We investigate the time variation of the long wavelength gravitational field of Mars due to mass redistribution associated with the annual cycle of CO2 exchange between the atmosphere and polar caps. Our analysis utilizes simulated monthly estimates of atmospheric pressure and CO2 polar frost as calculated by the NASA Ames general circulation model (GCM) for a typical year. We show that the deposition and sublimation of CO2 polar frost is expected to be the dominant effect on the gravity field at all low degrees. Variations in the amplitude and phase of time variations of gravity are sensitive to multiple factors, including polar elevation and the eccentricity of the Martian orbit. Phase effects associated with additive and competing influences at the summer and winter poles dictate that odd-numbered harmonic coefficients produce an annual signal in the gravity field while even-numbered coefficients show a semiannual signal. The predicted changes in the planetary flattening and pear-shaped terms of the field are at or above the noise level of expected X band Doppler tracking observations from Mars Global Surveyor and so could conceivably be detected from an orbital spacecraft. ¿ 1999 American Geophysical Union