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New research has revealed a previously unknown geological history of the Moon.
Among scientists who study the Moon, it is generally agreed that the formation of the Moon occurred after a small planet collided with the early Earth some 4.5 billion years ago. A large quantity of molten rock was ejected from the surface of the Earth after this collision, and eventually coalesced into what we call the Moon. Because it formed so quickly, early after its formation it was covered by an ocean of molten rock. The melt cooled and solidified, but it did so quickly enough that denser rocks formed on the surface while lighter ones solidified beneath. Thus, the Moon’s structure was out of equilibrium. As such, it would be expected that the heavy surface rocks would sink and the less dense material below them would rise, but the specifics of how this would happen are not at all clear. The new research takes observational evidence, such as the composition of lunar rocks recovered by the Apollo missions and data on the location of gravitational anomalies discovered by a NASA probe called GRAIL (which provide information about what lies beneath the lunar surface), and results of geologic modeling to determine the most likely mechanism to explain the Moon in its present state.
The last molten rock on the surface of the early Moon would have solidified to form the mineral ilmenite, a dense mineral rich in iron and titanium. Lunar rocks brought back to the Earth are found to contain unexpectedly large quantities of titanium, though only on the side of the Moon that faces the Earth (the near side). (Why isn’t there an even distribution of titanium across the surface of the Moon? It is hypothesized that an impact on the far side concentrated the titanium-rich material on the near side.) The general idea is that the dense surface material then sank, remelted and combined with material in the mantle, which then resulted in titanium-rich lava flows we observe on the surface today. Still, there are several scenarios whereby this process could take place. The modeling of one particular scenario produced results that corresponded well with the Moon’s gravitational anomalies.
In a previous study, led by Nan Zhang at Peking University in Beijing, who is also a co-author on the latest paper, models predicted that the dense layer of titanium-rich material beneath the crust first migrated to the near side of the moon, possibly triggered by a giant impact on the far side, and then sunk into the interior in a network of sheetlike slabs, cascading into the lunar interior almost like waterfalls. But when that material sank, it left behind a small remnant in a geometric pattern of intersecting linear bodies of dense titanium-rich material beneath the crust.
"When we saw those model predictions, it was like a lightbulb went on," said Andrews-Hanna, "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."
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The authors found that the gravity signatures measured by the GRAIL mission are consistent with ilmenite layer simulations, and that the gravity field can be used to map out the distribution of the ilmenite remnants left after the sinking of the majority of the dense layer.
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The team's observations also constrain the timing of this event: The linear gravity anomalies are interrupted by the largest and oldest impact basins on the near side and therefore must have formed earlier. Based on these cross-cutting relationships, the authors suggest that the ilmenite-rich layer sank prior to 4.22 billion years ago, which is consistent with it contributing to later volcanism seen on the lunar surface.
This is a wonderful example of how science should work: Computer modeling based on the reigning theory of the Moon’s origin is confirmed and complemented by observational data to form a clear picture of how the Moon came to be the way it currently looks.
Plus, it’s pretty cool to realize that the Moon turned itself inside-out.
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