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D. P. O'Brien, P. Geissler, R. Greenberg (Lunar and Planetary Laboratory, University of Arizona)
Several current lines of geological evidence suggest that Europa's ice shell is on the order of a few km thick [1,2]. This is much thinner than previous estimates of around 20 km [3], which depend sensitively on assumed parameters, namely Q, and were conservative in that their main point was to demonstrate the existence of liquid water. We have scaled the observed heat flux of Io to Europa's size, mass, and orbital parameters (a, e and n) and find a heat flux for Europa of around 0.14 W/m2 (a total heat production of around 4e12 W). This implies an equilibrium ice shell thickness of around 5 km.
Thin ice is consistent with the appearence of chaos regions as melt-through sites. For melt-through to occur, the ice must be raised to the melting point and its latent heat must be overcome: A Conamara sized melt would require all of Europa's tidal heat for over 10 years [4]. Using a detailed 2-dimensional axisymmetric model, we compute the heat flow through an ice shell heated from below in order to account for conduction of heat away from the melting region. We find that melting proceeds much more quickly than conduction can remove heat when even a small fraction of Europa's total heat is applied. If 1/4 of Europa's heat is concentrated locally (plausible if there is seafloor volcanism) a Conamara-sized melt occurs in 250 years---much shorter than the time required for ice to flow viscously back into the melted region [5]. This melting requires that the heat be concentrated at the base of the ice, which may be facilitated if the total water layer is thin [6] and/or seafloor topography reaches close to the base of the ice [6]. Moreover, we are considering models in which oceanic convection transports heat to localized sites below the ice.
[1] Greenberg et. al. Icarus 141:263-286 (1999). [2] Hoppa et. al. Icarus 141:287-298 (1999). [3] eg. Squyres et. al. Nature 301:225-226 (1983). [4] Collins et. al. JGR 105E1:1709-1716 (2000). [5] Stevenson, D. LPSC 2000 (abstract no. 1506). [6] R. C. Ghail. LPSC 1997 (abstract no. 1040).