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 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 moderately volatile elements 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 permil. This isotopic composition is consistent with evaporation of Rb into a vapor medium that was ~99 percent 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 alpha-disk model, the alpha-viscosity needed to explain the ~99% saturation calculated here is 0.001 to 0.01, which is consistent with a vapor disk where viscosity is controlled by magnetorotational instability.

28 pages, 8 figures, 2 tables

References in corpus (5)

• Equilibration in the Aftermath of the Lunar-Forming Giant Impact
• Melting and Mixing States of the Earth's Mantle after the Moon-Forming Impact
• Gallium isotopic evidence for extensive volatile loss from the Moon during its formation
• A Model of the Primordial Lunar Atmosphere
• A hot big bang theory: magnetic fields and the early evolution of the protolunar disk

Cited by in corpus (4)

• Collision Chains among the Terrestrial Planets. III. Formation of the Moon
• A Magnetized, Moon-Forming Giant Impact
• Effect of Equation of State and Cutoff Density in Smoothed Particle Hydrodynamics Simulations of the Moon-Forming Giant Impact
• The Limited Role of the Streaming Instability During Moon and Exomoon Formation