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.