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

Investigate the Chemistry of Giant Planet Atmospheres
To help connect the solar system’s giant planets to those around other stars and to appreciate the
constraints internal and atmospheric composition place on planetary interior and formation models, we
need to better understand the chemistry of the “local” giants Jupiter, Saturn, Uranus, and Neptune. Giant
planets by definition have a major mass component derived from the gaseous nebula that was present
during the planetary system’s first several million years, the same nebula from which Earth formed. This
major component, primarily hydrogen and hydrides plus helium and other noble gases, offers the
possibility of remote and in situ access to sensitive diagnostics of processes that governed the early
nebular phase of solar system evolution. At the same time this mass, and its chemistry, can be modified
by interactions with the environment and the host star.
More than 15 years ago, the Galileo 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) would be present in solar abundances, as all were expected to accrete
with hydrogen during the gravitational capture of nebular gases. The probe made a surprising discovery:
Ar, Kr, and Xe appear to be significantly more abundant in the jovian atmosphere than in the Sun, at
enhancements generally comparable to what was seen for chemically active volatiles such as N, C, and S.
Neon, in contrast, was depleted; recent studies have implicated helium-neon rain as an active mechanism
for Jupiter to explain the depletion of neon detected by the Galileo probe. 19
Various theories have attempted to explain the unexpected probe results for Ar, Kr, and Xe.
Their enhanced abundances require that these noble gases were separated from hydrogen in either the
solar nebula or Jupiter’s interior. One way this could be done would be by condensation onto nebular
grains and planetesimals at very low temperatures, probably no higher than 25 K. 20 Such a scenario
would seem to require that much or most of Jupiter’s core mass accreted from these very cold objects,
otherwise the less volatile N, C, and S would be significantly more abundant than Ar, Kr, and Xe. Other
pathways towards the enhancement of the heavy noble gases have also been postulated. The noble gases
could have been supplied to Jupiter and Saturn via clathrate hydrates. 21,22 An alternative theory 23 suggests
that jovian abundance ratios are due to the relatively late formation of the giant planets in a partially
evaporated disk. A completely different possibility is that Jupiter’s interior excludes the heavier noble
gases, sulfur, nitrogen, and carbon more or less equally, so that in a sense Jupiter would have an
outgassed atmosphere.
These theories each make specific, testable predictions for the abundances of the noble gases. The
only way to address noble gas abundances in giant planets is by in situ measurements (abundances of N,
C, and S can be measured remotely using optically active molecules such as NH3, CH4, and H2S). A
Saturn probe provides an excellent test of the competing possibilities. For instance, the clathrate hydrate
hypothesis 24 uses a solar nebula model to predict that Xe is enhanced on Saturn due to its condensation,
whereas Ar and Kr are not since they would need lower temperatures to condense. The cold condensate
hypothesis 25 , in contrast, predicts that Ar and Kr, as well as Xe, would be more than twice as abundant in
Saturn, based on evidence that carbon in Saturn is more than twice as abundant as it is in Jupiter.
Discrimination among various models will profoundly influence our understanding of solar nebular
evolution and planet formation.
Some Important Questions
Some important questions concerning the chemistry of giant planet atmospheres include the
following:
• How did the giant planet atmospheres form and evolve to their present state?
• What are the current pressure-temperature profiles for these planets?
• What is the atmospheric composition of the ice giants?
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