Effect of impact velocity and acoustic fluidization on the simple‐to‐complex transition of lunar craters

Abstract

We use numerical modeling to investigate the combined effects of impact velocity and acoustic fluidization on lunar craters in the simple‐to‐complex transition regime. To investigate the full scope of the problem, we employed the two widely adopted Block‐Model of acoustic fluidization scaling assumptions (scaling block size by impactor size and scaling by coupling parameter) and compared their outcomes. Impactor size and velocity were varied, such that large/slow and small/fast impactors would produce craters of the same diameter within a suite of simulations, ranging in diameter from 10 to 26 km, which straddles the simple‐to‐complex crater transition on the Moon. Our study suggests that the transition from simple to complex structures is highly sensitive to the choice of the time decay and viscosity constants in the Block‐Model of acoustic fluidization. Moreover, the combination of impactor size and velocity plays a greater role than previously thought in the morphology of craters in the simple‐to‐complex size range. We propose that scaling of block size by impactor size is an appropriate choice for modeling simple‐to‐complex craters on planetary surfaces, including both varying and constant impact velocities, as the modeling results are more consistent with the observed morphology of lunar craters. This scaling suggests that the simple‐to‐complex transition occurs at a larger crater size, if higher impact velocities are considered, and is consistent with the observation that the simple‐to‐complex transition occurs at larger sizes on Mercury than Mars.

Transitional impact craters on the Moon: Insight into the effect of target lithology on the impact cratering process

“…), the formation of terraces and/or flat floors remains poorly described by present models (e.g., Silber et al. ).…”

“…Although Silber et al. () found notable morphological differences between the craters that have the same diameter, but are formed by projectiles with varying size/velocity, they noted that projectile characteristics cannot account for all the observed variations in crater morphologies on different targets. Their study suggests, however, that higher impact velocities may lead to a higher onset diameter for complex structures, which is consistent to observations on Mercury.…”

Impact Crater Morphology and the Structure of Europa's Ice Shell

“…The average impact velocity on Europa is 26 km/s (Zahnle et al, ), higher than that implemented in this study and other numerical studies (e.g., Bray et al, ; Cox & Bauer, ). Moreover, the morphology (e.g., onset of flat floors and central peaks) of impact craters, especially in the simple‐to‐complex transition regime, is highly sensitive to impact velocity, as shown by recent study on lunar craters (Silber et al, ). Silber et al () modeled lunar craters ( D = 10–26 km) forming as a result of impacts at velocities 6–15 km/s.…”

Geologic Analyses of the Causes of Morphological Variations in Lunar Craters Within the Simple‐to‐Complex Transition

“…Thus, similar-sized craters on targets with similar properties can be formed from a similar combination of impactor properties. However, impactor density, size, velocity, impact angle, and other potentially relevant parameters can vary significantly within that combination (see Table 2 in Silber et al, 2017) and possibly influence the shape and morphology of the craters. It is known from impact simulations and experimental studies that highly oblique impacts (12°or less; Bottke et al, 2000) create elliptical craters (Elbeshausen et al, 2013;Gault & Wedekind, 1978;Herrick, 2014) with asymmetric ejecta patterns and downrange offset of the central peak (Elbeshausen et al, 2009;Shuvalov & Dypvik, 2004).…”

“…It is known from impact simulations and experimental studies that highly oblique impacts (12°or less; Bottke et al, 2000) create elliptical craters (Elbeshausen et al, 2013;Gault & Wedekind, 1978;Herrick, 2014) with asymmetric ejecta patterns and downrange offset of the central peak (Elbeshausen et al, 2009;Shuvalov & Dypvik, 2004). A numerical modeling study suggested that the transition from simple to complex crater morphologies occurs at smaller diameters with an increase in impactor size or decrease in impactor velocity for acoustically fluidized targets (Silber et al, 2017).…”