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

Future Directions for Investigations and Measurements
Inside the solar system, one of our two gas giant planets does not fit within the simple
homogeneous picture of planetary cooling, and neither of our ice giants is well understood. Given the
abundance of extra-solar ice giants, the internal structure and atmospheric composition of Uranus and
Neptune are of particular interest for exoplanet science. For Uranus and Neptune, however, our
understanding is very limited of 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 N and O in the deep interior are
unknown. 14
The best approach to truly understand 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. 15
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 top priority of
the Giant Planets Panel in the last decadal survey. Juno will constrain H2O and possibly sense deep
convective perturbations of the gravitational field. The Jupiter Europa Orbiter (JEO), NASA’s
contribution to the proposed NASA-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 will yield only
limited information to supplement Juno’s determination of gravitational moments and the nature of the
inner magnetic field. Jupiter is our 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, for which far less is known. 16
A Saturn probe coupled with Cassini data (remote sensing and gravitational information from its
final phase) can test the He differentiation hypothesis through measurement of the He 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. 17
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 our understanding of
ice giant properties and processes, yielding significant insight into their evolutionary history.
Throughout the next decade, research and analysis 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 excruciatingly long time between spacecraft
visits: this necessitates regular observations from state-of-the-art Earth-based facilities to provide longterm
context for the short-duration spacecraft encounters. 18
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