The first American satellites to orbit the Moon, in the late 60s, ran into an unexpected problem: their orbits drifted in a systematic way as they passed over certain regions. The cause was concentrations of mass below the surface, named mascons (mass concentrations).
Mascons are regions where lunar gravity is stronger than average. The difference isn't dramatic (a few fractions of a percent), but it's enough to shift satellite trajectories and require continuous correction. Without understanding mascons, low-orbit missions would decay unpredictably.
Most mascons line up with lunar maria, the dark plains formed by ancient lava. Mare Imbrium, Mare Serenitatis, and Mare Crisium are among the most pronounced.
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