Friday 26 August 2005

G8
1000-1130 hours

489
Mars long wavelength gravity field time variations : a new solution from MGS tracking data
Balmino, Georges¹, Duron, Julien², Marty, Jean-Charles¹
1 CNES, Toulouse, France
2 Observatoire Royal De Belgique
Author email: Georges.Balmino@cnes.fr
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 carbone 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 J² and J3 coefficients show similar patterns as in the anterior US solutions, also in accordance with the variations estimated from the outputs of various GCMs or from the HEND instrument on board Mars Odyssey. Annual, semiannual and triannual terms have been derived for comparisons and further evaluation in terms of global constraints put on such planetary mass transfer. Finally the value found for k2 (in the range 0.13-0.15 depending on the solution strategy) is discussed.

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