More than 4.2 billion years ago, the moon turned itself inside out to create the lunar surface that has become familiar to humanity.
Most scientists agree that the moon was formed about 4.5 billion years ago, when another massive body in the solar system struck Earth, ejecting molten material into space, which coalesced to form our natural satellite.
How the moon's birth unfolded after this violent start has been described as "more of a choose-your-own-adventure novel" by a team of scientists at the University of Arizona's Lunar and Planetary Laboratory (LPL).
They say there are many possible paths that Earth's natural satellite could have taken to fully form, ultimately leading to the moon-Earth system we see today. The team obviously has its own ideas about the key events that could have shaped the moon. The researchers say that rock samples collected during the Apollo mission, for example, could indicate that there was a time when the moon was "turned inside out."
If true, this result could also solve a lingering mystery about the moon's composition.
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"Our moon literally turned itself inside out," co-author and LPL associate professor Jeff Andrews-Hanna said in a statement. "But there is little physical evidence that can shed light on the exact sequence of events during this critical phase of lunar history, and there is much disagreement about the details of what literally happened."
Titanium on the near side of the moon?
Basaltic lava rocks brought back from the moon have shown surprisingly high concentrations of titanium. In addition, satellite observations have revealed that titanium-rich volcanic rocks are mainly located on the near side of the moon. This left scientists scratching their heads as to how these particular rocks got there and not spread more widely.
The University of Arizona team suggests that the moon formed quickly, initially leaving it completely covered in a hot magma ocean. As this ocean cooled and hardened, the moon's outer layers would have formed, including the mantle and crust. Yet the young moon would still have been in turmoil in the lower layers.
Models of moon formation suggest that the last remnants of this giant lunar ocean crystallized into dense materials, including ilmenite, a mineral rich in iron and titanium.
"Because these heavy minerals are denser than the underlying mantle, they create a gravitational instability, and you would expect this layer to sink deeper into the moon's interior," says study leader and former LPL PhD student Weigang Liang.
Questions remain: Would this material sink all at once as a single "blob" after the moon solidified, or little by little as smaller blobs? And if it sank into the moon's interior on a global scale, how did some of it rise to transport titanium to the moon's near side?
"Without evidence you can choose your favorite model," says study co-lead authorand German Aerospace Center scientist Adrien Broquet said in the statement. "Each model has profound implications for the geological evolution of our moon."
Co-author and scientist Nan Zhang of Beijing University previously developed models suggesting that a giant impact on the moon could have caused a dense layer of titanium-rich material beneath the crust to shift toward the near side. Once there, this material would have sunk, formed plate-like slabs and flowed into the moon's interior, leaving a remnant beneath the crust in the form of intersecting bodies of dense titanium-rich deposits.
"When we saw those model predictions, it was like a light bulb went off," Andrews-Hanna said. "We see exactly the same pattern when we look at subtle variations in the moon's gravitational field, revealing a network of dense material lurking beneath the crust."
The GRAIL of moon formation models
To support its molten theories about titanium-rich ilmenite material and observations of the moon, the team turned to data surrounding lunar gravity anomalies detected by NASA's Gravity Recovery and Interior Laboratory (GRAIL) twin spacecraft mission.
"By analyzing these variations in the moon's gravitational field, we were able to look beneath the moon's surface and see what lies beneath it," Broque said.
This confirmed that GRAIL data agree with simulations of ilmenite layers.
Such confirmation also showed that observations in the gravitational field could be used to trace the distribution of ilmenite remnants left behind after most of the dense layer sank into the moon's deep interior.
"Our analyzes show that the models and data tell one remarkably consistent story," Liang said. "Ilmenite materials migrated to the near side and sank inland in sheet-like cascades, leaving behind a remnant that causes anomalies in the moon's gravitational field, as seen by GRAIL."
The team was also able to determine when the moon turned inside out. They say the interruption of gravity anomalies by large and ancient lunar impact basins indicates that the ilmenite-rich layer sank before these impacts. This 'transverse' meaning means that the sinking event would have occurred earlier than 4.22 billion years ago, indicating that the sinking could have caused volcanism, which was observed over the moon's surface at later times.
This research also adds nuance to an interesting image of the moon we see today. The collapse of the moon's mantle billions of years ago would have led to the creation of a dark region known as the Oceanus Procellarum region, as well as on the side of the moon close to Earth.
This part of the moon lies lower and has a thinner crust that is largely covered by lava flows, in contrast to the thicker crust on the far side of the moon. It also has a higher concentration of rare elements such as titanium and thorium. "The moon is fundamentally lopsided in every respect," Andrews-Hanna said. 'For the first time, we have physical evidence showing us what happened inside the moon during this critical phase of its evolution, and that's really exciting.
"It turns out that the moon's earliest history is written beneath its surface, and it just took the right combination of models and data to reveal that story."
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Broquet added: 'The remains of early lunar evolution are present beneath the crust today, which is fascinating.
"Future missions, such as with a seismic network, would allow better investigation of the geometry of these structures."
The findings could also help inform future studies of our faithful lunar companions if and when, in 2025, NASA's Artemis III mission returns humanity to the moon for the first time since the Apollo missions ended fifty years ago.
"When the Artemis astronauts finally land on the moon, a new era of human exploration begins," Liang concluded. "We will have a very different understanding of our neighbor than when the Apollo astronauts first set foot on it."
The team's research has been published in the journal Nature Geoscience.
