The detailed gravity field of Mars has been obtained principally from the tracking data of Mariner-9, Viking-1, and Viking-2 [ Rosborough and Lemoine, 1991]. Gravity field models developed from this data in the early 1980s were complete only to degree 18 or less in spherical harmonics. In preparation for the Mars Observer mission, Smith et al. [1993] performed a thorough reanalysis of these same data and developed a gravity model of 50th degree (Goddard Mars Model 1---GMM-1) using the a priori constraint technique discussed earlier. The superior spatial resolution of this model has resulted in an improved understanding of the geophysics of Mars as well as improved orbit determination accuracies for satellites orbiting Mars, an important element in the measurement of Martian topography using satellite altimeters. Little improvement is likely until tracking data are obtained from NASA's planned Mars Global Surveyor (MGS) mission. It is unfortunate that the Mars Observer mission failed to achieve orbit, as its low circular orbit would have resulted in a dramatic improvement in the knowledge of the Martian gravity field.
Gravity models for the Martian moons have largely been obtained
from imaging data assuming a homogenous composition. Thomas
[1993] and Rubincam et al. [1994] have successfully used
this technique to compute geopotential terms for Phobos and Deimos.
This technique has also been used to determine the gravity parameters
of asteroids [ Thomas et al., 1994].