Venus gravity - A harmonic analysis and geophysical implications

An improved model of Venusian global gravity has been obtained by fitting a tenth degree and order spherical harmonic series to 78 orbital arcs of Doppler tracking data from Pioneer Venus Orbiter. Maps of the free-air anomaly and its formal error are presented, as are “geoid” and isostatic anomaly maps. The gravitational variance spectrum of Venus has a form very similar to that observed previously on the earth, moon, and Mars. In contrast to the earth, Venus exhibits a significant positive correlation between topography and gravity. The gravity/topography admittance function is consistent with a global average effective depth of isostatic compensation of 200 ± 40 km. The second-degree harmonics determine the orientation of the principal inertial axes and thereby reveal two unusual features. First, the axis of greatest inertia departs from the rotation axis by 3°. However, the extremely slow rotation of Venus makes the resulting large amplitude wobble energetically comparable to the 0.3 arc sec Chandler wobble of the earth. Second, the orientation of the axis of least inertia relative to the sub-earth point at times of particularly close inferior conjunctions is strongly suggestive of a spin-orbit resonance with earth, albeit not the resonance that has usually been considered in the past.