Composition, structure, and origin of the Moon

Abstract

Extensive sampling of the Moon, both directly and remotely, has afforded a unique view into its composition and structure. In the canonical model, the proto-Earth is struck by a Mars-sized body, where the Moon is derived predominantly from the impactor. However, the isotopic compositions of a variety of elements that show variability among planetary materials, initially O, but subsequently Cr and Ti, attest to a near-identical match between the composition of the Earth's mantle and the Moon. These observations, together with the low Fe content of the bulk Moon (∼7.5wt%) compared to that of the Earth (33wt%) previously inferred from geophysical observations, have spurred the development of new physical models for its generation. Here we critically examine the geophysical and geochemical properties of the Moon in order to identify the extent to which dynamical scenarios satisfy these observations. New joint inversions of existing lunar geophysical data (mean mass, moment of inertia, and tidal response) assuming a laterally and vertically homogeneous lunar mantle show that, in all cases, a core with a radius of 300 ± 20km (∼0.8%–1.5% the mass of the Moon) is required. However, an Earth-like Mg# (0.89) in the lunar mantle results in core densities (7800 ± 100kg/m3) consistent with that of Fe-Ni alloy, whereas FeO-rich compositions (Mg#=0.80–0.84) require lower densities (6100 ± 800kg/m3). Consequently, an FeO-rich Moon implies the existence of a core with light (e.g., S, C) and/or exotic (e.g., ilmenite) components, whereas Earth's mantle-like compositions typically result in an elastically more rigid planet in contradiction with the observed tidal response and may require a partially molten layer surrounding the core. Geochemically, we use new data on mare basalts to reassess the bulk composition of the Moon for 70 elements, and show that the lunar core likely formed near 5GPa, 2100K, and ∼1 log unit below the iron-wüstite buffer, based on the depletions of Ni, Co, Mo, and W in the lunar mantle. Moreover, the Moon is depleted relative to the Earth's mantle in elements with volatilities higher than that of Li, with this volatile loss likely having occurred at low temperatures (1400 ± 100 K). These temperatures are also able to reproduce the extent of mass-dependent stable isotope fractionation observed in moderately volatile elements (e.g., Zn, K, Rb), provided equilibrium was attained between the vapor and the condensed phase, which, owing to the low temperatures, was likely solid. On this basis, and given the low 87Sr/86Sr of ferroan anorthosites (FANs) and high 238U/204Pb in mare volcanics, additional volatile loss relative to the Earth must have been associated with Moon formation. The identical nucleosynthetic (O, Cr, Ti) and radiogenic (W) isotope compositions of the lunar and terrestrial mantles (following subtraction of the late veneer from the latter) strongly suggest the two bodies were made from the same material, rather than from an Earth-like impactor. The W isotope homogeneity among lunar rocks, coupled with only mild 142Nd variations, indicates that lunar mantle differentiation occurred at ∼4350Ma, providing a minimum age for the formation of the Moon, whereas Rb-Sr in FANs and Lu-Hf and Pb-Pb zircon ages point to ∼4500Ma. Taken together, there is no unambiguous geochemical or isotopic evidence for the role of an impactor in the formation of the Moon, implying perfect equilibration between the proto-Earth and Moon-forming material or alternative scenarios for its genesis.