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
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?
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
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 in 2011, with an additional 1,200 candidates awaiting confirmation;a most of these are giants, 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.||Ground- and space-based telescopes|
|A spacecraft en route to Pluto observed Jupiter’s polar lightning, the life cycle of fresh ammonia 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.||New Horizons|
|Discoveries at Saturn include confirmation of the hot southern polar vortex, deep lightning, large 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.||Cassini|
|Uranus’s equinox in 2007 was observed with modern instruments (the most recent equinox was in 1965), revealing unprecedented cloud activity with both bright and dark atmospheric features, two new brightly colored rings, and several new small moons.||Ground-based telescopes|
|Neptune’s ring arcs shifted location and brightness in an unexplained fashion, and evidence emerged for a hot polar vortex on Neptune.||Ground-based telescopes|
|Three giant impacts on Jupiter have been recorded since June 2009; one of them was large enough to 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.||Ground-based telescopes|
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, location, 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 stability, as an aid to understanding similar processes and activities on Earth and other planets.
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 numbering 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
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 significantly 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 exhibiting detectable seasonal variation (discussed further below), which is also linked to energy balance, but the driving causes are not well understood.
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 Telescope 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 mechanisms, 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 balance will illuminate our understanding of exoplanet processes as well.
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
The best approach to truly understanding giant-planet heat flow and radiation balance would be a systematic 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
limited information to supplement Juno’s determination of gravitational moments and the nature of the inner magnetic 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 abundances. 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 probing 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 understanding 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
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 spacecraft 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 diagnostics 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 enhancement 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 hypothesis33 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?
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 measuring 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 conductivity, yet they cannot be directly observed (magnetospheres are discussed in more detail below). However, precise measurements 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 observations 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.
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 mission 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 important 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
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.
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 measurements 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 estimated 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 interaction 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
star-planet interaction could result if the star’s rotation rate is rapid compared with the planet’s orbital period: the planet’s motion through the star’s magnetic field would generate a large electric potential across the planet, driving a strong current between the planet and the star. This could result in a “starspot,” analogous to a sunspot. Observations of starspots may be a promising approach for remotely sensing the electrodynamic interaction of exoplanets with their stars.
The giant planets in the solar system have strong magnetic fields and giant magnetospheres, leading to solar wind interactions quite different from what is seen at Earth. The size scales are much larger, and the timescales are much longer; still, the aurora on both Jupiter and Saturn are affected by changes in the solar wind. Unlike Earth, Jupiter’s magnetosphere and aurora are dominated by internal sources of plasma, and the primary energy source is the planet’s rotation. Saturn is an intermediate case between Earth and Jupiter: it has a large, rapidly rotating magnetosphere with internal sources of plasma, yet the aurora and nonthermal radio emissions consistently brighten when a solar wind shock front arrives at the planet. The ice giants have substantially tilted magnetospheres that are significantly offset from the planets’ centers, configurations that differ completely from those of Jupiter and Saturn.
The understanding of the magnetospheric environments of Jupiter and Saturn has deepened since the 2003 decadal survey. The Galileo mission at Jupiter has concluded. Cassini passed Jupiter, entered orbit around Saturn, and successfully completed its nominal mission. NASA’s Infrared Telescope Facility (IRTF) has obtained infrared images of Saturn’s aurorae. A large Hubble Space Telescope program to observe the ultraviolet auroral emissions from Jupiter and Saturn has been conducted; coupled with New Horizons measurements at Jupiter and with Cassini measurements at Saturn, this program has shown the extent of solar wind control over giant-planet aurorae. There have been no comparable missions to Uranus or Neptune, however; thus knowledge of ice-giant magnetospheres is limited to data from Voyager 2 flybys more than two decades ago, supplemented by subsequent, scant, Earth-based observations. The scarcity of close-range measurements of ice giants has seriously limited the advance of our knowledge of their magnetospheres and plasma environments. New measurements from Uranus and/or Neptune therefore have a high priority in the outer planet magnetospheres community.
Some Important Questions
Some important questions concerning the properties and processes in planetary magnetospheres include the following:
• What is the nature of the displaced and tilted magnetospheres of Uranus and Neptune, and how do conditions vary with the pronounced seasonal changes on each planet?
• What is the detailed plasma composition in any of these systems, particularly for ice giants?
• What causes the enormous differences in the ion-to-neutral ratios in these systems?
• What can understanding of the giant-planet magnetospheres tell us about the conditions to be expected at extrasolar giant planets?
Future Directions for Investigations and Measurements
Despite concentrated observations of Jupiter and Saturn both in situ and from Earth-based facilities, there remain many unanswered questions. These can be addressed in part by measurements from JEO during its initial orbital phase, including observations of auroral emission distribution and associated properties from close polar orbital passes, as well as of upper-atmosphere energetics.53,54
Nearly nothing is known about the magnetic fields and magnetospheres of Uranus and Neptune aside from what was learned through the brief Voyager encounters more than two decades ago. Either a flyby spacecraft or an orbiter could address some aspects of ice-giant magnetospheric science, but an orbiter would tremendously advance understanding of ice-giant magnetospheres.55,56,57
Extrapolation of the solar system’s magnetospheres to those expected for exoplanets can 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 parameters. Most giant-planet missions could contribute to answering these questions; our knowledge is most lacking for the ice giants.
Use Ring Systems as Laboratories for Planetary Formation Processes
Investigations of planetary rings can be closely linked to studies of circumstellar disks. Planetary rings are accessible analogs in which general disk processes such as accretion, gap formation, self-gravity wakes, spiral waves, and angular-momentum transfer with embedded masses can be studied in detail. The highest-priority recommendation on rings in the 2003 decadal survey58 was accomplished: to operate and extend the Cassini orbiter mission at Saturn.59,60 Progress has also come from Earth-based observational and theoretical work as recommended by the 2003 decadal survey and others.61,62
Saturn’s Ring System
Cassini data, supported by numerical and theoretical models, have revealed a wealth of dynamical structures in Saturn’s rings, including textures in the main rings produced by interparticle interactions and patterns generated by perturbations from distant and embedded satellites. Its observations of the orbits of embedded “propeller” moons in Saturn’s rings reveal surprisingly robust orbital evolution on approximately 1-year timescales, possibly due to gravitational or collisional interactions with the disk.63 Collective interparticle interactions produce phenomena including what are now termed self-gravity wakes (elongated, kilometer-scale structures formed by a constant process of clumping counterbalanced by tidal shearing), radial oscillations in the denser parts of the rings that may be due to viscous overstability, and straw-like textures seen in regions of intense collisional packing such as strong density waves and confined ring edge.
