The unique lunar composition and its bearing on the origin of the Moon

Abstract

The noble gas abundance patterns of Earth, Venus and Mars suggest that accretion of the terrestrial planets took place after the loss of H and He from the inner solar nebula. Since most meteorites also have noble gas patterns which differ from “solar” abundances, such dispersal must have occurred close to T0 (4.55 Ae). An initial depletion of volatile elements, as shown by low KU ratios of the inner planets, is ascribed to the same time. This depletion of volatiles occurred out to a “snow line” around 5 A.U. Formation of the inner planets, through accretion of planetesimals in a gas-free environment, occurred on timescales of 107–108 years. Large bodies (100 Moon-sized, 10 Mercury-sized and several Mars-sized objects) characterised the later stages of planetesimal sweep-up. Over 50% of the mass of the Earth may have previously existed and have undergone metal-silicate fractionation in these large precursor objects. Evidence for the existence of massive planetesimals comes from the obliquities of the planets and the slow retrograde motion of Venus. The Moon has a unique composition among satellites and planets. It is strongly depleted in the very volatile elements (e.g. Bi, Tl) by factors of several hundred relative to the solar nebula. FeO values of 13% in the bulk moon contrast with 36% in CI and 8% in the terrestrial mantle. Trace siderophile elements are depleted in order of their metal-silicate partition coefficients, consistent with their removal into a small lunar core. There is a probable enrichment of refractory elements (e.g. Ca, Al, Ti, REE, U, Th) based on geochemical balance considerations, heat flow, the presence of a thick aluminous crust, and the need to satisfy seismic velocity profiles. A major part of the Moon was molten shortly after accretion, followed by extensive differentiation. Capture, fission or double planet hypotheses for the origin of the Moon all fail to account for the chemical and dynamical problems, including the anomalously high angular momentum of the Earth-Moon system. Only the impact of a massive Mars-sized (>0.1 Earth mass) planetesimal appears adequate to account for both chemical and dynamical aspects. Ejection of a portion of the mantle of the impactor at least partly in a vapor phase accounts for the extreme depletion in volatile elements. The large impactor hypothesis appears to be the only current hypothesis which accounts for the singular composition of the Moon and the unique nature of the Earth-Moon system.