and New Horizons these worlds and their parent planet; of particular interest are the origin and evolution of volatile species. Orbital evolution, and its intimate connections to tidal heating, provide a major influence on satellite evolution. 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? • What determines the abundance and composition of satellite volatiles? • How are satellite thermal and orbital evolution and internal structure related? • What is the diversity of geologic activity and how has it changed over time? Subsequent sections examine each of these objectives in turn, identifying important questions to be addressed and future investigations and measurements that could provide answers. What Were the Conditions During Satellite Formation? The properties of the existing regular satellite systems provide clues about the conditions in which they formed. The regular satellites of Jupiter, Saturn, and Uranus orbit in the same planes as the planets’ equators, suggesting that the moons likely formed in an accretion disk in the late stages of planet formation. 1 Neptune has one large irregular satellite, Triton, in an inclined and retrograde orbit (opposite from the direction of Neptune’s rotation). Triton may be a captured Kuiper belt object, and moons that might have formed in a neptunian accretion disk were probably destroyed during the capture. Each of the regular systems has unique characteristics. Jupiter has four large satellites (the Galilean satellites), the inner two of which are essentially rocky bodies while the outer two moons are rich in ice. The saturnian system has a single large satellite, whereas closer to Saturn there are much smaller, comparably sized icy moons. The regular uranian satellites lie in the planet’s equatorial plane that is tilted by 97 ° to the ecliptic (i.e., the plane of Earth’s orbit). The outer planet satellites have also been modified by endogenic (e.g., internal differentiation and tides) and exogenic (e.g., large impacts) processes that have strongly influenced what is seen today. Although the present orbital dynamical, physical, and chemical states of the satellites preserve information about their origins, such information can have been hidden or erased by processes occurring during the evolution of the moons. The Cassini mission has opened our eyes to the wonders of the saturnian satellites. Titan’s surface is alive with fluvial and aeolian activity,2 yet its interior is only partially differentiated,3 has no magnetic field, and probably has no metallic core. On tiny Enceladus, water vapor plumes have been discovered emanating from south polar

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222 VISION AND VOYAGES FOR PLANETARY SCIENCE fissures, warmed by an unusual amount of internal heat.4 These observations together with the satellite’s density have important implications for the interior of Enceladus that in turn impose limitations on its formation and evolu- tion. Iapetus is remarkably oblate for its size, and its ancient surface features a singular equatorial belt of mountains, providing unique constraints on its early history. Cassini observations of the other saturnian moons—Rhea, Dione, Mimas, and Tethys—have increased our knowledge of their surfaces, compositions, and bulk properties. Measurements of volatile abundances are enabling the reconstruction of the planetesimal conditions at the time of accretion of the satellites, but those conditions are still far from understood. Titan’s dense atmosphere makes it especially interesting: The dominance of molecular nitrogen and the absence of the expected accompanying abundance of primordial argon are important results that constrain its origin. 5 Important Questions Some important questions concerning the conditions during satellite formation include the following: • Why are Titan and Callisto apparently imperfectly differentiated whereas Ganymede underwent complete differentiation? • Why did Ganymede form an iron-rich core capable of sustaining a magnetic dynamo? • What aspects of formation conditions governed the bulk composition and subsequent evolution of Io and Europa? • In what ways did the formation conditions of the saturnian satellites differ from the conditions for the jovian satellites? • Is it possible to discern in the uranian satellites any evidence of a very different origin scenario (a giant impact on Uranus, for example), or is this satellite system also the outcome of a process analogous to processes by which the other giant-planet satellites originated? • What features of Triton are indicative of its origin? Future Directions for Investigations and Measurements An investigation key to understanding the conditions during satellite formation is to establish the thermo- dynamic conditions of satellite formation and evolution by determination of the bulk compositions and isotopic abundances. These results would directly constrain conditions of formation, for example the radial temperature profile in the planetary accretion disk from which the regular satellites formed. Two other crucial areas of inves- tigation are to better constrain the internal mass distributions of many of the satellites by measuring the static gravitational fields and topography and to probe the existence and nature of internal oceans by measuring tidal variations in gravity and topography and by measuring electromagnetic induction in the satellites at multiple fre- quencies. Internal oceans may date to a satellite’s earliest history, given that they can be difficult to re-melt tidally once frozen, and thus their presence constrains formation scenarios. 