7

The Giant Planets: Local Laboratories and
Ground Truth for Planets Beyond

Jupiter, Saturn, Uranus, and Neptune are the giants of the solar system (Figure 7.1). These four planets define the dominant characteristics of our planetary system in multiple ways—for example, they contain more than 99 percent of the solar system’s mass and total angular momentum. Their formation and evolution have governed the history of the solar system. As the 2003 planetary exploration decadal survey articulated, “the giant planet story is the story of the solar system.”1

One of the most significant advances (Table 7.1) since the 2003 decadal survey has been the discovery that giant planets also reside in the planetary systems discovered around other stars. To date, the vast majority of known planets around other stars (exoplanets) are giants close to their parent stars, although observational bias plays a role in the statistics. This chapter discusses the four local giant planets, placing them in the context of the growing population of exoplanets and understanding of the solar system. Both remote and in situ measurements of their outer atmospheric compositions are discussed, as well as external measurements that probe their deeper interiors both through their gravity fields and through their magnetic fields and magnetospheric interactions with the Sun. This chapter also addresses the ring systems and smaller moons of these worlds, which together with the larger moons effectively constitute miniature solar systems. It explicitly excludes a discussion of the largest moons of the giant planets, which are addressed in Chapter 8 of this report.

Studying the giant planets is vital to addressing many of the priority questions developed in Chapter 3. For example, central to the theme, building new worlds, is the question, How did the giant planets and their satellite systems accrete, and is there evidence that they migrated to new orbital positions? The formation and migration of the giant planets are believed to have played a dominant role in the sculpture and future evolution of the entire solar system. The giant planets are particularly important for delving into several key questions in the workings of solar systems theme, for example, the question, How do the giant planets serve as laboratories to understand Earth, the solar system, and extrasolar planetary systems? Most of the extrasolar planets that have been discovered to date are giants, with a spectrum of types that include our own ice and gas giants; close-up study of the giants of the solar system provides crucial insights about what astronomers are seeing around distant stars. Our giant planets, particularly Jupiter, are central to the question, What solar system bodies endanger Earth’s biosphere, and what mechanisms shield it? In fact Jupiter may shield Earth from impact (Figure 7.2). The atmospheres of the giant planets provide important laboratories in addressing the question, Can understanding the roles of physics, chemistry, geology, and dynamics in driving planetary atmospheres and climates lead to a better understanding of climate change on Earth? Finally, harboring most of the mass and energy of our planetary system, the giant planets are a



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 175
7 The Giant Planets: Local Laboratories and Ground Truth for Planets Beyond Jupiter, Saturn, Uranus, and Neptune are the giants of the solar system (Figure 7.1). These four planets define the dominant characteristics of our planetary system in multiple ways—for example, they contain more than 99 percent of the solar system’s mass and total angular momentum. Their formation and evolution have governed the history of the solar system. As the 2003 planetary exploration decadal survey articulated, “the giant planet story is the story of the solar system.”1 One of the most significant advances (Table 7.1) since the 2003 decadal survey has been the discovery that giant planets also reside in the planetary systems discovered around other stars. To date, the vast majority of known planets around other stars (exoplanets) are giants close to their parent stars, although observational bias plays a role in the statistics. This chapter discusses the four local giant planets, placing them in the context of the growing population of exoplanets and understanding of the solar system. Both remote and in situ measurements of their outer atmospheric compositions are discussed, as well as external measurements that probe their deeper interiors both through their gravity fields and through their magnetic fields and magnetospheric interactions with the Sun. This chapter also addresses the ring systems and smaller moons of these worlds, which together with the larger moons effectively constitute miniature solar systems. It explicitly excludes a discussion of the largest moons of the giant planets, which are addressed in Chapter 8 of this report. Studying the giant planets is vital to addressing many of the priority questions developed in Chapter 3. For example, central to the theme, building new worlds, is the question, How did the giant planets and their satellite systems accrete, and is there evidence that they migrated to new orbital positions? The formation and migration of the giant planets are believed to have played a dominant role in the sculpture and future evolution of the entire solar system. The giant planets are particularly important for delving into several key questions in the workings of solar systems theme, for example, the question, How do the giant planets serve as laboratories to understand Earth, the solar system, and extrasolar planetary systems? Most of the extrasolar planets that have been discovered to date are giants, with a spectrum of types that include our own ice and gas giants; close-up study of the giants of the solar system provides crucial insights about what astronomers are seeing around distant stars. Our giant planets, particularly Jupiter, are central to the question, What solar system bodies endanger Earth’s biosphere, and what mechanisms shield it? In fact Jupiter may shield Earth from impact (Figure 7.2). The atmospheres of the giant planets provide important laboratories in addressing the question, Can understanding the roles of physics, chemistry, geology, and dynamics in driving planetary atmospheres and climates lead to a better understanding of climate change on Earth? Finally, harboring most of the mass and energy of our planetary system, the giant planets are a 175

OCR for page 175
176 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 7.1 The giants planets—Neptune, Uranus, Saturn, and Jupiter—exhibit a diversity of properties and processes relevant to planetary science both in our local neighborhood of the solar system and in planetary systems discovered around nearby stars. SOURCE: NASA; available at http://www.astrophys-assist.com/educate/robot/page11.htm. major element in understanding the question, How have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time? SCIENCE GOALS FOR THE STUDY OF GIANT PLANETS Giant planets dominated the history of planetary evolution: the processes of their formation and migration sculpted the nascent solar system into the habitable environment of today. The materials that comprise the giant planets preserve the chemical signatures of the primitive nebular material from which the solar system formed. Understanding the interiors and atmospheres of these planets and their attendant moons, rings, and fields both gravitational and magnetic illuminates the properties and processes that occur throughout the solar system. A key

OCR for page 175
177 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND TABLE 7.1 Major Accomplishments by Ground- and Space-Based Studies of Giant Planets in the Past Decade Major Accomplishment Mission and/or Technique The census of known exoplanets increased dramatically, from about 50 in 2000 to more than 520 Ground- and space-based in 2011, with an additional 1,200 candidates awaiting confirmation;a most of these are giants, telescopes with increasing evidence that ice-giant-size planets are more abundant than Jupiters; the first compositional measurements were acquired; and complex multi-planet systems were discovered. A spacecraft en route to Pluto observed Jupiter’s polar lightning, the life cycle of fresh ammonia New Horizons clouds, the velocity of extensive atmospheric waves, boulder-size clumps speeding through the planet’s faint rings, and the path of charged particles in the previously unexplored length of the planet’s long magnetic tail. Discoveries at Saturn include confirmation of the hot southern polar vortex, deep lightning, large Cassini equatorial wind changes and seasonal effects, ring sources and shepherd moons, propeller-like ring structures as well as spokes and wakes, and the likely source of Saturn’s kilometric radio emissions. Uranus’s equinox in 2007 was observed with modern instruments (the most recent equinox was in Ground-based telescopes 1965), revealing unprecedented cloud activity with both bright and dark atmospheric features, two new brightly colored rings, and several new small moons. Neptune’s ring arcs shifted location and brightness in an unexplained fashion, and evidence emerged Ground-based telescopes for a hot polar vortex on Neptune. Three giant impacts on Jupiter have been recorded since June 2009; one of them was large enough to Ground-based telescopes create a debris field the size of the Pacific Ocean; Jupiter also exhibited planet-wide cloud and color changes for the first time in two decades. a W. Borucki and the Kepler Team, Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data. Astrophysical Journal 736(1):19, 2011. lesson from studying giant planets is this: there is no such thing as a static planet; all planets (including Earth) constantly change owing to internal and external processes. Giant planets illustrate these changes in many ways, including weather, response to impacts, aurorae, and orbital migration. Researchers also must understand the properties and processes acting in the solar system in order to extrapolate from the basic data that astronomers have on exoplanetary systems to understand how they formed and evolved. In the solar system and possibly in other planetary systems, the properties of the giants and ongoing processes driven by the giants can ultimately lead to the formation of a biosphere-sustaining terrestrial planet. 2,3 Currently, Earth is the single known example of an inhabited world, and the solar system’s giants hold clues to how Earth came to be. Bearing this in mind, the committee articulates three overarching goals for giant-planet system exploration, each of which is discussed in more depth in subsequent sections. • Giant planets as ground truth for exoplanets. Explore the processes and properties that influence giant planets in the solar system (including formation, orbital evolution, migration, composition, atmospheric structure, and environment) in order to characterize and understand the observable planets in other planetary systems. • Giant planets’ role in promoting a habitable planetary system. Test the hypothesis that the existence, loca- tion, and migration of the giant planets in the solar system have contributed directly to the evolution of terrestrial planets in the habitable zone. • Giant planets as laboratories for properties and processes on Earth. Establish the relevance of observable giant-planet processes and activities, such as mesoscale waves, forced stratospheric oscillations, and vortex stabil- ity, as an aid to understanding similar processes and activities on Earth and other planets.

