Figure 10. Schematic diagram of our mass-wasting model for Sites 1 and 2. Older and younger regolith layers are shown by green and red, respectively. (a) In upslope areas, the surface regolith layer is thin, and boulders are more easily generated by meteorites striking the megaregolith bedrock. (b) Younger regolith generated in upslope regions moves downward by lunar gravity, and ground shaking caused by meteorite impacts triggers boulder falls. The resulting craters and boulder trails are clear in the thick regolith layer in gently sloping areas. (c) Repetition of the processes shown in (a) and (b) makes the slope gentler and the surface layer thicker and eventually results in the degradation of the crater wall.

5. Conclusions

We investigated the distributions of boulder sources, boulder trails, small craters, and OMAT values at Site 1 in the Schrödinger basin and Site 2 in Laue crater and found the following features: (1) boulder sources are located in upslope areas, and some boulder trails are superimposed by craters whereas other crosscut craters; (2) there are few small craters and boulder trails in areas with large slope angles; and (3) OMAT values are high in boulder source areas. Our estimated ages of the slopes at both Sites 1 and 2, based on small crater density, are clearly younger than the formation ages of the Schrödinger basin and Laue crater. Therefore, craters have been erased from these slopes by subsequent mass-wasting processes. The correlation between starting points of boulder falls and maximum acceleration due to impacts at Site 1 suggests that boulder falls were triggered by ground shaking caused by meteorite impacts. A moonquake on 3 January 1975 was thought to trigger boulder falls at Site 2. However, the distributions of craters with and without boulder falls within an epicentral distance of about 200 km showed no dependence on epicentral distance. Therefore, boulder falls at and around Site 2 were not triggered by the moonquake. Using our estimates of the density of small craters, slope angles, and OMAT values, we proposed a mass-wasting model for slopes at Sites 1 and 2 as follows: Meteorite impacts fracture the megaregolith bedrock in upslope areas where the surface regolith layer is thin, thereby producing boulder sources. Ground shaking due to meteorite impacts causes the boulders to move down the slopes; boulder trails and small impact craters are clearly visible in downslope areas, where regolith derived from upslope areas has caused a thick regolith layer to accumulate. Repetition of these processes makes the slopes gentler and the surface regolith layer thicker, and results in degraded craters without boulder falls. The results of this study strongly suggest that boulder falls in these areas are not caused by shallow moonquakes along lobate scarps but by ground shaking due to meteorite impacts. In future studies, the distributions of boulder falls, boulder sources, small craters, and OMAT values should be systematically investigated in other areas to evaluate the universality of the mass-wasting model proposed by this study.

Acknowledgments

We thank Sei-ichiro Watanabe, Sinichi Sirono, and Hiroaki Katsuragi for useful comments and discussions. This work was partly supported by Nagoya University Interdisciplinary Frontier Fellowship and JSPS KAKENHI grant 20H00194.

Data Availability Statement

The NAC image data are available at https://wms.lroc.asu.edu/lroc/search by entering the image names listed in Table 1 (M154370363LC, M1097122291LC, M1390708014LC, M1244314600RC, M141446269LC, M1206659832RC, M139071213LC, and M1206659832LC) into the “Product ID” field. The MI and DEM data are available at https://darts.isas.jaxa.jp/planet/pdap/selene/product_search.html by entering “SELENE MOON MI 5 MAP V3.0” and “SELENE MOON TC 5 DTM MAP SEAMLESS V2.0” into the “Data set name” field, respectively, with latitude and longitude ranges shown in the image names in Table 1. Our estimated data of small craters, boulder trails, and boulder sources, densities of small craters, and OMAT values can be found online (https://1drv.ms/u/s!Au3OlkoCl34TalfovoJGus7iNoY?e=g499Kv). This link is for the peer review only, and the data will be reposited in the Harvard Dataverse (https://dataverse.harvard.edu) after the acceptance of this manuscript.