Boulder Distributions Around Young, Small Lunar Impact Craters and Implications for Regolith Production Rates and Landing Site Safety

Abstract

We use Lunar Reconnaissance Orbiter Camera Narrow Angle Camera images to characterize boulder populations around six small (<1 km), young (<200 Ma) impact craters near spacecraft landing sites. The Narrow Angle Camera boulder counts are used to analyze how boulder distributions vary around craters of different sizes and ages. These comparisons inform how various properties affect the distance to which boulders are ejected and the size and density of boulders produced by an impact event. The counts show that boulder population densities decrease with crater age, with few boulders remaining at craters older than a few hundred million years, consistent with results of other studies of boulder degradation rates on the Moon. Variations in boulder distributions around younger craters may provide information regarding impact conditions; South Ray crater has a larger population of small boulders than the larger North Ray crater, which could be explained by variations in impact velocity. Large craters generally excavate more boulders than smaller craters, and the size of the largest boulder ejected is related to crater size by a power‐law function. Larger boulders occur closer to the crater rim (within 2–4 crater radii), whereas smaller boulders occur at all distances. The density of boulders is greater near the crater rim and decreases with increasing radial distance; this data can aid in establishing safe landing zones for future missions. Analyzing boulder distributions across craters of varying ages allows us to test models of boulder breakdown rates, with implications for understanding the Moon's regolith production rate.

Prolonged Rock Exhumation at the Rims of Kilometer‐Scale Lunar Craters

“…Those prior studies used high-resolution LROC images to manually count the boulders present on various impact ejecta blankets and compare those distributions with the modeled age of the impact crater to establish boulder lifetimes. Results generally agree that even the largest boulders on the lunar surface should be completely broken down in no longer than ∼300 Myr with a median boulder survival time of ∼40-80 Myr (e.g., Basilevsky et al, 2013;Watkins et al, 2019). While boulders are present in other locations on the lunar surface such as rilles, wrinkle ridges, and domes, impact craters and associated ejecta blankets have been the main study sites for examining boulder lifetimes on the lunar surface.…”

“…Those prior studies used high‐resolution LROC images to manually count the boulders present on various impact ejecta blankets and compare those distributions with the modeled age of the impact crater to establish boulder lifetimes. Results generally agree that even the largest boulders on the lunar surface should be completely broken down in no longer than ∼300 Myr with a median boulder survival time of ∼40–80 Myr (e.g., Basilevsky et al., 2013; Watkins et al., 2019).…”

Lunar Secondary Craters and Estimated Ejecta Block Sizes Reveal a Scale‐Dependent Fragmentation Trend

“…Krishna and Kumar (2016) studied the boulder field around the bright-rayed, 3.8-km-diameter Censorinus crater and found large blocks near the rim were~20-50 m in diameter, and maximum block sizes of~10 m closer to the edge of the ejecta blanket. Watkins et al (2019) measured boulders around small primary craters near spacecraft landing sites ranging in diameter from 0.2 to 0.95 km. These craters (and boulder fields) had a range of degradation states but they report a linear relationship of d block,max ¼ 0.022 D. Our measured block sizes are in accordance with these previous works.…”

Particle Size-Frequency Distributions of the OSIRIS-REx Candidate Sample Sites on Asteroid (101955) Bennu

“…The goal of determining PSFDs is often to assess candidate landing or sampling sites (e.g., [4][5][6][7]) and to understand the geophysical processes responsible for shaping a planetary surface (e.g., [8][9][10][11][12][13]). For example, on the Moon, PSFDs have been used to understand rock breakdown and regolith evolution rates (e.g., [14,15]). PSFDs are also often used to understand impact processes, including the formation of secondary craters (e.g., [16][17][18]).…”