Topography (top) and surface gravity (bottom) for Mare Serenitatis. The gravity shows a plateau and thus a substantial near-surface contribution to the mascon. Also apparent is the smaller mascon Mare Vaporum in the lower left and center part of the figure. Data from Lunar Prospector.
A mascon is a region on the surface of the Moon where the gravitational pull is slightly higher than normal due to the presence of dense rock. 'Mascon' is short for 'mass concentration.' Mascons were first detected by spacecraft orbiting the Moon and are associated with basalt-filled impact basins such as Mare Imbrium.