Mass concentration
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A mass concentration or mascon is a region of a planet or moon's crust that contains a large amount of material that is denser than average for that body. The result is a minute but measurable increase in the local gravity.
Mascons can have a variety of causes. On Earth, they frequently indicate processess occurring in the upper mantle such as plumes of magma. On the Moon, most mascons are thought to be remnants of asteroids that impacted the surface. The Moon's mascons are sufficiently variable that low uncorrected orbits around the Moon are unstable on a timescale of months or years, being distorted by the varying gravity until the satellite impacts the surface.