Topographic Degradation Processes of Lunar Crater Walls Inferred from
Boulder Falls
Ayame Ikeda1, Hiroyuki Kumagai1, and
Tomokatsu Morota2
1Graduate School of Environmental Sciences, Nagoya
University, Nagoya, Japan
2Graduate School of Science, The University of Tokyo,
Tokyo, Japan
Corresponding author: Ayame Ikeda (ikeda.ayame@k.mbox.nagoya-u.ac.jp)
Key Points:
- Distributions of boulder falls, small craters, and optical maturity
values on lunar crater walls were correlated with crater slope angles.
- Boulder falls were triggered by small meteorite impacts near boulders
rather than by shallow moonquakes.
- We proposed a model for the generation and transport of boulders and
regolith on crater walls resulting in the degradation of craters.
Abstract
Recent explorations by lunar orbiters have shown that boulder falls are
distributed over the entire lunar surface. To quantitatively evaluate
the effects of moonquakes and meteorite impacts on boulder falls, we
performed detailed surveys at two sites: one in the southern part of the
Schrödinger basin (Site 1) and the other in Laue crater (Site 2). Using
images and topography data from the Lunar Reconnaissance Orbiter and
KAGUYA, we estimated the detailed distributions of boulder falls, small
craters, slope angles, the optical maturity parameter (OMAT), and
maximum acceleration due to impacts at these sites. In steeply sloping
areas at both sites, we found that the density of small craters was
small and areas with high OMAT values corresponded to boulder sources,
where many boulders exist. At Site 1, the starting points of boulder
falls and acceleration due to impacts were correlated. In addition,
craters with boulder falls at and around Site 2 were distributed
independently of the epicentral distance from a shallow moonquake that
occurred in 1975 near Site 2, which was previously inferred to have
triggered boulder falls at the site. Our results suggest that boulder
falls at these sites were triggered not by moonquakes but by meteorite
impacts. We propose a model for the generation and transport of boulders
and regolith on slopes by meteorite impacts, which may be directly
related to the degradation of crater slopes on the Moon.
Plain Language Summary
Large rocks (a few to tens of meters in size) or boulders with
accompanying trails on crater slopes have been widely found on the Moon
from high-resolution images taken by recent lunar orbiters. These
observations indicate that crater slopes experienced large ground
shaking that triggered boulder falls. There are two ideas to explain how
boulder falls occurred: one by seismic events or moonquakes and the
other by meteorite impacts. In order to investigate the cause of boulder
falls, we performed detailed analysis of image data at two sites, where
boulders and boulder falls were found on slopes of large craters. Our
results show that there are many boulders in steeply sloping areas near
the edges of the large craters, where fresh materials are exposed. We
found that the starting points of boulder falls exist in large shaking
areas during meteorite impacts that produced small craters on the
slopes, but no clear evidence to support that boulder falls were
triggered by moonquakes. These results indicate that boulders were
generated in the upslope areas and their falls were triggered by
meteorite impacts. Our findings contribute to understand how topography
changes with the movement of rocks and soil on crater slopes.
1. Introduction
Mass wasting, which occurs over the entire surface of the Moon, is
fundamental to understanding topographic degradation and recent
near-surface activity on the Moon (e.g., Xiao et al., 2013). Although
our understanding of the physical mechanisms of mass-wasting phenomena
such as rock or boulder falls and landslides on the lunar surface have
been hindered by limited observations, recent explorations by lunar
orbiters have improved our knowledge of mass-wasting processes. The
Japanese lunar orbiter SELENE, which is known in Japan by its nickname
KAGUYA, is equipped with a multi-band imager, laser altimeter, and
terrain camera and provided global image data during 2007−2009 that
clarified the detailed topography of the entire lunar surface (Araki et
al., 2009; Haruyama et al., 2008) and the compositions of lunar rocks
(Ohtake et al., 2009). The Lunar Reconnaissance Orbiter (LRO) operated
by NASA from 2009 to the present has provided high-resolution images
taken by the Lunar Reconnaissance Orbiter Camera (LROC) (Robinson et
al., 2010) in which small topographic features such as lobate scarps and
boulder falls can be identified (e.g., Kumar et al., 2016; Watters et
al., 2010). The huge LROC image data archive has been analyzed by using
a deep learning approach to estimate global boulder distributions (e.g.,
Bickel et al., 2020) and temporal topographic changes by comparing
images taken at different times (e.g., Robinson et al., 2015). Lobate
scarps, which are widely distributed on the lunar surface (Watters et
al., 2015, 2019), may be formed by tidal stresses (Watters et al., 2019)
or contraction of the Moon (Watters et al., 2010). Boulders and boulder
falls with accompanying trails (e.g., Kumar et al., 2016; 2019) have
been identified across the lunar surface, especially on crater walls
(Bickel et al., 2020, 2021).
