Crater-ray formation through mutual collisions of hypervelocity-impact induced ejecta particles

Abstract

We investigate the patterns observed in ejecta curtain induced by hypervelocity impact (2−6 km/s) with a variety of the size and shape of target particles. We characterize the patterns by an angle, defined as the ratio of the characteristic length of the pattern obtained by Fourier transformation to the distance from the impact point. This angle is found to be almost the same as that obtained by the reanalysis of the patterns in the previous study at lower impact velocities (Kadono et al., 2015, Icarus 250, 215-221), which are consistent with lunar crater-ray systems. Assuming that the pattern is formed by mutual collision of particles with fluctuation velocity in excavation flow, we evaluate an angle at which the pattern growth stops and show that this angle is the same in the order of magnitude as the ratio of the fluctuation velocity and the radial velocity. This relation is confirmed in the results of experiments and numerical simulations. Finally, we discuss the dependence of the patterns on impact conditions. The experiments show no dependence of the angle on impact velocity. This indicates that the ratio between the fluctuation and radial velocity components in excavation flow does not depend on impact velocity. Moreover, the independences on particle size and particle shape suggest that the angle characterizing the structure of the patterns does not depend on cohesive force. Since cohesive forces should be related with elastic properties of particles, the structure does not depend on elastic properties, though inelastic collisions are important for the persistence and contrast of the patterns.

Modeling Early Clustering of Impact-induced Ejecta Particles Based on Laboratory and Numerical Experiments

“…Our simulations presented in the previous section confirm the expectation by Kadono et al (2015Kadono et al ( , 2020 that the mesh pattern in ejecta evolves in two stages: initial growth through inelastic particle collisions and subsequent geometric expansion with no particle collision. However, it is yet to be addressed when the initial growth stage is terminated and what initial conditions set the final size of the clusters constituting the pattern.…”

“…Moreover, the snapshots at 76 and 91 μs after the impact show that the pattern simply expands with the curtain (see the red circles in the lower panels of Figure 2). Kadono et al (2020) also observed the expansion of ejecta curtain patterns in their similar experiments, but at ∼10 ms after impact. Our experimental results presented here indicate that pattern formation occurs and is completed much earlier, on a timescale comparable to the expansion timescale of the ejecta curtain.…”

“…To avoid this issue, we use a monochromatic light source and a bandpass filter, which enable us to image an ejecta curtain as early as 10 μs after the hypervelocity impact of a millimeter-sized projectile. For comparison, Kadono et al (2020) conducted similar experiments but were only able to image the ejecta curtains ∼10 ms after the impacts.…”

“…While there are several studies characterizing crater rays on a variety of planetary bodies (e.g., Trask & Guest 1975;McEwen et al 2005), only a few studies have addressed the physical mechanisms responsible for crater-ray formation (e.g., Shuvalov 2012; Kadono et al 2015Kadono et al , 2020Sabuwala et al 2018). Shuvalov (2012) proposed that interaction of impact-induced shock waves with preexisting craters can produce crater rays.…”

“…More recently, Kadono et al (2015Kadono et al ( , 2020 proposed an alternative idea, which we focus on in this study, that crater rays are formed by an enhanced contrast in the particle number density in an ejecta curtain. They conducted impact experiments on granular targets and found that the resulting ejecta curtains have mesh-like patterns.…”