The seasonal carbon dioxide (CO2) cycle on Mars results in a time-variable global redistribution of mass. These large-scale variations are associated with changes in the gravity field, mainly in the two zonal gravity coefficients $bar{C}_{20}$ and $bar{C}_{30}$, which have been recently evaluated from Doppler tracking data of the Mars Global Surveyor (MGS) spacecraft. In the present study, we calculated these variations from the mass redistribution obtained from outputs of two general circulation models (GCM) as well as from CO2 thickness measurements by the High Energy Neutron Detector (HEND) instrument on board the Mars Odyssey spacecraft and compared them to the observations. Tracking observations provide one of the most direct measures of the global-scale atmospheric mass cycle. However, the associated uncertainties are relatively large, partly because the low-degree zonals obtained from a single orbiter tracking analysis are contaminated by higher-degree harmonics which are shown to have nonnegligible seasonal variations. Thus we investigated possibilities to improve the determination of the time-variable gravity field by means of simulated geodesy experiments. Additional radio tracking of a second spacecraft with suitable orbital characteristics was shown to be able to separate the higher-degree geodetic signatures. Radio links between landers on the Martian surface and a near-polar orbiter can further better estimate especially the even zonals.