Vision and Voyages for Planetary Science in the - Solar System ...

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/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 via probes.
In the future, directly imaged planets around young stars will provide a wealth of new data. Key
goals in studying exoplanets planets include: 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. Most missions to the giant planets would be capable of contributing to
answering these questions with the proper instrumentation. 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, identifies 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
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
7-5