Vapor Drainage in the Protolunar Disk as the Cause for the Depletion in Volatile Elements of the Moon

Abstract

Lunar rocks are severely depleted in moderately volatile elements (MVEs) such as Rb, K, and Zn relative to Earth. Identifying the cause of this depletion is important for understanding how the Earth-Moon system evolved in the aftermath of the Moon-forming giant impact. We measured the Rb isotopic compositions of lunar and terrestrial rocks to understand why MVEs are depleted in the Moon. Combining our new measurements with previous data reveals that the Moon has an ⁸⁷Rb/⁸⁵Rb ratio higher than Earth by +0.16 0.04‰. This isotopic composition is consistent with evaporation of Rb into a vapor medium that was ∼99% saturated. Evaporation under this saturation can also explain the previously documented isotopic fractionations of K, Ga, Cu, and Zn of lunar rocks relative to Earth. We show that a possible setting for achieving the same saturation upon evaporation of elements with such diverse volatilities is through viscous drainage of a partially vaporized protolunar disk onto Earth. In the framework of an α-disk model, the α-viscosity needed to explain the ∼99% saturation calculated here is 10^-3-10^-2, which is consistent with a vapor disk where viscosity is controlled by magnetorotational instability.