11:30 AM - 11:45 AM
[PPS03-04] Impact cratering experiments on glass beads with bead size frequency distribution: Implications for cratering process on asteroid Ryugu
Keywords:cratering mechanism, asteroids, boulders, size frequency distribution, crater scaling law
We conducted cratering experiments by using a one-stage light gas gun at Kobe University and a two-stage light gas gun at ISAS. We used glass beads having the diameters of 0.1, 1, 3, and 10 mm as a target and prepared four types of target; two targets consist of one size of glass bead (0.1 or 3 mm glass beads) and the others consist of mixing three (1-10 mm) or four (0.1-10 mm) different-sized glass beads with the same mass ratio. We used only four glass bead mixing target at ISAS. At Kobe University, the stainless-steel spheres with a diameter of 2 and 3 mm were used as a projectile and the impact velocity was changed from 53 to 181 m/s. At ISAS, the aluminum projectile with a diameter of 1 mm was used and the impact velocity was changed from 1.2 to 4.4 km/s. The collisional phenomena were recorded by high speed camera to observe the rebound of the projectile and the ejection of glass beads.
We observed the ejection of glass beads from the target surface. In the case of 0.1-mm bead target, the beads were ejected radially and the corn-shaped ejecta curtain was observed. In the case of 3-mm bead target, some intact beads were ejected at lower speed. In the case of four bead mixing target, some 0.1-mm beads were ejected like a smoke passing through other beads. Furthermore, the projectile rebounded from the target surface in most cases of 3-mm bead target and two mixing bead targets. When the projectile collided with a 10-mm bead, the amount of the ejected beads decreased drastically.
We applied our results to the crater scaling law showing as πR=a･π2-b, where πR is the non-dimensional parameter related to the crater radius, π2 is the non-dimensional parameter related to the gravity, the projectile size, and the impact velocity, and a and b are constants. As a result, the πR of the 0.1-mm bead target was largest. The πR of the 3-mm bead target and two mixing bead targets matched each other and a little smaller than that of the 0.1-mm bead target when the projectile did not collide with a 10 mm glass bead. Furthermore, the πR of mixing bead targets was much smaller when the projectile collided with a 10-mm bead.
Finally, we revised the crater scaling law of mixing bead targets by using the scaling law of the 0.1-mm bead target (a=0.58 and b=0.21). In this study, we assumed that the projectile collided with a 3-mm or 10-mm bead and the momentum of the projectile was transferred to the impacted bead, then the bead penetrated into the target slowly and the cavity was formed. On this assumption, we constructed the revised crater scaling law by using the coefficient of the restitution of projectile, e, and the momentum conservation law. As a result, we could obtain e=0.5-1 to match our experimental results with the revised crater scaling law.
 Satakani et al. (2018), 2nd Aqua Planetology Symposium, Kobe University.  Tatsumi & Sugita (2018), Icarus 300, 227-248.