Origin of the Moon: New Results and Open Issues

A primary scientific outcome of the Apollo program was the giant impact theory for lunar origin, in which a collision at the end of Earth’s main accretionary phase creates a disk from which the Moon forms.  In the past decade, the nature of a Moon-forming impact has become highly debated, driven by increasingly precise sample analyses that show that the Earth and Moon have essentially identical isotopic compositions across all non-volatile elements. Giant impacts usually produce disks that originate primarily from the impactor (“Theia”) rather than from the target protoearth.  Meteorites that originate from Mars, and nearly all those from parent bodies in the asteroid belt, have quite different isotopic compositions than the Earth.  If Theia had been similarly non-Earth like, and the pre-lunar disk originated primarily from Theia, one would then most naturally expect measurable differences between the Earth and Moon.  Instead, they are indistinguishable across most elements.  Many new scenarios have been proposed to resolve this fundamental dilemma, involving different Theia compositions, protolunar disk evolutions, and/or early Earth-Moon dynamical histories.  I will discuss several of these, as well as key remaining uncertainties and prospects for relevant new constraints from future lunar exploration.