Cumulative Mass-velocity Distribution of Impact Ejecta in Oblique Impacts

Introduction: The cumulative mass M(>ve) of impact ejecta with velocities higher than ve (hereafter we call this cumulative mass-velocity (CMV) distribution) is one of the most important parameters for understanding quantitatively the morphology of ejecta blankets around craters or the surface evolution of planetary bodies. A scaling law on CMV distribution has been formulated based on the data measured by vertical impact experiments [1,2,3,4]. However, the impact angle dependence on the scaling law on CMV distribution is unclear. This is because there are few data on CMV distribution for oblique impacts, especially for the lowvelocity ejecta (ve< a few m/s). In this study, we measured M(>ve) for the ejecta with ve of a few m/s for various impact angles, and studied the impact angle dependence on the scaling law on CMV distribution. Experiments: Polycarbonate projectiles with mass of 0.49 g and diameter of 10 mm were accelerated by a single-stage light-gas gun. Impact velocity of the projectile was 192±5 m/s. We can tilt the gas gun at various angles (θ=15°, 30°, 45°, 60°, and 90° to the target), so that we can do oblique impacts. We prepared soda-lime glass spheres with mean diameter of 220μm as the target material. A stainless basin (40 cm diameter and 15 cm depth) filled by the glass spheres was placed in a vacuum chamber with the ambient pressure < 90 Pa. We measured CMV distribution M(>ve) in a way of the collector method [e.g., 2,5], which is as follows: After experiment, we measured the mass of the glass spheres deposited in glass cylindrical collectors (2.7 cm diameter and 4.6 cm height) set around the glass sphere target. Then we can derive the total mass M(>r) of the glass spheres deposited beyond the distance r from the impact site. In order to estimate the ejection velocities ve from the distance r, we need to know the ejection angle α and the launch position L for each ejecta. However, it is difficult experimentally to determine α and L for each ejecta. Therefore, we assumed constant values for α and L for all ejecta: We measured that the angle of ejecta curtain ranged from 45° to 60°, irrespective of θ. We can consider the case with α=60° and L=0 as the maximum ejection velocities ve1 and α=45° and L=R (R is the apparent crater radius) as the minimum ejection velocities ve2, respectively. Then we defined the ejection velocity as ve=(ve1+ ve2)/2. From M(>r) and ve, we can estimate CMV distribution. 1 2 4 6 8 10 2 4 6 8 100 2