by Over 4.2 billion years ago, the moon turned itself inside out to create the lunar surface known to mankind.
Most scientists would agree that the moon was formed about 4.5 billion years ago, when another massive body in the solar system smashed into Earth, sending molten material into space that coalesced as our natural satellite.
However, a team of scientists from the University of Arizona’s Lunar and Planetary Laboratory (LPL) described how the birth of the moon proceeded after this violent beginning.
They say that there are many possible ways that Earth’s natural satellites could form as a whole, leading to the Moon-Earth system we see today. Of course, the team has its own ideas about the major events that the moon could lead to. The researchers say that rock samples collected during the Apollo missions, for example, could indicate a time when the moon “turned inside out”.
This finding, if true, could solve a long-standing mystery about the composition of the moon.
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“Our moon went inside out,” research co-author and LPL associate professor Jeff Andrews-Hanna said in a statement. “But there was little physical evidence to shed light on the precise sequence of events during this critical phase of the moon’s history, and there is much disagreement about the details of what went down—literally.”
Titanium on the side next to the moon?
Basaltic lava rocks brought back from the moon have shown remarkably high concentrations of titanium. In addition, satellite observations have shown that titanium-rich volcanic rocks are located mainly on the lunar side. This has left scientists scratching their heads as to how these particular rocks came to be and are not more widely distributed.
The University of Arizona team suggests that the moon formed quickly, leaving it completely covered in an ocean of hot magma to begin with. As this ocean cooled and solidified, it would form the outer layers of the moon, including its mantle and crust. But, at lower levels, the infant moon would still be in turmoil.
Models of lunar formation suggest that the last remnants of this large lunar ocean crystallized into dense material including ilmenite, a mineral rich in iron and titanium.
“Because these heavy minerals are denser than the mantle below, they create gravitational instability, and you would expect this layer to penetrate deeper into the lunar interior,” said the research leader and former LPL doctoral candidate Weigang Liang, said.
Questions remain: Would this material sink all at once as one “blob” after the moon solidified, or a little at a time as smaller blocks? And, if it sank into the interior of the moon on a global scale, how did some of the titanium rise to be transported to the nearest side of the moon?
“Without evidence, you can pick your favorite model,” co-author researcher and German Aerospace Center scientist Adrien Broquet said in the statement. “Each model has major implications for the geological evolution of our moon.”
Co-author and Peking University scientist Nan Zhang previously developed models that suggested a massive impact on the moon could have caused a dense layer of titanium-rich material beneath the crust to move to its near side. When it was there, this material would have been spent, made into slabs like sheets and its cascade to the interior of the moon, leaving a remnant under the crust in the form of a transverse body of dense deposits rich in titanium.
“When we saw those model predictions, it was like a light bulb went on,” Andrews-Hanna said. “We see the exact same pattern when we look at subtle variations in the moon’s gravity field, revealing a network of dense matter lurking beneath the crust.”
GRAIL models of moon formation
To corroborate their theories about the titanium-rich ilmenite content of the melt and the moon’s observations, the team turned to data on lunar gravity anomalies discovered by NASA’s two-spacecraft Gravity Recovery and Interior Laboratory (GRAIL) mission.
“Analyzing these variations in the moon’s gravity field allowed us to look beneath the surface of the moon and see what lies beneath,” said Broque.
This confirmed that the GRAIL data match the ilmenite layer simulations.
Such confirmation also showed that gravity field observations could be used to trace the distribution of the ilmenite remnants left behind after most of the compact layer had sunk deep into the moon’s interior.
“Our analysis shows that the models and the data tell one remarkably consistent story,” Liang said. “Ilmenite materials were transported to the near side and penetrated into the interior in sheet-like outcrops, leaving behind a vestige that creates anomalies in the moon’s gravity field, as seen by GRAIL.”
The team was also able to determine when the moon turned inside out. They say disruption of gravity anomalies at large and ancient lunar impact basins indicates the ilmenite-rich layer sank before the impacts. This “cross section” means that the sinking event would have occurred earlier than 4.22 billion years ago, indicating that the sinking may have triggered volcanism, which was later seen across the surface of the moon.
This research also adds nuance to an interesting picture of the moon that we see today. The retreat of the moon’s mantle billions of years ago would have created a dark region known as the Oceanus Procellarum region, as well as the side of the moon close to Earth.
This part of the moon is at a lower elevation and has a thinner crust that is mostly covered by lava flows unlike the thicker crust in the far side regions of the moon. It also has a higher concentration of rare elements such as titanium and thorium. “The moon is basically lopsided in every way,” said Andrews-Hanna. “For the first time, we have physical evidence that shows us what was happening inside the moon during this critical stage of its evolution, and that’s amazing.
“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 reveal that story.”
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Broquet added: “The remnants of early lunar evolution are present below the crust today, which is surprising.
“Future missions, for example with a seismic network, would allow a better investigation of the geometry of these structures.”
The findings could also aid future investigations 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 50 years ago.
“When Artemis astronauts finally land on the moon to usher in a new era of human exploration,” Liang said. “We will have a very different understanding of our neighbor than we did when the Apollo astronauts first set foot on it.”
The team’s research is published in the journal Nature Geoscience.
