Phase behavior and thermodynamic modeling of ices - implications for the geophysics of icy satellites. (Invited)

Ground-based observations and space missions to the outer Solar System (Voyager, Galileo, Cassini-Huygens) have evidenced recent geologic activity on many satellites of the giant planets. The diversity in surface expression of these icy moons’ activity is striking: from a scarred and young surface on Europa,1 with hydrated salts that may originate from a liquid layer buried at depth,2 to the South Polar plumes of Enceladus,3 where water ice particles are expelled along with a myriad of more complex molecules,4 to Titan, largest satellite of Saturn, with a dense atmosphere and a hydrocarbon cycle similar to the hydrological cycle on Earth.5 Large icy moons, i.e. with a radius greater than 500 km, share two particularities: a high content in water (on the order of a 30-70% bulk composition), and an interior segregated between a water-dominated mantle and a silicate-dominated core. The many forms water may have beneath the surface (ice polymorphs, liquid, hydrated compounds) bear a crucial role in the detected or alleged activity, and in the potential for astrobiological relevance. Indeed, any endogenic activity can only be approached through geophysical modelling of the internal structure and the thermal evolution. Current internal structure models for the icy moonse.g.,6 rely mainly on the contribution of each internal layer to the moment of inertia, generating non-unique solutions due to the large variability in density of H2O-bearing phases. Thermal evolution models,e.g.,7 can help constrain further the internal structure and geophysical activity, by starting with a given initial composition and state and investigating the thickening of icy layers through time. However, such models require both observational datasets and a precise description, as a function of pressure, temperature, and composition, of the thermophysical properties of the individual layers. Over the past century, experimental studies have provided a comprehensive view of the phase diagram of pure water, and of more complex chemical systems relevant to icy moons (sulfate salts, ammonia, volatiles). This presentation will review phases of interest, their physical properties, and their influence on the geophysical behavior of icy satellites. Then the focus will shift toward thermodynamic tools (equations of state, thermodynamic models), which can provide the inputs required by geophysical models. The specific case of the modeling of the water and water-ammonia phase diagrams will be presented,8,9 along with its planetary implications, and anticipated developments to address the chemical complexity of icy satellites. The author acknowledges support from a NASA Postdoctoral Program Fellowship, administered by Oak Ridge Associated Universities. This work has been conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. Government sponsorship acknowledged. References : 1Greeley et al., Icarus, 1998. 2McCord et al., J. Geophys. Res., 1999. 3Porco et al., Science, 2006. 4Waite et al., Nature, 2009. 5Toon et al., Icarus 1988. 6Sohl et al., Icarus 2002. 7Tobie et al., Icarus, 2005. 8Choukroun and Grasset, J. Chem. Phys, 2007. 9Choukroun and Grasset, J. Chem. Phys., in press.