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

• Why are Titan and Callisto apparently imperfectly differentiated whereas Ganymede
underwent complete differentiation?
• Why did Ganymede form an iron-rich core capable of sustaining a magnetic dynamo?
• What aspects of formation conditions governed the bulk composition and subsequent
evolution of Io and Europa?
• In what ways did the formation conditions of the saturnian satellites differ from the
conditions for the jovian satellites?
• Can we discern any evidence in the uranian satellites of a very different origin scenario (a
giant impact on Uranus for example) or is this satellite system also the outcome of a process analogous to
the other giant planet satellite origins?
• What features of Triton are indicative of its origin?
Future Directions for Investigations and Measurements
A key investigation to understanding the conditions during satellite formation is to establish the
thermodynamic conditions of satellite formation and evolution. This requires determination of the bulk
compositions and isotopic abundances. These results would directly constrain conditions of formation,
for example the radial temperature profile in the planetary accretion disk from which the regular satellites
formed. Two other crucial areas are to better constrain the internal mass distributions of many of the
satellites by measuring the static gravitational fields and topography and to probe the existence and nature
of internal oceans by measurements of tidal variations in gravity and topography and by measurements of
electromagnetic induction in the satellites at multiple frequencies. Internal oceans may date to a
satellite’s earliest history, as they can be difficult to re-melt tidally once frozen, and thus their presence
constrains formation scenarios.
What Determines the Abundance and Composition of Satellite Volatiles?
Volatiles on the outer planet satellites are mainly contained in ices, although volatiles can also be
retained in the rocky components (e.g., hydrated silicates on Europa or Io). Clathration (i.e., the
incorporation of gas molecules within a modified water-ice structure) is a likely process for retention of
many volatiles in satellite interiors, and it helps to explain the current composition of Titan. 6 Alternatives
like trapping of gases in amorphous ice have also been suggested.
The building blocks for the satellites may have originated from the solar nebula or formed in the
planetary subnebula. 7 In either case, the thermodynamic conditions and composition of the gas phase
determine the formation conditions of ices.
Huygens probe results and Cassini results have motivated a great deal of modeling of the
formation conditions for the Saturn system and Titan in particular. Planetesimal formation in the solar
nebula with only modest subnebula processing may be representative of the satellite formation process in
the Saturn system, 5 and clathration may have had an important role, presumably aided by collisions
between planetesimals to expose “fresh” ice. This may be different from the Galilean satellites where
extensive processing in the jovian subnebula may have occurred. However, the formation conditions of
the Galilean satellites are not well constrained at this time, due to the lack of measurements of volatiles
for these satellites including noble gases and their stable isotopes. The origin and evolution of methane
on Titan is receiving much attention, with some workers favoring ongoing outgassing from the interior to
balance the continual destruction over geologic time. The argon content of the atmosphere implies that
nitrogen arrived as ammonia instead of molecular nitrogen; yet how ammonia evolved into molecular
nitrogen is unknown.
Ground-based spectroscopy continues to expand our knowledge of volatile inventories on satellite
surfaces, for instance with the discovery of carbon dioxide ice on the uranian satellites. 8
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