Icy Satellites

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)

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