Differentiation of the South Pole-Aitken basin impact melt sheet: Implications for lunar exploration

Abstract

We modeled the differentiation of the South Pole-Aitken (SPA) impact melt sheet to determine whether noritic lithologies observed within SPA formed as a result of the impact. Results indicate differentiation of SPA impact melt can produce noritic layers that may accommodate observed surface compositions but only in specific scenarios. One of nine modeled impact melt compositions yielded layers of noritic materials that account for observations of noritic lithologies at depths of ∼6 km. In this scenario, impact occurred before a hypothesized lunar magma ocean cumulate overturn. The 50 km deep melt sheet would have formed an insulating quenched layer at the surface before differentiating. The uppermost differentiated layers in this scenario have FeO and TiO2 contents consistent with orbital observations if they were subsequently mixed with the uppermost quenched melt layer and with less FeO- and TiO2-enriched materials such as ejecta emplaced during younger impacts. These results verify that noritic lithologies observed within SPA could have formed as a direct result of the impact. Therefore, locations within SPA that contain noritic materials represent potential destinations for collecting samples that can be analyzed to determine the age of the SPA impact. Potential destinations include central peaks of Bhabha, Bose, Finsen, and Antoniadi craters, as well as walls of Leibnitz and Schrödinger basins. Additionally, potential remnants of the uppermost quenched melt may be preserved in gabbroic material exposed in "Mafic Mound." Exploring and sampling these locations can constrain the absolute age of SPA, a task that ranks among the highest priorities in lunar science. Key Points SPA impact melt differentiation accommodates observed noritic surface materials SPA formed before LMO overturn to produce noritic material at the lunar surface Norite-bearing materials represent key samples for dating the age of SPA ©2014. American Geophysical Union. All Rights Reserved.