The long-wavelength variations in the Martian and terrestrial potential and topography have been compared in an attempt to understand the origin of the Mars gravity field, and to estimate the average lithospheric thickness of Mars. The geopotential and topographic power spectra of Mars contain more absolute power in the low degree harmonics (l<7) than the corresponding terrestrial spectra and, unlike the earth, the two quantities correlate positively for low degrees. The two planets also differ from each other in that density anomalies in the Mars crust and lithosphere contribute more to the long wavelength gravity field than they do in the earth. The maximum shear stresses set up by a fully compensated Martian topography, including the Tharsis, plateau, are somewhat less than those occurring on earth but the stresses set up by the observed gravity, assuming that the gravity originates from density anomalies wholly within the lithosphere, appear to be excessive. Excluding the second and third degree harmonics, the part of gravity that correlates with topography and that is assumed to be a consequence of isostatically compensated density anomalies, results in stresses that are comparable to those found on earth if the thickness of the layer is of the order 100--200 km. The low degree harmonics l=2, 3 in the gravity and topography describe essentially the Tharsis complex and they require a layer of about 400 km thickness with a finite strength of about 500 bars, if this plateau is supported statically. If these values are excessive, the Tharsis region requires an explanation that differs from the rest of the planet. Partial isostatic compensation of this region is improbable as the stresses become excessive. Without further information, the contributions to gravity density anomalies in the lithosphere and mantle cannot be separated but it does appear that the Martian lithosphere is much thicker than the earth's and that its mantle may be relatively less heterogeneous than the terrestrial mantle.