Gravity Field of Venus: A Preliminary Analysis

The line-of-sight gravity field for Venus has been mapped by tracking the Pioneer Venus spacecraft in the vicinity of periapsis for a 45 ° swath of longitude eastward of 294 °. There are consistent and systematic variations in the gravity signature from orbit to orbit, attesting to the reality of observed anomalies. Orbit 93 passes over a large positive topographic feature, the "northern plateau," for which there is no corresponding gravity signature. If this region has no isostatic compensation, the gravity signal would exceed the noise level by a factor of 7. The results of simulation modeling indicate that the northern plateau must be compensated at depths of about 100 kilometers or less. The long-wavelength anomalies seen in the Venus gravity data have been Fourier-decomposed along the orbital tracks and compared to analogous spectra for Earth. The gross power in the two mean spectra is approximately, the same, but systematic variations among the harmonics suggest differences in dynamic processes or lithospheric behavior, or both, for the two planets.