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). Clathration (i.e., the incorporation of gas molecules within a modified water-ice structure) is a likely process for retention of many volatiles in satellite interiors, and it helps to explain the current composition of Titan.6 Alternatives like trapping of gases in amorphous ice have also been suggested. The building blocks for the satellites may have originated from the solar nebula or formed in the planetary subnebula.7 In either case, the thermodynamic conditions and composition of the gas phase determine the forma- tion conditions of ices. Huygens probe results and Cassini results have motivated a great deal of modeling of the formation condi- tions for the Saturn system and Titan in particular. Planetesimal formation in the solar nebula with only modest

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223 SATELLITES: ACTIVE WORLDS AND EXTREME ENVIRONMENTS subnebula processing may be representative of the satellite formation process in the Saturn system,8 and clathration may have had an important role, presumably aided by collisions between planetesimals to expose “fresh” ice. In the Galilean satellites, by contrast, extensive processing in the jovian subnebula may have occurred. However, the formation conditions of the Galilean satellites are not well constrained at this time, due to the lack of measurements of volatiles for these satellites, including noble gases and their stable isotopes. The origin and evolution of methane on Titan are receiving much attention, with some workers favoring ongoing outgassing from the interior to balance the continual destruction over geologic time. The argon content of the atmosphere implies that nitrogen arrived as ammonia rather than as molecular nitrogen, yet how ammonia evolved into molecular nitrogen is not known. Ground-based spectroscopy continues to expand our knowledge of inventories of volatiles on satellite surfaces, for instance with the discovery of carbon dioxide ice on the uranian satellites. 9 Important Questions Some important questions concerning the abundance and composition of satellite volatiles include the following: • In what ways do the highly volatile constituents differ between Callisto and Ganymede? • Are volatiles present at the surface or in the ice shell of Europa that are indicative of internal processing or resurfacing? • How, and to what extent, have volatiles been lost from Io? • What does the plume material from Enceladus tell us about the volatile inventory of that body? • Why does Titan uniquely have an exceptionally thick atmosphere? • What does the volatile inventory of Titan tell us about its history? In particular, how is the methane resupplied, given its rapid photochemical destruction in the upper atmosphere? Future Directions for Investigations and Measurements Investigations and measurements relevant to the abundance and composition of satellite volatiles include determination of the volatile composition of the ices, the stable isotope ratios of carbon, hydrogen, oxygen, and nitrogen, and the abundances of the noble gases to help untangle nebula and subnebula processes using highly precise remote and in situ determinations of atmospheric and surface compositions; improved observations of cur- rently active processes of loss of volatiles; and improved understanding of the thermodynamics of volatiles and the efficiency of clathration of volatiles as a function of the formation conditions. How Are Satellite Thermal and Orbital Evolution and Internal Structure Related? Like those of planets, the structure and evolution of satellites are strongly affected by mass and composi- tion. Unlike planets, satellites are very close to the central body and can therefore be greatly affected by tides and tidally mediated resonances (i.e., periodic mutual gravitational interactions). 10 This leads to a rich diversity of outcomes (Figure 8.2), understanding of which can reveal the history of the system and a satellite’s internal structure. At least three bodies (Io, Europa, and Enceladus) are thought to be currently undergoing large tidal heating, and others (Ganymede, Triton, possibly Titan, and maybe more) may have been heated in this way in the past. Tidal effects are ultimately limited by orbital evolution and the energy budget this allows. Unlike radiogenic heating caused by energy released from radioactive substances, the magnitude and the spatial and temporal vari- ability of tidal heating are very sensitive to the structure of a satellite. 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. The plume activity and estimates of thermal emission imply a level of tidal heating that is unexpectedly high for a

