INTERNAL STRUCTURE OF IO AND THE GLOBAL DISTRIBUTION OF ITS TOPOGRAPHY

The distribution of equatorial long-wavelength topography on Io consists of alternating basins and swells, similar to the pattern expected if tidal heating in a thin asthenosphere controls the elevation of the surface through a conductive lithosphere in isostatic equilibrium. The equatorial basin-swell pattern is shifted in longitude with respect to theoretical tidal heating patterns, however, making the correlation between theoretical heat flow and elevation unclear. We calculate global topography for multilayer Io models with dissipation occurring in a viscous asthenosphere and a solid mantle. Two models are considered: a "thermal swell" model in which topography and heat flow are positively correlated and a "differentiated lithosphere" model in which topography and heat flow are negatively correlated. Both models match the observed pattern of alternating basins and swells along the equator and moderate polar topography if about two-thirds of the tidal heating occurs in the asthenosphere and about one-third occurs in the underlying solid mantle. The mean thicknesses of the model lithospheres are 35 km in the differentiated model and 150 km in the thermal swell model. Polar topography and the hypsometric distribution of elevations in the differentiated lithosphere model match the observations better than the thermal swell model. In a differentiated lithosphere model consistent with the topography, the asthenosphere is at least 50 km thick with viscosity about 100 MPa sec and is about 7% more dense than the lithosphere; the mantle density exceeds the asthenosphere density by at least 10%. The shift of the equatorial basin-swell pattern suggests that Io's lithosphere has recently undergone a zonal rotation of about 25°. © 1990.