Why Moon Rocks Hold Fewer Volatiles Than Earth Rocks

Scientists have learned a bit more about moon rocks and why they're more depleted in volatile elements than those found on Earth. They've taken a closer look at models of the moon's formation to learn a bit more about the elements that make up the world.

"Explaining the moon's volatile depletion has been a long-standing mystery, and yet it is a key piece of evidence about how the Earth-Moon system formed," said Robin Canup, one of the researchers, in a news release.

Scientists believe that the moon formed from an Earth-orbiting disk of vapor and molten matter produced by a giant impact between Earth and another Mars-sized body about 4.5 billion years ago. Previously, scientists had considered that volatiles vaporized by the impact might have escaped before the moon formed.

"However, few volatiles may have actually been lost because the velocity needed to escape the Earth's gravity is quite high," said Canup. "The new research suggests instead that as the moon completed its growth, volatile-rich melt was preferentially deposited onto the Earth, rather than onto the growing moon."

The model shows that the moon acquired the final half of its mass from melt condensed in the inner portions of the disk, close to the Earth and just inside the moon's initial orbit. Over time, the moon's orbit expanded due to dynamical interactions with inner disk material. When the moon is distant enough, it can no longer efficiently accumulate inner disk melt, which is instead scattered inward and assimilated by Earth.

The findings reveal a bit more about how the moon-and Earth-formed. More specifically, it shows why the moon's material is far different from the Earth's.

The findings are published in the journal Nature Geoscience.

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