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W. B. Moore, G. Schubert (Dept. of Earth and Space Sciences, UCLA)
Ever since the remarkable prediction of Io volcanism by Peale, Cassen and Reynolds (1979) and the Voyager flybys, the presence of volcanic activity has indicated that things inside Io are quite different than expected for a body only slightly larger than the Moon. Volcanism on Io is somewhat concentrated at the equator, with minor longitudinal variation, although there is considerable temporal variation. This distribution supports a model in which tidal heating is limited to the upper few hundred kilometers of the mantle instead of a model with heating distributed throughout the mantle which would predict maximum heating at the poles. The degree of melting in Io's mantle depends on the balance between melt generation and melt transport, which are intimately tied to the processes of heat generation and transport. We have modeled the equilibrium established between the generation of heat and melt by tidal dissipation in Io and the transport of heat by convection and melt by porous flow. Tidal heating and heat transport are coupled through the temperature dependence of viscosity. A critical parameter turns out to be the temperature (or melt fraction) that maximizes tidal dissipation in the mantle, since the equilibrium temperature in a partially molten zone will not exceed this by more than a few tens of degrees. The equilibrium temperatures may be as high as the 1700~K temperatures inferred for lavas at the surface. The effects of melt transport through the lithosphere are also examined. Lithospheric thicknesses are likely limited to less than 50 km due to the loss of heat into the lithosphere by rising magma. Portions of this project were funded by NSF LExEn and NASA PG&G.