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Lunar rocks are severely depleted in moderately volatile elements such as Rb,K, and Zn relative to Earth. Identifying the cause of this depletion isimportant for understanding how the Earth-Moon system evolved in the aftermathof the Moon-forming giant impact. We measured the Rb isotopic compositions oflunar and terrestrial rocks to understand why moderately volatile elements aredepleted in the Moon. Combining our new measurements with previous data revealsthat the Moon has an 87Rb/85Rb ratio higher than Earth by +0.16 permil. Thisisotopic composition is consistent with evaporation of Rb into a vapor mediumthat was ~99 percent saturated. Evaporation under this saturation can alsoexplain the previously documented isotopic fractionations of K, Ga, Cu and Znof lunar rocks relative to Earth. We show that a possible setting for achievingthe same saturation upon evaporation of elements with such diverse volatilitiesis through viscous drainage of a partially vaporized protolunar disk ontoEarth. In the framework of an alpha-disk model, the alpha-viscosity needed toexplain the ~99% saturation calculated here is 0.001 to 0.01, which isconsistent with a vapor disk where viscosity is controlled by magnetorotationalinstability.

Vapor drainage in the protolunar disk as the cause for the depletion in volatile elements of the Moon