Giant Impact and Fission Hypotheses for the Origin of the Moon: A Critical Review of Some Geochemical Evidence

Abstract

Geochemical data for Ni, Co, Cr, V, and Mn have played an important role in theories for the Moon's origin. It has been argued that the data for these elements strongly support formation of the Moon as ejecta from the Earth, either as a result of one giant or numerous smaller impacts on the proto-Earth. These theories have come to be known as the “Giant Impact” and “Impact-triggered Fission” hypotheses, respectively, and the first of these has been the leading explanation for the origin of the Moon over the past decade. Data for these same “diagnostic” elements also have been used to argue for significant distinctions between the bulk compositions of the Moon and a eucrite (HED) parent body, which otherwise appear to be remarkably similar in their compositions. We review geochemical evidence pertaining to the origin of the Moon, focusing on the diagnostic elements, and find that there is no strong geochemical support for either the Giant Impact or Impact-triggered Fission hypotheses. We show that basalts produced in the Moon and a HED parent body (mare basalts and eucrites, respectively) were derived by the melting of source regions with similar compositions. Mare and eucrite basalts differ in Ni and Co abundances but lie on the same igneous fractionation trend. Chromium, Mg# (Mg/[Fe+Mg]), and V abundance systematics suggest a close similarity between mare and eucrite basalts, and a significant difference from terrestrial volcanic rocks, which are depleted in Cr. Mare and eucrite basalts differ in their Mn abundances and Fe/Mn ratios, but the same can be said for mare and terrestrial basalts. On the whole, the Moon appears to be more chemically similar to the HED parent body than to the Earth. This suggests that either: (1) the HED parent body (probably the asteroid Vesta) formed by an impact mechanism and is an escaped satellite, or (2) the Moon is a captured body that formed independently of Earth. Similar conclusions were reached long ago by Anders and colleagues (e.g., Anders, 1977), long before the Giant Impact Hypothesis attained popularity.