Moons embedded within the rings are observed to produce gaps, although the origins of many other gaps remain unknown. In Saturn’s F ring, Cassini images show evidence for active accretion triggered by close approaches of the nearby moon Prometheus,64 while recent accretion is inferred for other known ring moons.65,66,67 Data from Cassini’s spectrometers and other instruments are elucidating the composition and thermal properties of the icy particles in Saturn’s rings, as well as the characteristics of the regolith covering larger ring particles. These properties vary subtly between different regions of the rings, for reasons that are currently not understood.
The evident processes and properties of Saturn’s ring system provide essential clues as to how all rings and other disks of material behave (including circumstellar disks and protoplanetary disks). Nongravitational forces, including electromagnetism, drive the evolution of dusty rings such as Saturn’s E ring (as well as Jupiter’s rings and probably Uranus’s dusty zeta ring), but much work is still needed to clarify the processes involved. Cassini will continue tracking the orbits of propeller structures through the end of its Solstice mission in 2017. The direct detection of orbital migration remains a major goal for Cassini in the Solstice mission, either for nearby moons interacting gravitationally with the rings or for the embedded and unseen propeller moonlets. Cassini will also make further observations of the ring microstructure. Cassini results also have focused renewed attention on the origin and age of Saturn’s rings: the realization that the B ring may be much more massive than previously thought has the potential to ease a primary constraint on the rings’ age—namely, pollution by interplanetary mass infall; a continuing goal of Cassini is to measure that flux.
Other Ring Systems
The most remarkable features of Neptune’s rings are the azimuthally confined arcs embedded in the Adams ring. Although a resonance mechanism has been proposed for the confinement of these arcs, post-Voyager observations at the Keck Observatory have cast at least the details of this model into doubt.68 The same observations also reveal that the arcs are evolving on timescales as short as decades, consistent with our emerging perspective of the dynamic nature of diffuse rings. Further close-range observations of both the rings and their associated arcs will be necessary to resolve the outstanding questions regarding their nature, origin, and persistence.
The uranian ring system is massive, complex, and diverse, and much about it remains poorly understood. It includes several narrow and sharp-edged dense rings whose confinement mechanism remains unclear. The 2007 equinox of Uranus provided an unprecedented opportunity to study its ring system. During the edge-on apparition (which last occurred in 1965), two new diffuse uranian rings were discovered in Hubble images in 2005.69 They appear eerily similar to Saturn’s E and G rings in color and planetocentric distance70 but have yet to be characterized in any detail. Equinoctial Keck observations of Uranus also detected the diffuse dusty inner zeta ring for the first time since the 1986 Voyager flyby and revealed that it has changed substantially since then.71 The reason for this change is unknown; temporal effects of Uranus’s extreme seasons on the rings may be contributing factors (Figure 7.5).
Some important questions concerning ring systems as laboratories for planetary formation processes include the following:
• What can the significant differences among ring systems teach us about the differing origins, histories, or current states of these giant-planet systems?
• Can the highly structured forms of the Uranus and Neptune ring systems be maintained for billions of years, or are they “young”? Are their dark surfaces an extreme example of space weathering?
• What drives the orbital evolution of embedded moonlets; how do they interact with their disks?
• What drives mass accretion in a ring system?
Future Directions for Investigations and Measurements
The chemical and physical properties of uranian and neptunian ring particles remain almost completely unknown, beyond the former’s very low albedo. Observing the rings of Uranus and Neptune at close range in the near infrared would considerably enhance our understanding of their origin and composition. Observations at high phase angles inaccessible from Earth are the key to estimating particle sizes. Ground-based observations have detected changes in the rings of Neptune and the diffuse rings of Uranus on decadal timescales or shorter; Cassini likewise saw significant structural changes in Saturn’s D and F rings on similar timescales. The mechanisms behind these changes remain mysterious, and it is highly desirable to study these changing structures in detail. Therefore, orbiter missions to Uranus and/or Neptune represent the highest priority for advancing ring science in the next decade.72,73,74
The recent New Horizons flyby has demonstrated the value of continued observations of Jupiter’s rings, revealing a new structure that still is not fully understood. Missions to Jupiter, as well as any mission encountering Jupiter en route to another target, should observe this poorly characterized ring system as opportunities allow.75
The Cassini Solstice mission will continue to yield significant ring science, as articulated above. In future decades, a dedicated Saturn Ring Observer mission could potentially obtain “in situ” Saturn ring data with unprecedented spatial resolution and temporal coverage. Initial engineering studies for such a mission exist (Appendix G), but further technology development is required during the next decade to develop a robust mission profile (Figure 7.6).76,77,78
Nearly all constituent ring particles are too small to observe individually, even from an orbiting spacecraft, so a proper interpretation of any observations requires an understanding of the particles’ collective effects and behavior. Thus, for example, theoretical and numerical analyses of ring dynamics are essential to interpreting the photometry of Saturn’s rings as observed by Cassini and from Earth-based telescopes. Laboratory studies of potential ring-forming materials also are needed in order to understand ring spectroscopy.79,80,81,82
The solar system contains myriad objects—small and large—orbiting the Sun, and these bodies can directly affect the habitability of Earth. For example, large planetary impacts are an ongoing process, not merely a fact. Observations of Jupiter make this very clear: witness the spectacular impact of Shoemaker-Levy 9 with Jupiter in 1994. The effects of the jovian collisions prompted studies of, and surveys for, potentially hazardous asteroids in near-Earth space. The surprising second collision of a body with Jupiter in 2009, followed by two more jovian impacts in 2010, underscores the hazards in the interplanetary environment.
The Sun itself is highly variable, and the variability has potentially significant consequences. The explosive release of stored magnetic energy in the Sun’s atmosphere leads to extremely large solar “storms,” causing changes in emitted electromagnetic radiation at all energies, ejecting energetic particles, and enhancing the solar wind at Earth. The most prominent examples of the manifestations of solar storms include not only natural spectacles such as auroral displays, but also direct impacts on human activities such as catastrophic failures of electrical grids and spacecraft hardware. The aurorae of Jupiter and Saturn provide important data points in understanding the propagation of these storms across the solar system. Understanding these solar eruptions and their propagation to Earth and beyond plays an important role in contemporary solar physics and has generated its own field of space weather.
