Accretion of the Moon after a High-Energy, High-Angular Momentum Giant Impact

Different giant impact scenarios are being debated for lunar origin. However, the main observations being used to constrain lunar origin are geochemical and cannot be addressed by giant impact simulations alone. Understanding the chemical relationships between the Earth and Moon requires accretion models that predict the composition of the Moon. Here, we focus on understanding the accretion of a moon after a high-energy, high-angular momentum giant impact. Such impacts drive the Earth into a post-impact state that exceeds the hot spin stability limit (HSSL), which defines the maximum mantle entropy and angular momentum for a corotating body. In typical post-HSSL states, the mantle, atmosphere and disk form a dynamically and thermodynamically continuous structure. We present a new lunar accretion model based on combining numerical simulations of cooling highly-vaporized post-impact structures with geochemical calculations. We find that condensation at large radii quickly forms a lunar seed that orbits within the bulk silicate Earth (BSE) vapor structure. As the vapor structure continues to cool, condensates form and the pressure-supported structure contracts. The seed accretes condensed material, primarily derived from collapse of the low surface density regions of the structure at large radii. The lunar seed is heated by the vapor until the first major element (Si) begins to vaporize. The growing Moon equilibrates with BSE vapor at the temperature of Si vaporization and the pressure of the structure for an extended period of time. Eventually, the cooling structure recedes within the lunar orbit, truncating the main stage of lunar accretion. Our model links the pressure-temperature conditions of lunar accretion with the chemical composition of the Moon. We find that equilibration of the Moon with BSE vapor under a certain range of pressure-temperature conditions can establish the observed lunar isotopic composition and pattern of depletion in moderately volatile elements. Bulk equilibration at the inferred pressure-temperature conditions is not expected in the canonical giant impact, but a range of high-energy, high-angular momentum giant impacts can generate post-impact structures that satisfy the inferred range of vapor pressures for lunar origin.