Explaining the Moon’s composition

It’s widely thought that our Moon emerged out of a giant collision between Earth and a body the size of Mars (see the article by Dave Stevenson, Physics Today, November 2014, page 32 ). That now 40-year-old hypothesis was born in the aftermath of the Apollo missions, which returned hundreds of kilograms of lunar rock. Subsequent analysis of the material revealed a moon whose bulk chemical composition is largely the same as that of Earth’s mantle but for a striking depletion of relatively volatile elements, such as potassium and other alkali metals. Using computer simulations that track the evolution of an Earth-orbiting disk of melted and vaporized rock formed in the giant collision, Robin Canup of the Southwest Research Institute and her colleagues can now quantitatively explain the depletion. Outside a planet’s so-called Roche limit, self-gravitating satellites retain their integrity against tidal forces. In Canup’s simulations, which combine dynamical, thermal, and chemical models, melt that found itself beyond Earth’s Roche limit of roughly 3 Earth radii coalesced into thousands of moonlets within weeks; the first 40% of the Moon formed within months. Inside the Roche limit, though, any developing clumps were quickly sheared apart. What’s more, gravitational interactions in the disk scattered the moonlets outward and the inner-disk melt and gas inward. The inner-disk material eventually spread beyond the Roche limit and formed its own moonlets, which accreted onto the Moon over the next century or so. But by the time most volatile gases were finally cool enough to condense, the Moon had spiraled away, out of their reach; the volatiles fell back to Earth as rain. (R. M. Canup et al., Nat. Geosci., in press, doi:10.1038/ngeo2574 .)