SO₂-Rich Equatorial Basins and Epeirogeny of Io

The most concentrated deposits of SO₂ frost on Io occur within a series of large equatorial basins. About 30% of the surface is covered by SO₂ outside of the basins, increasing to more than 50% within the basins. This pattern is poorly expressed in the region from longitude 240° to 360° where bright areas are frequently buried by the fallout from the large Pele-type plumes. The fourfold pattern of alternating basins and swells in Io's equatorial region is similar to the heat-flow pattern predicted from tidal heating in a thin, partially molten asthenosphere. However, the topographic pattern is offset from the predicted heat-flow pattern; thus it is unclear whether topographic highs correspond to regions of higher or lower predicted heat flow. These two possibilities imply two very different models for Io's highlands: a thermal-uplift model or a continental-crust model. In the thermal-uplift model, the regions of enhanced asthenospheric heating cause lithospheric thinning and isostatic uplift, perhaps accompanied by uplift due to penetrative magmatism or basaltic underplating. In the continental-crust model, "continents" of differentiated crust float on low-density roots, the crust and lithosphere are approximately one and the same, and basal melting controls its thickness. Although both models are plausible, the thermal-uplift model best explains the SO₂ distribution. Cold trapping must be important for concentrating SO₂ frost in optically thick patches; thus either cold traps are preferentially initiated over large basin areas or they are preferentially removed from the highlands. The patchy distribution and approximately 30% SO₂ coverage of the highlands show that cold traps are abundant here, but not extensive; thus the SO₂ must be preferentially removed and/or buried. Higher heat flows in the highlands should lead to increased volatilization of SO₂ frost, and a greater frequency of relatively SO₂-poor volcanism should tend to bury frost patches. This model links asthenospheric tidal heating, large-scale heat flow and topography, volcanic activity, and the global distribution of surface SO₂, and it leads to several specific predictions for future observations.