Thermal convection in ice I shells of Titan and Enceladus

Details Thermal convection in ice I shells of Titan and Enceladus Thermal convection in ice I shells of Titan and Enceladus

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p12a..07m&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #P12A-07

Physics

6280 Saturnian Satellites, 6281 Titan

Scientific paper

Domes and flow-like features present on Titan are the surface manifestation of internal processes in the ice. And Enceladus exhibits a diversity of tectonized terrains. It has been suggested that refreezing of a liquid sublayer in the interior of Enceladus might cause its tectonic structures (Squyres et al., 1983), although Nimmo and Pappalardo (2006) proposed that the tectonism at the active south pole may result from an ice upwelling. Modest internal heat production variations can force an ice shell to switch between a conductive and convective state (Mitri and Showman, 2005; McKinnon, 2006). In the presence of an internal ocean, a conductive-convective transition can produce radial expansion of a cooling ice shell. Because these conductive-convective transitions cause a discontinuous jump in the heat flux delivered to the surface (Mitri and Showman 2005), the satellite expansion can occur much more quickly than it would in the absence of such a transition. The rapidity of these switches implies that stress buildup, hence extensive fractures, could occur. We explore this hypothesis for Titan and Enceladus. We present results of numerical simulations of convection in the ice I shells, with Newtonian rheology and temperature-dependent viscosity. We show that the thermal convection can occur under a range of conditions in the ice shells of Titan and Enceladus. Because of the dependence of Rayleigh number Ra on d3/η where d is the thickness of the ice shell and η is the viscosity at the base of the ice shell, and because the ammonia in the liquid layer strongly depresses the melting temperature, Ra is equal to its critical value for two thicknesses: for relatively thin ice shell with warm, low-viscosity base (Onset I) and for thick ice shell with cold, high-viscosity base (Onset II). Assuming that Enceladus has an internal ocean, we demonstrate that Onset I leads to sufficient thickening to produce tectonic stress of ~500 bars and fractures of several tens of km depth. During the cooling of Titan, the global expansion caused by the thickening ice I layer is, in general, counterbalanced by the global contraction caused by the thickening high-pressure ice layer.

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