Most of the moons of the outer planets, Jupiter, Saturn, Uranus, and
Neptune, lack a rocky crust but are surfaced by hard, rigid material: ices
of various kinds that behave like rocks. These moons are important objects
for comparative planetology. They offer insights on how the Earth and
other planet size bodies have evolved and what processes may occur on and
beneath their surfaces. Early in 2005 an unprecedented encounter with an
icy moon will happen. The Huygens probe, carried on board the Cassini
orbiter for eight years, will descend into Titan, Saturn's largest moon,
and perform an in-depth study of its clouds, atmosphere, and surface. The
Voyager-1 mission determined its mass and radius (within 2 km) and
established that its ice-to-rock mass ratio should be ~ 48:52. Models of
its present structure are uncertain (see Fig. 3); whether it is a
differentiated body or not it depends on whether radiogenic and
accretional heat were sufficient to allow differentiation into a rocky
core and an icy mantle. Observation of its surface "geology" by the
Huygens probe will likely answer this question. It has a rich organic
atmosphere and it is anticipated that the icy material includes
water-methane-clathrates. We plan to further advance our studies of
H2O-ice, address ice-disordered phases, and phase boundaries between
stable and metastable phases. This is a starting point for understanding
the internal structural of Titan and other bodies, since the depth and
thickness of their layers depend on these thermodynamic phase boundaries,
which are unknown in most cases. (Wentzcovitch, Baroni, Truhlar, and
Price)