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224 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 8.2 Schematic of the highly diverse interiors of the Galilean satellites, inferred from Galileo data. Io ( upper left), Europa (upper right), and Ganymede (lower left) have metallic cores surrounded by silicate mantles that, in the case of Europa and Ganymede, are overlain by water ice and subsurface oceans (blue). Europa’s ocean, unlike Ganymede’s, is in contact with the underlying silicate mantle. Callisto (lower right) also hosts an ocean but is only partially differentiated, and its interior consists primarily of mixed silicates and ice. Ganymede’s diameter is 5,262 km. SOURCE: NASA/JPL. body so small. Enceladus’s forced eccentricity and tidal heating may not, however, be constant through geologic time.12 Progress has also continued on a more complete understanding of Io and Europa, through continued analysis of Galileo data combined with ground-based and Earth orbit telescopic observations. Recent work appears to sup- port the idea that Io is in thermal but not orbital equilibrium.13 Cassini gravity data suggest that Titan is not fully differentiated,14 perhaps like Callisto but unlike Ganymede. 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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225 SATELLITES: ACTIVE WORLDS AND EXTREME ENVIRONMENTS • What is the history of the resonances responsible for the tidal heating, and how is this heating accomplished? • How does this heat escape to the surface? • How is this heat transfer related to the internal structure (thickness of an outer solid shell, or composition of the interior) and formation? • How hydrostatic are the satellites? There are also body-specific questions: • Does Io have a magma ocean, and what is the compositional range of its magmas? • What is the origin of the topography of Io? • What are the magnitude and the spatial distribution of Io’s total heat flow? • What are the thickness of Europa’s outer ice shell and the depth of its ocean? • What is the magnitude of Europa’s tidal dissipation, and how is it partitioned between the silicate interior and the ice shell? • What is the relationship between Titan’s surface morphology and its internal processes, particularly for the history of the methane budget and lakes or seas and possible replenishment of methane from the interior or subsurface? • Does Titan have an internal liquid-water ocean? • What is the spatial distribution of Enceladus’s heat output, and how has it varied with time? • Does Enceladus have an ocean or some other means of providing large tidal dissipation, and to what extent is its behavior dictated by its formation conditions (e.g., presence or absence of a differentiated core)? • What does the diversity of the uranian moons indicate about the evolution of small to medium-size icy satellites? What drove such dramatic endogenic activity on Miranda and Ariel? • What powers past or possible ongoing activity on Triton, which currently has negligible tidal heating? 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. The presence and nature of intrinsic magnetic fields also constrain internal thermal evolution and initial conditions. Another needed key investigation is subsurface sounding (e.g., radar) to investigate the structure of the upper lithosphere. Heat flow can be sufficiently large to be detected through thermal infrared techniques. This provides a powerful constraint on the satellite’s thermal state. Improved maps of composition and geology of the satellite surfaces will constrain the extent and nature of transport of heat from the interior. Critical to interpretations from these investigations are improved laboratory determinations of the thermophysical and mechanical properties of relevant candidate materials to better constrain interior processes. What Is the Diversity of Geologic Activity and How Has It Changed Over Time? The surfaces of solar system bodies provide important clues to their history and evolution. Collectively, outer planet satellites show the scars of almost every surface process, including impact cratering, tectonic deformation, cryovolcanism, and aeolian and fluvial erosion. Many of these processes are still mysterious. Icy-satellite tectonism is often extensional,15 sometimes bringing interior materials up to the surface, but strike-slip faulting is also observed. Compressive tectonism is less evident on icy satellites; however, it is likely responsible for Io’s tower- ing mountains.16 Much of Europa’s surface is disrupted by extensive and mysterious chaos regions. 17 Solid-state convection is likely to be an important driver of icy-satellite geology, but details are unclear. The large range of ages and processes provides a valuable window into solar system history, constraining thermal and compositional evolution and allowing a better understanding of how planetary systems form and evolve.

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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), and evidence for seasonal variations. 18 Images of Enceladus reveal a long and complex geologic history that continues to the present day, and includes ridges that are morphologically similar to Europa’s ubiquitous double ridges. 19 Wispy features on Dione and Rhea’s trailing hemisphere have been revealed to be huge cliffs, evidence of a tectonically active past. 20 Images of Iapetus show FIGURE 8.3 False-color Cassini radar image of a methane sea near Titan’s north pole, fed by dendritic drainage channels. The image is 360 km from top to bottom. SOURCE: NASA/JPL/USGS.

