The Composition and Evolution of a Geophysically Reasonable Moon Produced by a Giant Impact

Introduction: The magma ocean model for the early Moon’s evolution has been one of the most important and influential hypotheses to come out of the Apollo program. Recognizing that the earliest stages of a terrestrial planet’s evolution involve widespread melting, and attempting to understand what such a planetary-scale melting episode implies for geophysical and geochemical evolution, has been fundamental in our understanding not only of the Moon but of the major terrestrial planets and the HED parent body. In the 1970s Cameron first suggested that the Moon formed as the result of a Mars-sized planesimal impacting onto the Earth. Since the 1980s this model has gained significant favor. It is consistent with our understanding of how planetesimals form, and we now know that a large number of such giant impacts did occur elsewhere in the solar system. It can explain the depletion of iron metal in the Moon, and might explain the identical oxygen isotope abundances between Earth and Moon rocks. It can explain the Moon’s nearly circular but mildly inclined orbit. And, frankly, no other viable theory for the origin of the Moon exists. However, as modeled by Cameron and by Canup et al. [1], the nature of the impact event has profound implications for both the physical origin of any magma ocean, and for the Moon’s bulk composition.