These maps of the Moon show the "Bouguer" gravity anomalies as measured by NASA's GRAIL mission. Bouguer gravity is what remains from the gravity field when the attraction of surface topography is removed, and therefore represents mass anomalies inside the Moon due to either variations in crustal thickness or crust or mantle density. Red areas have stronger gravity, while blue areas have weaker gravity. // Credit: NASA/JPL-Caltech/CSM