By studying the giant planets in the context of processes that occur throughout the solar system, we gain a deeper understanding of how those processes play out here on Earth. This is illustrated with specific examples about energy balance, interactions with the Sun’s magnetic field, and how the surfaces in giant-planet systems are “weathered.”
Specific objectives associated with the goal of exploring the giant planets’ role in crafting a habitable planetary system include the following:
• Search for chemical evidence of planetary migration,
• Explore the giant planets’ role in creating our habitable Earth through large impacts, and
• Determine the role of surface modification through smaller impacts.
Subsequent sections examine each of these objectives in turn, identifying important questions to be addressed and future investigations and measurements that could provide answers.
Search for Chemical Evidence of Planetary Migration
In the past, various models have been proposed for the formation of planetary systems in general and specifically for the solar system. All of these models made basic assumptions concerning the condensation of planet-forming components and the manner in which they were accumulated by the planets. In the past two decades, increased computing power has led to a rejection of some models and increased support for a model in which Jupiter and Saturn interacted to perturb the planets into their current configuration. The degree to which the planets were formed by collisional impacts of volatile-bearing bodies or by the collapse of gases onto larger bodies should have left behind evidence that can be found within the compositional makeup of the surviving bodies. Thus, the determination of the chemical composition (i.e., the D/H ratio, other isotopic abundances, noble gases, water) will discriminate among models that will constrain initial conditions and illuminate how the planets have evolved.
The distribution of the heavy elements (atomic mass greater than 4) as a function of distance from the Sun can provide strong constraints for discriminating among theories and dynamical models of solar system formation and
evolution. One of the predictions of the models is that the central core mass of the giant planets should increase with distance from the Sun. This should result in a corresponding increase in the abundances of the heavier elements. Currently the only element measured for all four planets is carbon, increasing from 3-times solar at Jupiter to about 30-times solar at Neptune. In order to discriminate among formation models, abundances are needed for the heavy elements (nitrogen, sulfur, oxygen, and phosphorus), helium and the other noble gases and their isotopes, and isotope ratios of hydrogen, helium, nitrogen, oxygen, and carbon.
Although the isotopic information is limited, modeling efforts have produced divergent theories of the formation of the solar system. Models that placed the formations of Uranus and Neptune at their current positions were unable to produce adequate ice-giant cores before the proto-nebula dissipated. Faced with this stumbling block, dynamic modelers have been led to conclude that the outer planets have significantly changed their orbital positions since their original formation. The “Nice model”—the currently accepted standard solar system formation scenario—proposes that during the first several hundred million years after the formation of the planets, Neptune was less than 20 AU from the Sun.83,84 As the orbits evolved, Saturn and Jupiter entered a 2:1 mean motion resonance and the resulting perturbation to Saturn’s eccentricity drove the orbits of Uranus and Neptune outward, leading to the current configuration of giant planets.
Many variations of the dynamical formation scenario have been proposed.85,86,87,88,89,90 Other approaches to the birth of the solar system address the manner in which the heavy elements were delivered to the giant planets, as discussed above. To help distinguish among these theories or to generate others, we require in situ measurements of heavy element abundances and isotopic ratios in the well-mixed atmospheres of the giant planets.
Some Important Questions
Some important questions concerning chemical evidence of planetary migration include the following:
• How and why do elemental and isotopic abundances vary as a function of distance from the Sun?
• How and why do the abundances of the heavy elements and their isotopes, the deuterium/hydrogen ratio, the hydrogen/helium ratio, and noble gases differ between the two classes of giant planets represented in the solar system?
Future Directions for Investigations and Measurements
Shallow entry probes (<10 bars at Saturn) will enable the determination of the abundances for most of the required species. The elemental abundances and isotopic information gained from the shallow probes will provide major constraints for the plethora of solar system formation models and a guide to extend models to exoplanetary systems. To reach deep below the water cloud on Saturn and determine the water abundance in a well-mixed region would be desirable, but far more technically challenging.91,92,93
Coupling a probe with an orbiter, particularly for one of the relatively unexplored ice giants, will substantially advance giant-planet science. An orbiter could provide the global distribution of disequilibrium species and ortho/para hydrogen ratios by means of infrared remote sensing with high-frequency resolution. This would yield a framework for interpreting the in situ elemental and isotopic probe results.94,95,96,97,98
Explore the Giant Planets’ Role in Creating Our Habitable Earth Through Large Impacts
On average, 1-km-or-larger-diameter comets and asteroids impact Earth about once every 100,000 years. Impacts of this size and larger yield major tsunamis if an ocean is struck, and they can destroy areas of land equivalent to moderate-size states. Larger impacts, like the Cretaceous/Paleogene impactor 65 million years ago, can dramatically affect life over the entire surface of Earth. Although the impactor hazard to life on Earth is not negligible, it is less than might otherwise be the case without the giant planets. Around 1 million asteroids are believed to be larger than 1 km, and there are likely far more comets of this size and larger. All of these objects represent potential Earth impactors.
The solar system’s giant planets, particularly Jupiter, exert a major influence on the orbits of such objects. Asteroids or comets on elliptical orbits that would bring them to the inner solar system must cross the orbit of Jupiter. Close encounters with Jupiter can dramatically alter a potentially dangerous body’s orbit, possibly sending it out of the solar system. Although in some cases Jupiter might cause an otherwise harmless object to take a dangerous turn, some n-body simulations suggest that in other cases Jupiter protects Earth.99
The number and timing of jovian impacts provide insight into the rate at which Jupiter deflects small bodies. Each impact delivers species to Jupiter’s stratosphere that would not be produced by internal jovian processes; a better inventory of jovian stratospheric composition along with improved atmospheric models and numerical models of asteroid and comet orbits would constrain impact history.
More importantly, these events serve as laboratories for the physics of large airbursts. We now have an open, unclassified source of Earth bolide observational data, but these are for relatively small events. Those who have studied the subject of Earth impacts estimate intervals of hundreds of years between events the size of the Tunguska impact in Siberia in 1908. Yet, as of this writing astronomers have observed four such impact events on Jupiter in the past 16 years (counting the demise of Shoemaker-Levy 9 in 1994 as “one” event). By understanding the physics of large airbursts, better estimates of their threat to Earth can be made.
Some important questions concerning the giant planets’ role in creating a habitable Earth by means of large impacts include the following:
• What is the current impact rate on Jupiter?
• To what extent can Jupiter’s current atmospheric composition be utilized as a record of the impact history?
• What are the characteristics of bolides and large airbursts on Jupiter, and how do they compare with known bolides and airbursts on Earth?
