Asymmetric crustal growth on the Moon indicated by primitive farside highland materials

The Moon's nearside and farside differ in topography(1), crustal thickness(2), mare volcanic activity(3) and elemental concentrations(4). The origin of this dichotomy is still unclear(5-7). It is also unknown whether the characteristics of the oldest crust, the anorthositic lunar highlands, reflect a different magmatic evolution of nearside and farside crust. Based on analyses of nearside highland rocks(8,9), it has been suggested that nearside crustal growth occurred from an evolved, iron-rich magma ocean(10), but information from the farside highlands is lacking. Here we apply an empirical algorithm to lunar reflectance spectra(11) from the Kaguya Spectral Profiler and report that magnesium contents relative to iron of primitive crustal highland rocks on the farside are higher than on the nearside. Our findings indicate that the farside crust consists of rocks that crystallized from less-evolved magma than the nearside crust. We conclude that the lunar dichotomy is directly linked to crystallization of the magma ocean and suggest that the composition of the magma ocean was more primitive at the time of crustal growth than previously estimated.