• Mass concentrations detected below the surface of the Moon by their effect on the orbits of the Lunar Orbiter spacecraft. A dozen Mascons have been found underlying the nearside Maria, and they seem to indicate a high density for Maria basalt. They provide data about the physical state of the Moon’s interior – if the interior were molten they would sink instead of being at the surface. Anomalous regions of high gravity have also been detected by Mars orbiters.