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

• Is there evidence for life on the satellites?
Subsequent sections examine each of these objectives in turn, identifying key questions to be
addressed and future investigations and measurements that could provide answers.
Where Are Subsurface Bodies of Liquid Water Located, and What Are Their Characteristics?
A fundamental requirement for habitability is the presence of liquid water. Several of the larger
satellites are thought to possess at least some liquid water in their interiors. 9 In the coming decade, two
key objectives will be to further characterize the known subsurface oceans, and to determine whether
other bodies also possess such oceans.
One of the key results of the Galileo mission was the use of Jupiter’s tilted magnetic field to
detect subsurface oceans via magnetic induction on Europa, Ganymede and Callisto. 40 However, neither
the thickness nor the conductivity (and thus composition) of these oceans can be uniquely determined
with the current observations.
The plumes on Enceladus include salt-rich grains, for which the most likely source is a salty
subsurface body of liquid. 41 A global ocean which permits greater tidal flexing and heating of the ice
shell is also suggested by the observed surface heat flux; however, a regional “sea” beneath the South
Pole is also possible.
Because of Titan’s size and the likely presence of ammonia, a subsurface ocean is plausible 42 and
expected to be a long-lived feature.
Important Questions
Some important questions concerning the location and characteristics of subsurface bodies of
liquid water include the following:
• What are the depths below the surface, thickness and conductivities of the subsurface oceans
of the Galilean satellites? The depth of the ocean beneath the surface is important because it controls the
rate of heat loss from the ocean, and the probability of material exchange with the surface. The thickness
indicates the likely ocean lifetime, and for Ganymede and Callisto constrains the ocean temperature.
• Which satellites elsewhere in the solar system possess long-lived subsurface bodies of liquid
water? Titan and Enceladus are obvious candidates, but other mid-sized icy satellites, including those of
Uranus and Neptune, could in theory have retained internal oceans to the present day. 43 Triton in
particular, with its geologically young surface and current geysering, is another interesting candidate.
• For all satellites, what is the lifetime of potential oceans? Ocean lifetime is a key to
habitability. If Enceladus is only intermittently active, for instance, as suggested by several lines of
evidence, and thus only intermittently supports liquid water, it is less attractive as a potential habitat. 44
Future Directions for Investigations and Measurements
Important investigations and techniques for exploring subsurface liquid water include further
characterization of the Galilean satellite oceans with satellite orbiters that can measure the induction
response at both Jupiter’s spin frequency and the satellite’s orbital frequency. With two frequencies, both
the ocean depth and conductivity (which constrains composition) can be solved for independently. 45 For
Saturn’s satellites, the negligible tilt of Saturn’s magnetic field precludes induction studies by flybys, but
studies may be possible from satellite orbit by exploiting the satellite’s orbital eccentricity. A flyby
detection of an ocean would be possible at Triton or the uranian satellites. Measurement of tidal flexing,
for example at Europa, can provide strong constraints on the thickness of the overlying ice shell and the
presence of an ocean. Geodetic studies of the rotation states of these bodies might provide additional
constraints on ocean characteristics. Other important investigations and measurements for probing
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