Fachbereich 14 - Geowissenschaften
Institut für Planetologie
Planetenphysik

Oceans in Icy Satellites

Equilibrium models of heat transfer by heat conduction and thermal convection show that Europa, Ganymede, Callisto and Titan may have internal oceans underneath ice shells tens of kilometers to more than a hundred kilometers thick. A wide range of rheology and heat transfer parameter values and present day heat production rates have been considered. The rheology was cast in terms of a reference viscosity calculated at the melting temperature and the rate of change of viscosity with inverse homologous temperature. The temperature dependence of the thermal conductivity of ice I has been taken into account by calculating the average conductivity along the temperature profile. Heating rates are based on a chondritic radiogenic heating rate of 4.5 pW kg^-1, but have been varied around this value over a wide range. The phase diagrams of H₂O (ice I) and H₂O + 5 weight-% NH₃-ice have been considered. The ice I models are worst-case scenarios for the existence of a subsurface liquid water ocean because ice I has the highest possible melting temperature and the highest thermal conductivity of candidate ices and the assumption of equilibrium ignores the contribution to ice shell heating from deep interior cooling. In the context of ice I models, we find that Europa is the satellite most likely to have a subsurface liquid ocean. Even with radiogenic heating alone the ocean is tens of kilometers thick in the nominal model. If tidal heating is invoked, the ocean will be much thicker and the ice shell will be a few tens of kilometers thick. Ganymede, Callisto, and Titan have frozen their oceans in the nominal ice I models, but since these models represent the worst-case scenario, it is conceivable that these satellites also have oceans at the present time. The most important factor working against the existence of subsurface oceans is contamination of the outer ice shell by rock. Rock increases the density and the pressure gradient and shifts the triple point of ice I to shallower depths where the temperature is likely to be lower then the triple point temperature. According to present knowledge of ice phase diagrams, ammonia produces one of the largest reductions of the melting temperature. If we assume a bulk Previous models have suggested that efficient convection in the ice will freeze any existing ocean. The present conclusions are different mainly because they are based on a parameterization of convective heat transport in fluids with strongly temperature-dependent viscosity rather than a parameterization derived from constant viscosity convection models.

Projektdauer:

2001 - 2002

Drittmittelgeber:

Deutsche Forschungsgesellschaft

Beteiligter Wissenschaftler:

Prof. Dr. Tilman Spohn

Veröffentlichungen:

Spohn, T. and G. Schubert 2003: Oceans in the icy Galilean satellites of Jupiter?, Icarus, 161, 456-467.