Gravity Drilling of Mass Differentiated Planets

Introduction: The internal structures of terrestrial planets are commonly constrained by seismic data and geochemical considerations. We suggest that these constraints may be augmented by gravity drilling that focuses on interpreting satellite altitude free-air gravity observations for boundary undulations of the internal density layers related to mass flow. This approach involves separating the free-air gravity anomalies into terrain-correlated and -decorrelated components based on the correlation spectrum between the anomalies and the gravity effects of the terrain [1]. The terrain decorrelated gravity anomalies are largely devoid of the long wavelength interfering effects of the terrain gravity and hence provide enhanced constraints for modeling mass variations of the mantle and core. For the Earth, subcrustal interpretations of the terrain-decorrelated anomalies are constrained by radially stratified densities inferred from seismic observations [e.g., 2, 3]. For other differentiated bodies like the Moon, Mars and Venus, seismic soundings are largely lacking so that geochemical and moments-of-inertia considerations must be invoked to constrain the radial density structures in interpreting the terrain-decorrelated freeair anomalies. These anomalies, with frequencies that clearly decrease as the density constrasts deepen, facilitate mapping mass flow patterns related to the thermodynamic state and evolution of the interior.