Three-dimensional simulations of mantle convection in Io

Io has very high surface heat flow and an abundance of volcanic activity, w hich are thought to be driven by nonuniform tidal heating in its interior. This nonuniform heat is transported to the base of the lithosphere by very vigorous convection in Io's silicate mantle, the form of which is presumabl y responsible for the distribution of surface features such as volcanoes an d mountains. We here present three-dimensional spherical calculations of ma ntle convection in Io, in order to ascertain the likely form of this convec tion and the resulting distribution of heat flow at the surface and core-ma ntle boundary. Different models of tidal dissipation are considered: the en dmember scenarios identified by M. N. Ross and G. Schubert (1985, Icarus 64 , 391-400) of dissipation in the entire mantle, or dissipation in a thin (s imilar to 100-km-thick) asthenosphere, as well as the "preferred" distribut ion of M, N. Ross et al. (1990, Icarus 85, 309-325) comprising 1/3 mantle a nd 2/3 asthenosphere heating. The thermal structure of Io's mantle and asth enosphere is found to be strongly dependent on tidal heating mode, as well as whether the mantle-asthenosphere boundary is permeable or impermeable. R esults indicate a large-scale flow pattern dominated by the distribution of tidal heating, with superimposed small-scale asthenospheric instabilities that become more pronounced with increasing Rayleigh number. These small-sc ale instabilities spread out the surface heat flux, resulting in smaller he at flux variations with increasing Rayleigh number. Scaled to Io's Rayleigh number of O(10(12)), variations of order a few percent are expected. This small but significant variation in surface heat flux may be compatible with the observed distributions of volcanic centers and mountains, which appear fairly uniform at first sight but display a discernible distribution when suitably processed. The observed distribution of volcanic centers is simila r to the asthenosphere heating distribution, implying that most of the tida l heating in Io occurs in an asthenosphere. (C) 2000 Academic Press.