Definition
Early bombardment of the Moon by planetesimals, from geocentric and heliocentric orbits, is the possible cause of the lunar crustal thickness asymmetries. The Earth’s presence also causes the Moon to be bombarded nonuniformly. A very early, yet intense bombardment like this could have redistributed lunar crustal materials, thinning certain lunar regions. This period is suggested to be >3.9 Gyr ago.
Possible Bombardment Chronology
In the early solar system, if low-eccentricity heliocentric planetesimals were abundant during this time, local concentrations of impacts would have occurred on the western part of the Moon (Wood 1973), thinning this area and redistributing crustal materials. This would also cause a change in the position of the principal inertial axes, and reorientation of the Moon, making flooding of lavas prefer towards thin-crusted, low-lying areas. Bombardment asymmetrically by interplanetary meteoroids is expected due to the “gravitational focusing” effect...
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Ahrens, C. (2023). Early Bombardment. In: Cudnik, B. (eds) Encyclopedia of Lunar Science. Springer, Cham. https://doi.org/10.1007/978-3-319-14541-9_138
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DOI: https://doi.org/10.1007/978-3-319-14541-9_138
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