VENUS CENTER OF FIGURE - CENTER-OF-MASS OFFSET

Magellan altimetry data reveal that the center of figure (CF) of Venus is displaced approximately 280 m from its center of mass (CM) toward 4.4 degrees S, 135.8 degrees E, a location in Aphrodite Terra. This of fset is smaller than those of other terrestrial planets but larger tha n the estimated error, which is no more than a few tens of meters. We examine the possibility that the CF-CM offset is related to specific g eologic provinces on Venus by deriving three simple models for the off set: a thick-crust model, a hotspot model, and a thick-lithosphere mod el. The offset caused by a region of thick crust depends upon the regi on's extent, the crust-mantle density contrast, and the thickness of e xcess crust. A hotspot-related offset depends on the extent of the the rmally anomalous region and the magnitude of the thermal anomaly. Offs et due to a region of thick lithosphere depends upon the extent of the region, the average temperature contrast across the lithosphere, and the amount of excess lithosphere. We apply the three models to Venus p lateau-shaped highlands, volcanic rises, and lowlands, respectively, i n an attempt to match the observed CF-CM offset location and magnitude . The influence of most volcanic rises and of Ishtar Terra on the CF-C M offset must be quite small if we are to explain the direction of the observed offset. The lack of influence of volcanic rises can be expla ined if the related thermal anomalies are limited to a few hundred deg rees or less and are plume-shaped (i.e., characterized by a flattened sublithospheric ''head'' with a narrow, cylindrical feeder ''tail''). The unimportance of Ishtar Terra is most easily explained if it lies a top a significant mantle downwelling. (C) 1994 Academic Press, Inc.