[P-PS05] 月の科学と探査

[PPS05-03] 月のボルダー崩れから推定されるクレータ斜面の物質移動過程

*池田 あやめ¹、熊谷 博之¹、諸田 智克² (1.名古屋大学大学院環境学研究科地球環境科学専攻、2.東京大学理学系研究科地球惑星科学専攻)

キーワード：月、物質移動、ボルダー崩れ

Mass wasting is important to understand topographic degradation processes and present geologic activity on the lunar surface. Moonquakes and/or ground shaking due to impacts may trigger mass movement, but its mechanism remains unknown. Recently, lunar orbiters such as Kaguya and Lunar Reconnaissance Orbiter (LRO) explored the lunar surface, and they found boulder falls and lobate scarps, which are distributed in the lunar surface. Kumar et al. (2016, 2019) indicated that shallow moonquakes might have triggered boulder falls, but they did not consider another effect such as ground shaking due to impacts. In this study, we quantitatively evaluated the effects of both moonquakes and impacts to investigate the cause of boulder falls at the following two sites: Site 1 in a southern part of the Schrödinger basin and Site 2 near the Lorentz basin. Previous hypocenter determinations using waveform data from the Apollo seismic network suggest that a shallow moonquake occurred near Site 2 in 1975 (Nakamura et al., 1979). We investigated boulder falls in craters located within an epicentral distance of about 200 km from the moonquake. The craters with boulder falls on their walls were distributed independent of the epicentral distance, indicating that the shallow moonquake cannot explain the distribution of the boulder falls. We investigated the following surface features at the two sites using image and topography data obtained by LRO and Kaguya: (1) the distributions of boulder falls, small craters, and optical maturity parameter (OMAT), (2) the relationship between slope angles and densities of small craters, and (3) the relationship between acceleration of ground shaking induced by impacts and starting points of boulder falls. We estimated the ages of the crater walls at Sites 1 and 2 to be about 3.5 and 10 Ma, which are younger than the ages of the craters. Steep slope areas have small densities of small craters and high OMAT values or immature surfaces. These areas correspond to boulder sources, where many boulders are exposed. We found the correlation between starting points of boulder falls and acceleration of ground shaking induced by impacts inferred from the spatial distribution of small craters at Site 1. This result strongly suggests that the boulder falls were induced by ground shakings due to impacts. Based on these results we propose mass wasting processes depending on the slope angle to explain the distributions of boulder falls at Sites 1 and 2 as follows. In steep slope areas, regolith falls down the slopes by lunar gravity, and thus the regolith layer is thin. When meteorites hit steep areas, they fracture megaregolith or bedrocks beneath regolith layers and generate boulders, in which clear traces or small craters are not left. When meteorites hit gentle slope areas, where thick regolith layers exist, craters and boulder falls are clearly left. Thus, we propose that the lunar topographic degradation is controlled by such heterogeneities in regolith thickness and surface strength depending on the slope angle, which may not be described by a simple diffusion process.