A Pull Towards the Unexpected
The Moon is the most gravitationally "lumpy" major body in the Solar System. Its largest mass concentrations, or "mascons," can alter the local gravity enough to make a plumb bob hang a third of a degree off vertical. These invisible regions of high gravity were first discovered in 1968 by Paul M. Muller and William L. Sjogren at NASA's Jet Propulsion Laboratory. They noticed that the pre-Apollo Lunar Orbiter spacecraft would dip and accelerate as they passed over certain areas, revealing an unexpectedly uneven gravitational field. This phenomenon posed a significant navigational hazard; early orbital calculations were off by factors of 100, and it was quickly understood that uncorrected low lunar orbits are unstable, leading to spacecraft crashing into the surface within months or even weeks. The subsatellite released by Apollo 16, for instance, was expected to orbit for a year and a half but crashed after only 35 days. This gravitational "lumpiness" had to be accounted for to ensure the precise landings of subsequent Apollo missions, starting with Apollo 12.
The mascons are primarily located under the large, circular impact basins on the Moon's near side, such as Mare Imbrium, Mare Serenitatis, and Mare Crisium. These basins are themselves topographic depressions, where a negative gravity anomaly would be expected. The presence of a strong positive anomaly indicates a significant excess of mass lurking beneath the surface. The largest mascon, under Mare Imbrium, has an anomalous mass of about 10^21 grams. This is equivalent to an additional layer of basalt rock 3 kilometers thick. These dense regions are powerful enough to increase the local force of gravity by half a percent.
Forging a Mascon
The origin of these mass concentrations was a subject of debate for decades. Data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, which mapped the Moon's gravity in unprecedented detail from 2011 to 2012, provided a much clearer picture. The prevailing theory, confirmed by GRAIL data, is that mascons are a direct result of massive asteroid or comet impacts billions of years ago, during a period when the Moon's interior was much hotter.
The process begins with a colossal impact that excavates a huge basin. The shock of the impact causes the dense, iron-rich mantle material beneath the lighter crust to rebound upwards. Simultaneously, the surrounding crust collapses inward. Over millions of years, the area is flooded with dense volcanic basalt lava, which fills the basin to form the dark maria we see today. The combination of the uplifted mantle plug and the thick layers of dense lava creates the powerful positive gravity anomaly. The structure often resembles a bull's-eye, with a central positive anomaly, a surrounding ring of negative gravity, and another outer ring of positive gravity caused by ejected material. Understanding this process on the Moon shows how major impacts modified the crusts of other rocky planets like Mars and Mercury, where mascons have also been identified.