Large‐scale topography of Io: Implications for internal structure and heat transfer

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Abstract
New stereographic techniques were applied to Voyager 1 images to determine the shape and large‐scale topography of Io. The best‐fit triaxial figure has semiaxes of 1830.0, 1818.7 and 1815.3 kilometers with random errors of about 200 meters. Systematic navigation and optical system errors are comparable. The resulting figure is consistent with a differentiated satellite in hydrostatic equilibrium. The deviation from homogeneity could be explained by the presence of an iron core with a radius of about 800 km. Most elevations over areas larger than ≈500 km diameter deviate from the triaxial figure by no more than ±1 km; we suggest that many of these broad topographic swells and basins may be due to isostatic responses to thermal changes in the lithosphere ‐ asthenosphere system. In the simplest picture, increasing the heat flow converts the basal lithosphere into lower‐density asthenosphere, resulting in isostatic uplift. Lithospheric thicknesses ranging from 5 to 100 km may result in elevation variations of up to 1 km. If the large‐scale topography is positively correlated with heat flow, then the global pattern of basins and swells suggests tidal dissipation in a molten or partially molten asthenosphere rather than in the mantle. Areas higher than 1 km may consist of low‐density crustal materials. Media Regio, the largest such region, is centered near the equator at longitude 70°.