The Moon — Lunar Mascons: The Mystery of Concentrated Mass
The Moon — Lunar Mascons: The Mystery of Concentrated Mass
Discovery
Mascons — short for mass concentrations — are regions beneath the lunar surface where the gravitational field is stronger than the surrounding area, indicating localised concentrations of denser material. They were discovered in 1968 by analysis of the orbits of the Lunar Orbiter spacecraft.
As the Lunar Orbiters tracked the Moon, mission controllers noticed systematic deviations in the spacecraft's trajectories — deviations consistent with regions of higher gravitational pull beneath specific areas of the lunar surface. The deviations were too regular and too localised to be explained by topographic variations alone. Something beneath the surface was denser than the surrounding material.
The Distribution Pattern
Mascons are found predominantly beneath the large mare basins on the Moon's near side — the dark volcanic plains that give the Moon its characteristic face. The major maria (Mare Imbrium, Mare Serenitatis, Mare Crisium, Mare Nectaris, Mare Humorum) all have mascons beneath them.
This distribution is anomalous in itself: the mascons are not distributed randomly across the lunar surface but are concentrated in the mare basins, and those basins are concentrated on the near side of the Moon. The far side, with its thicker crust and almost no mare, has far fewer mascons.
What Mascons Are (Scientific Explanation)
The most widely accepted explanation for mascons combines several processes:
1. Dense mantle material uplift: Large impacts that created the mare basins excavated so much material that the lunar mantle below partially rebounded upward. This mantle material is denser than the crustal rock it replaced, creating the mass concentration.
2. Dense impact melt sheet: The enormous energy of the impacts that created the major basins melted a large volume of rock. This impact melt may have formed a dense sheet at the base of the excavated zone.
3. Volcanic filling: The basins were subsequently flooded with dense basaltic lava that erupted from the mantle. This volcanic rock is denser than the original crustal rock.
The Anomalous Dimension
The mascons create anomalies that remain imperfectly explained:
The gravity field is backwards: For a simple impact basin, you would expect lower gravity above the excavated zone (you removed material) and higher gravity at the rim (you deposited it as ejecta). Instead, mascons produce higher gravity directly above the basin floor — exactly where the most material was removed. The combination of mantle uplift, impact melt, and volcanic filling apparently overcompensates for the excavation, but the magnitude of the overcompensation in some mascons exceeds what models predict.
Orbits are unpredictable: Mascons create gravitational irregularities that make low lunar orbits inherently unstable. Spacecraft in low lunar orbit without active station-keeping will drift toward the surface over time due to mascon perturbations. NASA discovered this empirically — tracking stations noted that mascons could pull spacecraft into unplanned trajectories. This is one reason why establishing a permanent lunar orbit requires significant fuel expenditure for station-keeping.
The far side deficit: The concentration of mascons on the near side, beneath the maria that cover only the near side, is part of the broader near-side/far-side asymmetry that planetary scientists have not fully explained.
The Time Magazine Gravity Revelation
An anecdote in the hollow moon literature: a Time Magazine article inadvertently revealed, through a published distance to the Earth-Moon gravitational null point, a calculation that implied the Moon's surface gravity relative to Earth was anomalous — suggesting a higher gravity than the standard density calculation would predict for a solid body. This observation has been used by hollow moon theorists to argue that the Moon has a higher surface mass density than an internally uniform body of its bulk density would produce — consistent with a dense outer shell surrounding a less-dense or hollow interior.
