Mechanism of impact cratering on the Moon: A key to improve the precision of lunar impact flux

The Moon is the calibration plate for reconstructing the impact history of the inner Solar System. On the basis of samples returned by the Apollo and Luna missions to the Moon, the overall trend of lunar impact history since magma ocean differentiation has become clear. However, there are significant discrepancies among existing models of time evolution of lunar impact flux, and major controversies exist for impact flux during key geological periods, causing long-standing doubts about the reliability of estimating model ages using crater statistics. Refining the impact history of the Moon is an important part of lunar science. Recently, the successful implementation of the Chang'e-5 and Chang'e-6 sample return missions has verified the overall reliability of the classic Neukum impact flux model, and multiple new impact flux calibration points were also established. In addition to high-precision compositional and isotopic ages of lunar samples, the geological events represented by the samples and the spatial density of primary craters formed in a given geological unit are also core factors determining the accuracy of impact flux calibration points, and their uncertainties are key reasons for discrepancies between existing lunar crater chronology models. During geological evolution of the Moon, extensive and continuous impact processes mix materials from different provenances and alter their physical and chemical properties, posing challenges to tracing provenances of lunar samples. Meanwhile, secondary craters formed by re-impact of impact ejecta, topographic degradation of impact craters, and the equilibrium effect of crater populations affect reliable extraction of spatial densities of primary craters. Impact ejection and ejecta landing directly affect the spatial distribution of ejecta and their gardening effect of lunar surface materials. Recent studies have shown that the non-radial excavation during impact cratering, heterogeneous distribution of ejecta, and the diversity of their landing modes yield uncertainties to interpretations of both provenances of lunar samples and spatial densities of primary craters. Therefore, insufficient understanding of impact cratering mechanisms is an important reason for the significant discrepancies between lunar crater chronology models. Research of impact cratering mechanisms is the core to improve both the provenance tracing accuracy of extraterrestrial samples and the observation accuracy of primary craters, and it is the key pathway to advancing lunar impact flux models.