How a cataclysmic collision turned the moon inside out

About 4.5 billion years ago, Earth experienced a cataclysmic collision with a smaller planet. This violent event sent molten rock hurtling into space, where it gradually came together, cooled, and crystallized to form our moon. 

This narrative of the moon’s creation is widely endorsed by the scientific community. Yet, as researchers from the University of Arizona Lunar and Planetary Laboratory (LPL) have pointed out in a recent publication, the intricacies of this process resemble “more of a choose-your-own adventure novel.” 

The research presents the first geophysical evidence that a young, unstable moon turned itself inside out during the course of its evolution.

Insights into moon evolution

The study provides crucial insights into the evolution of the lunar interior, with broader implications for understanding the development of other celestial bodies, like Earth or Mars.

The primary evidence supporting the moon’s origin story has been derived from rock samples collected by Apollo astronauts over fifty years ago and from theoretical models.

These samples, mainly basaltic lava rocks, revealed unexpectedly high levels of titanium, mainly situated on the moon’s nearside, sparking questions about their origin and distribution.

The moon’s formation was rapid and intensely hot, resulting in a global magma ocean covering its surface. As this ocean cooled and solidified, it gave rise to the moon’s mantle and the luminous crust visible during a full moon. Below this surface, however, the young moon was anything but stable. 

Dense material sank into the moon, then resurfaced

Theoretical models have suggested that the remnants of the magma ocean crystallized into dense minerals, including ilmenite, rich in titanium and iron.

“Because these heavy minerals are denser than the mantle underneath, it creates a gravitational instability, and you would expect this layer to sink deeper into the moon’s interior,” said lead author Weigang Liang, who conducted the study during his doctoral degree at LPL.

Over time, this dense material descended into the moon’s depths, mingling with the mantle, melting, and eventually resurfacing as the titanium-rich lava flows observed today.

The moon turned itself inside out 

“Our moon literally turned itself inside out,” said co-author Jeff Andrews-Hanna, an associate professor at LPL. “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 study explores various hypotheses regarding how and when this material sank, whether gradually or all at once, whether it moved globally before surfacing on the near side, and whether it descended in one large mass or several smaller ones.

Geologic evolution of the moon

“Without evidence, you can pick your favorite model. Each model holds profound implications for the geologic evolution of our moon,” said co-lead author Adrien Broquet, a planetary geologist at the German Aerospace Center in Berlin.

In an earlier study published in the journal Nature Geoscience, scientists predicted that the dense layer of titanium-rich material beneath the crust first moved to the moon’s near side – possibly due to a giant impact on the far side – then sunk into the interior in a cascading manner, leaving behind geometric patterns of dense material.

“When we saw those model predictions, it was like a light bulb went on, because we see the exact same pattern when we look at subtle variations in the moon’s gravity field, revealing a network of dense material lurking below the crust,” said Andrews-Hanna.

Remarkably consistent story 

By comparing simulations with gravity anomalies detected by NASA’s GRAIL mission, the authors confirmed the presence of ilmenite remnants below the crust. This is consistent with their simulations of the moon turning itself inside out.

“Our analyses show that the models and data are telling one remarkably consistent story,” said Liang. “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.”

This investigation also sheds light on the timing of these events, suggesting the ilmenite-rich layer sank before the formation of the moon’s oldest impact basins, hinting at its role in subsequent volcanic activity on the lunar surface.

“Analyzing these variations in the moon’s gravity field allowed us to peek under the moon’s surface and see what lies beneath,” Broquet added, highlighting the study’s ability to bridge model predictions with physical evidence.

Additional layers to a complex narrative 

The identification of anomalies in the moon’s gravitational field not only substantiates the descent of a dense layer into the lunar depths but also enables a finer determination of the timing and mechanics of this phenomenon. Moreover, the moon’s surface characteristics introduce additional layers of complexity to this narrative.

“The moon is fundamentally lopsided in every respect,” Andrews-Hanna said, highlighting the disparities between the moon’s near side, which faces Earth – particularly the darker Oceanus Procellarum area – and its far side.

The near side is characterized by a lower elevation, a thinner crust, extensive lava flows, and elevated levels of rare elements such as titanium and thorium, contrasting sharply with the far side’s features. 

The reorganization of the lunar mantle is believed to be intricately linked to the distinct structure and historical dynamics of the Oceanus Procellarum region, a topic that has sparked significant scientific discourse.

First geophysical evidence 

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

“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,” Andrews-Hanna added.

“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,” he concluded.

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

Liang concluded by noting, “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.”

The study is published in the journal Nature Geoscience.

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