Like a twice-baked potato, the moon itself might have required a multi-part recipe, forming in two distinct stages that ultimately produced inner and outer layers with different compositions, BBC News and Astronomy Magazine explained. The study could explain why the moon, which is in other ways similar to the Earth, is depleted in volatiles like water and sodium.
The new model, which was developed by researchers at the Southwest Research Institute (SwRI) in Colorado along with colleagues at universities in Iowa and Missouri, begins by building on an existing theory that an impact between the proto-Earth and a Mars-sized body approximately 4.5 billion years ago left a disk of vapor and molten material orbiting around the planet.
However, to address the issue of the missing volatiles, the study authors deviated from previous explanations and added models for how the temperature and chemical composition of these disk materials would have evolved over time into a computer simulation. The results suggest that the vaporized volatiles had not actually been lost prior to the moon’s formation.
Why the outer layers are volatile-poor
As lead author Dr. Robin Canup, associate VP at the SwRI’s Space Science and Engineering Division, explained in a statement, it is not likely that much of the missing water, sodium, and potassium were lost in this way due to the velocity needed to escape Earth’s gravity. Rather, as the moon grew, its volatile-rich melt was simply deposited onto the planet.
In the months and years immediately following the impact, roughly half of the moon’s mass was compressed into a ball at the edge of the disk, near the Earth, and just inside the initial orbit of the moon. As the moon’s orbit expanded over time, it became so distant that it was no longer able to efficiently accumulate inner disk melt, which instead wound up being assimilated by Earth.
Since the early material came from the fringe of the disk, it was cool and was higher in volatile element content. The outer half of the moon that was formed later, and the molten material from the inner portion of the disk, would have been too hot for the volatile elements to condense with it. As a result, the outer layers of the moon ended up being “volatile-poor”, the BBC said.
“What we find is that the initial half of the Moon, say 50 percent of its mass, may well have retained its volatile species. But for the last half, as that material accreted on to the Moon, it was consistently too hot to contain the volatile species,” Dr Canup told the BBC. After accumulating both layers, the moon ended up travelling further away from Earth.
“Thus, the portion of the Moon derived from the inner disk is expected to be volatile depleted,” she and her co-authors wrote. “We suggest that this mechanism may explain part or all of the moon’s volatile depletion, depending on the degree of mixing within the lunar interior.”
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Feature Image: JD Hancock/Flickr
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