11:00 AM - 1:00 PM
[PPS03-P13] Impact experiments on a target simulating the surface of a rubble-pile asteroid : Effects of weak-strength coarse-grained regolith on crater size and impact-induced seismic shaking
Keywords:crater, impact-induced seismic shaking, rubble-pile asteroid, impact, coarse, low strength
Impact crater formation on a non-cohesive regolith layer of asteroids is widely accepted to be controlled by gravity. Impact cratering experiments in the gravity regime have been carried out for a non-cohesive granular target simulating a regolith layer under various impact conditions, and several scaling laws were proposed[e.g.,1]. However, recent planetary explorations show that rubble-pile asteroids are covered with boulders with size distributions[2], and low strength[3]. Therefore, a detailed study to elucidate the effects of the size distribution and the boulder strength would be necessary to investigate the impact craters observed on rubble-pile asteroids. However, a conventional crater size scaling law in the gravity regime is not enough to include these effects, and so we focus on the following effects: one is a projectile to target grain size ratio, another is a weak strength of boulders. Seismic shaking induced by a high-velocity impact is one of the most important surface geologic processes that occurs on an asteroid surface. Such shaking could modify the asteroid surface topography and dissipate small impact craters. Therefore, we conducted impact experiments on granular targets composed of low strength and coarse-grains to study the effects of size distribution and strength on the crater size scaling law and the impact-induced seismic shaking processes on asteroids.
Cratering experiments were conducted by using vertical gas gun sets at Kobe University and ISAS. Granular targets were prepared by using weathered tuff granules with the size of 1 to 4 mm (small particle) and the size of 1 to 4 cm (large particle). The crush strength of these tuff particles was measured to be about 60 kPa and 30 kPa, respectively. A spherical projectile with the size of 3mm (stainless steel, zirconia, alumina, glass, and nylon) was launched at the impact velocity from 40 to 200m/s, and a spherical projectile with the size of 2mm (tungsten carbide, copper, stainless steel, zirconia, titan, aluminum, and nylon) was launched at the impact velocity from 1.2 to 6.3 km/s. These projectiles were impacted on the target surface at the normal direction. Impact cratering phenomena were observed by a high-speed camera at the frame rate of 103-105 fps. After each shot, the crater morphology was observed by using the 2D laser displacement, and the diameter and the depth were measured. Impact-induced seismic waves were measured by using three accelerometers (a specific frequency is 30kHz) at different positions from the impact point, and a data logger was used to record the seismic data through charge amplifiers (the data acquisition rate was 100 kHz).
The crater size was found to increase with the projectile kinetic energy (Ek) at lower than 0.14J and at higher than 0.63J, so that the crater size was almost constant among them, and more the crater size obtained at very high Ek was almost on the extrapolated line from the relationship given by the low Ek data. This trend did not depend on the projectile materials but the crater radius of the large particle target was smaller than that of the small particle target at the same Ek.
Cratering experiments were conducted by using vertical gas gun sets at Kobe University and ISAS. Granular targets were prepared by using weathered tuff granules with the size of 1 to 4 mm (small particle) and the size of 1 to 4 cm (large particle). The crush strength of these tuff particles was measured to be about 60 kPa and 30 kPa, respectively. A spherical projectile with the size of 3mm (stainless steel, zirconia, alumina, glass, and nylon) was launched at the impact velocity from 40 to 200m/s, and a spherical projectile with the size of 2mm (tungsten carbide, copper, stainless steel, zirconia, titan, aluminum, and nylon) was launched at the impact velocity from 1.2 to 6.3 km/s. These projectiles were impacted on the target surface at the normal direction. Impact cratering phenomena were observed by a high-speed camera at the frame rate of 103-105 fps. After each shot, the crater morphology was observed by using the 2D laser displacement, and the diameter and the depth were measured. Impact-induced seismic waves were measured by using three accelerometers (a specific frequency is 30kHz) at different positions from the impact point, and a data logger was used to record the seismic data through charge amplifiers (the data acquisition rate was 100 kHz).
The crater size was found to increase with the projectile kinetic energy (Ek) at lower than 0.14J and at higher than 0.63J, so that the crater size was almost constant among them, and more the crater size obtained at very high Ek was almost on the extrapolated line from the relationship given by the low Ek data. This trend did not depend on the projectile materials but the crater radius of the large particle target was smaller than that of the small particle target at the same Ek.