A new mechanism that may explain observed geochemical differences between the Earth and Moon is reported in a study published online in Nature Geoscience.
Chemical similarities between the Earth and Moon support the theory that they share a common origin: the collision of a Mars-sized body into the proto-Earth. According to this hypothesis, the giant impact would have produced an Earth-orbiting disk of vaporized and melted debris that coalesced due to gravity to form the Moon. However, compared with the Earth, the Moon is depleted in volatile elements-those that more readily vaporize at lower temperatures, such as potassium, sodium and zinc-even though the disk of debris from which the Moon formed was rich in such elements.
Robin Canup and colleagues combined dynamical simulations of the evolution of the impact debris disk with a model of the disk's thermal and chemical evolution. The Moon initially coalesces from material in the outer part of the disk, which is sufficiently cool for volatile elements to condense and be incorporated. However, in their simulations, the authors find that more than half of the Moon's mass comes from melt from the inner part of the disk that is still too hot for volatile elements to condense from vapour; thus, the part of the Moon that accretes from inner disk material has relatively low abundances of volatiles.
They show that, although the inner disk eventually cools to temperatures at which volatile elements can condense, this occurs after the Moon's orbit has expanded away from the disk and after the growth of the Moon has ceased. The authors suggest that the volatile-rich material from the inner disk instead accretes to Earth.
In an accompanying News & Views article, Steven Desch writes, "The result is a single, comprehensive model that successfully reproduces the volatile element abundances in the Moon."