Oceans and Internal Structures of the Large Icy Satellites

We predict water ice freezing and constrain geothermal gradients in the interiors of the large icy satellites Ganymede, Callisto, and Titan using thermodynamic and phase-boundary data for MgSO4 and ammonia solutions, including new results for ammonia. Accounting for available measurements of gravitational moments of inertia leads to estimates of the depths to silicate boundaries. In the case of Ganymede we also compute the size of an iron-bearing core. The new equations of state allow us to assess the influence of ocean salinity on the thickness of layers of ice I-III-V-VI in the interiors of these objects, and on associated ocean dynamics. Ocean compositions with salt and ammonia have less high-pressure ice, and can exist in the presence of ice III. In some model oceans high-pressure ice phases become buoyant relative to surrounding fluids, implying frazil-like upward snows, interlayered liquids and ices, and fluids in direct contact with rock. We discuss the relative roles of dissolved constituents in the large icy satellites, the consequences for their habitability, and prospects of future missions for testing these predictions. Schematic of interior structure for Ganymede showing dense fluids under high pressure ices and directly in contact with rock. We use available thermodynamic properties for relevant fluids and solids materials us to calculate self-consistent depths for the various layers (approximate values shown here).