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Register a new userPattern of Impact-Induced Ejecta from Granular Targets with Large Inclusions
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We performed impact experiments to observe patterns in an ejecta curtain with targets consisting of small sand particles and large inclusions comparable to or smaller than the size of the projectiles. The spatial intensity distributions in the ejecta at early stages of crater formation depend on the size of the inclusions. Our numerical simulations of radially spreading particles with different sizes support this result. Based on the results, we proposed a procedure for evaluating the subsurface structures of celestial bodies from the images of ejecta curtains obtained from space-impact experiments.
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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.
When a dense granular jet hits a target, it forms a large dead zone and ejects a highly collimated conical sheet with a well-defined opening angle. Using experiments, simulations, and continuum modeling, we find that this opening angle is insensitive to the precise target shape and the dissipation mechanisms in the flow. We show that this surprising insensitivity arises because dense granular jet impact, though highly dissipative, is nonetheless controlled by the limit of perfect fluid flow.
The collapse of an inclined cohesive granular layer triggered by a certain perturbation can be a model for not only landslides on Earth but also relaxations of asteroidal surface terrains. To understand such terrain dynamics, we conduct a series of experiments of a solid-projectile impact onto an inclined wet granular layer with various water contents and inclination angles. As a result, we find two types of outcomes: crater formation and collapse. The collapse phase is observed when the inclination angle is close to the maximum stable angle and the impact-induced vibration at the bottom of wet granular layer is sufficiently strong. To explain the collapse condition, we propose a simple block model considering the maximum stable angle, inclination angle, and impact-induced vibrational acceleration. Additionally, the attenuating propagation of the impact-induced vibrational acceleration is estimated on the basis of three-dimensional numerical simulations with discrete element method using dry particles. By combining wet-granular experiments and dry-granular simulations, we find that the impact-induced acceleration attenuates anisotropically in space. With a help of this attenuation form, the physical conditions to induce the collapse can be estimated using the block model.
For a fundamental understanding of terrain relaxation occurring on sloped surfaces of terrestrial bodies, we analyze the crater shape produced by an impact on an inclined granular (dry-sand) layer. Owing to asymmetric ejecta deposition followed by landsliding, the slope of the impacted inclined surface can be relaxed. Using the experimental results of a solid projectile impact on an inclined dry-sand layer, we measure the distance of centroid migration induced by asymmetric cratering. We find that the centroid migration distance $x_mathrm{mig}$ normalized to the crater minor-axis diameter $D_mathrm{cy}$ can be expressed as a function of the initial inclination of the target $tantheta$, the effective friction coefficient $mu$, and two parameters $K$ and $c$ that characterize the asymmetric ejecta deposition and oblique impact effect: $x_mathrm{mig}/D_mathrm{cy}=K tantheta/(1-(tantheta/mu)^2)+c$, where $K=0.6$, $mu=0.8$, and $c=-0.1$ to $0.3$. This result is consistent with a previous study that considered the effect of asymmetric ejecta deposition. The obtained results provide fundamental information for analyzing the degradation of sloped terrain on planetary surfaces, such as crater-shape degradation due to the accumulation of micro-impacts.
Although a large number of astronomical craters are actually produced by the oblique impacts onto inclined surfaces, most of the laboratory experiments mimicking the impact cratering have been performed by the vertical impact onto a horizontal target surface. In previous studies on the effects of oblique impact and inclined terrain, only one of the impact angle $varphi$ or target inclination angle $theta$ has been varied in the experiments. Therefore, we perform impact-cratering experiments by systematically varying both $varphi$ and $theta$. A solid projectile of diameter $D_{rm i}=6$~mm is impacted onto a sand surface with the range of impact velocity $v_{rm i}=7$--$97$~m~s$^{-1}$. From the experimental result, we develop scaling laws for the crater dimensions on the basis of $Pi$-group scaling. As a result, the crater dimensions such as cavity volume, diameter, aspect ratio, and depth-diameter ratio can be scaled by the factors $sin varphi$ and $cos theta$ as well as the usual impact parameters ($v_{rm i}$, $D_{rm i}$, density of projectile, and surface gravity). Finally, we consider the possible application of the obtained scaling laws to the estimate of impact conditions (e.g., impact speed and impact angle) in natural crater records.
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