Modeling the dynamic component of the geoid and topography of Venus

Kurzfassung

We analyze the Venusian geoid and topography to determine the relative importance of isostatic, elastic and dynamic compensation mechanisms over different degree ranges. The geoid power spectrum plotted on a log-log scale shows a significant change in its slope at about degree 40, suggesting a transition from a predominantly dynamic compensation mechanism at lower degrees to an isostatic and/or elastic mechanism at higher degrees. We focus on the dynamic compensation in the lower-degree interval. We assume that (1) the flow is whole mantle in style, (2) the long-wavelength geoid and topography are of purely dynamic origin, and (3) the density structure of Venus' mantle can be approximated by a model in which the mass anomaly distribution does not vary with depth. Solving the inverse problem for viscosity within the framework of internal loading theory, we determine the families of viscosity models that are consistent with the observed geoid and topography between degrees 2 and 40. We find that a good fit to the data can be obtained not only for an isoviscous mantle without a pronounced lithosphere, as suggested in some previous studies, but also for models with a high-viscosity lithosphere and a gradual increase in viscosity with depth in the mantle. The overall viscosity increase across the mantle found for the latter group of models is only partially resolved, but profiles with a ∼100-km-thick lithosphere and a viscosity increasing with depth by a factor of 10–80, hence similar to viscosity profiles expected in the Earth's mantle, are among the best fitting models.