Forming the Moon from terrestrial silicate-rich material

Abstract

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 1029–1030J almost instantaneously generated additional energy if the angular momentum of the proto-Earth was similar to that of the Earth–Moon system today. The only known source for this additional energy is nuclear fission. We show that it is feasible to form the Moon through the ejection of terrestrial silicate material triggered by a nuclear explosion at Earth's core–mantle boundary (CMB), causing a shock wave propagating through the Earth. Hydrodynamic modelling of this scenario shows that a shock wave created by rapidly expanding plasma resulting from the explosion disrupts and expels overlying mantle and crust material. Our hypothesis straightforwardly explains the identical isotopic composition of Earth and Moon for both lighter (oxygen, silicon, potassium) and heavier (chromium, titanium, neodymium and tungsten) elements. It is also consistent with the proposed Earth-like water abundances in the early Moon, with the angular momentum and energy of the present-day Earth–Moon system, and with the early formation of a ‘hidden reservoir’ at Earth's CMB that is not present in the Moon.