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227 SATELLITES: ACTIVE WORLDS AND EXTREME ENVIRONMENTS an ancient equatorial belt of mountains, a remnant of Iapetus’s early evolution. Cassini images have shown unusual impact crater morphologies on Hyperion. Continued analysis of Galileo images has constrained the population of primary and secondary impactors in the outer solar system21 and provides continued new insights into the remark- able geology of Europa. Important Questions Some important questions about the diversity of geologic activity and how it has changed over time include the following: • One of the key missing pieces in the understanding of satellite surface geology is adequate knowledge of the cratering record in the outer solar system.22 What are the impactor populations in the outer solar system, and how have they changed over time, and what is the role of secondary cratering? • What are the origins of tectonic patterns on Europa, including the ubiquitous double ridges (Figure 8.4) and chaos regions? • How much non-synchronous rotation has Europa’s ice shell undergone, and how have the resulting stresses manifested at the surface? • How is contraction accommodated on Europa? • Has material from a subsurface Europa ocean been transported to the surface, and if so, how? • What caused Ganymede’s surface to be partially disrupted to form grooved terrain, and is the grooved terrain purely tectonic or partly cryovolcanic in origin? • Did Ganymede suffer a late heavy bombardment that affected its appearance and internal evolution? • What is the age of Titan’s surface, and have cryovolcanism and tectonism been important processes? Have there been secular changes in the surface methane inventory? • Why is Enceladus’s geology so spatially variable, and how has activity varied with time? • What geologic processes have created the surfaces of the diverse uranian moons, particularly the dramatic tectonics of Miranda and Ariel? • Has viscous extrusive cryovolcanism occurred on icy satellites, as suggested by features on Ariel and Titan? • 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) of selected targets to understand details of their formation and structure. In particular, understanding of tidally induced tectonics requires such global maps. Improved knowledge about subsurface structure is essential to constrain the nature and extent of endogenic geologic processes, for example the lithospheric thickness, fault penetration depths, porosity, thermal structure, and the presence of subsurface liquid. Maps of compositional variations at high spatial and spectral resolution and over a broad range of wavelengths are key to understanding how surface materials are emplaced and evolve. Critical to accurate interpretation of such spacecraft data are better laboratory reflectance and emission spectra of materials relevant to the outer solar system (some of which do not exist at standard temperature and pressure). A comprehensive spectral database of ices and minerals covering a wide temperature range would have wide-ranging applications to outer solar system satellites. WHAT PROCESSES CONTROL THE PRESENT-DAY BEHAVIOR OF THESE BODIES? Many planetary satellites are highly dynamic, alive with geologic and/or atmospheric activity, and even the more sedate moons have active chemical and physical interactions with the plasma and radiation environments that

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246 VISION AND VOYAGES FOR PLANETARY SCIENCE 3. Determine global surface compositions and chemistry, especially as related to habitability. 4. Understand the formation of surface features, including sites of recent or current activity, and identify and characterize candidate sites for future in situ exploration. 5. Understand Europa’s space environment and interaction with the magnetosphere. 6. Conduct Jupiter system science (Jupiter’s atmosphere, magnetosphere, other satellites, and rings). Launched in 2020, JEO would enter the Jupiter system in 2026, using Io for a gravity assist prior to Jupiter orbit insertion. This strategy increases the delivered mass to Europa by significantly decreasing the required Jupiter orbit insertion propellant in exchange for a modest increase in the radiation shielding of the flight system. The JEO mission design features a 30-month jovian system tour, which includes four Io flybys, nine Callisto flybys (including one near-polar), six Ganymede flybys, and six Europa flybys along with ~2.5 years of observing Io’s volcanic activity and Jupiter’s atmosphere, magnetosphere, and rings. After the jovian tour phase, JEO would enter orbit around Europa and spend the first month in a 200-kilometer circular orbit before