月の昼夜の温度差がクレーター斜面の地形を変える －斜面の微地形と岩塊分布の統合解析から月面の地形変化に迫る－

Complex topographic features on crater slopes recently revealed by lunar orbiter explorations suggest the existence of rapid mass-wasting phenomena, and their contribution to long-term crater degradation remains poorly understood. We investigated Flamsteed crater, where regions of large rock abundance (RA) derived from the Lunar Reconnaissance Orbiter Diviner Radiometer are widely exposed. We found that large RA regions corresponded to densely distributed boulders (boulder fields) with large contents of FeO and TiO₂ that were heterogeneously distributed in the northern to southwestern slopes. There are horizontally layered cliffs in upslope positions above boulder fields. Topographic profiles estimated from digital terrain model data showed microtopographic features characterized by bulges and depressions with undulations of 10−20 m in the southwestern slope, and depression locations roughly coincided with RA peak positions. We found that the amounts of depressions increased with increasing RA. Based on these results, we considered that boulders supplied from cliffs formed boulder fields and that the boulders have been gradually fragmented into small pieces and regolith. We developed a model to explain the formation of boulder fields by fragmented rocks supplied from cliffs, and estimated the boulder supply rate to be 0.7 × 10³ m³/Myr. This estimate was two orders of magnitude larger than that expected from fracturing of a rock layer by meteorite impacts, and such rapid processes required weakening and fracturing due to thermal fatigue. We propose that RA is related to the microtopography of crater slopes and that thermal fatigue plays an important role in rock fragmentation.