Future Directions for Investigations and Measurements
The best approach to determining the rate and characteristics of jovian atmospheric impacts is through continuous observation of Jupiter. Today, such work relies on a small number of highly motivated amateur these unfunded volunteer observers, however, cannot cover Jupiter at all times. Small, automated, planetary monitoring telescopes could provide a comprehensive survey of future impacts on Jupiter, perhaps as a National Science Foundation (NSF) project. The Large Synoptic Survey Telescope (LSST) or other survey telescopes may also provide observational constraints on moving objects that could be on Jupiter impact trajectories.100,101
Determine the Role of Surface Modification Through Smaller Impacts
A number of important external processes govern the size, structure, and dynamics of the giant planets, their ring systems, and their satellites (in addition to the obvious role of solar illumination). Many of these processes are analogous to those that operate on terrestrial planets and in the Earth-Moon system. Impacts by kilometer-size asteroidal and/or cometary objects have long been recognized as a dominant process in sculpting the surfaces of most bodies in the solar system.
Less obvious and much less understood is the role played by smaller impactors, down to dust size, in modifying the surface composition and texture. Examples in the outer solar system include the neutral-colored material that darkens the C ring and Cassini Division at Saturn102 and the dark material that coats the leading side of Iapetus, thought to be derived from Phoebe or the other outer satellites.103 More speculative are the long-term effects on the structure and lifetime of outer-planet ring systems owing to ring-particle collisions and collisions of external impactors.
Some important questions concerning the role of surface modification by means of smaller impacts include the following:
• What are the flux, size distribution, and chemical composition of the various populations of impactors, from late-stage planetesimals 4 billion years ago to present-day interplanetary dust?
• What are the surface modification mechanisms for low-temperature, smaller icy targets?
Future Directions for Investigations and Measurements
Sophisticated dust detectors carried by spacecraft such as Galileo, Ulysses, and Cassini have already refined—and in some cases revolutionized—knowledge of interplanetary dust, and much more remains to be learned here. Near-infrared spectral studies of the Galilean and saturnian moons and rings have led to new models for dust “contamination” of icy surfaces, but definitive identification of the chemical species involved remains elusive and may require in situ sampling. Near-infrared spectral studies of the rings and small moons in the ice-giant systems are needed to fully characterize the differences of the dust populations in the more distant regions of the solar system.104,105,106,107
The planet that matters most to humankind is Earth. The planet’s health and ecologic stability are of paramount importance to us all. Earth, however, is a notoriously difficult planet to understand. The atmosphere interacts in a complex fashion with the lithosphere, hydrosphere, cryosphere, and biosphere (surfaces that are, respectively, rocky, liquid, icy, or biologically active). Yet knowledge of this interplay is critical for understanding the processes that determine conditions of habitability within the thin veneer of Earth’s surface.
Giant planets, though larger than Earth, are in many respects simpler than Earth. The physics and chemistry driving the processes in their thick outer atmospheres can be understood without reference to a lithosphere, cryosphere, hydrosphere, or biosphere. The processes in giant-planet ring systems at times resemble pure examples of Keplerian physics, with added interactions from collisions, resonances, and self-gravity. In a very real sense, the giant planets and their environs can serve as laboratories for the fundamental physical processes that affect all planetary atmospheres and surfaces.
Fundamental objectives associated with the goal of using the giant planets as laboratories for properties and processes of direct relevance to Earth include the following:
• Investigate atmospheric dynamical processes in the giant-planet laboratory,
• Assess tidal evolution within giant-planet systems,
• Elucidate seasonal change on giant planets, and
• Evaluate solar wind and magnetic-field interactions with planets.
Subsequent sections examine each of these objectives in turn, identifying critical questions to be addressed and future investigations and measurements that could provide answers.
Investigate Atmospheric Dynamical Processes in the Giant-Planet Laboratory
On the giant planets, jet streams dominate the atmospheric layers accessible for remote sensing and in situ measurements. Visible and infrared data generated with spacecraft (e.g., Voyager, Galileo, Cassini, New Horizons) and large Earth- and space-based telescopes (e.g., Hubble Space Telescope, Keck, Gemini, the Very Large Telescope [VLT]) established two distinct regimes of atmospheric circulation: Jupiter and Saturn show strong eastward equatorial jets and alternating poleward east-west jets, whereas Uranus and Neptune display westward equatorial
winds and broad mid-latitude prograde jets. These results, in conjunction with the inferred differences in bulk composition, imply that distinct giant-planet regimes are represented within the solar system: that of gas giants (90 percent hydrogen by mass) with deep-seated convective regions and that of ice giants (bulk compositions are dominated by heavier elements) that support a structure in which water becomes a supercritical fluid with depth. Studies of both types of planets, to sample a range of obliquities, insolation values, and internal heat-flow values, are needed to untangle the role of each forcing mechanism.
Within the atmospheres of the gas giants, the jets blow in the east-west (i.e., zonal) directions. Alongside the jet streams, vortices large and small pepper the visible layers; some appear as textbook fluid dynamics turbulent features, which are also seen on Earth. The steady existence of large-scale atmospheric features combined with infrequent close-range spacecraft observations and long seasonal cycles of the outer planets has given the general impression that the giant planets are static (Figure 7.7).
Long temporal baseline data and ever-improving observational capabilities prove that these worlds are as dynamic as Earth is. In the past decade, Jupiter went through a global upheaval, during which the colors of the clouds changed in multiple zonal bands associated with the jet streams. On Saturn, Cassini and Hubble measurements in 2003 showed that the equatorial jet at the cloud level had slowed compared with the winds seen during the Voyager flybys in 1980-1981.108 A long-term campaign by the NASA Infrared Telescope Facility, combined with Cassini data, has revealed a new stratospheric temperature oscillation on Saturn analogous to those on Jupiter, Earth, and Mars.109,110
A new generation of ground-based telescopes armed with adaptive optics (including the Keck, Gemini, and Subaru) has enabled discoveries of many dynamic cloud features on Uranus and Neptune, revealing the changing seasons on these slowly orbiting planets. These facilities have also revealed that Saturn, Uranus, and Neptune have hot poles (Figure 7.8),111,112,113 although Jupiter’s poles have never been imaged (Jupiter’s rotation axis has little tilt, making Earth-based observations difficult; missions that have flown by or orbited Jupiter have stayed near the equatorial plane). Together, these new observations show that the giant-planet atmospheres are dynamic and evolving.
