Size–Frequency Distributions of Centimeter- to Meter-scale Rock Fragments at the Chang’e-3–6 Landing Sites

The distribution of boulders on the Moon is critical for understanding regolith evolution and cratering processes. Using high-resolution descent imagery from the Chang’e-3 to -6 (CE-3 to CE-6) missions, we compared the size–frequency distributions (SFDs) and spatial patterns of centimeter- to meter-scale boulders across diverse geological settings. Results show that local boulder SFDs are dictated by proximal impacts and regolith thickness. At the young CE-3 and CE-5 sites, the power-law α ranges between −2.43 and −2.06. Here, hundred-meter-scale impacts penetrated the regolith, ejecting fresh bedrock and driving rapid regolith accretion. In contrast, the ancient CE-4 and CE-6 sites exhibit α ranging between −3.41 and −3.17. At these older sites, the lack of large-scale excavation slows regolith thickening, while continuous micrometeorite gardening drives a rapid decline in boulder abundance. Notably, the thinner regolith at CE-6 allows meter-scale impacts to excavate bedrock. However, as kinetic energy is limited, the exponent of the scaling law for maximum boulder size versus crater diameter (0.52) is significantly shallower than for large craters. Additionally, observations at the CE-5 and CE-6 sites confirm that significant spatial heterogeneity in boulder distributions is driven by oblique impacts. By uncovering the potential mechanisms behind varying boulder SFDs across these four sites, this study provides new observational constraints on the boulder–regolith coevolution model.