OCR for page 175
178 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 7.2 Left: This Hubble Space Telescope image of Jupiter shows the aftermath of an impact in 2009. The collision created a debris field the size of the Pacific Ocean (dark region on lower right), similar to the sites created by the impacts of the Shoemaker-Levy 9 fragments in 1994. Right: Yet another impact occurred on Jupiter in 2010, this time seen only during its fireball phase (bright spot on right edge of planet). Two amateur astronomers each individually captured the meteor on video, but the impact left no detectable trace in the atmosphere, even in observations with the world’s most powerful telescopes, including Hubble and Gemini. SOURCE: Left: Courtesy of NASA, ESA, M.H. Wong, H.B. Hammel, and the Hubble Impact Team. Right: Courtesy of Anthony Wesley. GIANT PLANETS AS GROUND TRUTH FOR EXOPLANETS As of this writing, the previous sample size of four giants (our “local” giants: Jupiter, Saturn, Uranus, and Neptune) had grown to include more than 520 planets orbiting other stars (“exoplanets” or “extrasolar planets”), with a thousand-plus planet candidates waiting in the wings.4 Hundreds of these exoplanets are giants. Dozens reside in multi-planet systems, and their orbits range from circular to elliptical; some giants even exist in retrograde orbits. Emerging evidence suggests a continuum in planet properties, from massive Jupiters (easiest to find with most techniques) to Neptune-size ice giants (or water worlds), and beyond to even smaller planets; an Earth-size planet may be within our grasp during the period covered by this decadal survey.5 The results of planet searches by means of transits6 and microlensing7 suggest that ice giants, like Neptune and Uranus, are very common among exoplanets. Indeed, evidence is mounting that ice giants are at least three times more prevalent than gas giants beyond the planetary disk snow line.8 The recent discovery of a planetary system with five Neptune-mass planets, as well as two others including one mass of about 1.4 times that of Earth, underscores this result. 9 To date, transiting planets have been most amenable to further physical characterization, specifically through their positions on a mass-radius diagram. Prior to 1999, only solar system planets could be so plotted. As of this writing, more than 80 known transiting exoplanets have been added; the Kepler mission has a candidate list num- bering more than 300, and the Convection Rotation and Planetary Transits (CoRoT) spacecraft also continues to find candidates. Such large numbers of objects enable correlations of mass with bulk composition in a statistical sense, opening a new window into processes of planet formation. Giant planets in the Jupiter-mass range (100 to 300 Earth masses) are primarily composed of hydrogen and helium captured from nebulae that were present in the first few millions of years of planet formation. Smaller

OCR for page 175
179 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND planets such as Uranus and Neptune (about 15 Earth masses), or still smaller terrestrial planets, such as Earth, are depleted overall in light gases. Uranus and Neptune, although essentially water-dominated, retain deep hydrogen- helium envelopes that were likely captured from the Sun’s early nebula. Three confirmed transiting exoplanets similar to Uranus and Neptune have been discovered as of this writing, and many more are apparent in the Kepler mission early-release data. 10 More such objects will be discovered, permitting us to map out the efficiency of capture of nebular gas, planetary formation and migration processes, and variations in bulk composition with planetary mass. The science return on such statistical information is sig- nificantly enhanced by combining it with highly detailed data on the giant planets in the solar system, which can be visited by spacecraft and studied in situ by means of atmospheric entry probes. In the future, directly imaged planets around young stars will provide a wealth of new data. Key goals in studying exoplanets include the following: determining atmospheric and bulk composition variation with orbital distance, mass, and the properties of the primary star, as well as understanding the physical and chemical processes that affect atmospheric structure, both vertically and globally. 11 Our knowledge of solar system giants directly informs exoplanet studies because we can study local giants with exquisite spatial resolution and sensitivity as well as with in situ analyses by planetary probes. Thus, key goals in studying Jupiter, Saturn, Uranus, and Neptune mirror those stated above for exoplanets, with further examples including studies of internal structure and evidence for cores, stratospheric heating mechanisms, the role of clouds in shaping reflected and emitted spectra, and the importance of photochemistry and non-equilibrium atmospheric chemistry. The extrapolation of the solar system’s magnetospheres to those expected for exoplanets can also be addressed by gaining knowledge of comparative magnetospheres; these should include Earth and the four giant planets, and scaling relations should be determined between magnetospheric size, density, strength of interaction with the solar/stellar wind, and other properties. With the proper instrumentation, most missions to the giant planets would be capable of contributing to answering these questions. Our knowledge is most lacking for the ice giants. Objectives associated with the goal of using giant planets as ground truth for exoplanets include the following: • Understand heat flow and radiation balance in giant planets, • Investigate the chemistry of giant-planet atmospheres, • Probe the interiors of giant planets with planetary precession, • Explore planetary extrema in the solar system’s giant planets, • Analyze the properties and processes in planetary magnetospheres, and • Use ring systems as laboratories for planetary formation processes. Subsequent sections examine each of these objectives in turn, identifying key questions to be addressed and future investigations and measurements that could provide answers. Understand Heat Flow and Radiation Balance in Giant Planets As giant planets age, they cool. A giant planet’s atmosphere not only controls the rate at which heat can be lost from the deep interior, it responds dynamically and chemically as the entire planet cools. Atmospheric energy balance depends on the depth and manner in which incident solar energy is deposited and the processes by which internal heat is transported to the surface. Also, vertically propagating waves likely play a role in heating the upper atmospheres, since giant-planet ionospheres are hotter than expected. This overall view of giant-planet evolution has been well understood since the early 1970s, and it seems to describe Jupiter’s thermal evolution very well. Saturn, however, is much warmer today than simple evolutionary models would predict. A “rain” of helium may be prolonging the planet’s evolution, keeping it warmer for longer. As the helium droplets separate out and rain from megabar pressures, eventually redissolving at higher pressures and temperatures, helium is enhanced in the very deep interior. A credible, complete understanding of the thermal evolution of Saturn cannot be claimed until the atmospheric helium abundance is known in Saturn and this mechanism can be tested. Saturn is exhibit- ing detectable seasonal variation (discussed further below), which is also linked to energy balance, but the driving causes are not well understood.

OCR for page 175
180 VISION AND VOYAGES FOR PLANETARY SCIENCE The evolution of Uranus and of Neptune is likewise poorly understood, in part because knowledge of the energy balance of their atmospheres is limited. Data from the Voyager 2 encounter with Neptune showed that the intrinsic global heat flow from Neptune’s interior is about 10 times larger than radioactive heat production from a Neptune mass of chondritic material. Voyager 2’s radiometric data for Uranus placed an upper limit on that planet’s intrinsic heat flow that was about a factor of three lower than the Neptune value, about three times higher than the chondritic value.12,13 Determination of the actual Uranus heat-flow value rather than an upper limit would greatly constrain interior structure by means of thermal history models, and it would clarify the difference in heat flow as compared with Saturn and Neptune. Of particular interest is whether composition gradients in the ice-giant mantles may be inhibiting cooling and influencing the morphology of the magnetic field. Hints of seasonal or solar-driven changes are emerging for the ice giants as well. More precise infrared and visual heat-balance studies of these planets would better constrain their thermal histories. Outside of the solar system, the “standard” theory of giant-planet cooling fails again, this time in explaining the radii of the transiting hot Jupiters. The radii of more than 50 transiting planets have now been measured, and approximately 40 percent of these planets have radii larger than can be accommodated by standard cooling models. A better understanding of solar system giant-planet evolution will inform characterization and interpretation of the process of planetary evolution, leading to a better understanding of why such a substantial number of exoplanets seem to be anomalous. In addition to questions about the global heat flow and evolution of extrasolar giant planets, questions remain about the radiation balance and heating mechanisms within their detectable atmospheres. The Spitzer Space Tele- scope turned out to be extraordinarily adept at detecting atmospheric thermal inversions (hot stratospheres) on exoplanets; thus the study of exoplanet atmospheric thermal structure has received great attention. Varied mecha- nisms, including absorption by equilibrium and non-equilibrium chemical species, likely play a role in exoplanet stratospheric heating. A better understanding of solar system giant-planet atmospheric chemistry and energy bal- ance will illuminate our understanding of exoplanet processes as well. Important Questions Some important questions associated with the objective of understanding heat flow and radiation balance in giant planets include the following: • What is the energy budget and heat balance of the ice giants, and what role do water and moist convection play? • What fraction of incident sunlight do Uranus and Neptune absorb, and how much thermal energy do they emit? • What is the source of energy for the hot coronas/upper atmospheres of all four giant planets? • What mechanism has prolonged Saturn’s thermal evolution? • Does helium rain play a role in reducing the H/He ratio in Saturn’s molecular envelope? • Why and how do the atmospheric temperature and cloud composition vary with depth and location on the planet? • Which processes influence the atmospheric thermal profile, and how do these vary with location? Future Directions for Investigations and Measurements Inside the solar system, one of the two gas giant planets does not fit within the simple homogeneous picture of planetary cooling, and neither of the ice giants is well understood. Given the abundance of extrasolar ice giants, the internal structure and atmospheric composition of Uranus and Neptune are of particular interest for exoplanet science. For Uranus and Neptune, however, understanding is very limited regarding their atmospheric thermal structures and the nature of their stratospheric heating, particularly compared to what is already known for Jupiter and Saturn. Atmospheric elemental and isotopic abundances are poorly constrained, and the abundances of nitrogen and oxygen in the deep interior are not known (Figures 7.3 and 7.4). 14,15,16,17

