The Mars seasonal CO2 cycle and the time variation of the gravity field: Ageneral circulation model simulation

We investigate the time variation of the long wavelength gravitational fiel d of Mars due to mass redistribution associated with the annual cycle of CO 2 exchange between the atmosphere and polar caps. Our analysis utilizes sim ulated monthly estimates of atmospheric pressure and CO2 polar frost as cal culated by the NASA Ames general circulation model (GCM) for a "typical" ye ar. We show that the deposition and sublimation of CO2 polar frost is expec ted to be the dominant effect on the gravity field at all low degrees. Vari ations in the amplitude and phase of time variations of gravity are sensiti ve to multiple factors, including polar elevation and the eccentricity of t he Martian orbit. Phase effects associated with additive and competing infl uences at the summer and winter poles dictate that odd-numbered harmonic co efficients produce an annual signal in the gravity field while even-numbere d coefficients show a semiannual signal. The predicted changes in the plane tary flattening and pear-shaped terms of the field are at or above the nois e level of expected X band Doppler tracking observations from Mars Global S urveyor and so could conceivably be detected from an orbital spacecraft.