Origin of the lunar isotopic crisis from solidification of a stratified lunar magma ocean

Abstract

According to the giant impact theory, the Moon formed through accreting the debris disk produced by a collision between Theia and the proto-Earth. The giant impact theory can explain most of the properties of the Earth-Moon system, however, simulations with respect to giant impact between a planetary embryo and the growing proto-Earth show that more than 40 percent of the materials in the circum-terrestrial debris disk produced by the giant impact originates from the impactor. Thus, the giant impact theory has difficulty explaining the Moon's Earth-like isotopic compositions, which is referred to as the lunar isotopic crisis. With the assumption that Theia may have possessed an iron-rich mantle compared with proto-Earth's mantle, here we show that, after the formation of the stratified lunar magma ocean, solidification of the stratified lunar magma ocean would result that the upper solid layer is composed of proto-Earth's mantle and the lower solid layer is made of Theia's mantle, meaning that the Moon's Earth-like isotopic composition is a natural result of the giant impact. The theory proposed here may provide a way of explaining the lunar isotopic crisis.