The Galileo probe set limits on critical isotopic abundances in Jupiter’s atmosphere and revealed zonal winds increasing with depth, and we are beginning to understand the nature of the wind fields and vortices. Yet our knowledge of gas giants is not complete; we need in situ measurements of Saturn’s winds. Uranus and Neptune are even less well understood: due to the scarcity of observational data, the life cycles of large and small vortices are unknown, and the temporal dynamics of the zonal winds, as well as their horizontal and vertical structures, have not been examined. Unlike the case of Earth, for these planets we do not have true three-dimensional information at high spatial resolution over reasonable timescales to allow for full comparison with laboratory and theoretical models.
Some important questions concerning atmospheric dynamical processes in the giant planets include the following:
• What processes drive the visible atmospheric flow, and how do they couple to the interior structure and deep circulation?
• What are the sources of vertically propagating waves that drive upper-atmospheric oscillations, and do they play a role on all planets?
• Are there similar processes on Uranus and Neptune, and how do all these compare with Earth’s own stratospheric wind, temperature, and related abundance (ozone, water) variations?
• How does moist convection shape tropospheric stratification?
• What are the natures of periodic outbursts such as the global upheaval on Jupiter and the infrequent great white spots on Saturn?
Future Directions for Investigations and Measurements
To answer the important questions listed above, we must resolve the three-dimensional structure of the atmospheric flow fields, including polar regions, with high spatial and temporal resolution. The vertical motions in the troposphere involve the fast, localized motions caused by moist convection and the slow, global, overturning meridional circulation caused by the predicted (Hadley-like) belt-zone convection cells.
Determining atmospheric motion and coupling includes the study of atmospheric waves and the stratospheric responses to the wave forcing (oscillations); such waves may have a chemical signature. The stratospheric oscillations that have been discovered on Earth, Mars, Jupiter, and Saturn provide a rare stage for conducting comparative planetologic investigation between terrestrial and giant planets. It may also be possible to detect similar oscillations on Uranus and Neptune.
Finally, there is a need to explore polar phenomena. Jupiter’s poles exhibit numerous small vortices, and, to date, their zonal mean-flow structure has not been observed in detail. Saturn’s north pole has a circumpolar jet that meanders in a hexagonal shape, whereas the south pole has a hurricane-like structure with a well-defined eye wall. Little is known about the polar regions of Uranus and Neptune.
Some of the above atmospheric objectives may be addressed by Juno and JEO for Jupiter, and to some extent by Cassini for Saturn. Significant advances on Jupiter by JEO would require long temporal observations, adequate spatial resolution on Jupiter, and relevant instrumentation. For Uranus or Neptune, these atmospheric objectives are poorly constrained by Earth-based data. An orbiter would be optimal for investigating such phenomena. Significant theoretical and modeling research should also be supported to infer the atmospheric structures underlying the observed layers and to advance the understanding of shear instabilities.114,115,116,117,118,119
Juno may achieve measurements of gravitational signatures of deep zonal flows for Jupiter, and the Cassini mission may do likewise for Saturn during its final proximal orbits. This information will reveal the basic structure of the deep flow driven by internal convection and will yield information about the internal heat transport. Juno and Cassini should place useful limits on the higher-order moments of the internal magnetic fields and potentially detect some temporal evolution (i.e., secular variation). For an ice giant, a flyby could moderately improve our understanding, whereas an orbiter with a low periapse approach would greatly advance the scientific understanding of the interiors and magnetic fields of the ice giants.120
Assess Tidal Evolution Within Giant-Planet Systems
A ubiquitous example of an external process within planet systems is the tide raised on a planet by an inner satellite, and the resulting transfer of angular momentum from the planet’s spin to the orbit of the moon (or vice versa in the case of retrograde or subsynchronous satellites such as Triton and Phobos). Such tidal torques are thought to have established the orbital architectures of the inner satellite systems of Jupiter, Saturn, and Uranus—including their numerous orbital resonances—as well as the current states of the Earth-Moon and Pluto-Charon systems. Tides raised by giant planets on their satellites, in concert with eccentricities driven by orbital resonances, are responsible for significant heating in Io and probably also in Europa and Enceladus. Although the theory of tidal evolution is well known, the precise nature and level of tidal energy dissipation within jovian planets (which in turn determines the timescale for tidal evolution) are much less certain: estimates range over many orders of magnitude for Jupiter.
Some important questions concerning tidal evolution within giant-planet systems are as follows:
• How far have the various satellites evolved outward from their sites of formation?
• To what extent do the observed eccentricities and inclinations of satellites reflect this evolution?
Future Directions for Investigations and Measurements
Advances in understanding of tidal influences on the Moon and Mars have come from the ability to track surface landers, either with laser ranging or Doppler tracking. Accurate measurements of satellite orbital evolution offer the only realistic avenue to measure the dissipation rates inside the giant planets—for example, by the accurate tracking of multiple spacecraft flybys (Cassini at Titan) or satellite orbiters (the proposed JEO).121,122 Recent work suggests that direct detection of orbital expansion for the inner jovian moons may be possible with spacecraft imagery spanning many decades—for example, from Voyager to JEO.123 The inner moons at Uranus and Neptune may offer similar opportunities for orbiters at these planets.
Elucidate Seasonal Change on Giant Planets
The seasonal variation of Earth’s atmosphere is well understood; the extent to which seasonal change impacts the atmospheres of the giant planets is a field of intense speculation. Observations at any one epoch cannot be interpreted properly if long-term variability is not understood. In the past decade, the ongoing interpretation of the Galileo and the Hubble Space Telescope data has provided constraints for dynamical models of Jupiter.124 Juno promises to supply additional constraints concerning the jovian water abundance and global distribution that were not obtained with the Galileo probe.
Saturn’s zonal flow exhibits detectable variation that may be seasonal in nature.125,126,127 We are also beginning to understand the effects of ring shadow on insolation and atmospheric response, an added complication for Saturn.128 Infrared imaging with Cassini’s VIMS instrument has revealed that under the overlying high cloud cover, the saturnian atmosphere is highly convective and latitudinally constrained. The extension of the Cassini mission to summer solstice in the northern hemisphere provides an opportunity for detailed observations of Saturn. Similar deep wind and composition information is needed for Saturn, however, which requires an atmospheric probe.
Understanding how seasonal changes are driven on ice giants as opposed to gas giants is necessary for a fuller understanding of weather and climate processes. With no flight missions to Uranus or Neptune since 1989, progress in understanding these processes has been challenging and is exacerbated by the extreme observational requirements presented by these distant cold bodies: high spatial resolution, moving target tracking, and (particularly in the molecular-rich infrared regime) high sensitivity.
