How did the moon turn itself inside out?

More of a choose-your-own adventure novel.

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Almost 4.5 billion years ago, a minor planet collided with Earth, throwing molten rock into space. As time passed, this debris came together to create our moon. The details are like a “choose-your-own-adventure novel,” according to researchers from the University of Arizona Lunar and Planetary Laboratory, even though experts agree on this core plot.

The study sheds light on the evolution of the lunar interior and, potentially, of planets such as Earth or Mars.

Whatever is known about the moon’s formation comes from the study of rock samples brought by Apollo astronauts 50 years ago. Their analysis revealed surprisingly high concentrations of titanium in the samples. Subsequent satellite observations revealed that these volcanic rocks rich in titanium are mostly found on the moon’s nearside, but it is still unknown how and why they got there.

Moon formation took place fast and hot; hence, it was covered by a global magma ocean. The magma gradually cooled down and solidified with time, forming the moon’s mantle. However, the young moon was drastically out of balance underneath the surface. According to models, the remaining remnants of the magma ocean solidified into minerals with a high density, such as ilmenite, which is a mineral with iron and titanium.

Weigang Liang, who led the research as part of his doctoral work at LPL, said, “Because these heavy minerals are denser than the mantle underneath, they create gravitational instability, and you would expect this layer to sink deeper into the moon’s interior.”

The thick material gradually dissolved as it sunk into the moon’s interior and combined with the mantle. The structures we see today were formed when this melted material re-emerged as titanium-rich lava flows.

“Our moon turned itself inside out,” said co-author and LPL associate professor Jeff Andrews-Hanna. “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.”

The thick material gradually dissolved as it sunk into the moon’s interior and combined with the mantle. The structures we see today were formed when this melted material re-emerged as titanium-rich lava flows.

When the moon solidified, did this material sink all at once or gradually as it formed? Did it migrate to the near side first and then sink, or did it sink entirely into the interior before rising on the near side? Was it submerged in a single, enormous mass or multiple, smaller ones? Scientists are still discussing these issues.

According to earlier research using simulations, there may have been a significant impact on the moon’s far side that caused the thick layer of titanium-rich material beneath its crust to first migrate towards the moon’s near side. Subsequently, it submerged itself in flat, lunar-like slabs, cascading inward like waterfalls. But as it sunk, this material left behind a tiny trace of intersecting linear bodies of dense material rich in titanium beneath the crust.

In this new study, the researchers matched a set of linear gravity anomalies detected by NASA’s GRAIL mission to simulations of a sinking layer rich in ilmenite. Between 2011 and 2012, GRAIL’s two spacecraft orbit the moon to measure minute variations in its gravitational pull. A large dark region surrounded by volcanic flows known as mare (Latin for “sea”) sits on the moon’s near side. These linear anomalies surround this area.

The researchers discovered that the GRAIL mission’s gravity patterns agree with models of the sinking of the ilmenite layer. They also found that once most of the dense layer sank, the leftover ilmenite fragments may be located using the moon’s gravity as a map.

The study reveals a consistent narrative between the models and the data. GRAIL discovered signs of ilmenite materials that flowed to the near side and sunk into the interior in sheet-like flows, affecting the moon’s gravity field.

The team’s discoveries also provide insight into the timing of this process: the most significant and oldest impact basins on the near side disrupt the linear gravity anomalies, indicating an earlier formation date. Based on their analysis of these correlations, the scientists hypothesize that the ilmenite-rich layer sunk 4.22 billion years ago, consistent with its involvement in later volcanic activity observed on the lunar surface.

Through analyzing these gravitational differences, scientists have been able to probe beneath the moon’s surface and discover what is hidden there. They showed that the gravitational field anomalies on the moon are consistent with zones of thick, titanium-rich material predicted by computer models of lunar overturn.

The moon is fundamentally lopsided in every respect,” Andrews-Hanna said, explaining that the near side facing the Earth, and particularly the dark region known as Oceanus Procellarum region, is lower in elevation, has a thinner crust, is primarily covered in lava flows, and has high concentrations of typically rare elements like titanium and thorium. The far side differs in each of these respects. The overturn of the lunar mantle is related to the unique structure and history of the near-side Procellarum region. But the details of that overturn have been a considerable debate among scientists.

“Our work connects the dots between the geophysical evidence for the interior structure of the moon and computer models of its evolution,” Liang added. 

“For the first time, we have physical evidence showing us what was happening in the moon’s interior during this critical stage of its evolution, and that’s really exciting,” Andrews-Hanna said. “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.” 

“The vestiges of early lunar evolution are present below the crust today, which is mesmerizing,” Broquet said. “Future missions, such as with a seismic network, would allow a better investigation of the geometry of these structures.” 

Liang added“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.”

Journal Reference:

  1. Liang, W., Broquet, A., Andrews-Ha6nna, J.C., et al. Vestiges of a lunar ilmenite layer following mantle overturn revealed by gravity data. Nat. Geosci. (2024). DOI: 10.1038/s41561-024-01408-2

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