descending to a 100-kilometer circular orbit for another 8 months. The mission would end with impact onto Europa. Flagship-class missions historically have a greatly enhanced science return compared to that of smaller missions—the whole is greater than the sum of the parts—and so the higher cost of a flagship mission compared to a New Frontiers-class mission is well justified. Europa remains the highest priority for satellite exploration, and a Europa mission deserves sufficient resources to realize its phenomenal scientific potential. Therefore, a Europa mission should take precedence over smaller missions to outer solar system targets during the next decade. If ESA’s Jupiter Ganymede Orbiter also flies, then the science return will be even higher. The intense jovian radiation environment remains the largest challenge for the JEO spacecraft and its instru- ments, although thanks to extensive study of the issue in the past decade, the risks and mitigation strategies are now well understood. This work has included characterization of the radiation hardness of key electronic components (including development of an “approved parts and materials list” for use by instrument developers), improved modeling of expected radiation fluxes, and detailed consideration of shielding strategies. NASA should continue to work closely with instrument developers to understand and mitigate the impact of radiation on JEO instruments, prior to final payload selection. Io Observer Io provides the ideal target to study tidal dissipation and the resulting variety of volcanic and tectonic processes in action, with fundamental implications for the thermal co-evolution of the Io-Europa-Ganymede system as well as for habitable zones around other stars. As such, an Io mission is of high scientific priority, 72 as highlighted in the 2003 planetary science decadal survey73 and subsequent 2008 New Frontiers recommendations.74 An Io mission was studied in detail at the committee’s request. The study (Appendix G) and subsequent cost and technical evalu- ation (CATE) analysis (Appendix C) 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 energy- exchange 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. Two baseline options were studied; one used ASRGs and the other was solar powered. Each was a Jupiter orbiter carrying a narrow-angle camera, ion-neutral mass spectrometer, thermal mapper, and magnetometers

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247 SATELLITES: ACTIVE WORLDS AND EXTREME ENVIRONMENTS and performing ten Io flybys. A floor mission with reduced payload and six flybys was also studied, as was an enhanced payload including a plasma instrument. A high-inclination orbit (~45°) provides polar coverage to better constrain the interior distribution of tidal heating and significantly reduces accumulated radiation: the total radiation dose is estimated to be half that of the Juno mission. No new technology is required. All science objectives are addressed by the floor mission and accomplished to much greater extent by the baseline and enhanced missions. This mission provides complementary science to that planned by JEO (which is limited by JEO’s low-inclination orbit, less Io-dedicated instrumentation, and small number—three—of Io science flybys, plus one non-science Io flyby). The Io Observer mission could also, with the addition of suitable particles and fields instrumentation (perhaps funded separately), 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. 80 Titan Saturn System Mission Many Titan mission concept studies have been conducted over the past decade including the most recent outer planet flagship mission study.81 In that study, completed in 2009, NASA and ESA worked jointly to define a flagship-class mission that would achieve the highest priority science. The resulting concept is called the Titan Saturn System Mission (TSSM) and has three overarching science goals: 1. Explore and understand processes common to Earth that occur on another body, including the nature of Titan’s climate and weather and their time evolution, its geologic processes, the origin of its unique atmosphere, and analogies between its methane cycle and Earth’s water cycle. 2. Examine Titan’s organic inventory, a path to prebiotic molecules. This includes understanding the nature of atmospheric, surface, and subsurface organic chemistry, and the extent to which that chemistry might mimic the steps that led to life on Earth. 