Three-Dimensional Simulations of Mantle Convection in Io

Io has very high surface heat flow and an abundance of volcanic activity, which 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 presumably responsible for the distribution of surface features such as volcanoes and mountains. We here present three-dimensional spherical calculations of mantle convection in Io, in order to ascertain the likely form of this convection and the resulting distribution of heat flow at the surface and core-mantle boundary. Different models of tidal dissipation are considered: the endmember scenarios (identified by M. N. Ross and G. Schubert) of dissipation in the entire mantle, or dissipation in a thin (approximately 100-km-thick) asthenosphere, as well as the 'preferred' distribution of M. N. Ross et al. comprising 1/3 mantle and 2/3 asthenosphere heating. The thermal structure of Io's mantle and asthenosphere is found to be strongly dependent on tidal heating mode, as well as whether the mantle-asthenosphere boundary is permeable or impermeable. Results 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-scale instabilities spread out the surface heat flux, resulting in smaller heat flux variations with increasing Rayleigh number. Scaled to Io's Rayleigh number of O(10(exp 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 similar to the asthenosphere heating distribution, implying that most of the tidal heating in Io occurs in an asthenosphere.