Mars Long Wavelength Gravity Field Time Variations: A New Solution from MGS Tracking Data.

Several modern solutions of the Mars gravity field have already been obtained from the Mars Gobal Surveyor (MGS) mission, by different NASA teams working at GSFC and at JPL, which also have shown that degree two and three lumped zonal coefficients exhibit time variations related to the seasonal cycle of carbon dioxide exchange between the planet surface and its atmosphere. A new solution of these time variations has been obtained by a third team working in Europe with a totally independent software. Five years of one and two way Doppler, and range tracking data collected by the Deep Space Network have been processed in three day arcs, taking into account all disturbing forces of gravitational and non gravitational origins; for each arc the state vector, drag and solar pressure model multiplying factors, and angular momentum dump parameters are adjusted. The zonal harmonics up to degree five and the k2 Love number are solved for. The zonals are estimated every ten days, or every thirty days in some variants, with a priori uncertainties either on their values or on their changes. The lumped C20 and C30 coefficients show similar patterns as in the anterior US solutions, also in accordance with the variations estimated from the output of a Global Circulation Model and from the HEND instrument on board Mars Odyssey. Annual and semiannual terms have been derived for comparisons and future evaluation in terms of global constraints put on such planetary mass transfer. Finally the values found for k2 (in the range 0.10 - 0.15 depending on the solution strategy) are discussed. [ABSTRACT FROM AUTHOR]