3. Explore Enceladus and Saturn’s magnetosphere—clues to Titan’s origin and evolution. This includes investigation of Enceladus’s plume for clues to the origin of Titan ices and a comparison of its organic content with that of Titan, and understanding Enceladus’s tidal heating and its implications for the Saturn system. The purpose of Goal 1 is characterization of the physical processes, many of which are similar to those on Earth, that shape Titan’s atmosphere, surface, and evolution. Goal 2 motivates investigation of Titan’s rich organic chemistry. An extensive study is particularly important because it will elucidate the chemical pathways that occur in two environments, which may resemble those of early Earth. Measurements of the composition of the thermosphere will determine whether amino acids are made in the upper atmosphere. The chemical pathways that lead to these prebiotic molecules will be investigated to determine whether this formation mechanism is typical, and whether prebiotic molecules are common in irradi- ated methane- and nitrogen-rich atmospheres, perhaps typical of early Earth. Measurements of the surface will investigate the progress of Titan’s organic chemistry over longer time periods. Goal 3 involves investigation of Enceladus, whose plumes provide a unique view of the composition and chem- istry of the interior, which is likely representative of the same types of icy materials that formed Titan. This goal could possibly be addressed by a separate Enceladus mission as described below, but Enceladus science remains a high priority for a Titan mission, if Enceladus is not targeted separately. The TSSM mission design includes Enceladus flybys prior to Titan orbit insertion, but some Titan mission architectures, such as aerocapture directly

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248 VISION AND VOYAGES FOR PLANETARY SCIENCE from heliocentric orbit, might preclude Enceladus science unless the spacecraft subsequently left Titan orbit for Enceladus, and these trade-offs require further study as mission concepts are developed further. The study of such a complex system requires both orbital and in situ elements, and the TSSM concept includes three components—an orbiter, a balloon, and a lander (Figure 8.13). The TSSM science was rated by both NASA and ESA science review panels as being on a level equivalent to the science of the Europa Jupiter System Mission. The science was rated as excellent and science implementation rated as low risk, although the need for continued technology development for TSSM was noted. Based on techni- cal readiness, a joint NASA-ESA recommendation in 2009 prioritized EJSM first, followed closely by TSSM. The multi-element mission architecture is appropriate because it enables complementary in situ and remote-sensing observations. The TSSM study demonstrated the effectiveness of such an approach for accomplishing the diverse science objectives that are high priorities for understanding Titan. However, the details of such an implementation are likely to evolve as studies continue. Technology needs for Titan, including surface sampling, balloons, and aerocapture, which may enable delivery of additional mass to Titan, were prominent in OPAG’s technology recommendations. 82 Technology development priorities for this mission are those needed to address the mission design risks identified by the outer planet flag- ship review panel.83 Specific components highlighted as requiring development include the following: • In situ elements enabling extensive areal coverage. The Montgolfière (hot-air) balloon system proposed for TSSM is a promising approach,84,85 but an aircraft, which could use an ASRG rather than an MMRTG,86 might be more appropriate if there is a limited supply of plutonium-238; • Mature in situ analytical chemistry systems that have high resolution and sensitivity; and • Sampling systems that can operate reliably in cryogenic environments. (See Chapter 11 for additional details.) Furthermore, mission studies have shown that any future mission to Saturn will require the use of suitable radioisotope power sources, thus placing a high priority on the completion of the ASRGs and the restart of the plutonium production program by the U.S. Department of Energy. The committee commissioned a detailed study of a Titan Lake Probe (Appendixes D and G) for consider- ing the mission and instrument capabilities needed to examine the lake-atmosphere interaction as set forth in the TSSM study report.87 In addition, the Titan Lake Probe study evaluated the feasibility and value of additional capability to directly sample the subsurface and lake bottom. The integrated floater/submersible concepts in that study were designed to make measurements at various lake depths and even sample the sediment on the bottom of FIGURE 8.13 The three elements of the 2009 TSSM mission architecture: a Titan orbiter, a lake lander, and a hot-air balloon. SOURCE: ESA/NASA.

