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Abstract

The significant power in the Martian gravitational field due to the Tharsis rise may mask or modify gravitational signatures that contain important information on Martian geophysical processes. In order to isolate gravity signals in regions where the field is significantly affected by Tharsis as well to investigate characteristics of the global field, we present a numerical technique to "remove" Tharsis from the Martian gravitational field. Our analysis will show that to first order Tharsis can be represented as a sixth-degree spherical harmonic zonal gravitational feature in a reference frame in which the axis of symmetry runs through the center of the province. We produced the "Mars without Tharsis" (MWT) field by subtracting the gravitational signature of Tharsis from the full field and rotating the spherical harmonics back to the original coordinate system, in which the z-axis is coincident with the planetary rotation axis. Our study yields limits on the moment of inertia factor of 0.361 <C/MR2<0.366 for the range of allowable non-hydrostatic contributions to the flattening. The analysis indicates that the formation of Tharsis may have resulted in reorientation of the spin axis, though if the planet retained rotational fossil bulge due to a thick elastic lithosphere then changes in the global figure could have been inhibited. The zonal and tesseral components of the degree 2 field in the GMM-1 and rotated configurations indicates that Tharsis is-5 times more axisymmetric about its "pole" than Mars as a whole is about the rotation axis. With the removal of Tharsis' long wavelength gravitational power, regional structures become better defined in a spatial sense while maintaining their local dynamic range. The Olympus Mons volcanic shield displays clear evidence of a gravitational flexural moat. Our method for removing Tharsis may also be applied to analyze other global-scale geophysical features that may be considered axisymmetric to first order, such as mantle plumes, major volcanic constructs, or large impact basins. 1. IntrodUction The Martian gravitational field contains significantly more power at long wavelengths than does Earth's [Phillips and Lambeck, 1980; Balmino et al., 1982]. The difference reflects the fact that topographic excursions on Mars are greater than on Earth, because Mars is a smaller planet with a smaller mass, Topographic variations are characterized by large gravity and geoid anomalies, of which the most prominent planet-scale feature is the Tharsis bulge. Tharsis, which is conspicuous in the global Martian geoid in Figure 1, is the dominant locus of volcanism and tectonism on Mars and rises up to 10 km above its surroundings [Carr, 1981]. The widespread and diverse distribution of tectonic features and the high topography represent evidence that Mars has undergone complex thermal and stress histories which have been influenced on both the global and regional spatial scale by Tharsis [Banerdt et al., 1992]. The large amplitude, long wavelength gravity and geoid anomalies associated wi...