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 <0.5 Ga is abundant; while as depth increases older melt is increasingly present. This may indicate that the excess of impact melt <0.5 Ga in lunar samples from the near surface is caused by the cumulative mixing of small impacts. A comparison of the age distribution of melt derived from craters of different sizes with that of impact glass constrains the size of spherule‐forming impacts. Our model is consistent with observations if most impact glass spherules from the near surface are produced by <100 m craters and >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.

Key Points: A numerical model is developed to investigate the effect of small impact gardening on ancient melt in the lunar near‐surface. Gardening of craters <100 m in diameter likely lead to the excess of datable impact melt <0.5 Ga in lunar regolith samples. Distribution of radiometrically datable melt in the regolith and the megaregolith is analyzed.

Deutche Forschungsgemeinshaft (DFG, German Research Foundation)