LITHOSPHERIC THICKNESS AND MANTLE LITHOSPHERE DENSITY CONTRAST BENEATH BETA-REGIO, VENUS

The spatial variation of the geoid/topography ratio over the large Ven usian volcanic highland Beta Regio is suggestive of thermal compensati on, i.e., support of the highland's topography by lithospheric thinnin g. Both the thickness of the lithosphere and the density contrast at i ts base can be inferred from a quadratic regression of suitably filter ed (600 km < wavelength < 4000 km) geoid vs. topography data. The regr ession yields a mean lithospheric thickness of 270 km and a density co ntrast of magnitude 2.5% to 3.0%. Simple isostatic balance of the long -wavelength topography at Beta Regio requires thinning of the lithosph ere by 50-60% beneath the rise.