Formation of Small Craters in the Lunar Regolith: How Do They Influence the Preservation of Ancient Melt at the Surface?

The impact melt that records the formation time of basins is essential for the understanding of the lunar bombardment history. To better understand melt distribution on the Moon, this study investigates mixing of melt by small impacts using a Monte Carlo numerical model. The obtained mixing behavior is then integrated into a larger scale model developed in previous work. While large impacts produce most of the melt volume in both the regolith and megaregolith, we find that the dominant source of melt near the surface is small impacts. Material in the top meter is affected mainly by impacts that form craters <5 km in diameter. In the uppermost 10 cm, melt with age 100 m craters do not contribute abundant spherules. The distribution of the datable melt with depth is also analyzed, which is essential for future sampling missions. Excavated materials of young and large craters (>100 m on highlands; >10 km on maria) appear to be the most fruitful targets. Plain Language Summary Hypervelocity impact events on the Moon generate great energy that melts materials in the near-surface. The generated melt products record the age of impact craters. The abundance of impact melts of different ages is therefore essential for our understanding of the lunar bombardment history. Most of the returned samples are derived from the near-surface, where the material composition has been significantly affected by the frequent gardening of small impacts. Improving our understanding of how impact processes change the material composition is helpful for sample interpretations. Here, we build a numerical model to investigate this issue. The simulation results show that craters <100 m likely lead to the excess of datable impact melt <0.5 Ga that has been found in returned samples. In addition, we delineate the distribution of datable melt in various depths. It provides insight into future lunar missions aiming to collect melt that can be easier to date. We suggest that ejecta blankets of young and large craters (>100 m on highlands; >10 km on maria) would be the optimal targets.