The moon is a big oddity in the sky, and scientists may have just figured out part of why. Its surface is chemically asymmetrical, and new evidence shows that the Moon’s mantle flipped over when the Moon was young. The thing on top of Earth’s satellite has come down; What happened below was revealed.

Lunar scientists have been considering the collapse of the lunar mantle for decades. We now have new evidence to support this. A research team led by planetary scientists Weigan Liang and Adrien Brocke of the University of Arizona found that the Moon’s gravity map best fits this model of mantle overturning.

In a series of simulations, the team showed how gravitational anomalies on the near side of the moon are consistent with the presence and location of dense rocks containing dense minerals, preserved from the moon’s early days.

“This interpretation is supported by the compelling similarity between the observed pattern, magnitude, and extent of gravity anomalies and those predicted by geodynamic models of ilmenite-bearing cumulate inclusions,” the researchers wrote in their paper.

At best, researchers have pinpointed the exact date when this upheaval occurred: about 4.22 billion years ago, shortly after the Moon formed from a piece of Earth that broke off in a violent collision early in the solar system’s history.

One of the strangest things about the Moon has to do with its surface. There is a region on the far side of the Moon that can be described as “geochemically strange.” Known as KREEP Terrane, it is rich in specific and unexpected metals: potassium, rare earth elements and phosphorus.

KREEP terrain also overlaps the Lunar Sea, large basalt plains that are the result of volcanic activity. This basalt is rich in a mineral called ilmenite. Composed mainly of titanium and iron, ilmenite is extremely dense, like the rock that supports it. This is surprising because the rocks underneath are less dense. Depending on density, accumulated ilmenites, or IBCs, can be expected to sink to the Moon, while less dense rocks can be expected to rise to the surface.

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

In this scenario, when the magma reaches the end of the cooling and crystallization process, dense minerals such as ilmenite form at the crust-mantle interface and KREEP elements concentrate in the liquid reservoir.

These dense minerals are expected to sink into the Moon’s core. But scientists couldn’t be sure that this happened without modeling the physical processes that occurred in the middle of the Moon during its formation. It is also possible that after the initial subduction, the IBC heated up and rose again, shedding the mantle in the process. This could explain both the KREEP Terrane and the titanium-rich basalts brought to the surface through volcanism.

Under this scenario, both sets of elements should be more or less evenly distributed throughout the lunar mantle, but that’s not what scientists found. One explanation concerns the South Pole-Aitken Basin on the far side of the Moon; This massive impact left a crater covering more than a quarter of the Moon’s surface.

This collision created a hotspot that could cause KREEP and ilmenite to move away from the impact zone and concentrate on the near side of the Moon. We can’t see it because it’s below the surface; But researchers realized that such a migration would have to leave clear gravitational signatures.

Researchers built models of the ascent of ilmenite-rich materials on the Moon to observe the gravitational patterns created by IBC concentrations beneath the lunar crust. Their results gave a clear polygonal pattern of linear gravity anomalies. They compared this to data collected by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) orbiters; this was a pair of spacecraft that spent more than a year in space, mapping surface gravity on the moon.

The patterns created by the overturning of the lunar mantle matched observations collected by GRAIL. The model also showed how long it took for the model to develop; This limited the duration of the uprising to at least 4.22 billion years ago.

“Gravity anomalies support the prediction that the eventual subsidence of these dense materials took the form of sheet-like downflows, and that these features may persist throughout geological history while constraining the timing of toppling,” the researchers write. “The Moon’s gravitational field thus preserves critical information about the collapse of the lunar mantle, which is considered one of the decisive events in the early history of the Moon, but the details of which were hitherto unknown.”