Martian Gravity Field Mean Model and its Time Variations From Mars Global Surveyor and Odyssey Data

The MGS and Odyssey missions have provided a wealth of orbit tracking data which are used for modeling the Martian gravity field and its time variations which are due to seasonal mass transfers between the planet surface and its atmosphere. We collected most observations covering the entire MGS orbital phase (1998-2006) and the Odyssey mapping mission between 2002 and mid 2007. Doppler and range measurements over those 4.5 Martian years have been processed in arcs of a few days duration each, using improved standards and background models. A static spherical harmonic model complete to degree and order 95 has been computed, together with the k2 Love number and time variations of the second and third degrees lumped zonal coefficients at ten day interval. The solution is compared to previously derived models by means of statistical and orbital quality tests and correlations with topography. The new solution is more robust than the ones derived in 2005 and 2006 by the authors, mostly due to the addition of Odyssey data and also the longer data span. The value of k2 is stable with respect to various parameterization and regularization strategies and supports the hypothesis of a fluid core of Mars.