OCR for page 175
181 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND FIGURE 7.3 Uranus at equinox in 2007 reveals complex atmospheric detail in these images from the Keck 10-meter telescope at 1.6 microns (Voyager camera’s longest band-pass was 0.6 micron). Some Keck images contain more than three dozen discrete features, three times more than were seen in the entire Voyager Uranus encounter. These images were selected to show the rapidly evolving structure of one particular large cloud complex in the southern ( leftmost) hemisphere. Images in the top row were obtained in 2007, the year of Uranus’s equinox. The bottom row shows an image in 2008 ( far left) and two images in 2009. Note the asymmetric banded pattern; ground-based photometric observations indicate that this asymmetry is seasonally driven. SOURCE: Courtesy of I. de Pater, L. Sromovsky, and H. Hammel. The best approach to truly understanding giant-planet heat flow and radiation balance would be a system- atic program to deliver orbiters with entry probes to all four giant planets in the solar system. The probes would determine the composition, cloud structures, and winds as a function of depth and location on each planet. They would be delivered by capable orbiting spacecraft that provide remote sensing of the cloud deck in visible light as well the near- and thermal-infrared regimes, and would yield detailed gravitational measurements to constrain planetary interior structure.18,19 The Galileo mission began this program at Jupiter. Indeed, Jupiter has been well studied by seven flyby missions, as well as by the Galileo spacecraft that spent almost 8 years in jovian orbit and delivered an in situ atmospheric probe. Jupiter is also the target of the Juno mission, the current incarnation of the top priority of the Giant Planets Panel in the 2003 decadal survey.20 Juno will constrain the water abundance and possibly sense deep convective perturbations of the gravitational field. The Jupiter Europa Orbiter (JEO), NASA’s contribution to the proposed NASA-European Space Agency (ESA) Europa Jupiter System Mission, might provide some confirmation of thermal and visible albedo measurements taken by Cassini and from Earth depending on final instrumentation. However, the selected orbit of JEO and the need to protect the craft from the jovian radiation belts will yield only

OCR for page 175
182 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 7.4 The intrinsic specific luminosities of some solar system objects are arrayed horizontally by predomininant regimes of internal heat production at the present epoch. The Sun (blue) derives quasi-steady heating through thermonuclear fusion, while heating from tidal flexure is important in Io (light blue) and possibly in some exoplanets. Jupiter, Saturn, and Neptune (red) predominantly release primordial accretional heat. In Earth and other rocky objects of roughly chondritic composition (black), radioactive decay is important for heat production. SOURCE: Courtesy of William Hubbard. limited information to supplement Juno’s determination of gravitational moments and the nature of the inner mag- netic field. Jupiter is the best-studied and best-understood analog for exoplanet formation. Further jovian studies would benefit most by the development of a more complete scientific understanding of the other giant planets, about which far less is known.21,22 A Saturn atmospheric-entry probe coupled with Cassini data (remote sensing and gravitational information from its final phase) can test the helium differentiation hypothesis through measurement of the helium abundance. Such a measurement by a Saturn entry probe would resolve a decades-old, fundamental question in solar system science. The probe would also provide atmospheric elemental and isotopic abundances, including methane abun- dances. Such measurements address formation history and help to better constrain atmospheric opacity for gas giant evolutionary modeling.23,24 An ice-giant entry probe will likewise measure atmospheric elemental and isotopic abundances—hence prob- ing formation mechanisms—and again measure methane abundances and thermal profiles necessary for ice-giant evolutionary modeling. An ice-giant orbiter—providing high-precision bolometric and Bond albedo measurements, phase functions, and mid- and far-infrared thermal luminosity—will provide significant advances in understand- ing energy balance in ice giant atmospheres. An orbiter with ultraviolet capability can address the issue of the hot corona by observing the altitudinal extent of the upper atmosphere. A mission combining an orbiter and a probe will revolutionize understanding of ice-giant properties and processes, yielding significant insight into their evolutionary history. Throughout the next decade, research and analysis (R&A) support should be provided to interpret spacecraft results from the gas giants and to continue ongoing thermal and albedo observations of the ice giants. The latter

OCR for page 175
183 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND is of particular importance because of the extremely long time between spacecraft visits: this necessitates regular observations from state-of-the-art Earth-based facilities to provide long-term context for the short-duration space- craft encounters.25,26,27 Investigate the Chemistry of Giant-Planet Atmospheres To help connect the solar system’s giant planets to those around other stars and to appreciate the constraints that internal and atmospheric composition place on planetary interior and formation models, we need to better understand the chemistry of the local giants Jupiter, Saturn, Uranus, and Neptune. Giant planets by definition have a major mass component derived from the gaseous nebula that was present during the planetary system’s first several million years, the same nebula from which Earth formed. This major component, primarily hydrogen and hydrides plus helium and other noble gases, offers the possibility of remote and in situ access to sensitive diag- nostics of processes that governed the early nebular phase of solar system evolution. At the same time this mass, and its chemistry, can be modified by interactions with the environment and the host star. More than 15 years ago, the Galileo atmospheric-entry probe provided the only in situ measurements of a giant planet to date. Prior to the probe’s measurements, it had been generally expected that the heavier noble gases (argon, krypton, and xenon) would be present in solar abundances, as all were expected to accrete with hydrogen during the gravitational capture of nebular gases. The probe made a surprising discovery: argon, krypton, and xenon appear to be significantly more abundant in the jovian atmosphere than in the Sun, at enhancements generally comparable to what was seen for chemically active volatiles such as nitrogen, carbon, and sulfur. Neon, in contrast, was depleted; recent studies have implicated helium-neon rain as an active mechanism for Jupiter to explain the depletion of neon detected by the Galileo probe.28 Various theories have attempted to explain the unexpected probe results for argon, krypton, and xenon. Their enhanced abundances require that these noble gases were separated from hydrogen in either the solar nebula or Jupiter’s interior. One way that this could be done would be by condensation onto nebular grains and planetesimals at very low temperatures, probably no higher than 25 K.29 Such a scenario would seem to require that much or most of Jupiter’s core mass accreted from these very cold objects; otherwise the less volatile nitrogen, carbon, and sulfur would be significantly more abundant than argon, krypton, and xenon. Other pathways toward the enhance- ment of the heavy noble gases have also been postulated. The noble gases could have been supplied to Jupiter and Saturn by way of clathrate hydrates.30,31 An alternative theory32 suggests that jovian abundance ratios are due to the relatively late formation of the giant planets in a partially evaporated disk. A completely different possibility is that Jupiter’s interior excludes the heavier noble gases, sulfur, nitrogen, and carbon more or less equally, so that in a sense Jupiter would have an outgassed atmosphere. These theories each make specific, testable predictions for the abundances of the noble gases. The only way to address noble gas abundances in giant planets is by in situ measurements (abundances of nitrogen, carbon, and sulfur can be measured remotely using optically active molecules such as NH3, CH4, and H2S). A Saturn probe provides an excellent test of the competing possibilities. For instance, the clathrate hydrate hypothesis 33 uses a solar nebula model to predict that xenon is enhanced on Saturn owing to its condensation, whereas argon and krypton are not since they would need lower temperatures to condense. The cold condensate hypothesis, 34 in contrast, predicts that argon and krypton, as well as xenon, would be more than twice as abundant in Saturn, based on evidence that carbon in Saturn is more than twice as abundant as it is in Jupiter. Discrimination among various models will profoundly influence understanding of solar nebular evolution and planet formation. Some Important Questions Some important questions concerning the chemistry of giant-planet atmospheres include the following: • How did the giant-planet atmospheres form and evolve to their present state? • What are the current pressure-temperature profiles for these planets? • What is the atmospheric composition of the ice giants?

