Revealing lunar secrets: How the Moon's 'inverted' geology puzzles scientists

Scientists from the University of Arizona (USA) have discovered that computer simulations combined with data from space probes have helped solve the mystery of the Moon's "inverted" geology. They indicate that rocks, originally hidden inside the Moon, surfaced at some point and sank back into the centre.

According to the widely accepted theory, the Moon was formed about 4.5 billion years ago when a small planet collided with a young Earth. The force of the impact ejected molten lava into space, which eventually solidified, creating Earth's natural satellite. However, many details of this story remain unknown.

The discovery by scientists from the University of Arizona sheds new light on this story. Our knowledge about the origin of the Moon comes mainly from the analysis of rocks collected during the Apollo missions and from theoretical models. These rocks contain an unusually high concentration of titanium. Satellite studies have shown that these volcanic, titanium-rich rocks are mainly located on the side of the Moon facing Earth.

The reason for such a distribution of rocks has remained a mystery until now. Scientists believe that the Moon was covered with a magma ocean in the early stages of its existence. Slowly cooling and solidifying, it formed the mantle and crust we observe today.

According to scientific models, deeper-located deposits were supposed to form dense minerals, such as ilmenite, containing large amounts of titanium and iron. However, these minerals, being denser than the mantle, should have been located in the deep layers of the Moon, not on its surface. Experts explain that the dense material settled deeper and deeper, mixed with mantle rocks, melted again, and returned to the surface as titanium-rich lava eruptions.

Professor Jeff Andrews, the author of the publication that appeared in the magazine "Nature Geoscience," says: "Our moon literally turned itself inside out. But there has been little physical evidence to shed light on the exact sequence of events during this critical phase of lunar history, and there is a lot of disagreement in the details of what went down – literally".

For example, it was not known whether materials containing titanium sank slowly during their formation or quickly after the Moon's surface solidified. It was also unclear whether these minerals sank globally, in all regions, and then returned to the surface on one side or if they sank similarly, asymmetrically. Researchers also did not know whether they sank as one large mass or many small ones.

One of the scientists, Adrien Broquet, emphasizes: "Without evidence, you can pick your favorite model. Each model holds profound implications for the geologic evolution of our moon".

An earlier analysis developed by Nana Zhang from Peking University, one of the authors of the new publication, indicated that dense, titanium-rich material present under the crust first surfaced on the hemisphere facing Earth. That was likely due to a strong impact on the opposite side. Then, this material was supposed to have sunk into the depths of the Moon in formations resembling thin plates falling downward, a bit like water in a waterfall. However, a small amount of this material was supposed to remain just below the crust.

This model aligns accurately with irregularities in the Moon's gravitational field – emphasize the scientists.

In the new study, scientists compared simulations of sinking ilmenite-rich layers with data on gravitational anomalies obtained from the NASA GRAIL satellite. They discovered that the gravitational data matched the simulation results and that data from GRAIL could be used to predict the distribution of ilmenite that remained after most of this material had sunk.

Leading the work, Weigang Liang says: "Our analyses show that the models and data are telling one remarkably consistent story. Ilmenite materials migrated to the near side and sunk into the interior in sheetlike cascades, leaving behind a vestige that causes anomalies in the moon's gravity field, as seen by GRAIL".

The analysis also provides information about the timing of these events. Linear gravitational anomalies are interrupted by the largest and oldest impact craters, visible on the side facing Earth. This means they must have formed earlier. The titanium-rich layer, therefore, sank more than 4.2 billion years ago, which also matches volcanic traces.

The unusual structures on the Moon's surface add to its mystery. Professor Jeff Andrews-Hanna, one of the researchers, says: "The Moon is fundamentally lopsided in every respect". The scientist explains that the side visible from Earth, especially the dark region known as the Ocean of Storms, is clearly lower, has a thinner crust, and is heavily covered with solidified lava streams. It also has a high concentration of rare elements, such as titanium and thorium. The far side is different in these respects.

The new model also explains these differences. Dr Liang says: "Our work connects the dots between the geophysical evidence for the interior structure of the moon and computer models of its evolution".

"For the first time we have physical evidence showing us what was happening in the moon’s interior during this critical stage in its evolution, and that's really exciting. It turns out that the moon’s earliest history is written below the surface, and it just took the right combination of models and data to unveil that story," says Prof. Andrews-Hanna.

"When the Artemis astronauts eventually land on the moon to begin a new era of human exploration, we will have a very different understanding of our neighbor than we did when the Apollo astronauts first set foot on it," adds Dr Liang.