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

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, 27 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 while 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 our 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. 28
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. 29 The efficiency and thus viability of this mechanism
hinges 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 1994, 30 and the subsequent impacts in 2009 31
and 2010. 32 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. 33 Ground and space-based observations of the aftermath of such impacts
provide data on the pyrolytic products created in the impact event.
Important Questions
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 (energy dissipation) 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.
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
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