Unveiling the Moon’s hidden past: How it turned itself inside out

More than 50 years ago, Apollo astronauts brought basaltic lava rocks back from the moon with surprisingly high concentrations of titanium. Later, satellite observations found that these titanium-rich volcanic rocks are primarily located on the moon's nearside - but how and why they got there has remained a mystery – until now. Credit: NASA.

Researchers at the University of Arizona Lunar and Planetary Laboratory, in a recent study published in Nature Geoscience, have revealed new insights into the moon’s complex interior and its early formation process.

Their findings highlight a dramatic “inside-out” transformation of the moon, which has implications for understanding not just the moon, but other planetary bodies like Earth and Mars.

The widely accepted theory of the moon’s origin suggests that about 4.5 billion years ago, a colossal collision between the young Earth and a Mars-sized body scattered debris into space. This debris eventually coalesced to form the moon.

Initially, the moon was a molten mass, characterized by a global magma ocean. As this ocean cooled, it solidified to form the moon’s mantle and the crust visible from Earth.

However, beneath the moon’s surface, the situation was chaotic. The last remnants of the magma ocean solidified into dense minerals rich in titanium and iron, such as ilmenite. These heavy minerals created a layer that, due to its density, was unstable atop the lighter mantle below.

Lead researcher Weigang Liang explains that this dense layer eventually began to sink into the moon’s interior.

Over millennia, this material mixed with the mantle, melted again, and rose back to the surface. This process resulted in the volcanic flows rich in titanium seen on the moon today, particularly on its near side.

The study’s co-author, Jeff Andrews-Hanna, describes this phenomenon by saying, “Our moon literally turned itself inside out.” This statement underscores the lack of direct evidence that has puzzled scientists regarding the specific sequence of these events.

The research team, including Adrien Broquet of the German Aerospace Center, used sophisticated models to simulate the sinking of the titanium-rich layer.

These models predicted that after a possible giant impact on the moon’s far side, the dense layer migrated toward the near side, sinking in sheet-like slabs almost resembling waterfalls.

This model was tested against data from NASA’s GRAIL mission, which orbited the moon in 2011-2012. GRAIL measured subtle variations in the moon’s gravity field, which revealed linear patterns of dense material beneath the crust.

The match between the GRAIL data and the simulation models confirmed the researchers’ hypothesis about the ilmenite layer’s behavior.

These findings are crucial as they provide a timeline for these processes, suggesting the dense layer sank before the formation of the moon’s oldest and largest impact basins, around 4.22 billion years ago. This timing is consistent with later volcanic activity observed on the lunar surface.

Additionally, the study highlights the fundamental asymmetry of the moon. The near side, which faces Earth, features low elevations, a thin crust, extensive lava flows, and high concentrations of elements like titanium and thorium, unlike the far side.

The researchers believe that the overturn of the lunar mantle is connected to these unique features of the near side, particularly the Oceanus Procellarum region.

Andrews-Hanna notes, “The moon is fundamentally lopsided in every respect,” pointing out the distinct differences between the moon’s two hemispheres. This asymmetry and the events that led to it are still a subject of intense study and debate among scientists.

Looking ahead, Liang expresses excitement about the future, particularly with the upcoming Artemis missions, which aim to return humans to the moon. These missions will provide new opportunities to explore and understand the moon’s complex geologic history, building on the groundbreaking insights revealed by their research.

This study not only connects geophysical evidence with advanced computer models but also opens up new avenues for exploring the evolutionary histories of other planetary bodies in our solar system.