OCR for page 175
184 VISION AND VOYAGES FOR PLANETARY SCIENCE Future Directions for Investigations and Measurements Accurate and direct determination of the relative abundances of hydrogen, helium and other noble gases, and their isotopes in the atmospheres of Saturn and the ice giants is a high-priority objective that directly addresses fundamental processes of nebular evolution and giant-planet origin. This objective is best addressed by in situ measurements from a shallow (up to ~10 bar) entry probe. An in situ probe is the only means of definitively mea- suring the pressure-temperature profile below the 1-bar level.35,36,37,38,39 To understand the fundamentals of atmospheric radiation balance in ice-giant atmospheres, a mission is required that can provide high-spatial-resolution observations of zonal flow, thermal emission, and atmospheric structure. An ice-giant orbiter can best achieve these observations. Probe the Interiors of Giant Planets with Planetary Precession Interior dynamic processes directly affect heat transport and the distribution of interior electrical con ductivity, yet they cannot be directly observed (magnetospheres are discussed in more detail below). However, precise mea- surements of high-order structure (and possible time variation) of giant-planet gravity fields can yield important constraints on these processes. Such measurements may also elucidate the degree of internal differentiation (i.e., presence or absence of a high-density core), related to the planets’ mode of formation and subsequent thermal history. Orbiter-based measurements of planetary pole position and gravity anomalies can now be carried out to precisions exceeding 1 part in 107. When combined with temporal baselines over years to decades, such observa- tions bring geophysical data on the solar system’s giant planets into a realm comparable to that of the terrestrial planets, furnishing detailed “ground truth” for the much cruder observations of exoplanets. Important Questions Some important questions concerning probing the interiors of giant planets with planetary precession include the following: • What are the pole precession rates for giant planets? • How much do they constrain models of the internal structure of the giant planets? Future Directions for Investigations and Measurements Determining the internal structure of Jupiter is a key measurement objective for Juno. The Cassini orbiter mission to Saturn has finished the Equinox mission and has started the Solstice mission. Together, the Juno mis- sion and the end-of-life plans for Cassini will address, for Jupiter and Saturn, many of the precision geophysical measurements advocated above. A single measurement with the year-long Juno orbiter mission is unlikely to provide adequate constraints, but it could ultimately be combined with measurements from other epochs to yield the jovian angular momentum and hence a model-independent value for the jovian axial moment of inertia. Explore Planetary Extrema in the Solar System’s Giant Planets Solar system giant planets provide valuable planet-scale laboratories that are relevant to understanding impor- tant physical processes found elsewhere in the solar system and in exoplanetary systems. One example is the balance between incident solar flux and internal heat flux. Hot Jupiters seen around other stars inhabit a regime where the internal heat flux is trivial compared to the huge incident flux. Young Jupiter-mass planets at large separation from their stars, such as the three planets imaged around the star HR 8799,40 inhabit the opposite extreme, where incident flux is trivial compared to the internal heat flow. Intriguingly, the internal heat flow of Uranus also is at best a tiny fraction of the incident flux, whereas at Jupiter the two energy fluxes are comparable. The large obliquity of Uranus, which imposes extreme seasonal changes, further makes this ice giant an excellent test subject for studying

OCR for page 175
185 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND planetary extrema. Understanding how planets respond to such extremes, both in terms of thermal structure and global dynamic state, is thus invaluable to understanding exoplanets. Indeed the same general circulation models of atmospheric winds that are used to study solar system giants have also been applied to the transiting exoplanets. Contributions to understanding will come from a better knowledge of both the internal heat flow of Uranus and Neptune and their atmospheric dynamics and winds as a function of altitude and latitude. 41,42,43,44,45 Another example is the radii of many extrasolar planets, which are much larger than expected on the basis of traditional planetary structure models. One explanation for this anomaly is that as the planet migrates and its orbit becomes more circularized, tidal dissipation in the interior of the giant provides a heat pulse, prolonging the evolution of the planet.46 The efficiency and thus viability of this mechanism hinge on the ratio of energy stored to energy dissipated (the so-called tidal Q factor) of the planet. A final example of a local extremum is the transient, highly shocked conditions achieved during the impact of an object into Jupiter’s atmosphere. We now understand that such impacts are not rare, having witnessed both the Shoemaker-Levy 9 impacts in 199447 and the subsequent impacts in 200948 and 2010.49 Studying the dark impact debris (highly shocked jovian “air” that has reached temperatures of thousands of degrees) helps test models of jovian thermochemistry that are used to model the atmospheres of the hot Jupiters. 50 Ground- and space-based observations of the aftermath of such impacts provide data on the pyrolytic products created in the impact event. Important Questions Some important questions concerning planetary extrema include the following: • How do giant planets respond to extreme heat-balance scenarios, both in terms of thermal structure and global dynamic state? • How is energy dissipated within giant planets? Future Directions for Investigations and Measurements Studies of the interior structures of solar system giants help to constrain the internal energy dissipation. For Jupiter, Juno will attempt to measure jovian tidal bulges produced by Io and Europa, measurements that will provide new data on Jupiter’s interior. Direct measurement of Jupiter’s tidal Q factor from the corresponding tidal-phase lags would require considerably more precision than Juno gravity data can deliver, but high-precision measure- ments of Galilean satellite orbits (perhaps from JEO) might be able to detect associated secular changes in orbital periods and thus constrain the tidal Q factor. A Neptune or Uranus orbiter will provide better knowledge of the internal heat flow of an ice giant, as well as critically needed information about ice-giant atmospheric dynamics and winds as a function of altitude and latitude. Analyze the Properties and Processes in Planetary Magnetospheres Giant exoplanets orbiting close to their parent stars exist in an extreme regime of physical conditions. They are expected to have much stronger interactions with the stellar winds than does Jupiter or Earth; in fact, detecting exoplanets through their auroral emissions has often been discussed. 51,52 The four giant planets and Earth provide us with an understanding of the basic physics and scaling laws of the interactions with a stellar wind needed to understand exoplanets. Exoplanet internal magnetic-field strengths are not known, but they can be roughly esti- mated if the planet rotation rate equals its orbital period due to tidal torques. Exoplanets’ hot atmospheres may well extend beyond the magnetopause and be subject to rapid loss in the stellar wind, important for estimating the lifetime of these objects. The interaction of an exoplanet magnetosphere with its host star could take many forms. A Venus-like inter- action with rapid mass loss from the top of the atmosphere could result if the planet’s internal magnetic field is weak. An Earth-like auroral interaction could result if the internal field is stronger, or a Jupiter-like interaction if the planet is rapidly rotating and its magnetosphere contains a large internal source of plasma. A much stronger

OCR for page 175
206 VISION AND VOYAGES FOR PLANETARY SCIENCE extended period of time during Jupiter approach that is suitable for low-phase-angle observations of the jovian atmosphere and for Jupiter system observations that will enable time-domain science, including fluid dynamics studies. After Jupiter orbit insertion, there is a further 2- to 3-year period that could be dedicated to Jupiter system observations before each spacecraft achieves its final satellite orbit. With the available extended time and with jovian-atmosphere-specific instrumentation, these observations could provide significant insights into several remaining questions and poorly understood atmospheric phenomena, such as aurora and polar haze structure and interactions, wave-induced dynamical processes, and coupling across atmospheric boundary layers. Although the Science Definition Team report157 expanded the mission science objectives to include some valuable Jupiter and ring science, Europa remains the focus and priority (see Chapter 8). The huge gaps in our knowledge of the Uranus and Neptune systems, combined with the narrower advances in Jupiter science, together put JEO at a lower priority for giant-planet science than a mission to an ice giant. Jupiter Polar Orbiter with Probes The Juno mission was selected for the second of the New Frontiers launch opportunities. Although it was not possible to include atmospheric probes on Juno, the mission is responsive to the 2003 decadal survey’s call for a New Frontiers mission to Jupiter, fulfilling a majority of the jovian science goals laid out for the Jupiter Polar Orbiter with Probes mission described in the 2003 decadal survey report New Frontiers in the Solar System.158 Due to launch in 2011 and to arrive at Jupiter in 2016, Juno will study the planet’s deep interior structure, abundance and distribution of water, and polar magnetic environment. Combined with results from the Galileo probe and orbital mission, a number of spacecraft flybys, and the future EJSM mission, Juno will complete a comprehensive assessment of Jupiter, making it the best studied of the giant planets. New Missions: 2013-2022 Flagship Missions Uranus Orbiter and Probe An ice-giant mission was identified as a deferred priority mission in the 2003 planetary decadal survey. 159 The specific mission considered by the survey focused on the Neptune system but did not have the benefit of detailed mission studies or the independent cost and technical evaluations (CATEs). For the current survey, the committee’s studies identified significant challenges and risks associated with a Neptune mission that are not at play for a Uranus mission in the next decade. (Included are risks associated with aerocapture at Neptune, the lack of optimal launch windows for Neptune in the upcoming decade, and long flight times incompatible with the Advanced Stirling Radioisotope Generator [ASRG] system lifetimes.) The mission studies (Appendix G) and CATEs (Appendix C) performed for this decadal survey indicate that it is possible to launch a Uranus mission within the next decade that will insert a fully equipped instrument package into orbit for a multi-year mission to study the atmosphere, rings, magnetic field, and magnetosphere, as well as to deploy a small atmospheric in situ probe and conduct a tour of the larger satellites. A Uranus mission will permit in-depth study of a class of planets glimpsed only briefly during a flyby mission carrying 1970s-era technology. Moreover, the CATE analysis indicated that much of the risk associated with this mission can be retired by studies of the ASRG power systems and proper preparations for probe entry. The prioritized science objectives for a Uranus Orbiter and Probe mission are as follows: • High-Priority Science Objectives 1. Determine the atmospheric zonal winds, composition, and structure at high spatial resolution, as well as the temporal evolution of atmospheric dynamics. 2. Understand the basic structure of the planet’s magnetosphere as well as the high-order structure and temporal evolution of the planet’s interior dynamo.

