W0209

Planetary Ices. David J. Stevenson, Caltech 150-21, Pasadena, CA 91125.

Water is the most abundant condensate in the universe and the most common constituent of many bodies in the outer solar system. There are other cryogenic condensables of interest, notably NH₃, CH₄, CO, CO₂, and N₂. An understanding of the physical and chemical properties of these ices is needed to interpret the nature of these bodies as we see them. There are three important aspects: (i) Thermochemistry and phase equilibria (melting, sublimation): We need to understand which constituents are likely, whether they can condense as planetary bodies form, and their melting curves (including multicomponent systems). Recent evidence for oceans in the satellites Europa, Ganymede and Callisto will be discussed and understood in light of expected phase diagrams, especially the unusual (negative) dependence of H₂O melting point on pressure. (ii) Equation of state including solid-solid phase transitions. In order to interpret the expected composition deep within a body such as Ganymede or Titan, we need to know which phases are present. An example of recent interest is the possible presence of high pressure modifications of methane clathrate, which may influence the outgassing ("volcanism") and hydrocarbon "aquifer" of Titan. (iii) Rheological properties of ice. We need to know how ice flows and fractures. Ice viscosity is a central parameter in estimating internal thermal structure since it relates temperature to heat flow. Moreover, the interpretation of surface features depends on knowing ice deformation properties. This is the least well understood aspect. Examples of morphologies exhibited in Galileo images will be discussed.