During the more than 20 years since the last flyby of an ice giant, we have built databases with time lines long enough to begin to study seasonal change on the giant planets (the years on Saturn, Uranus, and Neptune are approximately 29, 84, and 165 terrestrial years, respectively). Both spatial resolution and sensitivity necessitate the use of the best (and therefore most difficult to acquire) telescopic resources: Hubble and Keck. No other facilities—for example, VLT and Gemini—have the capability to produce comparable high-resolution visible and near-infrared imaging on these objects with their current laser guide star adaptive optics capability; furthermore, Hubble’s lifetime is now limited. Using Hubble and Keck (supplemented by Gemini, VLT, and Subaru in the mid-infrared, the Very Large Array [VLA] at radio wavelengths, and lower-resolution observations from Lowell Observatory, the NASA IRTF, and other facilities), seasonal changes are beginning to emerge on Uranus and Neptune,129,130,131 and we are beginning to glean insight on the lifetime and behavior of large- and medium-scale atmospheric features.132,133,134 Yet some of the most basic physical properties of these ice giant planets remain unknown, and planetary missions are the only means of uncovering those properties.
One important question among many relating to seasonal change on giant planets is this:
• How do variations in insolation and temperature (i.e., heat balance) drive changes in dynamics and composition?
Future Directions for Investigations and Measurements
A systematic effort is desired that would deliver multiple entry probes to all four planets to determine the composition and cloud structure and winds as a function of depth and location on the planet. A capable orbiting spacecraft would deliver these probes and would also provide remote sensing of the cloud deck in infrared and visible light, as well as detailed gravitational measurements to constrain the interior structure. However, the cost of such an approach is prohibitive.135,136
More realistically, the next logical steps for significant progress in studies of giant planets are a Saturn atmospheric-entry probe and an orbiting mission with an entry probe to Uranus or Neptune. For Jupiter, a second shallow probe is unlikely to refine our understanding further, and the Juno mission (constraining water and possibly sensing deep convective perturbations on the gravitational field) will continue to return new Jupiter data.137,138,139,140,141
At the same time, research and analysis support will allow the interpretation of Cassini Saturn results and allow for ongoing studies of weather and climate on the ice giants. More precise infrared and visual heat-balance studies of all of these planets would better constrain their thermal histories. Some, but not all, of this work can be done with Earth-based facilities.
Evaluate Solar Wind and Magnetic-Field Interactions with Planets
For comparison with Earth, the giant planets are the only solar system examples of planets with strong magnetic fields interacting with the solar wind. The dimensions of most planetary magnetospheres are set by a competition between solar wind ram pressure and the energy density in the planet’s own magnetic field. Many of the observed phenomena in the outer regions of a magnetosphere are controlled in part by interactions with the solar wind. These phenomena include the spectacular auroral displays seen near the magnetic poles of Earth, Jupiter, and Saturn, which are fed by magnetospheric plasma. In the case of Earth, most of the magnetospheric plasma is actually derived from trapped solar wind, but at Jupiter and Saturn the main sources appear to be Io and either the rings or the icy satellites (especially Enceladus).
At Earth these interactions have important consequences for human civilization. Strong currents can flow through the ionosphere in response to solar-wind-induced storms in the magnetosphere, resulting in disruptions in both power distribution networks on the ground and satellite communications in space (including cellular phones
and the Global Positioning System). These storms generally occur when shock fronts in the solar wind arrive at Earth, often generated by coronal mass ejections, and the detailed forecasting of these events is the subject of the NSF-funded Center for Integrated Space Weather Modeling. Understanding the interactions at all planets aids our understanding of physical processes at Earth. Solar system objects furthermore provide our only opportunity to make in situ measurements of plasma processes. Such processes are important in all areas of astrophysics;142 solar system plasma observations thus play a role in shaping our ideas about hosts of other cosmic systems.
In addition to the global magnetospheres, space weathering is the collection of physical processes that erode and chemically modify surfaces directly exposed to the space environment, either the solar wind or a planetary magnetosphere. Understanding space weathering effects is critical to the correct interpretation of surface observations from remote sensing and in situ studies. Space weathering exposure results in a thin patina of material that covers and sometimes obscures the endogenic surface materials that are often the principal interest of remote-sensing observations. Space weathering also encompasses surface-removal processes such as sputtering by energetic particles, micrometeoroid erosion, and photon-stimulated desorption; a less-well-recognized (but potentially important) process is electron-stimulated desorption. These processes may be more important for planetary rings than they are for icy satellites, where they can result in relatively short lifetimes for dusty rings such as those at Jupiter and Uranus. The chemical products of space weathering could also affect subsurface processes on small satellites. Finally, irradiation can affect the electrostatic and magnetic environment of airless bodies through the buildup of static charge. The effect of the buildup of charged dust on the Mars rover solar panels is well known; whether or not such effects are important on the surfaces of small icy satellites or in ring systems is not known.
Some important questions concerning solar wind and magnetic field interactions with planets include the following:
• How do magnetospheres interact with the solar wind?
• How is surface material modified exogenically (e.g., by processes such as magnetospheric interactions and impacts) versus being pristine or relatively unmodified?
Future Directions for Investigations and Measurements
For all planetary magnetospheres, from Earth to the outer solar system, in situ measurements of local magnetic fields and plasma environments should be combined with remote observations of the global magnetosphere. Local measurements should include both fields and particles in order to determine clearly the local density, currents, and large-scale flows. Global measurements may be a combination of auroral imaging and spectroscopy (Earth-orbital ultraviolet and/or ground-based infrared measurements), observations of nonthermal radio emissions, and measurements of energetic neutral atoms. In situ measurements require missions to the planets. Remote observations can be provided from Earth or other vantage points—for example, a spacecraft en route to Neptune or Uranus could observe Jupiter and Saturn. In all cases, however, sufficient time coverage is essential for complete context.
Space weathering processes are interdisciplinary and “universal” in nature, requiring grounding in laboratory and theoretical studies, as well as simulation facilities.
Connections with Other Parts of the Solar System
Comparative planetary studies offer great potential to improve our understanding of planetary systems in general. Knowledge gained about any of the terrestrial planets helps us to understand the origin and evolution of Earth-like planets in general. In the same vein, missions to the giant planets will help us to understand the basic physical properties of gas- and ice-giant planets as a class. In terms of the origin and evolution of the giant-planet
systems, learning about the planets, their satellites, and even other regions of the outer solar system (Kuiper belt objects, comets, and so on) help us to understand how conditions began and evolved over the lifetime of the solar system. Several aspects of giant-planet science have connections to terrestrial planets, including but not limited to polar vortices, stratospheric oscillations, effects of planetary migration and volatile delivery, and the physics of large airbursts.