OCR for page 175
207 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND • Medium-Priority Science Objectives 3. Determine the noble gas abundances (helium, neon, argon, krypton, and xenon) and isotopic ratios of hydrogen, carbon, nitrogen, and oxygen in the planet’s atmosphere and the atmospheric structure at the probe descent location. 4. Determine internal mass distribution. 5. Determine the horizontal distribution of atmospheric thermal emission, as well as the upper-atmospheric thermal structure and changes with time and location at low resolution. 6. Determine the geology, geophysics, surface composition, and interior structure of large satellites. • Low-Priority Science Objectives 7. Measure the magnetic field, plasma, and currents to determine how the tilted/offset/rotating magneto - sphere interacts with the solar wind over time. 8. Determine the composition, structure, particle-size distribution, dynamical stability, and evolutionary history of the rings, as well as the geology, geophysics, and surface composition of small satellites. 9. Determine the vertical profile of zonal winds as a function of depth in the atmosphere, in addition to the presence of clouds as a function of depth in the atmosphere. New Frontiers Missions The New Frontiers line is an essential component of NASA’s portfolio. Missions of this scope can achieve highly focused goals that can be combined with results from flagship missions to advance scientific progress sig- nificantly. However, the committee’s detailed mission studies revealed that the current cost cap of New Frontiers precluded nearly all outer solar system exploration. One exception was a Saturn Probe mission. Saturn Probe For a mission like the Saturn Probe, the current operating systems and protocols (extant paradigms and analyses of likely risk and cost) dictate that launching and operating an empty rocket (zero payload) to fly past the Saturn system would just barely fit within the 2009 New Frontiers cost cap. This is true for any mission beyond Saturn as well: similar results surfaced for other New Frontiers mission concepts to targets in the outer solar system. The Saturn Probe study was particularly illustrative because it was stripped down to almost an empty rocket, and yet it still substantially exceeded $1 billion including launch costs (the committee examined a single-probe mission design (see Appendixes C and G); multiple probes would further enhance the science yield). For reference, an extremely capable payload is a small fraction of the cost of the rocket (and thus the mission): Phase A through D costs of the probe, including aeroshell and payload, are only on the order of 10 percent of the total mission cost; the science payload itself is only on the order of 3 percent. The prioritized science objectives for a Saturn Probe mission under the expanded New Frontiers cost cap recommended in Chapter 9 are as follows: • Higher-Priority Science Objectives 1. Determine the noble gas abundances and isotopic ratios of hydrogen, carbon, nitrogen, and oxygen in Saturn’s atmosphere. 2. Determine the atmospheric structure at the probe descent location acceleration. • Lower-Priority Science Objectives160 3. Determine the vertical profile of zonal winds as a function of depth at the probe descent location(s). 4. Determine the location, density, and composition of clouds as a function of depth in the atmosphere.

OCR for page 175
208 VISION AND VOYAGES FOR PLANETARY SCIENCE 5. Determine the variability of atmospheric structure and presence of clouds in two locations. 6. Determine the vertical water abundance profile at the probe descent location(s). 7. Determine precision isotope measurements for light elements such as sulfur, nitrogen, and oxygen found in simple atmospheric constituents. The Saturn shallow probe targets very specific science goals. Retrieved elemental compositions from Saturn can be combined with those from the Jupiter/Galileo probe to constrain solar system formation models; in situ Saturn observations can leverage the results of remote sensing obtained with the Cassini mission. When a Saturn Probe mission is combined with a Uranus Orbiter and Probe mission, the understanding of planetary formation will be greatly advanced in the next decade. Discovery Missions Missions to the outer solar system are expensive and risky, and therefore rare. Although such missions acquire measurements unobtainable in any other way, their extended spacing in time severely limits the development of our understanding of giant-planet systems. New knowledge of these planets has increasingly come from ground- and space-based telescopes. Advances in telescope technology (especially AO imaging) and focal-plane instrumentation have greatly expanded the capabilities of ground-based facilities. Observations from large facility-class telescopes such as Hubble, Herschel, Chandra, and Spitzer have shed light on numerous problems in giant-planet science. Similarly, telescopic missions with tightly focused science goals have been groundbreaking in astrophysics (Far Ultraviolet Spectroscopic Explorer, Wilkinson Microwave Anisotropy Probe), in some cases garnering Nobel Prizes (e.g., Cosmic Background Explorer). Remote-sensing observations provide scientific advances at a fraction of the cost of deep-space missions; they are also shared facilities with other disciplines, further reducing cost. Young scientists trained on these facilities will be available to participate in the deep-space missions of the future, when scientists trained on Voyager, Galileo, and Cassini have retired. Ultraviolet and x-ray planetary observations require a telescope above 110 km altitude, where imaging and spectroscopy can be accomplished undistorted by the atmosphere. After the Hubble and Chandra missions con- clude sometime in the coming decade, such observations will no longer be possible. The scientific case for remote multi-wavelength observations of single solar system objects has been made in numerous Small Explorer (SmEx) and Discovery proposals, with at least two Phase A SmEx studies. The science case is strengthened greatly by the inclusion of multiple planets, satellites, and small bodies, yet there is currently no program in NASA in which such a mission—for observations of solar system objects in general—can be proposed, since Discovery-class missions are defined as focused on single systems. Presentations to the committee suggested that a highly capable planetary space telescope in Earth orbit could be accomplished as a Discovery mission. Such a facility could support all solar system scientific research, not just that involving giant planets. Concluding Thoughts The painful reality of giant-planet exploration is that even the revised New Frontiers cost cap proposed in this survey (see Chapter 9) severely restricts mission options within the Saturn system and precludes any mission to an ice giant. If NASA wants to explore beyond the orbit of Jupiter, NASA must accept that there are risks associ- ated with that exploration (long timescales, limited power options, and so on) and that there are concomitant costs associated with those risks. The good news is that we need not wait for a huge flagship to make substantial scientific gains in the outer solar system. The committee identified two missions that balance the challenge of deep-solar-system exploration with the risks and cost: a scientifically compelling New Frontiers candidate within the Saturn system and a scientifically rich mission to the Uranus system that costs much less than past flagships. Exploration of giant-planet systems offers rich connections to missions whose primary focus is the satellites of those systems; likewise, most satellite missions have the potential for giant-planet system science. The very name of the Europa Jupiter System Mission evokes this synergy. Likewise, any mission to an ice-giant system will

OCR for page 175
209 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND offer significant opportunities for satellite science. A Saturn Probe mission may have limited satellite capability, but depending on carrier instrumentation and specific trajectories, some Titan science might be feasible. Because some satellites of the giant planets are often captured objects (e.g., Triton, Phoebe), there is linkage to the primi- tive bodies community as well. Summary To achieve the primary goals of the scientific study of giant-planet systems as outlined in this chapter, the following objectives will have to be addressed. • Flagship missions—As discussed in this chapter and in the 2003 decadal survey, a comprehensive mission to study one of the ice giants offers enormous potential for new discoveries. The committee investigated mis- sions to both Uranus and Neptune and determined that the two systems offered equally rich science return. The Uranus mission is preferred for the decade 2013-2022 both because of the more difficult requirements of achieving Neptune orbit and because of the availability of favorable Uranus trajectories in the coming decade. The Jupiter Europa Orbiter component of the NASA-ESA Europa Jupiter System Mission will advance studies of the giant planets provided that it does the following:161 1. Maintains Jupiter system science as high priority by allowing Jupiter-specific instrumentation and investigations; 2. Designates Jupiter system science as the top-ranked priority during the approach and early jovian tour phases and devotes spacecraft resources accordingly (e.g., data volume and observing time); and 3. Incorporates Jupiter system science specific needs, such as lighting conditions and viewing geometry, into jovian tour design decisions. • New Frontiers missions—The current New Frontiers cost cap is too restrictive to permit many of the mis- sions of the highest interest—even those with highly focused science goals. A possible exception to this is the Saturn Probe mission, if the payload is lean and the New Frontiers cost cap is expanded slightly. The Saturn Probe mission will make important contributions to addressing giant-planet goals in the period 2013-2022 by providing measurements of noble gas abundances that can be obtained in no other way and thus placing Saturn into context relative to Jupiter and the Sun. • Discovery missions—Proposals should be permitted for targeted and facility-class orbital space telescopes in response to future Discovery Announcements of Opportunity. The science addressed by such facilities needs to be listed as a priority for the Discovery program. • Technology development—Developments need to be continued in the following prioritized areas: power needs, thermal protection systems for atmospheric probes, aerocapture and/or nuclear-electric propulsion, and robust deep-space communications capabilities. • Research support—Robust programs of synoptic observations of the giant planets, data analysis, laboratory work, theoretical studies, and computational development need to be maintained. • Observing facilities—Access to large telescopes needs to be ensured for giant-planet systems science obser- vations. The long timescales between giant-planet missions require substantial support of ground-based facilities for mission planning. The extreme distances to the giant planets necessitate very high spatial resolution and high sensitivity, requiring the largest and most sensitive astronomical facilities on Earth and in space. • Data archiving—The ongoing effort to evolve the Planetary Data System from an archiving facility to an effective online resource for the NASA and international communities needs to be supported. NOTE AND REFERENCES 1 . National Research Council. 2003. New Frontiers in the Solar System: An Integrated Exploration Strategy. The National Academies Press, Washington, D.C., p. 93.

