Improvement Of The Determination Of The Seasonal Variations Of Mars Gravity Field Using Both MGS And Mars Express Tracking Data

Mars is known to show a seasonal cycle due to condensation/sublimation of atmospheric carbon dioxide at polar caps. These processes deal with about one third of the atmospheric mass and provide detectable signatures on the time-varying first zonal components of the gravity field. The Mars Global Surveyor (MGS) spacecraft has allowed detecting these signatures through the determination of the fine perturbations of its orbit. However, the precision on such a detection is not enough to provide additional constraints to the Global Circulation Model (GCM) of Mars atmosphere. This lack of precision is partly due to the fact that the tracking of a single orbiter cannot permit to separate properly each even and each odd zonal components of the gravity field. One way to separate these contributions is to use radio-tracking of two orbiters with different orbital parameters, especially different inclination or eccentricity. Such a situation is actually occurring with the Mars Express (MEX) mission, which orbit eccentricity is quite different from the MGS one (0.6 instead of 0.01). The MaRS experiment provides MEX radio-tracking data that we propose to use, along with the MGS ones, in order to improve the determination of seasonal variations of zonal components. We present preliminary results based on the first months of MEX radio-tracking. We use the GINS software, primary developed at the CNES (French space agency), for precise orbit determination.