9:00 AM - 9:15 AM
[PPS02-01] Impact experiments for crater size scaling laws on asteroids covered with low-strength coarse-grained regolith
Keywords:regolith, crater, scaling law, armoring
Recent planetary explorations showed that rubble-pile asteroids were covered with boulders with size distributions and low strength. Moreover, it was found that the number of craters on Itokawa and Eros was small. The craters less than 10 m in the diameter were absent. The mechanism for the lack of small craters is thought to be an armoring effect. The reduction of the crater formation efficiency due to the armoring effect is caused when the impacts occur under the following condition: The target grain size is comparable or larger than an impactor size and the target grains are disrupted during the impact. However, the SCI crater formed on asteroid Ryugu were the same size as a crater typically formed on sand. Ryugu were covered with boulders having size distributions, low strength and high porosity, and these characteristics might cause differences in the armoring effects between the Itokawa and Ryugu surfaces. Therefore, in this study, in order to quantify the armoring effect by examining the effects of boulder strength, porosity, and the size ratio of the boulder to the impactor, we conducted impact experiments on granular targets composed of low strength, high porosity coarse-grains.
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 grains) and the size of 1 to 4 cm (large grains). The crush strength of these tuff particles was measured to be about 60 kPa and 13 kPa, respectively. A spherical projectile with the size of 3 mm (stainless steel, zirconia, alumina, glass, and nylon) was launched at the impact velocity from 40 to 200 m/s, and a spherical projectile with the size of 2 mm (tungsten carbide, copper, stainless steel, zirconia, titan, aluminum, polycarbonate, and nylon) was launched at the impact velocity from 1.2 to 4.5 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.
As a result, the crater radius increases with increasing kinetic energy of a projectile (Ek). However, in the region of the Ek between 0.1 and 0.6 J, the crater radius was almost constants. In this region, the kinetic energy of the projectile is considered to be used to disrupt the target grains. These trend did not depend on the projectile materials. The π-scaling law was applied to our experimental results. We found that the data of the small particle targets was separated into two regions with a clear offset, depending on the impact velocity and the projectile material. Therefore, π-scaling law for crater size should be improved to include the mechanism for the reduction in the crater formation efficiency due to the disruption of the target grains.
We developed π-scaling law for crater size that includes the effect of the grain disruption. We followed the idea proposed by Mizutani et al. (1983), which introduced a late-stage effective energy, I. Assuming that I is used for the disruption of target grains and the growth of crater pores due to the movement of target grains, the following relationship between crater size D and I was obtained by introducing the number of target grains disrupted during the impact, α:k1ρgD4+k2εαδtd3=I . The experimental relationship between crater size D and I can be reproduced by changing α. The relationship between I and α was obtained from the experimental results which can be organized as n. A=13.2,n=1.0 for the small grains targets and A=0.34,n=0.99 for the large grains targets. The π-scaling law for crater size including the decreasing factor f of crater formation efficiency due to armoring effects can be written as follows: πR_d=k4 f π2-a π4b, f={1-k3Aδt(d/D*)3}1/4 .The reduction factor became a constant and was about 0.7 for both the small and large grains targets. Thus, it was found that the effect of target grains disruption reduced the crater size by about 30%.
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 grains) and the size of 1 to 4 cm (large grains). The crush strength of these tuff particles was measured to be about 60 kPa and 13 kPa, respectively. A spherical projectile with the size of 3 mm (stainless steel, zirconia, alumina, glass, and nylon) was launched at the impact velocity from 40 to 200 m/s, and a spherical projectile with the size of 2 mm (tungsten carbide, copper, stainless steel, zirconia, titan, aluminum, polycarbonate, and nylon) was launched at the impact velocity from 1.2 to 4.5 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.
As a result, the crater radius increases with increasing kinetic energy of a projectile (Ek). However, in the region of the Ek between 0.1 and 0.6 J, the crater radius was almost constants. In this region, the kinetic energy of the projectile is considered to be used to disrupt the target grains. These trend did not depend on the projectile materials. The π-scaling law was applied to our experimental results. We found that the data of the small particle targets was separated into two regions with a clear offset, depending on the impact velocity and the projectile material. Therefore, π-scaling law for crater size should be improved to include the mechanism for the reduction in the crater formation efficiency due to the disruption of the target grains.
We developed π-scaling law for crater size that includes the effect of the grain disruption. We followed the idea proposed by Mizutani et al. (1983), which introduced a late-stage effective energy, I. Assuming that I is used for the disruption of target grains and the growth of crater pores due to the movement of target grains, the following relationship between crater size D and I was obtained by introducing the number of target grains disrupted during the impact, α:k1ρgD4+k2εαδtd3=I . The experimental relationship between crater size D and I can be reproduced by changing α. The relationship between I and α was obtained from the experimental results which can be organized as n. A=13.2,n=1.0 for the small grains targets and A=0.34,n=0.99 for the large grains targets. The π-scaling law for crater size including the decreasing factor f of crater formation efficiency due to armoring effects can be written as follows: πR_d=k4 f π2-a π4b, f={1-k3Aδt(d/D*)3}1/4 .The reduction factor became a constant and was about 0.7 for both the small and large grains targets. Thus, it was found that the effect of target grains disruption reduced the crater size by about 30%.