Kumar et al. (2016) investigated the detailed distribution of boulder
falls in the southern part of the Schrödinger basin, about 8 km from
nearby lobate scarps. Kumar et al. (2019) studied an area in Laue
crater, where a shallow moonquake was recorded on 3 January 1975 by the
Apollo lunar seismograph network (Nakamura et al., 1979). Mohanty et al.
(2020) mapped boulder falls in the Orientale basin, where abundant
tectonic structures such as normal faults along basin rings and grabens
are found. They concluded that, in these areas, shallow moonquakes at
lobate scarps and tectonic faults triggered boulder falls. However, it
is not known whether lobate scarps are active faults that generate
moonquakes radiating high-frequency seismic waves and triggering boulder
falls. Kokelaar et al. (2017) and Houston et al. (1973) noticed that
ground shaking due to impacts is an important contributor to mass
wasting on the Moon. Xiao et al. (2013) indicated that mass wasting can
be triggered by both impact cratering and moonquakes, and that cratering
induces seismic shock waves and crushes subsurface bedrock, causing the
formation of fractured zones beneath the crater floor. Xiao et al.
(2013) also noted that when moonquakes occur repeatedly, the accumulated
damage may promote various mass-wasting phenomena, even though a single
moonquake may not be strong enough to cause mass wasting. Bickel et al.
(2020, 2021) suggested that impacts during the Late Heavy Bombardment
event, about 3.9 billion years ago (e.g., Head et al., 2010; Tera et
al., 1974), fractured the bedrock, and the resultant rock fragments have
been brought to the lunar surface as boulders over billions of years by
continuous meteorite impacts. Kumar et al. (2019) inferred that a single
moonquake triggered boulder falls in Laue crater, but they did not
quantitatively evaluate ground shaking due to impacts in their studies
(Kumar et al., 2016, 2019).
Boulder falls provide invaluable information on how ground shaking
occurs on the Moon, and this information is fundamentally important in
investigations of dynamic processes associated with mass wasting. In
this study, we quantitively evaluated the effects of both moonquakes and
meteorite impacts on boulder falls in the two areas studied by Kumar et
al. (2016, 2019) to improve understanding of the physical mechanisms of
mass-wasting processes. Using images obtained by the LRO and KAGUYA, we
systematically estimated the distributions of boulder falls, impact
craters, and associated ground shaking, slope angles, and the optical
maturity parameter (OMAT) at the two sites. The results of our
comparison of these distributions strongly suggest that boulder falls at
our study sites were caused by small impacts on crater slopes.
Considering these results, we propose a model for the formation and
downslope movement of boulders and regolith by such impacts that can
explain the degradation of crater slopes at not only these sites but
also other crater sites on the Moon.
