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

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) is much less certain: estimates range over many orders of
magnitude for Jupiter.
Important Questions
Some important questions concerning tidal evolution within giant planet systems are as follows:
• How far have the various satellites evolved outwards 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 our 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,
e.g., by the accurate tracking of multiple spacecraft flybys (Cassini at Titan) or satellite orbiters (the
proposed JEO). 79 Recent work suggests that direct detection of orbital expansion for the inner jovian
moons may be possible with spacecraft imagery spanning many decades, e.g., from Voyager to JEO. 80
The inner moons at Uranus and Neptune may offer similar opportunities for orbiters at these planets.
Elucidate Seasonal Change on Giant Planets
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 last decade,
ongoing interpretation of the Galileo and HST data has provided constraints for dynamical models of
Jupiter. 81 Juno promises to supply additional constraints concerning the jovian water abundance and
global distribution that was not obtained with the Galileo probe.
Saturn’s zonal flow exhibits detectable variation that may be seasonal in nature. 828384 We are also
beginning to understand the effects of ring shadow on insolation and atmospheric response, an added
complication for Saturn. 85 Infrared imaging with Cassini (VIMS) 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 them 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 >20 years since the last flyby of an ice giant, we have built databases with longenough
timelines to begin to study seasonal change on the giant planets (the years on Saturn, Uranus and
Neptune are ~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, e.g., VLT and Gemini, have the capability to produce comparable high-
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