Time variations of Mars' gravitational field and seasonal changes in the masses of the polar ice caps

Abstract

Tracking of the Mars Global Surveyor spacecraft has been used to measure changes in the long-wavelength gravity field of Mars and to estimate the seasonal mass of carbon dioxide that is deposited in the polar regions each fall and winter and sublimed back into the atmosphere every spring and summer. Observations spanning 4 Mars years have been analyzed. A clear and well-defined seasonal signal, composed of annual and semiannual periods, is seen in the lowest odd degree 3 coefficient but with less confidence in the lowest even degree 2, which is expected to be smaller and is also much more difficult to observe. Direct estimation of the seasonal mass exchange employing a simple, seasonally varying model of the size and height of each cap provides values that indicate some systematic departures from the deposition predicted by a general circulation model. Estimates are also obtained for the precession and nutation of the pole of rotation of Mars, the degree 2 tidal Love number, k 2, and the mass of Phobos, the larger of Mars' two natural satellites.