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249 SATELLITES: ACTIVE WORLDS AND EXTREME ENVIRONMENTS the lake. The findings indicated that such a system that includes floater and submersible components and enhanced instrumentation would result in significantly increased science, but also significantly increased mass relative to the simpler TSSM lake lander concept. Following from the results of that study, further studies are needed to refine lander concepts as part of a flagship mission. Stand-alone lake lander concepts, independent of TSSM, were also studied (Appendixes D and G) but were judged to be less cost-effective than a lake lander integrated with TSSM. Enceladus Orbiter Enceladus, with its remarkable active cryovolcanic activity, including plumes that deliver samples from a potentially habitable subsurface environment, is a compelling target for future exploration, 88,89,90,91 and OPAG recommended study of mid-size Enceladus missions for the coming decade. 92 Mission studies commissioned by the committee indicated that a focused Enceladus orbiter mission is both scientifically compelling and would cost less than Europa or Titan flagship missions (Appendix C). Enceladus orbiters have been the subject of several previous mission studies, most recently in 2007 (Figure 8.14). 93 The most important science goals for an Enceladus mission, in priority order, are the following: 1. What is the nature of Enceladus’s cryovolcanic activity, including conditions at the plume source, the nature of the energy source, delivery mechanisms to the surface, and mass-loss rates? 2. What are the internal structure and chemistry (particularly organic chemistry) of Enceladus, including the presence and chemistry of a global or regional subsurface ocean? 3. What is the nature of Enceladus’s geologic history, including tectonism, viscous modification of the surface, and other resurfacing mechanisms? 4. How does Enceladus interact with the rest of the saturnian system? 5. What is the nature of the surfaces and interiors of Rhea, Dione, and Tethys? 6. Characterize the surface for future landing sites. The committee commissioned a broad study of possible mission architectures including flybys, simple and flagship-class orbiters, landers, and plume sample return missions, and concluded that a simple orbiter would provide compelling science (Appendix G). A follow-up detailed study (Appendix G) found that the above science goals could be addressed well using a simple orbiter with a payload consisting of a medium-angle camera, thermal mapper, magnetometer, mass spectrometer, dust analyzer, and radio science. Sophisticated use of leveraged flybys of Saturn’s mid-size moons before Enceladus orbit insertion was found to reduce delta-V requirements, and thus mass and cost, compared to previous studies.94 The mission requires plutonium for power, in the form of ASRGs, but requires little other new technology development. However, planetary protection is an issue for Enceladus because of the possibility of contamination of the probable liquid-water subsurface environment, and mission costs could increase somewhat if it proves necessary to sterilize the spacecraft to meet planetary protection guidelines. Ice-Giant Orbiters The exploration of the uranian satellites could potentially be accomplished by the Uranus Orbiter and Probe mission discussed in Chapter 7. The proposed satellite tour (Appendix G), which includes two targeted flybys of each of the five major satellites, would help to fill a major gap in understanding of planetary satellites, because the sides opposite to those seen by Voyager 2 would be illuminated, flybys would be closer than those of Voyager (for instance potentially enabling magnetic sounding of satellite interiors), and because of instrumentation improve- ments relative to Voyager. Neptune orbiter and flyby missions (Appendixes D and G) 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. The likely science return from both the Europa and Titan missions would be very

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250 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 8.14 The Enceladus Orbiter mission concept, studied by the committee, flying above Enceladus’s active south pole. SOURCE: NASA/JPL/Caltech. 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. The biggest technical issue for the Europa mission, the high radiation dose, remains challenging but has been mitigated by the extensive preparatory work. The Titan Saturn System Mission concept is considerably less mature, with more potential for the emergence of unanticipated problems. Also, the Titan mission is much more dependent for achievement of its

