The origin of the depletion of moderately volatile elements (MVEs) in the Moon.
The prevailing view for the formation of the Moon is that it was derived from a disk created by the collision of a Mars-sized body with the protoEarth (the so-called “giant impact” hypothesis). Although the hypothesis can explain many features of the Moon, it cannot readily explain why the Moon is depleted in MVEs (e.g, K, Rb, Cu, Zn and Ga) relative to Earth. We investigated this question using a new isotope tracer of Rb, as this element is very sensitive to volatilization processes, but is largely immune to lunar core formation. We measured high-precision Rb isotope data on selected lunar samples recovered by the Apollo missions. The new data showed that lunar volatile elements were very likely lost to the Earth through viscous drainage from the protolunar disk at a vapor saturation level of ~99%; this is a natural outcome if the viscosity of the vapor disk was controlled by magnetorotational instability. The model quantitatively explains the depletion and isotopic fractionation of MVEs in the Moon.