Forming the Moon for terrestrial silicate-rich material

Recent high-precision measurements of the isotopic composition of lunar rocks demonstrate that the bulk silicate Earth and the Moon show an unexpectedly high degree of similarity. This is inconsistent with one of the primary results of classic dynamical simulations of the widely accepted giant impact model for the formation of the Moon, namely that most of the mass of the Moon originates from the impactor, not Earth.Resolution of this discrepancy without changing the main premises of the giant impact model requires total isotopic homogenisation of Earth and impactor material after the impact for a wide range of elements including oxygen, silicon, potassium, titanium, neodymium, and tungsten. Isotopic exchange between partially molten and vaporised Earth and Moon shortly after the impact has been invoked to explain the identical oxygen isotopic composition of Moon and Earth but the effectiveness and dynamics of this process are contested. Even if this process could explain the O isotope similarity, it is unlikely to work for the much heavier, refractory elements. Given the increasing uncertainty surrounding the giant impact model in light of these geochemical data, alternative hypotheses for lunar formation should be explored.In this paper, we revisit the hypothesis that the Moon was formed directly from terrestrial mantle material, as first proposed in the 'fission' hypothesis (Darwin, 1879. On the bodily tides of viscous and semi-elastic spheroids, and on the ocean tides upon a yielding nucleus. Phil. Trans. Roy. Soc. (London) 170, 1-35). We show that the dynamics of this scenario requires on the order of 10