MARTIAN GLOBAL SCALE CO2 EXCHANGE FROM ORBITAL TRACK- ING DATA

CO2 in the Martian atmosphere condensates and sublimates on seasonal time scales, resulting in large mass exchanges between the atmosphere and the surface. As the mass redistribution is on global-scale, it mainly affects the long wavelength components of the gravity field. Chao and Rubincam(1990) showed that the timevariations of the gravity field could be large enough to have a measurable effect on the orbit of a spacecraft. There are several other possibilities to estimate the global scale mass exchanges over Mars (see, Zuber (2003). Detection of temporal changes in the long wavelength of Martian gravitational field provide one of the most direct methods as it does not need physical models related to the surface/atmosphere interactions or to the subsurface modelling. It also offers the advantage of continuous data for many years as tracking data will be available for any Mars orbiter. The low-degree zonal coefficients of the Martian time-variable gravity were derived from the tracking data of Mars Global Surveyor (MGS) spacecraft by Smith et al. (2001), Yoder et. al (2003) and most recently by Balmino et al. (2005). The perturbation of the MGS orbit due to the time-variable gravity field is at the edge of detectibility and the reported coefficients contain the influence of higher degree zonals since they were obtained from the tracking data of a single spacecraft. Nevertheless, Smith et al. (2001) calculated successfully the seasonal mass variations of polar caps by combining the ∆C20 solution with Mars Orbiter Laser Altimetry (MOLA). Orbital analysis of MGS allowed Yoder et. al (2003) to estimate the solid Love number k2 of Mars as well as to test the compatibility of several ice-cap models with the deduced time-variable gravity solution. Aharonson et al. (2004) calculated the density of seasonal polar deposits by combining Gamma Ray Spectroscopy (GRS) data of Mars Odyssey with the MOLA data as well as with the time-variable gravity data of MGS. Karatekin et al. (2005) investigated the influence of higher degree zonals on the reported time-variable gravity coefficients and compared ∆C20 and ∆C30 observations with those calculated from the mass redistributions given by GCM and the CO2 thickness measurements of High Energy Neutron Detector (HEND) onboard Mars Odyssey (Litvak et al. 2004). The use the tracking observations alone to obtain direct measure of the global scale mass redistribution has been challenging, mainly due to the low signal-tonoise ratio of the time-variable gravity signal (Smith and Zuber 2003, Karatekin et al. 2003). The objective of the present study is to determine the seasonal mass exchanges between the polar caps and atmosphere, as well as the global atmospheric pressure variations directly from the reported time-variable gravity field coefficients, ∆C20 and ∆C30.