Overtovel of the Lunar Mantle, the strange process carried out by the Moon 4,220 million years ago

New studies support the idea that the mantle of the Moon was put "upwards" when the satellite was still young. This would explain the chemical asymmetry that its surface presents.

lunar surface
Illustration showing the descent rich in ilmenite and the gravitational pattern produced on the lunar surface. (Adrien Broquet, LPL/Audrey Lasbordes)

A peculiar feature of our natural satellite, the Moon, is that its surface is chemically asymmetrical. New studies support the idea that this is due to the fact that the mantle of the Moon was put "upwards" when the satellite was still young. What was above her was below; and what was below, came to light.

Lunar scientists have been thinking for decades about the so-called overturning of the lunar mantle. Now, planetary scientists Weigang Liang and Adrien Broquet, from the University of Arizona, along with a team of researchers, have discovered that the gravitational map of the Moon is the one that best fits this model of mantle overturning.

According to ScienceAlert, the team demonstrated in a series of simulations how the gravitational anomalies on the near face of the Moon are consistent with the presence and location of dense rocks carrying dense minerals that have been preserved since the early days of the Moon.

"This interpretation is supported by the great similarity between the observed pattern, the magnitude and dimensions of the gravitational anomalies and those predicted by the geodynamic models of the remains of clusters containing ilmenite," the researchers write in their article.

In addition, the researchers accurately dated when this overturn occurred: about 4.22 billion years ago, shortly after the Moon was formed from a piece of Earth that broke off during a violent collision.

What happens on the surface of the Moon?

One of the most peculiar aspects of the Moon has to do with its surface. On the nearest side of the Moon, there is a region that can be described as "geochemically strange". Known as Terrana KREEP, rich in specific and unexpected metals: potassium, rare earth elements and phosphorus.

This region also overlaps with lunar seas (large basalt plains that are the result of volcanic activity). This basalt is rich in a mineral called ilmenite. Composed predominantly of titanium and iron, the ilmenite is quite dense like the rock that contains it.

This is disconcerting because the rocks underneath are less dense. Based on the density, it would be expected that the clusters containing ilmenite, or IBC, would have sunk into the Moon, and that the less dense rocks would have ascended to the surface.

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Both the KREEP terrana and the IBC can be explained by geodynamic processes that occurred shortly after the formation of the Moon. When it was still hot and humid, the Moon was probably covered by an ocean of molten magma that cooled to form the crust and mantle.

In this scenario, when the magma reaches the end of its cooling and crystallization process, dense minerals such as ilmenite are formed in a layer between the crust and the mantle, and the KREEP elements are concentrated in a liquid deposit.

The researchers accurately dated when this overturn occurred: about 4,220 million years ago.

These dense minerals are expected to sink towards the lunar core. But without modeling the physical processes that took place inside the Moon while it was forming, scientists could not be sure that that was what happened.

It is also possible that after this initial sinking inward, the IBCs warmed up and went up again, overturning the mantle in the process. This would explain both the KREEP Terrana and the titanium-rich basalts that came to the surface through volcanism.

In this scenario, both sets of elements should have ended up distributed more or less evenly by the lunar mantle, but this is not what scientists have found.

The model of the roll of the lunar mantle coincided with the observation

An explanation has to do with the South Pole-Aitken basin, on the hidden side of the Moon, a colossal impact that left a crater that covers more than a quarter of the lunar surface.

This impact produced a hot spot that could have seen the migration of the KREEP and the ilmenite far from the place of the impact, concentrating on the near face of the Moon. As it is below the surface, we can't really see it; but the researchers realized that such a migration should have left distinctive gravitational signatures.

lunar surface
An example of one of the gravitational patterns produced in the modeling of the team. (Liang & Broquet et al., Nat. Geosci., 2024)

The researchers built models of the lunar overturn of materials rich in ilmenite to observe the gravitational patterns generated by IBC concentrations under the lunar cortex.

The results showed a polygonal pattern of linear gravitational anomalies. They compared these data with those obtained by the orbiters of NASA's Interior and Gravity Recovery Laboratory (GRAIL), a pair of spacecraft that spent more than a year in space mapping the gravity of the Moon's surface.

The patterns produced by the model of the lunar mantle overturn coincided with the observations collected by GRAIL.

The model also revealed how long it takes for the pattern to evolve, limiting the time of the overturn to at least 4.220 million years ago.

Reference of the news:

Liang, W., Broquet, A., Andrews-Hanna, J.C. et al. Vestiges of a lunar ilmenite layer following mantle overturn revealed by gravity data. Nat. Geosci. (2024). https://doi.org/10.1038/s41561-024-01408-2