Complete Melting of the Moon Inferred from Hafnium Isotope Measurements

Abstract Investigations of lunar rocks demonstrate that the primordial crust and mantle of planetary bodies form through solidification of a magmatic layer, known as a magma ocean. However, the depths of planetary magma oceans are poorly constrained despite the fact that this information would provide critical constrains on the initial conditions under which planetary bodies form and evolve, as well as the geologic significance of measurements made on surficial materials. Here we use Hf and Nd isotopes measured on lunar basaltic rocks to demonstrate that the lunar magma ocean likely encompassed the entire Moon. Complete melting of the Moon implies planetary bodies are unlikely to preserve primitive unprocessed material in their interiors, that chemical equilibrium between planetary cores and mantles is probable, that samples collected on planetary surfaces can record primordial global-scale processes, and that heat associated with accretion is likely responsible for formation of magma oceans. These results have implications for the origin and evolution of all four terrestrial planets as well as all large geologically processed asteroids.