2. Study Areas, Data, and Methods
We explored two sites (Sites 1 and 2; Figure 1) previously studied by
Kumar et al. (2016, 2019). Site 1 (5 km NS × 7 km EW) is located on the
inner southern wall of the Schrödinger basin (79.35°−79.48°S,
128.4°−129.5°E), which is near the South Pole–Aitken basin, the largest
basin on the Moon. Lobate scarps occur to the north of Site 1; the
closest one is 8 km from our study area (Figure 1b). Site 2 is a small
crater (28.5°−29°N, 262.5°−263°E; diameter 8 km) located on the floor of
Laue crater. Lobate scarps are located on the Lorentz basin wall to the
south of Site 2 (Kumar et al., 2019) (Figure 1c). Kumar et al. (2019)
inferred that a moonquake with a seismic moment magnitude
(Mw ) of 4.1 occurred on 3 January 1975 along the
longest lobate scarp segment (Figure 1c).
We measured boulder falls and small craters on images obtained by the
LRO Narrow Angle Camera (NAC), which captured optical black and white
images with a resolution of 0.5 m/pixel (Robinson et al., 2010). We also
used digital terrain model (DTM) data created from stereo images taken
by the terrain camera (TC; resolution 10 m/pixel, Haruyama et al., 2008)
onboard KAGUYA to estimate slope angles, and we used multi-band image
data from the multi-band imager (MI) onboard KAGUYA to estimate OMAT
values. The MI data comprise nine bands in the 415−1000 nm (VIS) and
1000−1550 nm (NIR) wavelength ranges (Ohtake et al., 2009) with
resolutions of 20 m (VIS) and 62 m (NIR). The images and topographic
data used in this study are listed in Table 1.
Surface age was estimated by the crater counting method, which uses the
crater size–frequency distribution (CSFD), which was assumed to be
stable over time, and the cratering chronology model derived from the
relationship between crater density and the radiometric ages of lunar
samples (Neukum, 1983; Neukum et al., 2001). We used the CSFD described
by the following production function (Neukum, 1983):
,
where D is the crater diameter (km), N (D ) is the
cumulative number of craters with diameter larger than D per unit
area (km−2), anda 0−a 11 are polynomial
coefficients (see Neukum, 1983, for the coefficient values). The crater
chronology function is as follows (Neukum, 1983):
,
where N (1) is the cumulative number of craters with D = 1
km and T is time (Gyr). We used the Craterstats software (Michael
& Neukum, 2010) and equations (1) and (2) to determine surface ages in
the study areas.
To estimate the peak ground acceleration (PGA, cm/s2)
induced by meteorite impacts and moonquakes, we used the following
attenuation equation for earthquakes (Kanno et al., 2006):
,
where X is epicentral distance (km). This equation was derived
from strong-motion records of earthquakes with Mw> 5.5 occurring during 1996−2003 in Japan and other
countries. We extrapolated this equation for small meteorite impacts and
moonquakes, where PGA was normalized by the lunar gravitational
acceleration (g = 1.62 m/s2). Although faulting
that generates seismic waves is considered to be identical between
moonquakes and earthquakes, seismic structural differences, especially
in intrinsic and scattering attenuations (\(Q_{i}^{-1}\) and\(Q_{s}^{-1}\), respectively), between the Moon and Earth may result in
different seismic wave attenuations between moonquakes and earthquakes.Qi values in the upper mantle and crust of the
Moon and Earth have been estimated to be more than 4000 (Nakamura &
Koyama, 1982) and about 100−500 (Dziewonski & Anderson, 1981),
respectively, indicating that intrinsic attenuation is relatively small
on the Moon. On the other hand,
total
scattering coefficients (g 0) around a frequency
(f ) of 1 Hz are 1−2 orders larger on the Moon (about
10–3 m–1) than on the Earth (about
10–5 m–1) (Sato et al., 2012).
Accordingly, Qs = 2πf/(g 0β ) with an S -wave velocity
(β ) of 3000 m/s is estimated to be 2 on the Moon and 200 on
Earth; thus, scattering attenuation is relatively large on the Moon.
Therefore, equation (3) may not be suitable for estimating absolute PGA
values but can be used to estimate relative values within small
hypocentral distances, where intrinsic and scattering attenuation
effects are considered to be small.
Table 1. Images and topographic data used in this study.