Why Do Moon Rocks Contain Fewer Volatiles Than Earth Rocks?

A team of curious researchers created models of moon formations to determine why moon rocks have less volatile elements when compared to earth rocks, according to a study at Southwest Research Institute.

"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," Dr. Robin Canup, lead author of the study, said in a news release.

Volatiles are chemical elements and compounds that have low boiling points, and are mostly affiliated with the surfaces and atmosphere of a planet or moon. Lunar rocks appear almost identical to earth rocks. However, they have small amounts of volatile elements like zinc, potassium and sodium. Lunar rocks vaporize easily due to their depletion of volatile elements and low boiling points, according to the researchers.

"The new research suggests that as the Moon completed its growth, volatile-rich melt was preferentially deposited onto the Earth, rather than onto the growing Moon," Canup said.

Many scientists believed that the moon was formed from an earth orbiting disk of vapor and molten matter, which was created from a giant impact between Earth and another planet 4.5 billion years ago.

In their study, the researchers combined thermal, dynamical and chemical models of the Moon. They used an existing computer simulation of the Moon's accumulation from a disk, which was combined with models for how the temperature and chemical composition of the disk material evolve with time. They found that more than half of the moon's mass was obtained from melt condensed in the inner portions of the disk, close to the Earth.

"We find that the inner disk melt remains hot and volatile-poor as it accretes onto the Moon. Eventually the disk cools and volatiles condense. But by the time this occurs the Moon's accumulation from this inner disk region has essentially terminated," Canup said. "So the final materials the Moon accumulates are lacking in volatile elements, even in the absence of escape."

The findings of this study were published in Nature Geoscience.

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