VENUSIAN CRUSTAL AND LITHOSPHERIC PROPERTIES FROM NONLINEAR REGRESSIONS OF HIGHLAND GEOID AND TOPOGRAPHY

Geoid and topography observations are used to constrain the density st ructure of the venusian lithosphere under the assumptions that convect ive stresses are small and local isostasy prevails. Results are presen ted for 13 venusian highlands, including Ishtar Terra, In general, Ven us has a thick (200-400 km) thermal lithosphere which is thinned benea th volcanic highlands by as much as 80%, inducing slopes of 0.05 to 0. 18 in the base of the lithosphere. From scalings of geoid/topography r atio and lithospheric basal slope as a function of internal Rayleigh n umber and viscosity contrast, we estimate lower bounds to these parame ters of 10(7) and 10(5), respectively. Several lines of evidence point to values near these lower limits, which puts Venus in the sluggish-l id regime of convection, There also exist several accumulations of cru stal material, most notably Ovda Regio, where there may be as much as 90 km of crust. (C) 1997 Academic Press.