Connections with Heliophysics
The giant planets are the only solar system examples besides Earth of planets with strong internal magnetic fields interacting with the solar wind. Many of the observed phenomena in the outer regions of a magnetosphere—including the auroral displays seen near the magnetic poles of Earth, Jupiter, and Saturn—are controlled in part by interactions with the solar wind. In the case of Earth, most of the magnetospheric plasma is actually derived from trapped solar wind, but at Jupiter and Saturn the main sources appear to be, respectively, Io and either the rings or the icy satellites (especially Enceladus). At Earth, these interactions have important consequences for human civilization (e.g., disruption in power distribution networks and satellite communications). Understanding the interactions of the solar wind at all of the planets aids in understanding the physical processes at Earth.
Connections with Extrasolar Planets
The rapidly expanding fields of exoplanets and protoplanetary disks—fueled by data from space observatories as well as ground-based facilities—bring a wealth of new ideas regarding the processes that build and shape planetary systems. The majority of exoplanets discovered to date are giant planets, although the field is rapidly evolving as the Kepler, CoRoT, and other missions study hundreds of candidate objects.143 Current studies of the atmospheres and magnetospheres of the giant planets are increasingly performed with an eye toward the application to extrasolar giant planets. It is critically important to understand the basic physics of the giant planets in the solar system if we are to understand the more than 500 exoplanets that have been discovered around nearby stars, for which there is a small fraction of the data that we have about the local giants Jupiter, Saturn, Uranus, and Neptune (Figure 7.9).144,145
Giant-planet atmospheres exhibit both super- and subrotation with respect to their cores, but the driving mechanisms are not fully understood. If we cannot understand the origin and physics of atmospheric dynamics in the local giants’ atmospheres and clouds, our predictions for the circulation from dayside to nightside in an extrasolar giant that is phase-locked to its star will be of limited robustness. Understanding thermal balance and tidal effects is critical to understanding the evolution of extrasolar giant-planet atmospheres and orbits.
Remarkably, thermal profiles for many exoplanets have been measured, and hot stratospheres have been found to be ubiquitous. If we do not understand the energy sources of the hot stratospheres and coronal upper atmospheres of the local giant planets (and we do not), we will not be able to predict the conditions in the upper atmosphere of a jovian planet at an orbital distance less than that of Mercury from its star. This is critical for understanding the rapid escape into space of the atmospheres of extrasolar giants that approach too close to their host stars; such escape may set a limit to the minimum distance of a giant with a given mass (and hence gravity) from its host star.
The local giants all exhibit strong magnetic fields from dynamo action in their fluid interiors; the magnetic fields of Uranus and Neptune are offset and tilted in manners that have not been explained, and the physical origin and variability of their dynamos are not well understood. If we do not understand the basic principles of the local giants’ magnetic fields and plasma environments, we cannot predict the strength and orientation of the magnetic fields of extrasolar giants, which may be phase-locked with their host stars, nor can we know how they will interact with the expected strong stellar winds. This interaction is critical to the coupling of the star and planet and could potentially dominate mass loss from the planet as well as the rotational dynamics of the planet-star interaction.
Progress in studies of the giant planets must be made on multiple fronts in order to understand the numerous, intertwined processes operating inside these dynamic and complex systems. The specific examples discussed in this chapter are representative of just a subset of research and analysis efforts focused on giant planets. A single space mission lasts for a short time compared to the long orbital periods of the outer planets, and studying the processes with longer timescales requires research programs with long-term vision. Robust R&A programs, coupled with ground-based observations of giant planets and their attendant rings and moons, provide the foundation that links missions separated by decades.146
The challenges common to all giant-planet missions—large distances, long flight times, and stringent limitations on mass, power, and data rate—mean that all missions can benefit from technical advances in a number of broad areas. The breadth of technology needed for giant-planet exploration calls for an aggressive and focused technology development strategy. Specific technologies needed to enable future missions to the giant planets include power sources, thermal protection systems for atmospheric probes, aerocapture and/or nuclear electric propulsion, and robust deep-space communications capabilities.147,148
Low-mass and low-power electronics, as well as high-resolution and high-sensitivity instruments, are necessary in many applications including ground-based instrumentation. Support that is directed to instrument programs that contribute to these areas of development will be particularly beneficial.
Some of the most important advances in outer-planet research have come from access to facilities such as Gemini and the National Optical Astronomy Observatory (NOAO), as well as access to the Keck telescopes, through the NSF Telescope System Instrumentation Program (TSIP). The TSIP provides funding to develop new instruments to enhance the scientific capability of telescopes operated by private (non-federally-funded) observatories, in exchange for public access to those facilities. For example, much of the Uranus ring-plane crossing observational work was supported at Keck through NOAO/TSIP time.
Earth- and Space-Based Telescopes
The Hubble Space Telescope has been crucial for giant-planet research, especially high-resolution imaging of the ice giants. The study of auroral activity on the gas giants has been accomplished almost completely with Hubble’s ultraviolet capability. There is no ultraviolet-optical high-resolution alternative from the ground, and thus Hubble observations remain a high priority for giant-planet research through the mission’s remaining lifetime.149,150
The James Webb Space Telescope (JWST) is an infrared-optimized telescope to be placed at the second Sun-Earth Lagrangian point. Nonsidereal (moving target) tracking requirements have been identified and are currently being implemented. The JWST’s science working group is assessing the feasibility of observing Jupiter and Saturn, which may require restricting wavelengths or using subarrays; observations of Uranus and Neptune are planned, as are observations of their satellites and ring systems.151
The NASA 3-meter Infrared Telescope Facility is a major facility in giant-planet research: it provides support observations for spacecraft missions and produces original science data for research on a variety of giant-planet areas from the near infrared through the thermal infrared. The IRTF sponsors a visitor equipment program that provides unique capabilities and wavelength coverage outside the scope of the facility instruments, as well as training for new students in instrumentation.152
Telescopes of the larger-than-8-meter class are crucial for observations of giant planets. Large-aperture observations coupled with AO systems provide the only means of obtaining the spatial resolution needed for the detailed evolution of atmospheric features, for example. In particular, Keck 2’s AO system has been optimized for the extended planetary sources Uranus and Neptune, as well as Io, Titan, and Pluto. Laser-guide-star AO may someday allow other telescopes to rival Keck’s image quality. Since this is not a viable option to date, however, NASA time at Keck is critical for the proper planning of future space missions to these targets. The LSST, with its wide-field and synoptic capabilities, may provide observational constraints on objects in the vicinity of the giants, particularly for objects that may be on Jupiter impact trajectories.
