The systematic and large deviation of the gravitational equipotential surface (EPS) of Mars in terms of a spheroid nearly in isostatic equilibrium with an extra mass in the Tharsis region. The displacement from Mars and the shape of the spheroid are calculated by using this description and a Mars gravity model. The EPS is represented as a contour map of its height above the spheroid. This representation provides the first clear demonstration that the Hellas depression coincides with a depression in the EPS. The disequilibrium contribution of Tharsis to the coefficient J2 of the second-degree harmonics of gravitational potential of Mars is estimated to be ΔJ2= (126¿5) ¿106. Thus if the rigidity supporting Tharsis could be relaxed, the resulting body would have J2= (1829¿12) ¿106 and a polar moment of inertia O= (3654¿10) ¿10 4MR2. The optical flattening and dynamic flattening calculated with this model are in substantially better agreement than are those calculated in the usual way.