The Compositional Asymmetry of the Moon Driven by the South Pole-Aitken Impact Event and an Overturning Mantle

Credit: NASA.

The Moon has long been thought to have experienced large-scale melting, leading to a global magma ocean, early in its history. During solidification of the magma ocean, denser olivine-rich materials would have sunk into the deep interior, while less dense feldspar-rich materials floated to the top, forming an anorthositic crust. The solidification of a lunar magma ocean often leads to a prediction of a spherically symmetric compositional interior. However, topography and surface compositional data suggest a strong lunar nearside-farside asymmetry, with denser ilmenite-rich mare basalts and materials rich in KREEP (potassium, rare earth elements, and phosphorus) concentrated on the surface of the nearside. This suggests that these materials were derived from a local near-surface source, a finding largely incompatible with the classic lunar magma ocean hypothesis. Consequently, it has been suggested that the asymmetry is the result of a large-scale impact, such as the one that formed the ancient South Pole-Aitken (SPA) Basin. However, the exact mechanism by which a large impact could create the asymmetry has been unclear.

To address this question, Nan Zhang (Peking and Curtin Universities), Min Ding (Macau University of Science and Technology), and colleagues simulate a SPA impact on a post-magma ocean lunar mantle. Models of the impact event provide initial conditions for subsequent simulations of global thermochemical convection. Results of this work showed that a SPA-forming impact would have generated high-temperature anomalies within the mantle, inducing lateral migration of otherwise spherically symmetric KREEP- and ilmenite-rich materials away from the region of impact to coalesce on the nearside of the Moon. This global reorganization results in a chemical reservoir that could become the source region for ilmenite-rich basalts and KREEP-rich terrains later exposed through additional impacts. This work can help explain the wide range and compositionally diverse mare basalts observed on the surface of the Moon. READ MORE