Physics
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p12c..08s&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P12C-08
Physics
5430 Interiors (8147), 5455 Origin And Evolution, 6207 Comparative Planetology, 6218 Jovian Satellites
Scientific paper
Equilibrium models of heat transfer by heat conduction and thermal convection show that the three satellites of Jupiter, Europa, Ganymede and Callisto, 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. 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 and Callisto 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. Ammonia produces one of the largest reductions of the melting temperature. If we assume a bulk concentration of 5 weight-% ammonia we find that all the satellites have substantial oceans. For a model of Europa heated only by radiogenic decay, the ice shell will be a few tens of kilometers thinner than in the ice I case. The underlying rock mantle will limit the depth of the ocean to 80 - 100 km. For Ganymede and Callisto, the ice I shell on top of the H2O-NH3 ocean will be around 60-80 km thick and the oceans may be 200 to 350 km deep. 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. The present parameterization introduces a conductive stagnant lid at the expense of the thickness of the convecting sub-layer, if the latter exists at all. The stagnant lid causes the temperature in the sub-layer to be warmer than in a comparable constant viscosity convecting layer. We have further modified the parameterization to account for the strong increase in homologous temperature, and therefore decrease in viscosity, with depth along an adiabat. This modification causes even thicker stagnant lids and further elevated temperatures in the well-mixed sub-layer. It is the stagnant lid and the comparatively large temperature in the sub-layer that frustrates ocean freezing.
Schubert Gerald
Spohn Tilman
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