Lunar Mare - Distribution of Mare Basalts

Distribution of Mare Basalts

There are many common misconceptions concerning the spatial distribution of mare basalts.

1. Since many mare basalts fill low-lying impact basins, it was once thought that the impact event itself somehow caused the volcanic eruption. Given that mare volcanism typically occurred about 500 million years after the impact, a causal relationship is unlikely.
2. It is sometimes suggested that the gravity field of the Earth might preferentially allow eruptions to occur on the near side, but not far side. However, in a reference frame rotating with the Moon, the centrifugal acceleration is exactly equal and opposite to the gravitational acceleration of the Earth. There is thus no net force directed towards the Earth. The Earth tides do act to deform the shape of the Moon, but this shape is one of an elongated ellipsoid with high points at both the sub- and anti-Earth points. As an analogy, one should remember that there are two high tides per day on Earth, and not one.
3. Since mare basaltic magmas are denser than upper crustal anorthositic materials, basaltic eruptions might be favored at locations of low elevation where the crust is thin. However, the far side South Pole-Aitken basin contains the lowest elevations of the Moon and is yet only modestly filled by basaltic lavas. In addition, the crustal thickness beneath this basin is predicted to be much smaller than beneath Oceanus Procellarum. While the thickness of the crust might modulate the quantity of basaltic lavas that ultimately reach the surface, crustal thickness by itself can not be the sole factor controlling the distribution of mare basalts.
4. It is commonly suggested that there is some form of link between the synchronous rotation of the Moon about the Earth, and the mare basalts. However, gravitational torques that result in tidal despinning only arise from the moments of inertia of the body (these are directly relatable to the spherical harmonic degree-2 terms of the gravity field), and the mare basalts hardly contribute to this (see also tidal locking). (Hemispheric structures correspond to spherical harmonic degree-1, and do not contribute to the moments of inertia.) Furthermore, tidal despinning is predicted to have occurred quickly (on the order of 10s of millions of years), whereas the majority of mare basalts erupted about 1 billion years later.

The reason that the mare basalts are predominantly located on the near-side hemisphere of the Moon is still being debated by the scientific community. Based on data obtained from the Lunar Prospector mission, it appears that a large proportion of the Moon's inventory of heat producing elements (in the form of KREEP) is located within the regions of Oceanus Procellarum and the Imbrium basin, a unique geochemical province now referred to as the Procellarum KREEP Terrane. While the enhancement in heat production within the Procellarum KREEP Terrane is most certainly related to the longevity and intensity of volcanism found there, the mechanism by which KREEP became concentrated within this region is not agreed upon.

In 2011, planetary scientists Erik Asphaug and Martin Jutzi published a study proposing that the uneven distribution of maria occurred because the moon collided with another, smaller moon of the earth, a few million years after its formation. In this low-impact collision, the smaller moon was crushed into the surface of the moon, thickening the moon's crust on one side. Subsequent impacts broke through the crust and released lava, only on where the crust was thin, leading to the observed uneven distribution of maria.