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G. Mitri (Dept. of Planetary Sciences, LPL, U. Arizona and International Research School of Planetary Sciences, University G. d'Annunzio, Pescara, Italy), A. Showman (Dept. of Planetary Sciences, LPL, U. Arizona)
We investigate the response of conductive and convective ice shells on Europa to variations of heat flux and interior tidal-heating rate. We present numerical simulations of convection in Europa’s ice shell with Newtonian, temperature-dependent viscosity and tidal heating. Modest variations in the heat flux supplied to the base of a convective ice shell, can cause large variations of the ice-shell thickness. In contrast, for a conductive ice shell, large variations in the heat flux involves relatively small variations of the ice-shell thickness. We demonstrate that, for a fluid with temperature-dependent viscosity, the heat flux undergoes a finite-amplitude jump at the critical Rayleigh number. This jump implies that, for a range of heat fluxes relevant to Europa, two equilibrium states — corresponding to a thin, conductive shell and a thick, convective shell — exist for a given heat flux. We show that, as a result, modest variations in heat flux near the critical Rayleigh number can force the ice shell to switch between the thin, conductive and thick, convective configurations over a 10 Myr interval, with thickness changes of up to 10-30 km. Depending on the orbital and thermal history, such switches might occur repeatedly. However, existing evolution models based on parameterized-convection schemes have to date not allowed these transitions to occur. Rapid thickening of the ice shell would cause radial expansion of Europa, which could produce extensional tectonic features such as fractures or bands. Furthermore, based on interpretations for how features such as chaos and ridges are formed, several authors have suggested that Europa’s ice shell has recently undergone changes in thickness. Our model provides a mechanism for such changes to occur.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.