The Moon Likely Turned Itself Inside Out 4.2 Billion Years Ago

During its early years of formation, the Moon may have undergone a dramatic process as a dense layer of material sunk deeper into its interior, mixed with the lunar mantle, and made its way back to settle on the surface.

A team of researchers from the University of Arizona found new evidence that supports one of the wildest formation theories for the Moon, which suggests that Earth’s natural satellite may have turned itself inside out a few million years after it came to be. In a new study published Monday in Nature Geoscience, the researchers looked at subtle variations in the Moon’s gravitational field to provide the first physical evidence of a sinking mineral-rich layer.

In 2011, a pair of NASA spacecraft began orbiting the Moon to create a map of its gravitational field. The GRAIL mission (Gravity Recovery and Interior Laboratory) discovered gravitational anomalies on the Moon, whereby one of the spacecraft would accelerate while flying over certain regions.

“These gravity anomalies are indicating the presence of these dense rocks that are about 24 miles (40 kilometers) deep into the lunar interior,” Adrien Broquet, a researcher at the German Aerospace Center in Berlin and co-author of the study, told Gizmodo. “And so we linked these rocks to the whole evolution of the Moon, we think these rocks are the vestiges of the early dynamic evolution of the Moon.”

When it first formed, the Moon was covered in an ocean of magma. As this ocean cooled and solidified, the less dense layers at the top began crystalizing, forming the Moon’s mantle and the crust. Beneath the surface and closer to the Moon’s core, however, the more dense layers took longer to crystalize. Those layers were heavy with iron and titanium, and because they were more dense than the layers above them, they sunk deeper into the Moon’s interior. As they did that, the mineral-rich, dense layers mixed with the mantle of the Moon, melted, and then returned to the lunar surface as titanium-rich lava flows (which we still see today).

This theory of the Moon overturning has been around since the days of Apollo, when astronauts collected samples from the Moon and found high concentrations of titanium. The recent study, however, is the first to tackle the formation model using the Moon’s subtle gravitational anomalies.

As NASA prepares its astronauts to land on the Moon for the upcoming Artemis missions, the new crew will collect more evidence to help us understand how our natural satellite came to be.

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