With apertures of 30 meters and larger, future extremely large telescopes (ELTs) will play a significant role in outer planet research. A key advantage of ELTs is spatial resolution in the mid-infrared and longer wavelengths, which is mandated by the diffraction limit; even 8- to 10-meter telescopes have difficulty with the small angular sizes of Uranus and Neptune. Observations using a 30-meter telescope could resolve thermal emission from Neptune with resolution comparable to that obtained by the 3-meter IRTF for Saturn.
At the other end of the facility-size spectrum, small amateur telescopes play an increasing role in laying the groundwork for professionals. The 2009 and 2010 Jupiter impacts were discovered by amateurs, and within hours of each event, telescopes around the world were mobilized to follow up. Likewise, the monitoring of Uranus and Neptune for anomalous cloud activity is solidly within the amateur purview. NSF could play a role in supporting amateurs with a modest investment in, for example, equipment or filter sets; this would enhance the current synergy with the professional outer-planet community.153
The Stratospheric Observatory for Infrared Astronomy (SOFIA), a facility operated jointly by NASA and the German Aerospace Center (DLR), is a 2.5-meter telescope flying at 40,000 ft altitude. SOFIA will provide important infrared observations of the outer planets, observing all four giant planets across their full bolometric spectrum. SOFIA’s spectral coverage and resolution can discover and map many key molecules spatially and (by means of modeling of line profiles) vertically (Figures 7.10 and 7.11).
Significant planetary work can be done from balloon-based missions flying higher than 45,000 ft. This altitude provides access to electromagnetic radiation that would otherwise be absorbed by Earth’s atmosphere and permits high-spatial-resolution imaging unaffected by atmospheric turbulence. These facilities offer a combination of cost, flexibility, risk tolerance, and support for innovative solutions that is ideal for the pursuit of certain scientific opportunities, the development of new instrumentation, and infrastructure support. Given the rarity of giant-planet missions, these types of observing platforms (high-altitude telescopes on balloons and sounding rockets) can be used to fill an important data gap.154,155,156
The Very Long Baseline Array (VLBA) is able to determine spacecraft positions to high accuracy (which allows the refinement of planetary ephemerides). The VLBA has also assisted in tracking probe release and descent (Cassini’s Huygens spacecraft is an example).
In the microwave and submillimeter-wavelength regions, two ground-based facilities are of great importance to giant planets: the Atacama Large Millimeter Array (ALMA) and the Expanded VLA. The VLA expansion project will be completed this decade and, upon completion, it will produce high-fidelity, wide-band imaging of the planets across the microwave spectrum. With a full suite of X- and Ka-band receivers, the VLA also provides a backup downlink location to the Deep Space Network (Cassini has recently been successfully tracked with the VLA at Ka band). Mission studies performed for this decadal survey showed, for example, that the best downlink location for an ice-giant mission would be the Goldstone Deep Space Communication Complex; since the VLA is in the same footprint as Goldstone, it could provide a critical backup. The submillimeter array, ALMA, will also come online during the next decade, and it will provide unprecedented imagery of the giant planets in the relatively unexplored wavelength region from 0.3 to 3.6 millimeters (84 GHz to 950 GHz). ALMA will be an important tool for probing giant-planet atmospheres in altitude and latitude. For ice giants, ALMA will probe through the stratosphere into the troposphere and will have enough spatial sampling to get many resolution elements across each hemisphere.
Previously Recommended Missions
Cassini Extended Mission
The Cassini spacecraft has returned an unprecedented volume of data from the Saturn system. It completed its main mission in 2008, returning nearly 2 terabytes of data on the planet, magnetosphere, rings, and satellites. The mission has also completed its first extended mission, ending in mid-2010. During this time, many advances were made in our understanding of Saturn, including a new value for the most basic of quantities—its deep internal rotation rate. In addition, detailed observations showed the existence of a warm polar vortex, detailed 5-micron cloud structure, long-lived storms, and the presence of equatorial wind and temperature changes.
In the so-called Solstice mission, Cassini will continue its operations until a planned atmospheric entry in 2017. The value of this data set cannot be overestimated. The extended time base of observations is critical for understanding several aspects of Saturn’s atmosphere, including the much longer and larger seasonal variations (as compared with those of Jupiter), as well as its long-period equatorial oscillation. The Solstice mission results will provide many insights into the dynamics and circulation on this planet, as well as understanding of polar vortex formation, ring shadowing effects, and other atmospheric phenomena. It will also greatly extend the time baseline for the study of variable features in the rings, such as spokes, propellers, and noncircular ring edges, while permitting radio occultation probes of ring structure at many different incidence angles. In addition, the planned end-of-life scenario to place the craft into a Juno-like orbit (to constrain the internal mass distribution and higher-order magnetic-field components) adds an economic mini-mission that will allow comparison of the internal structures of Jupiter and Saturn.
Europa Geophysical Explorer
The Europa Geophysical Explorer recommended in the 2003 planetary decadal survey is now being studied in the context of a proposed joint NASA-ESA Europa Jupiter System Mission (EJSM). This cooperative venture combines a NASA-provided Jupiter Europa Orbiter with an ESA-provided Ganymede orbiter. There is an
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
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.
• 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 magnetosphere 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 significantly. 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.
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.
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.
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 conclude 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.
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 associated 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
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 primitive bodies community as well.
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 missions 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 missions 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 observations. 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.
1. National Research Council. 2003. New Frontiers in the Solar System: An Integrated Exploration Strategy. The National Academies Press, Washington, D.C., p. 93.
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. Astrophysical 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. Executive 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. Astrophysical 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 Astrophysics 528:A112.
10. W. Borucki for the Kepler Team. 2011. Characteristics of Kepler planetary candidates based on the first data set. Astrophysical 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, Executive 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 submitted 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.
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 submitted 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. Astrophysical 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.
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. Searching 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 submitted 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 Conference 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.
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 submitted 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 submitted 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 submitted 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.
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 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 temperatures. 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 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.
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 submitted 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. Astrophysical 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 submitted 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 submitted 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.
155. C.A. Hibbitts. 2009. Stratospheric Balloon Missions for Planetary Science. White paper submitted to the Planetary 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 submitted 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.