[P-PS06] 月の科学と探査

[PPS06-13] 月のクレータ斜面における熱疲労による岩塊の生成と破砕

*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：クレータ斜面、熱疲労、岩塊

In previous studies, crater degradation on the Moon has been described by a diffusion process resulting from cumulative effects of small impacts (e.g., Fassett & Thomson, 2014). Recent explorations by lunar orbiters have revealed various types of flows and slides of regolith and boulders (e.g., Xiao et al., 2013), indicating that crater degradation is affected by more complex processes. Bickel et al. (2021) suggested that thermal fatigue might cause bedrock fracturing and generate both regolith and boulders. Ikeda et al. (2022) proposed that the generation and transport of boulders and regolith on crater slopes by meteorite impacts. As such, crater degradation processes are not well understood. To investigate how boulders are formed and fragmented on crater slopes, we analyzed the detailed distributions of boulders, small craters, and rock abundance (RA) on lunar craters. We used the RA map derived by the Lunar Reconnaissance Orbiter (LRO) Diviner camera (Paige et al., 2011). From the RA map, we found 77 craters with large RA values where nine or more pixels spread continuously with a value larger than 0.4. We found that there were abundant boulders (boulder sources) in areas with large RA values located in the near-side maria. We used images taken by the LRO Camera (Robinson et al., 2010) to estimate the distributions of boulder sources and small craters with diameters larger than 5 m in Flamsteed crater. We found that boulder sources were distributed on the crater wall mainly in upper slope regions. Furthermore, some boulder sources were distributed in darker areas with multiple lobate structures, where few boulder falls were found. These boulder sources were produced from a dark layer with a thickness of several meters beneath the crater rim and spread widely down the slope. We estimated crater retention ages by the crater counting method to be 3−6 Ma in subdivided regions along the Flamsteed crater wall. We also calculated the RA density, which is defined by RA values accumulated within each region and divided by the area, with the estimated crater retention age in the corresponding region. Including our previous estimates in other craters, we found that the RA density tends to decrease with increasing crater retention age, indicating that the slopes with larger RA values tend to be resurfaced. The dark boulder sources may correspond to debris slides (Xiao et al., 2013) derived from the dark layer, which is a bedrock formed by magmatic activity and appeared on the crater slope due to subsequent excavation. This bedrock was highly fractured and served as source material for the debris slides. Fracturing of the thin layer along the steep slope is more effectively proceeded by thermal fatigue rather than small meteorite impacts. The bedrock is likely to be lava with small porosity, which can be selectively fractured by thermal fatigue (Cambioni et al., 2021). Regolith was also produced from fragmentation of boulders in the bedrock as well as on the boulder sources by thermal fatigue and/or meteorite impacts. Small craters on the slopes were erased by regolith movement, resulting in young crater retention ages. Our results suggest that thermal fatigue has an important role for the formation and fragmentation of boulders on lunar craters.