Vision and Voyages for Planetary Science in the Decade 2013-2022 (2011) / Chapter Skim
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7 The Giant Planets: Local Laboratories and Ground Truth for Planets Beyond
7 The Giant Planets: Local Laboratories and Ground Truth for Planets Beyond
Pages 175-216
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From page 175... ...
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.
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From page 176... ...
major element in understanding the question, How have the myriad chemical and physical processes that shaped the solar system operated, interacted, and evolved over time? SCIENCE GOALS FOR THE STUDY OF GIANT PLANETS Giant planets dominated the history of planetary evolution: the processes of their formation and migration sculpted the nascent solar system into the habitable environment of today.
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From page 177... ...
• 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)
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From page 178... ...
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.
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From page 179... ...
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.
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From page 180... ...
Important Questions Some important questions associated with the objective of understanding heat flow and radiation balance in giant planets include the following: • What is the energy budget and heat balance of the ice giants, and what role do water and moist convection play? • What fraction of incident sunlight do Uranus and Neptune absorb, and how much thermal energy do they emit?
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From page 181... ...
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)
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From page 182... ...
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
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From page 183... ...
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)
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From page 184... ...
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. Important Questions Some important questions concerning probing the interiors of giant planets with planetary precession include the following: • What are the pole precession rates for giant planets?
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From page 185... ...
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.
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From page 186... ...
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.
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From page 187... ...
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)
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From page 188... ...
. Important Questions Some important questions concerning ring systems as laboratories for planetary formation processes include the following:
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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 u nprecedented spatial resolution and temporal coverage.
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From page 190... ...
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.
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From page 191... ...
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.
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From page 192... ...
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)
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From page 193... ...
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.
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From page 194... ...
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.
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THE GIANT PLANETS: LOCAL LABORATORIES AND GROUND TRUTH FOR PLANETS BEYOND 195 FIGURE 7.7 Turbulent phenomena are a common feature of planetary atmospheres. These images of Jupiter, Saturn, and Earth (from Galileo, Cassini, and Landsat 7, respectively)
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. 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
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From page 197... ...
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)
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From page 198... ...
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 internal 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.
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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.
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From page 200... ...
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.
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From page 201... ...
Many objects, termed inflated giants, lie much higher than those lines, indicating that we do not yet understand giant-planet evolution under severe stellar insolation. The solar system's gas giants Jupiter and Saturn have been studied in some detail; its ice giants Uranus and Neptune are less well understood.
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From page 202... ...
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.
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From page 203... ...
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.
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, Earth's atmosphere transmission is reduced to zero throughout much of the infrared regime; many windows in the near infrared are blocked as well. Middle: The airborne Stratospheric Observatory for Infrared Astronomy (SOFIA)
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From page 205... ...
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)
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From page 206... ...
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.
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From page 207... ...
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.
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From page 208... ...
When a Saturn Probe mission is combined with a Uranus Orbiter and Probe mission, the understanding of planetary formation will be greatly advanced in the next decade. Discovery Missions Missions to the outer solar system are expensive and risky, and therefore rare.
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From page 209... ...
Summary To achieve the primary goals of the scientific study of giant-planet systems as outlined in this chapter, the following objectives will have to be addressed. • Flagship missions -- As discussed in this chapter and in the 2003 decadal survey, a comprehensive mission to study one of the ice giants offers enormous potential for new discoveries.
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White paper submitted to the Planetary Science Decadal Survey, . National Research Council, Washington, D.C.
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White paper submitted to . the Planetary Science Decadal Survey, National Research Council, Washington, D.C.
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Rings Research in the Next Decade. White paper submitted to the Planetary Science Decadal Survey, .
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White paper submitted to the . Planetary Science Decadal Survey, National Research Council, Washington, D.C.
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White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.
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White paper submitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.
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White paper sub mitted to the Planetary Science Decadal Survey, National Research Council, Washington, D.C.
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