The gardening process of lunar regolith by small impact craters: A case study in Chang'E-4 landing area

On January 3, 2019, Chang'E-4 (CE-4) probe successfully achieved soft landing on the farside of the Moon. A large number of small craters in the landing area can be found in the high-resolution remote sensing images. These small craters played a vital role in the evolution of lunar regolith because of the overturning and mixing of lunar surface material through excavating and ejecting during impact processes. However, the research of the gardening process by small craters and the evolution of farside lunar regolith are still insufficient. This research aims to make quantitative analysis to the evolution of the lunar regolith from regional small impacts and apply them in the CE-4 landing area. Firstly, the processes of excavation and ejection of a single small impact on the lunar surface are analyzed, during which the formation and distribution of lunar regolith are numerically modeled. Then, based on the remote sensing observation and the analysis of regional impact flux, a group of impact events with resulting lunar regolith are modeled by Monte Carlo method. Thirdly, the above analysis is applied in the CE-4 landing area, and the thickness, grain size distribution, and overturning times of the lunar regolith are calculated. Finally, the influencing factors are discussed and evolution process of lunar regolith is summarized. The results indicate that the formation of lunar regolith is controlled by larger impact events, while the surficial overturning and mixing of lunar regolith are mainly caused by smaller impact events. In addition, the lunar regolith is mostly formed in the earlier times because of the high impact flux at that time. For example, the absolute model age of the CE-4 landing area is about 3.5 Ga, while the lunar regolith had mostly been formed before 3.0 Ga and changed slowly afterwards. Our model beyond the observational capability of the Yutu-2's lunar penetrating radar (LPR), and the results can contribute to better understanding of the regional lunar regolith formation and evolution.