Scientists at Southwest Research Institute combined dynamical, thermal, and chemical models of the Moon’s formation to explain the relative lack of volatile elements in lunar rocks. Lunar rocks closely resemble Earth rocks in many respects, but Moon rocks are more depleted in volatile elements like potassium, sodium, and zinc, which tend to have lower boiling points and vaporise readily.
“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 Dr. Robin Canup, associate vice president in SwRI’s Space Science and Engineering Division and lead author of the Nature Geoscience paper detailing the findings.
Scientists think the Moon formed from an Earth-orbiting disc of vapour and molten matter produced by a giant impact between Earth and another Mars-sized body approximately 4.5 billion years ago. Previously, scientists had considered that volatiles vaporised 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.”
Canup’s team — which included researchers from SwRI, Dordt College, and Washington University — began with an existing computer simulation of the Moon’s accumulation from the disc. This was combined with models for how the temperature and chemical composition of the disc material evolve with time.
The models show that the Moon acquires about the final half of its mass from melt condensed in the inner portions of the disc, close to the Earth and just inside the Moon’s initial orbit. Over time, the Moon’s orbit expands due to dynamical interactions with inner disc material. When the Moon is distant enough, it can no longer efficiently accumulate inner disc melt, which is instead scattered inward and assimilated by the Earth.
“We find that the inner disc melt remains hot and volatile-poor as it accretes onto the Moon. Eventually the disc cools and volatiles condense. But by the time this occurs the Moon’s accumulation from this inner disc region has essentially terminated,” said Canup. “So the final materials the Moon accumulates are lacking in volatile elements, even in the absence of escape.”
The authors suggest that the materials the Moon initially accumulates from the outer disc could be volatile-rich, followed by a final 100- to 500-kilometre layer of volatile-poor material. In that case, the Moon’s volatile content could then increase with depth, depending on the extent of mixing in the Moon’s interior.