VENUSIAN HIGHLANDS - GEOID TO TOPOGRAPHY RATIOS AND THEIR IMPLICATIONS

Using Pioneer Venus line-of-sight gravity data and orbit simulation procedures, we have estimated apparent depths of isostatic compensation (ADCs) for twelve Venusian highland features: Asteria, Atla, Bell, Beta, Ovda, Phoebe, Tellus, Thetis and Ulfrun Regiones, and Nokomis, Gula and Sappho Montes. ADCs range from 50 km to 270 km; half of the values are less than 100 km. Using these ADCs, we estimate geoid to topography ratios (GTRs) for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots, swells and plateaus. The geoid is estimated in the wavenumber domain from the isostatic formula, using the topography and ADC for each region. In the space domain, the GTR is equal to the least squares slope of the linear fit of the geoid to the topography. The resulting GTR range is 7-31 m/km, which is much higher than terrestrial oceanic values (-1 to 5 m/km). The features fall into two distinct groups, one with a GTR range of 7-13 m/km, and one with a range of 19-25 m/km. The exception is Beta Regio, which has a GTR of 31 m/km. A model for thermal thinning of a 100 km thick lithosphere fits all values in the lower GTR group to within one standard deviation. Airy compensation could also be present, but cannot fully compensate these features. Partial dynamic compensation of the lower GTR group in combination with lithospheric mechanisms is also possible, but not required to fit the data. The upper GTR range, 19-25 m/km, can be fit with an upper mantle, constant viscosity convection model. The large GTR values are inconsistent with the presence of a low viscosity zone. If more than one compensation mechanism is present in the regions in the higher GTR group, the GTRs will be underestimated in terms of a dynamic interpretation. We thus fit the convection models to the upper end of the GTR range, 25 m/km. Rayleigh numbers in the range 10(4)-10(6) will produce a GTR of 25 m/km when combined with conductive lid thicknesses of 85-150 km. The 6 m/km range in both of the GTR groups is probably due to varying degrees of crustal and thermal compensation, combined with dynamic compensation in the upper GTR group. The difference between terrestrial and Venusian GTR ranges can be explained largely by the lack of a low viscosity zone on Venus.