Gravitational Surprises
The Moon is the most gravitationally "lumpy" major body in the Solar System. Orbiting spacecraft experience unexpected tugs that pull them off course or dip their altitude. These regions of stronger gravity are called mass concentrations, or "mascons". The effect is significant; the largest mascons can increase the local force of gravity by one-half percent.
The mascons were discovered in 1968 by Paul M. Muller and William L. Sjogren at NASA's Jet Propulsion Laboratory. They analyzed the Doppler tracking data from the Lunar Orbiter spacecraft and noticed consistent anomalies. This discovery was a practical concern for Apollo mission planners, who had to account for the gravitational lumpiness to ensure precise landings. Before the mascons were properly mapped, they caused low-orbiting satellites to become unstable and crash into the lunar surface in a matter of months, or even weeks. The Apollo 16 subsatellite, for instance, was expected to orbit for a year and a half but crashed after only 35 days.
Mascons are located under the large, circular basins—the dark "seas" or maria—that are visible from Earth. The coordinates for this point of interest are in the Mare Imbrium (the "Sea of Showers"), which contains the largest mascon on the Moon. This basin is a vast lava plain over 700 miles (1,145 km) wide, formed by a massive impact billions of years ago.
A Persisting Mystery
The origin of mascons remained a puzzle for decades. Initially, some thought the basins were simply filled with dense lava, but the observed lava flows were often too thin to explain the gravitational excess. Some mascon basins show no signs of volcanic activity at all.
Data from NASA's GRAIL (Gravity Recovery and Interior Laboratory) mission, which flew twin spacecraft named Ebb and Flow from 2011-2012, gave a much clearer picture. The mission mapped the Moon's gravity in unprecedented detail, revealing that mascons have a distinct bull's-eye pattern: a center of positive gravity, surrounded by a ring of negative gravity, and then another ring of positive gravity.
Current models suggest this pattern results from ancient, high-energy impacts. An asteroid collision excavated a massive crater, causing the Moon's crust to rebound and pulling up denser material from the mantle below. The heat from the impact melted surrounding rock, which became more concentrated and dense as it cooled. This combination of mantle uplift and dense, solidified melt rock creates the central mascon. The surrounding rings are artifacts of the crust collapsing, cooling, and settling after the initial shock.
The fact that these heavy masses haven't sunk back into the Moon's interior shows that the lunar lithosphere—its rigid outer layer—is thick and strong enough to support them. This shows the Moon's thermal history, suggesting it was much hotter and had a thinner crust when these impacts occurred billions of years ago.