OCR for page 175
210 VISION AND VOYAGES FOR PLANETARY SCIENCE 2 . H.F. Levison and C. Agnor. 2003. The role of giant planets in terrestrial planet formation. Astronomical Journal 125:2692-2713. 3 . R. Brasser, A. Morbidelli, R. Gomes, K. Tsiganis, and H.F. Levison. 2009. Constructing the secular architecture of the solar system II: The terrestrial planets. Astronomy and Astrophysics 507:1053-1065. 4 . W. Borucki for the Kepler Team. 2011. Characteristics of Kepler planetary candidates based on the first data set. Astro- physical Journal 728(2):117. 5 . J.I. Lunine, D. Fischer, H.B. Hammel, T. Henning, L. Hillenbrand, J. Kasting, G. Laughlin, B. Macintosh, M. Marley, G. Melnick, D. Monet, et al. 2008. Worlds beyond: A strategy for the detection and characterization of exoplanets. Execu- tive Summary of a Report of the ExoPlanet Task Force Astronomy and Astrophysics Advisory Committee, Washington, D.C., June 23, 2008. Astrobiology 8:875-881. 6 . W. Borucki for the Kepler Team. 2011. Characteristics of Kepler planetary candidates based on the first data set. Astro- physical Journal 728(2):117. 7 . T. Sumi, D.P. Bennett, I.A. Bond, A. Udalski, V. Batista, M. Dominik, P. Fouque, D. Kubas, A. Gould, B. Macintosh, K. Cook, et al. 2010. A cold Neptune-mass planet OGLE-2007-BLG-368Lb: Cold Neptunes are common. Astrophysical Journal 710:1641-1653. 8 . T. Sumi, D.P. Bennett, I.A. Bond, A. Udalski, V. Batista, M. Dominik, P. Fouque, D. Kubas, A. Gould, B. Macintosh, K. Cook, et al. 2010. A cold Neptune-mass planet OGLE-2007-BLG-368Lb: Cold Neptunes are common. Astrophysical Journal 710:1641-1653. 9 . C. Lovis, D. Ségransan, M. Mayor, S. Udry, W. Benz, J.-L. Bertaux, F. Bouchy, A.C.M. Correia, J. Laskar, G. Lo Curto, C. Mordasini, F. Pepe, D. Queloz, and N.C. Santos. 2011. The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: Probing the architecture of low-mass planetary systems. Astronomy and Astro- physics 528:A112. 10 . W. Borucki for the Kepler Team. 2011. Characteristics of Kepler planetary candidates based on the first data set. Astro- physical Journal 728(2):117. 11 . J.I. Lunine, D. Fischer, H.B. Hammel, T. Henning, L. Hillenbrand, J. Kasting, G. Laughlin, B. Macintosh, M. Marley, G. Melnick, D. Monet, et al. 2008. Worlds beyond: A strategy for the detection and characterization of exoplanets, Execu- tive Summary of a Report of the ExoPlanet Task Force Astronomy and Astrophysics Advisory Committee, Washington, D.C., June 23, 2008. Astrobiology 8:875-881. 12 . J. Pearl, B.J. Conrath, R.A. Hanel, J.A. Pirraglia, and A. Coustenis. 1990. The albedo, effective temperature, and energy balance of Uranus, as determined from the Voyager IRIS data. Icarus 84:12. 13 . B.J. Conrath, F.M. Flasar, and P.J. Gierasch. 1991. Thermal structure and dynamics of Neptune’s atmosphere from Voyager measurements. Journal of Geophysical Research 96:18931. 14 . W.A. Traub. Exoplanets and Solar System Exploration. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 15 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 16 . M. Hofstadter. 2009. The Atmospheres of the Ice Giants, Uranus and Neptune. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 17 . C.J. Hansen. 2009. Neptune Science with Argo—A Voyage through the Outer Solar System. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 18 . D.H. Atkinson. 2009. Entry Probe Missions to the Giant Planets. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 19 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 20 . National Research Council. 2003. New Frontiers in the Solar System: An Integrated Exploration Strategy. The National Academies Press, Washington, D.C., pp. 110-116. 21 . K.B. Clark. 2009. Europa Jupiter System Mission. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 22 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 23 . D.H. Atkinson. 2009. Entry Probe Missions to the Giant Planets. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 24 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.

OCR for page 175
211 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND 25 . M. Hofstadter. 2009. The Atmospheres of the Ice Giants, Uranus and Neptune. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 26 . C.J. Hansen. 2009. Neptune Science with Argo—A Voyage through the Outer Solar System. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 27 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 28 . H.F. Wilson and B. Militzer. 2010. Sequestration of noble gases in giant planet interiors. Physical Review Letters 104:121101. 29 . T. Owen, P. Mahaffy, H.B. Niemann, S. Atreya, T. Donahue, A. Bar-Nun, and I. de Pater. 1999. A low-temperature origin for the planetesimals that formed Jupiter. Nature 402:269-270. 30 . D. Gautier, F. Hersant, O. Mousis, and J.I. Lunine. 2001. Enrichments in volatiles in Jupiter: A new interpretation of the Galileo measurements. Astrophysical Journal Letters 550:L227-L230. 31 . F. Hersant, D. Gautier, G. Tobie, and J.I. Lunine. 2008. Interpretation of the carbon abundance in Saturn measured by Cassini. Planetary and Space Science 56:1103-1111. 32 . T. Guillot and R. Hueso. 2006. The composition of Jupiter: Sign of a (relatively) late formation in a chemically evolved protosolar disc. Monthly Notices of the Royal Astronomical Society: Letters 367(1):L47-L51. 33 . F. Hersant, D. Gautier, G. Tobie, and J.I. Lunine. 2008. Interpretation of the carbon abundance in Saturn measured by Cassini. Planetary and Space Science 56:1103-1111. 34 . T. Owen, P. Mahaffy, H.B. Niemann, S. Atreya, T. Donahue, A. Bar-Nun, and I. de Pater. 1999. A low-temperature origin for the planetesimals that formed Jupiter. Nature 402:269-270. 35 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 36 . D.H. Atkinson. 2009. Entry Probe Missions to the Giant Planets. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 37 . M. Hofstadter. 2009. The Atmospheres of the Ice Giants, Uranus and Neptune. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 38 . C.J. Hansen. 2009. Neptune Science with Argo—A Voyage through the Outer Solar System. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 39 . W.A. Traub. 2009. Exoplanets and Solar System Exploration. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 40 . C. Marois, B. Macintosh, T. Barman, B. Zuckerman, I. Song, J. Patience, D. Lafrenière, and R. Doyon. 2008. Direct imaging of multiple planets orbiting the star HR 8799. Science 322:1348. 41 . M. Hofstadter. 2009. The Atmospheres of the Ice Giants, Uranus and Neptune. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 42 . C.J. Hansen. 2009. Neptune Science with Argo—A Voyage through the Outer Solar System. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 43 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 44 . W.A. Traub. 2009. Exoplanets and Solar System Exploration. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 45 . C. Agnor. 2009. The Exploration of Neptune and Triton. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 46 . P. Bodenheimer, D.N.C. Lin, and R.A. Mardling. 2001. On the tidal inflation of short-period extrasolar planets. Astro- physical Journal 548:466-472. 47 . J. Harrington, I. de Pater, S.H. Brecht, D. Deming, V. Meadows, K. Zahnle, and P.D. Nicholson. 2004. Lessons from Shoemaker-Levy 9 about Jupiter and planetary impacts. Pp. 159-184 in Jupiter: The Planet, Satellites and Magnetosphere (F. Bagenal, T. Dowling, and W. McKinnon, eds.). Cambridge University Press, New York. 48 . A. Sánchez-Lavega, A. Wesley, G. Orton, R. Hueso, S. Perez-Hoyos, L.N. Fletcher, P. Yanamandra-Fisher, J. Legarreta, I. de Pater, H. Hammel, A. Simon-Miller, et al. 2010. The impact of a large object with Jupiter in July 2009. Astrophysical Journal Letters 210:L155-L159. 49 . R. Hueso, A. Wesley, C. Go, M.H. Wong, L.N. Fletcher, A. Sánchez-Lavega, M.B.E. Boslough, I. de Pater, G. Orton, A.A. Simon-Miller, S.G. Djorgovski, M.L. Edwards, H.B. Hammel, J.T. Clarke, K. Noll, and P. Yanamandra-Fisher. 2010. First Earth-based detection of a superbolide on Jupiter. Astrophysical Journal Letters 721:L129-L133.

