Degradation of the Lunar Surface by Small Impacts

Abstract

The surfaces of airless bodies like the Moon are bombarded by a steady stream of small impactors that lead to erosion of the topography over time. However, the rate of degradation from small impacts, a key parameter in interpreting the ages of present-day lunar surface features, is not well constrained. Here we demonstrate, using a numerical mass transport model, that impact erosion is a nonlinear diffusion process, in contrast to past studies of crater degradation that have assumed that the downslope mass flux of ejecta is linearly proportional to hillslope gradient. Nonlinearity is a consequence of the asymmetric shape of ejecta blankets on sloped surfaces, and as a result, the degradation rate on steep slopes is over 40% greater than on nearly flat surfaces. Using measurements of the morphology and formation rate of small primary and secondary craters, the kilometer-scale lunar landscape diffusivity is computed and compared to the value inferred from topographic profiles of degraded craters. We show that the abundance of decameter-scale craters forming on the Moon over the past decade is consistent with small impacts dominating the erosion of the lunar landscape, but only if the primary size−frequency distribution remains steep down to the submillimeter scale.

Depth-to-diameter Ratios of Fresh Craters on the Moon and Implications for Surface Age Estimates

“…Surfaces of bodies that lack an atmosphere (e.g., the Moon) are inundated with impacts, resulting in impact craters that influence the topography of the surface. On airless bodies throughout the solar system, smaller meter-scale impacts are much more common than larger impacts, and therefore sub-kilometer-scale impacts control more of the degradation and erosion of the lunar surface over time when compared to kilometer-scale impacts (Xie et al 2017;Fassett et al 2022;O'Brien & Byrne 2022;Fassett & Thompson 2014;Ross 1968;Soderblom 1970;Craddock & Howard 2000;Howard 2007).…”

“…Topographic diffusion on the Moon may be a result of several different diffusive processes, including thermal expansion and seismic shaking, but the dominant process is thought to be topographic degradation from small impact craters (e.g., O'Brien & Byrne 2022). To use a lunar topographic diffusion model, assumptions must be made regarding the initial d/D of small lunar craters (e.g., Fassett & Thomson 2014;Speyerer et al 2016;Stopar et al 2017;Xie et al 2017;Mahanti et al 2018;Fassett et al 2022;O'Brien & Byrne 2022). Variations seen in d/D values of craters can be caused by many different factors, including the presence of volatiles in the target material (e.g., Boyce et al 2005 for craters on Mars); variations in the target strength and material properties (e.g., Herrick & Hynek 2017;Krüger et al 2018;Osinski et al 2019); postimpact surface modification, degradation, or infilling of the crater (e.g., Fassett & Thomson 2014); and/or velocity of the impactor (e.g., Silber et al 2017;Prieur et al 2018).…”

“…It is important to further understand results reported in Stopar et al (2017) since several studies use their results for modeling, which has implications for lunar surface degradation rates and ages (e.g., Minton et al 2019;Xie et al 2019;O'Brien & Byrne 2022;Fassett et al 2022). Stopar et al (2017) expanded on previous work to more thoroughly investigate variations in d/D based on degradation states and to more rigorously investigate minimally modified, and therefore assumed-to-be-young, craters.…”