A New Model for Lunar Origin: Elemental and Isotopic Constraints

Introduction: The widely accepted model for the origin of the Earth-Moon system as the result of a Mars-sized giant impactor colliding with proto-Earth is inconsistent with a variety of new isotopic data. Generally, it is thought that the Earth and the Mars-sized Moon-forming impactor were isotopically different (for mass independent isotopic variations). Therefore, according to the “canonical” Moon forming SPH simulations, the Earth and the Moon should end up isotopically different from each other and that is inconsistent with observations. In contrast, isotopic studies have demonstrated that the Earth and Moon are remarkably similar in their isotopic compositions for many elements (notably mass independent isotopic compositions of O, Ti, Cr and W, and mass dependent isotope compositions of Mg and Si). This similarity between the Earth and Moon is unique in our Solar System when compared to other planetary bodies. In particular, the isotopic similarity in W isotopes and Hf/W ratios are unexpected, as this is a system that evolves with time after the formation of the solar system. It provides a fundamental constraint on the theory of the origin of the Moon; any successful model must properly explain such similarity between the Earth and Moon. We have developed a new model (see companion abstracts: [14]) that we believe is capable of meeting the isotopic, chemical and physical modeling constraints. In this model the Moon condenses out of bulk silicate Earth (BSE) vapor. The requirement is that the silicate Earth and impactor materials are shocked to a sufficiently high energy state such that upon adiabatic release to lower pressures this material forms a single phase fluid that bypasses the critical point and the Moon forms surrounded by 10’s of bars of BSE gas. During Moon formation, the most volatile components stay in the vapor resulting in the well-known volatile element depletion of the Moon. Many siderophile elements are more depleted in the Moon’s mantle compared to the Earth (Ni, Mo, Re) that has been taken as evidence for the formation of a small (few percent) lunar metal core after/during the formation of the Moon. We first discuss the constraints on the composition of the Moon and then we explore the case of the Hf-W evolution as many previous studies have reported measurements that suggest that the W/W isotopic compositions of the Earth and Moon are essentially the same; in addition their Hf/W ratios are similar. This is surprising in light of abundant evidence for a small lunar core. We show here that the inferred elemental composition of the Moon is consistent with the new model and that a small lunar core may only lead to insignificant effects in the Hf-W isotopic system while having major effects on elements like Mo. The Bulk Silicate Composition of the Moon: The bulk silicate Earth (BSE) composition is compared with different estimates of the bulk silicate Moon (BSM) in Figure 1 for the major, minor and selected trace elements. For the Earth we chose the widely used BSE composition of [5]. The gray band in Figure 1 shows a range of estimates of the bulk Moon composition (BSM) normalized to the bulk silicate Earth. Our range of estimates of the bulk Moon composition is based on data and discussions provided by [6-17].