Scientists Solve a Long-Standing Mystery Surrounding the Moon's 'Lopsided' Geology

taylorvich writes:

https://phys.org/news/2024-04-scientists-mystery-moon-lopsided-geology.html

About 4.5 billion years ago, a small planet smashed into the young Earth, flinging molten rock into space. Slowly, the debris coalesced, cooled and solidified, forming our moon. This scenario of how the Earth's moon came to be is the one largely agreed upon by most scientists. But the details of how exactly that happened are "more of a choose-your-own-adventure novel," according to researchers in the University of Arizona Lunar and Planetary Laboratory who published a paper in Nature Geoscience.

The findings offer important insights into the evolution of the lunar interior, and potentially for planets such as the Earth or Mars.

Most of what is known about the origin of the moon comes from analyses of rock samples, collected by Apollo astronauts more than 50 years ago, combined with theoretical models. The samples of basaltic lava rocks brought back from the moon showed surprisingly high concentrations of titanium. Later satellite observations found that these titanium-rich volcanic rocks are primarily located on the moon's nearside, but how and why they got there has remained a mystery-until now.

Because the moon formed fast and hot, it was likely covered by a global magma ocean. As the molten rock gradually cooled and solidified, it formed the moon's mantle and the bright crust we see when we look up at a full moon at night. But deeper below the surface, the young moon was wildly out of equilibrium. Models suggest that the last dregs of the magma ocean crystallized into dense minerals including ilmenite, a mineral containing 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 Weigang Liang, who led the research as part of his doctoral work at LPL.

Somehow, in the millennia that followed, that dense material did sink into the interior, mixed with the mantle, melted and returned to the surface as titanium-rich lava flows that we see on the surface today.

"Our moon literally 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."

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