Seasonal Cycles and the Gravity Field of Mars

M. T. Zuber (MIT), D. E. Smith (NASA/GSFC)

Motions within the atmosphere of Mars cause small but measurable effects on the gravity field and planetary dynamics. To estimate the magnitude of expected effects in order to assess their detectability, we have used the NASA Ames General Circulation Model (GCM) in a simulation of a typical Mars year. Mean atmospheric conditions were computed for twenty 33-day months from the GCM, which also utilized a recent geopotential topography model that has improved long wavelength control and fits the Viking lander site elevations better than previous models. From the GCM we determined the pressure field, precipitation and wind fields over the surface of Mars, and then calculated their effect on the low degree terms of the gravity field, the length of the Martian day, and the position of the pole of rotation. Approximately 25% of the CO tex2html_wrap_inline11 in the Mars atmosphere moves from one pole to the other as the seasons change; some of it condenses out as ice forming an additional mass layer on the surface, thus increasing the mass at the winter pole at the expense of mass at the equator and summer pole. This mass re-distribution changes the gravitational (dynamic) flattening ( tex2html_wrap_inline13 ) and the "pear shape" ( tex2html_wrap_inline15 ) terms in the gravity field of Mars. Our preliminary analysis of the magnitudes of these effects indicates precipitation of CO tex2html_wrap_inline11 at the winter pole is the single most important effect on the gravity field, while winds have the most important effect on the rotation. The polar cap-atmosphere mass exchange may possibly be detectable from Mars Global Surveyor X-band tracking observations.