OCR for page 175
212 VISION AND VOYAGES FOR PLANETARY SCIENCE 50 . K. Zahnle, M. Marley, and J. Fortney. 2009. Thermometric soots on warm Jupiters? Available at arXiv.org e-prints, arXiv:0911.0728 [astro-ph.EP]. 51 . P. Zarka. 2007. Interactions of exoplanets with their parent star and associated radio emissions. Planetary and Space Science 55(5):598-617. 52 . K. France, J. T. Stocke, H. Yang, J.L. Linsky, B.C. Wolven, C.S. Froning, J.C. Green, and S.N. Osterman. 2010. Search - ing for far-ultraviolet auroral/dayglow emission from HD 209458b. Astrophysical Journal 712(2):1277-1286. 53 . K.B. Clark. 2009. Europa Jupiter System Mission. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 54 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 55 . M. Hofstadter. 2009. The Atmospheres of the Ice Giants, Uranus and Neptune. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 56 . W.A. Traub. 2009. Exoplanets and Solar System Exploration. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 57 . C. Agnor. 2009. The Exploration of Neptune and Triton. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 58 . National Research Council. 2003. New Frontiers in the Solar System: An Integrated Exploration Strategy. The National Academies Press, Washington, D.C. 59. T. I. Gombosi and A.P. Ingersoll. 2010. Saturn: Atmosphere, ionosphere, and magnetosphere. Science 327(5972):1476-1479. 60 . J.N. Cuzzi, J.A. Burns, S. Charnoz, R.N. Clark, J.E. Colwell, L. Dones, L.W. Esposito, G. Filacchione, R.G. French, M.M. Hedman, S. Kempf, et al. 2010. An evolving view of Saturn’s dynamic rings. Science 327(5972):1470-1475. 61 . National Research Council. 2003. New Frontiers in the Solar System: An Integrated Exploration Strategy. The National Academies Press, Washington, D.C. 62 . M.K. Gordon, S. Araki, G.J. Black, A.S. Bosh, A. Brahic, S.M. Brooks, S. Charnoz, J.E. Colwell, J.N. Cuzzi, L. Dones, R.H. Durisen, et al. 2002. Planetary rings. Pp. 263-282 in The Future of Solar System Exploration, 2003-2013. Community Contributions to the NRC Solar System Exploration Decadal Survey. (M.V. Sykes, ed.). ASP Confer- ence Series 272. Astronomical Society of the Pacific, Orem, Utah. Available at http://www.aspbooks.org/a/volumes/ table_of_contents/?book_id=13. 63 . M.S. Tiscareno, J.A. Burns, M. Sremcevic, K. Beurle, M.M. Hedman, N.J. Cooper, A.J. Milano, M.W. Evans, C.C. Porco, J.N. Spitale, and J.W. Weiss. 2010. Physical characteristics and non-keplerian orbital motion of “propeller” moons embedded in Saturn’s rings. Astrophysical Journal Letters 718:L92-L96. 64 . K. Beurle, C.D. Murray, G.A. Williams, M.W. Evans, N.J. Cooper, and C.B. Agnor. 2010. Direct evidence for gravitational instability and moonlet formation in Saturn’s rings. Astrophysical Journal Letters 718:L176-L180. 65 . C.C. Porco, P.C. Thomas, J.W. Weiss, and D.C. Richardson. 2007. Saturn’s small inner satellites: Clues to their origins. Science 318:1602-1607. 66 . S. Charnoz, A. Brahic, P.C. Thomas, and C.C. Porco. 2007. The equatorial ridges of Pan and Atlas: Terminal accretionary ornaments? Science 318:1622-1624. 67 . S. Charnoz, J. Salmon, and A. Crida. 2010. The recent formation of Saturn’s moonlets from viscous spreading of the main rings. Nature 465:752-754. 68 . I. de Pater, S. Gibbard, E. Chiang, H.B. Hammel, B. Macintosh, F. Marchis, S. Martin, H.G. Roe, and M. Showalter. 2005. The dynamic neptunian ring arcs: Gradual disappearance of Liberté and a resonant jump of Courage. Icarus 174:263-272. 69 . M.R. Showalter and J.J. Lissauer. 2006. The second ring-moon system of Uranus: Discovery and dynamics. Science 311:973-977. 70 . I. de Pater, H.B. Hammel, S.G. Gibbard, and M.R. Showalter. 2006. New dust belts of Uranus: One ring, two ring, red ring, blue ring. Science 312:92-94. 71 . I. de Pater, H.B. Hammel, M.R. Showalter, and M.A. van Dam. 2007. The dark side of the rings of Uranus. Science 317:1888-1890. 72 . M.S. Tiscareno. Rings Research in the Next Decade. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 73 . C.J. Hansen. 2009. Neptune Science with Argo—A Voyage Through the Outer Solar System. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 74 . M. Hofstadter. 2009. The Atmospheres of the Ice Giants, Uranus and Neptune. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.

OCR for page 175
213 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND 75 . M.S. Tiscareno. 2009. Rings Research in the Next Decade. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 76 . M.S. Tiscareno. 2009. Rings Research in the Next Decade. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 77 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 78 . L.J. Spilker. 2009. Cassini-Huygens Solstice Mission. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 79 . M.S. Tiscareno. 2009. Rings Research in the Next Decade. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 80 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 81 . J.C. Castillo-Rogez. 2009. Laboratory Studies in Support of Planetary Geophysics. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 82 . M. Gudipati. 2009. Laboratory Studies for Planetary Sciences. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 83 . K. Tsiganis, R. Gomes, A. Morbidelli, and H.F. Levison. 2005. Origin of the orbital architecture of the giant planets of the solar system. Nature 435:459-461. 84 . A. Morbidelli, H.F. Levison, K. Tsiganis, and R. Gomes. 2005. Chaotic capture of Jupiter’s Trojan asteroids in the early solar system. Nature 435:462-465. 85 . J.A. Fernandez and W. Ip. 1984. Some dynamical aspects of the accretion of Uranus and Neptune: The exchange of orbital angular momentum with planetesimals. Icarus 58:109-120. 86 . R. Malhotra. 1993. Orbital resonances in the solar nebula: Strengths and weaknesses. Icarus 106:264. 87 . R. Malhotra. 1995. The origin of Pluto’s orbit: Implications for the solar system beyond Neptune. Astronomical Journal 110:420. 88 . J.J. Lissauer, J.B. Pollack, G.W. Wetherill, and D.J. Stevenson. 1995. Formation of the Neptune system. Pp. 37-108 in Neptune and Triton (D.P. Cruikshank, M.S. Matthews, and A.M. Schumann, eds.). 89 . E. Kokubo and S. Ida. 1998. Oligarchic growth of protoplanets. Icarus 131:171-178. 90 . P. Goldreich, Y. Lithwick, and R. Sari. 2004. Final stages of planet formation. Astrophysical Journal 614:497-507. 91 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 92 . D.H. Atkinson. 2009. Entry Probe Missions to the Giant Planets. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 93 . W.A. Traub. 2009. Exoplanets and Solar System Exploration. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 94 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 95 . D.H. Atkinson. 2009. Entry Probe Missions to the Giant Planets. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 96 . W.A. Traub. 2009. Exoplanets and Solar System Exploration. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 97 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 98 . M. Hofstadter. 2009. The Case for a Uranus Orbiter. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 99 . J. Horner, B.W. Jones, and J. Chambers. 2010. Jupiter—Friend or foe? III: The Oort cloud comets. International Journal of Astrobiology 9(1):1-10. 100 . G.S. Orton. 2009. Earth-Based Observational Support for Spacecraft Exploration of Outer-Planet Atmospheres. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 101 . A. Wesley. 2009. Ground-Based Support for Solar-System Exploration: Continuous Coverage Visible Light Imaging of Solar System Objects from a Network of Ground-Based Observatories. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 102 . P.R. Estrada and J.N. Cuzzi. 1996. Voyager observations of the color of Saturn’s rings. Icarus 122:251-272.

