Constraining the Source Craters of Apollo Impact Melts

Interpreting the age distribution of Apollo samples to determine the early bombardment history of the Moon has been fraught with controversy, and the question of how much material from large impact basins such as Imbrium is present in the Apollo sample collection remains unresolved. Here, we model impact melt production and transportation over lunar history to estimate the amount of melt that each basin should have contributed to the Apollo 14–17 sites. Our model results suggest that Imbrium contributes significantly to all four studied Apollo sites, but also that each site should contain more impact melt from basins older than Imbrium compared to what has been interpreted in the Apollo sample collection. We then use Bayesian inference to evaluate possible correlation between specific basins and groups of Apollo impact melt samples. We find that in every simulation, there are only a few basins sampled, but the specific basins (other than Imbrium) are different for each simulation. This suggests that most basins were not sampled in the suite of Apollo impact melts. Using a combination of numerical models and Bayesian statistics, we define four possible scenarios for the timing of basin emplacement. The impact flux for these scenarios varies from a smooth decline to a terminal “cataclysm.” The behavior of impacts in the time before Imbrium remains unconstrained, and specific samples are needed to better understand this period in lunar history.