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:
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 0a 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.