It’s widely thought that our Moon emerged out of an oblique 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. 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 water and relatively volatile elements such as potassium and other alkali metals.
Early on, researchers suggested that the volatiles may simply have boiled off and been lost as the Moon formed from the hot disk of molten rock and gas launched into Earth orbit, and the idea remained popular for decades. Recent studies, however, have shown that Earth’s gravity and frequent collisions among atoms in the protomoon’s viscous...