OCR for page 175
214 VISION AND VOYAGES FOR PLANETARY SCIENCE 103 . J.R. Spencer and T. Denk. 2010. Formation of Iapetus’ extreme albedo dichotomy by exogenically triggered thermal ice migration. Science 327:432-435. 104 . C.J. Hansen. Triton Science with Argo—A Voyage Through the Outer Solar System. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 105 . C. Agnor. 2009. The Exploration of Neptune and Triton. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 106 . M. Hofstadter. 2009. The Atmospheres of the Ice Giants, Uranus and Neptune. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 107 . M.S. Tiscareno. 2009. Rings Research in the Next Decade. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 108 . A. Sanchez-Lavega, S. Perez-Hoyos, J.F. Rojas, R. Hueso, and R.G. French. 2003. A strong decrease in Saturn’s equatorial jet at cloud level. Nature 423:623-625. 109 . G.S. Orton, P.A. Yanamandra-Fisher, B.M. Fisher, A.J. Friedson, P.D. Parrish, J.F. Nelson, A.S. Bauermeister, L. Fletcher, D.Y. Gezari, F. Varosi, A.T. Tokunaga, et al. 2008. Semi-annual oscillations in Saturn’s low-latitude stratospheric tem- peratures. Nature 453:196-199. 110 . T. Fouchet, S. Guerlet, D.F. Strobel, A.A. Simon-Miller, B. Bézard, and F.M. Flasar. 2008. An equatorial oscillation in Saturn’s middle atmosphere. Nature 453:200-202. 111 . G.S. Orton and P.A. Yanamandra-Fisher. 2005. Saturn’s temperature field from high-resolution middle-infrared imaging. Science 307:696-698. 112 . G. Orton et al. unpublished data. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif. 113 . H.B. Hammel, M.L. Sitko, G.S. Orton, T. Geballe, D.K. Lynch, R.W. Russell, and I. de Pater. 2007. Distribution of ethane and methane emission on Neptune. Astronomical Journal 134:637-641. 114 . K.B. Clark. 2009. Europa Jupiter System Mission. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 115 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities, and to some extent by Cassini for Saturn. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 116 . L.J. Spilker. 2009. Cassini-Huygens Solstice Mission. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 117 . G.S. Orton. 2009. Earth-Based Observational Support for Spacecraft Exploration of Outer-Planet Atmospheres. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 118 . M. Hofstadter. 2009. The Case for a Uranus Orbiter. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 119 . C. Agnor. 2009. The Exploration of Neptune and Triton; Wesley A. Traub, Exoplanets and Solar System Exploration. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 120 . L.J. Spilker. 2009. Cassini-Huygens Solstice Mission. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 121 . L. J. Spilker. 2009. Cassini-Huygens Solstice Mission. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 122 . K.B. Clark. 2009. Europa Jupiter System Mission. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 123 . V. Lainey and T. van Hoolst. 2009. Jovian tidal dissipation from inner satellite dynamics. European Planetary Science Congress 2009. EPSC Abstracts 4:EPSC2009-392. 124 . See, e.g., A.P. Ingersoll, T.E. Dowling, P.J. Gierasch, G.S. Orton, P.L. Read, A. Sanchez-Lavega, A.P. Showman, A.A. Simon-Miller, and A.R. Vasavada. 2004. Dynamics of Jupiter’s atmosphere. Pp. 105-128 in Jupiter: The Planet, Satellites and Magnetosphere (F. Bagenal, T. Dowling, and W. McKinnon, eds.). Cambridge University Press, New York. 125 . A. Sanchez-Lavega, S. Perez-Hoyos, J.F. Rojas, R. Hueso, and R.G. French. 2003. A strong decrease in Saturn’s equatorial jet at cloud level. Nature 423:623-625. 126 . F.M. Flasar, R.K. Achterberg, B.J. Conrath, J.C. Pearl, G.L. Bjoraker, D.E. Jennings, P.N. Romani, A.A. Simon-Miller, V.G. Kunde, C.A. Nixon, B. Bézard, et al. 2005. Temperatures, winds, and composition in the saturnian system. Science 307:1247-1251. 127 . S. Pérez-Hoyos and A. Sánchez-Lavega. 2006. On the vertical wind shear of Saturn’s equatorial jet at cloud level. Icarus 180:161-175.

OCR for page 175
215 THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND 128 . A.W. Brinkman and J. McGregor. 1979. The effect of the ring system on the solar radiation reaching the top of Saturn’s atmosphere: Direct radiation. Icarus 38:479-482. 129 . L.A. Sromovsky, P.M. Fry, S.S. Limaye, and K.H. Baines. 2003. The nature of Neptune’s increasing brightness: Evidence for a seasonal response. Icarus 163:256-261. 130 . H.B. Hammel and G.W. Lockwood. 2007. Long-term atmospheric variability on Uranus and Neptune. Icarus 186:291-301. 131 . L.A. Sromovsky, P.M. Fry, W.M. Ahue, H.B. Hammel, I. de Pater, K.A. Rages, M.R. Showalter, and M.A. van Dam. 2009. Uranus at equinox: Cloud morphology and dynamics. Icarus 203:265-286. 132 . L.A. Sromovsky and P.M. Fry. 2005. Dynamics of cloud features on Uranus. Icarus 179:459-484. 133 . H.B. Hammel, L.A. Sromovsky, P.M. Fry, K.A. Rages, M.R. Showalter, I. de Pater, M.A. van Dam, R.P. LeBeau, and X. Deng. 2009. The dark spot in the atmosphere of Uranus in 2006: Discovery, description, and dynamical simulations. Icarus 201:257-271. 134 . S.H. Luszcz-Cook, I. de Pater, H.B. Hammel, and M. Ádámkovics. 2010. Seeing double at Neptune’s south pole. Icarus 208(2):938-944. 135 . D.H. Atkinson. 2009. Entry Probe Missions to the Giant Planets. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 136 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 137 . D.H. Atkinson. 2009. Entry Probe Missions to the Giant Planets. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 138 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 139 . W.A. Traub. 2009. Exoplanets and Solar System Exploration. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 140 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 141 . M. Hofstadter. 2009. The Case for a Uranus Orbiter. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 142 . National Research Council. 2004. Plasma Physics of the Local Cosmos, The National Academies Press, Washington, D.C. 143 . W. Borucki for the Kepler Team. 2011. Characteristics of Kepler planetary candidates based on the first data set. Astro- physical Journal 728(2):117. 144 . J.J. Fortney. 2009. Planetary Formation and Evolution Revealed with Saturn Entry Probe. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 145 . W.A. Traub. 2009. Exoplanets and Solar System Exploration. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 146 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 147 . P.M. Beauchamp. 2009. Technologies for Outer Planet Missions: A Companion to the Outer Planet Assessment Group (OPAG). White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 148 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 149 . C.J. Hansen. 2009. Neptune Science with Argo—A Voyage Through the Outer Solar System. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 150 . M. Hofstadter. 2009. The Atmospheres of the Ice Giants, Uranus and Neptune. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 151 . G. Sonneborn. 2009. Study of Planetary Systems and Solar System Objects with JWST. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 152 . A. Tokunaga. 2009. The NASA Infrared Telescope Facility. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 153 . A. Wesley. 2009. Ground-Based Support for Solar-System Exploration: Continuous Coverage Visible Light Imaging of Solar System Objects from a Network of Ground-Based Observatories. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 154 . E.F. Young. 2009. Balloon-Borne Telescopes for Planetary Science: Imaging and Photometry. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.

OCR for page 175
216 VISION AND VOYAGES FOR PLANETARY SCIENCE 155 . C.A. Hibbitts. 2009. Stratospheric Balloon Missions for Planetary Science. White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 156 . W.B. McKinnon. 2009. Exploration Strategy for the Outer Planets 2013-2022: Goals and Priorities. White paper sub - mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C. 157 . K. Clark et al. 2009. Jupiter Europa Orbiter Mission Study 2008: Final Report—The NASA Element of the Europa Jupiter System Mission (EJSM). Jet Propulsion Laboratory, Pasadena, Calif. 158 . National Research Council. 2003. New Frontiers in the Solar System: An Integrated Exploration Strategy. The National Academies Press, Washington, D.C., pp. 110 and 195. 159 . National Research Council. 2003. New Frontiers in the Solar System: An Integrated Exploration Strategy. The National Academies Press, Washington, D.C., p. 111. 160 . Note that the mission studied by the committee and subject to CATE analysis did not include lower-priority science objectives. 161 . K. Clark et al. 2009. Jupiter Europa Orbiter Mission Study 2008: Final Report—The NASA Element of the Europa Jupiter System Mission (EJSM). Jet Propulsion Laboratory, Pasadena, Calif.