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251 SATELLITES: ACTIVE WORLDS AND EXTREME ENVIRONMENTS science goals on integration with non-U.S. mission components. Although the Jupiter Europa Orbiter is intended as an element of a multi-spacecraft mission, operating in tandem with the ESA-supplied Jupiter Ganymede Orbiter, the missions are launched and flown separately, and so integration issues are relatively minor. Also, the majority of the Jupiter Europa Orbiter science goals are achieved independently of the Jupiter Ganymede Orbiter. In contrast, many key science objectives of the Titan mission rely on the balloon and lander elements, which are in an early stage of development. The use of three spacecraft elements at Titan (orbiter, lander, and balloon) also increases the complexity of spacecraft integration and mission operations, and thus the associated risk. For these and other reasons, the NASA-sponsored evaluation of the 2008 flagship mission studies rated the Europa mission as having mission implementation and cost risk lower than those for the Titan mission. Costs to NASA as estimated for the decadal survey (Appendix C) were lower for the Jupiter Europa Orbiter ($4.7 billion in FY2015 dollars) than for the Titan Saturn System Mission (at least $5.7 billion, after subtraction of the estimated $1 billion cost of the ESA-supplied balloon and lake lander from the $6.7 billion estimate for an all-NASA mis- sion, and addition of any potential costs associated with dividing the mission between NASA and ESA). Finally, the outer planets community, as represented by the Outer Planets Assessment Group (OPAG), ranked the Europa mission as its highest-priority flagship mission, followed by the Titan mission. JEO is also given higher priority than the Enceladus Orbiter for two primary reasons. JEO’s flagship-class payload will return a greater breadth and volume of science data than would the more focused payload of the Enceladus Orbiter (see the discussion of mission size above). Also the severe limitations of the Galileo data set, due to Galileo’s low data rate and the older technology of its instrument payload, mean that knowledge of Europa and the Jupiter system is now poorer than knowledge of Enceladus and the Saturn system, giving a particularly high potential for new discoveries by JEO at Europa and throughout the Jupiter system. Among the smaller missions studied by the panel, the Enceladus Orbiter was given highest priority because of the breadth of science questions that it can address (with the potential for major contributions to understand- ing the chemistry, active geology and geophysics, and astrobiological potential of Enceladus), coupled with its relatively simple implementation, requiring little new technology. The Io Observer was chosen as a New Frontiers candidate because of its compelling science and because it was the only outer planet satellite mission studied for which cost estimates placed it plausibly within the New Frontiers cost cap. Of the other satellite missions studied (Appendix D) the stand-alone Titan Lake Lander was rated lower priority because of its relatively narrow science focus and relatively challenging technology requirements. The Ganymede Orbiter was rated as lower priority for a NASA mission because of the probability that ESA’s planned Jupiter Ganymede Orbiter will achieve most of the same science goals. 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. However, high data rates are required to obtain full benefit from the remote sensing that would be a key measurement goal of an aircraft or a balloon. High data rates are difficult to achieve without the use of a relay spacecraft, making aircraft or balloons less attractive as stand-alone mission candidates than the lake lander chosen for detailed study. One additional stand-alone mission, the Titan Geophysical Network, was proposed in a white paper96 but was not chosen for detailed study because the science goals, which go beyond those of TSSM, were judged to be of lower priority, and the required low- power radioisotope power supplies would entail significant additional development. A stand-alone Titan orbiter without the in situ elements might also be considered, but was not chosen for study because it was not proposed by community white papers, and because of the advantages of an integrated orbiter and in situ elements both for delivery to Saturn and for data relay. Summary To achieve the primary goals of the scientific study of the satellites of the giant-planet systems as outlined in this chapter, the following actions are needed.

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252 VISION AND VOYAGES FOR PLANETARY SCIENCE • Flagship missions—The planned continuation of the Cassini mission through 2017 is the most cost-effective and highest-priority way to advance understanding of planetary satellites in the near term. The highest-priority satellite-focused missions to be considered for new starts in the coming decade are, in priority order: (1) 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); (2) Titan Saturn System Mission, with both Titan- orbiting and in situ components; and (3) Enceladus Orbiter. JEO is synergistic with ESA’s JGO. However, JEO’s priority is independent of the fate of ESA’s JGO. The Uranus Orbiter and Probe mission discussed in Chapter 7 would return very valuable satellite science, but it is not prioritized here relative to the satellite-focused missions discussed in this chapter. • New Frontiers missions—An Io Observer, making multiple Io flybys from Jupiter orbit, is the high-priority medium-size mission. The Ganymede Orbiter concept studied at the committee’s request (Appendixes D and G) was judged to be of lower priority for a stand-alone NASA mission. • Technology development—After the development of the technology necessary to enable JEO, the next highest priority goes to addressing the technical readiness of the orbital and in situ elements of TSSM. Priority areas include the balloon system, low-mass and low-power instruments, and cryogenic surface sampling systems. • International cooperation—The synergy between the JEO, JGO, and Japan Aerospace Exploration Agency’s (JAXA’s) proposed Jupiter Magnetospheric Orbiter is great. 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