Figure 1: Long-wavelength geoid anomalies as measured by space-satellites for Earth (a), Venus (b) and Mars (c). (Figure adapted form M. Wieczorek, Treatise on Geophysics, vol. 10, Ch. 5, Elsevier, 2007).
Today the most effective method to measure the gravitational field of planet is to study the disturbances of a satellite’s orbit, which are caused by local gravity acceleration anomalies. The disturbances are then used to reconstruct the global gravitation usually in the form of geoid anomalies (or equipotential anomalies) referenced to a rotational ellipsoid (gravitational field of a rotating planet without mass anomalies). The precise knowledge of the gravity can then help us to better determine the topography relief which is the second important component in the gravimetrical inversion problems. After obtaining these data, i.e. gravity and topography field, together with estimates of the densities, one can make simplified but realistic lateral models of the crustal structure, the lithosphere thickness at the time of loading and in some cases even the mantle and core-mantle-boundary structure. In the case of the Moon, the analysis of the gravitational and topography field indicates a dichotomy of the crustal thickness with the far side of the Moon having a much thicker crust in comparison to the near side. Furthermore, areas of large mass concentrations - so called 'mascons' - in the centre of many big impact structures, have been identified. These mascons can be explained by an infill of basaltic lava which has a higher density than the primary crust. Like for the Moon, a crustal dichotomy has been observed for Mars from the analysis of its gravitational and topography field; the crust thickness is assumed to increase by about 30 km from the northern hemisphere into the southern hemisphere. A dominant feature in the Martian gravity and topography field is the Tharsis bulge; a huge volcanic construct close to the equatorial region. Gravity models suggest that the Tharsis bulge is not isostatically compensated but supported by a thick elastic lithosphere of more than 100 km. On Venus, the gravity is strongly correlated with the topography even at the largest scales. This correlation for the large scale features is quite unusual and a possible explanation is the support of the topography of Venus by its mantle convection together with the presence of a thin elastic lithosphere.