5:15 PM - 7:15 PM
[PPS07-P14] Atmospheric Effects on the Formation of Ejecta Morphology in Venusian Craters: High-Velocity Oblique Impact Experiments in the Atmosphere
Keywords:Impact Experiment, Atmosphere, Crater, Venus
High-velocity impact phenomena play an important role for the formation of surface topography on planetary bodies, so understanding their processes is important to elucidate the origin of surface topography on solid bodies. Investigating these phenomena offers valuable insights into the topography and surface evolution of not only Earth but also other planets, such as Mars and Venus. Some of the Venusian rampart craters show defects in the ejecta emplacement; this morphology may be due to the high atmospheric pressure of Venus and the low angle of oblique impact. Previous studies suggest that the missing sector angle increases with smaller impact angles [1]. A lot of studies have been conducted to estimate impact conditions from crater morphology [1,2,3], but accurate estimation is difficult because it is based on multiple assumptions such as impact velocity, impactor density and impact angle. However, few experimental studies have focused on the detailed observation of the interaction between the atmosphere and projectiles during high-velocity impacts under atmospheric pressure, as well as their effects on the ground surface. In this study, we conducted high-velocity impact experiments in the atmosphere on sand surface and made detailed observations for shock wave and trailing flow induced in the atmosphere using shadowgraph method. Then we compared our results with Venus craters and evaluated the atmospheric effects on the crater topography.
All experiments were performed using horizontal two-stage light gas gun at Kobe University. The impact velocities were used at 2 km/s to 5 km/s under ambient pressures from 350 Pa to 70 kPa. The projectile was polycarbonate spheres with the diameter of 4.7 mm, and the targets were quartz sand with a grain size of 100 µm, which simulated the surface of a solid body. The target was set at 20º to the projectile trajectory, so the oblique impact was conducted. All experiments were observed by two high-speed cameras, one using visible light and the other using a shadowgraph method that visualized density differences of the atmosphere. In some experiments, temperatures were also observed using a high-speed infrared camera.
At pressures above 30 kPa, the formation of shock waves such as a bow shock around the projectile prior to impact could be observed by the shadowgraph method. However, no clear effects on the target surface by direct impact of the shock wave could be observed. After the projectile passed, the wake of projectile formed. The width of the wake increases with increasing projectile velocity and tends to decrease with higher ambient pressure. The ejecta curtain spread concentrically in vacuum at ~1 ms after the impact, whereas in air a hole appeared on the ejecta curtain around the trajectory. In shadowgraph observation, the wake can only be observed up to about 2 ms after the projectile passes, but visible light observations indicate that the wake continues to influence the ejecta curtain even in the later stages of crater formation. Measurements of the ejecta curtain observed from the side showed that the uprange angle decreases with increasing impact velocity. To make a comparison with the craters on Venus, we normalized the uprange missing sector width of the rampart, i.e. defect in the ejecta emplacement by the impactor diameter. This is consistent with the value of the width of the wake obtained in this study normalized by the projectile diameter.
[1] Schultz. (1992) Journal of Geophysical Research, 97, 11623-11662
[2] Ivanov. (1986) Journal of Geophysical Research, 91(B4), 413–430
[3] Tauber & Kirk. (1976) Icarus, 28(3), 351–357
All experiments were performed using horizontal two-stage light gas gun at Kobe University. The impact velocities were used at 2 km/s to 5 km/s under ambient pressures from 350 Pa to 70 kPa. The projectile was polycarbonate spheres with the diameter of 4.7 mm, and the targets were quartz sand with a grain size of 100 µm, which simulated the surface of a solid body. The target was set at 20º to the projectile trajectory, so the oblique impact was conducted. All experiments were observed by two high-speed cameras, one using visible light and the other using a shadowgraph method that visualized density differences of the atmosphere. In some experiments, temperatures were also observed using a high-speed infrared camera.
At pressures above 30 kPa, the formation of shock waves such as a bow shock around the projectile prior to impact could be observed by the shadowgraph method. However, no clear effects on the target surface by direct impact of the shock wave could be observed. After the projectile passed, the wake of projectile formed. The width of the wake increases with increasing projectile velocity and tends to decrease with higher ambient pressure. The ejecta curtain spread concentrically in vacuum at ~1 ms after the impact, whereas in air a hole appeared on the ejecta curtain around the trajectory. In shadowgraph observation, the wake can only be observed up to about 2 ms after the projectile passes, but visible light observations indicate that the wake continues to influence the ejecta curtain even in the later stages of crater formation. Measurements of the ejecta curtain observed from the side showed that the uprange angle decreases with increasing impact velocity. To make a comparison with the craters on Venus, we normalized the uprange missing sector width of the rampart, i.e. defect in the ejecta emplacement by the impactor diameter. This is consistent with the value of the width of the wake obtained in this study normalized by the projectile diameter.
[1] Schultz. (1992) Journal of Geophysical Research, 97, 11623-11662
[2] Ivanov. (1986) Journal of Geophysical Research, 91(B4), 413–430
[3] Tauber & Kirk. (1976) Icarus, 28(3), 351–357