5:15 PM - 6:45 PM
[PPS07-P14] Experimental study of surface structure around impact craters formed in the atmosphere
Keywords:Impact Experiment, atmosphere, crater
Understanding of crater formation processes related to impact conditions is important for deepening our knowledge of ancient surface environments and planetary surface evolution. The morphologies around the impact craters on the solid surfaces in the Solar System are extremely diverse, and their appearance is believed to be the result of various impact conditions such as topographic features, porosity, and strength of the impactors and targets. The surrounding areas of impact craters on solid bodies with atmosphere, such as Earth, Mars, and Venus, have characteristic that cannot be seen on the bodies without atmosphere. Although various studies have been conducted about the causes, many things remain unclear.
In previous studies, low-velocity impact experiments under atmospheric pressure or high-velocity impact experiments under vacuum have been conducted [1], but high-velocity impact experiments under atmospheric pressure have not been conducted. In this study, we conducted high-velocity cratering experiments at a pressure close to Earth's atmospheric pressure and investigated the structural changes around the crater.
All experiment were performed using two-stage light gas gun at Kobe University. The impact velocities were varied from 2 km/s, 4 km/s, and 5 km/s under ambient pressures of 350 Pa and 0.3 MPa. The projectile were polycarbonate spheres with a 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. All experiments were observed by two high-speed cameras, one using visible light and the other using a shadowgraph method that visualized density differences. The crater diameter and depth were measured using a two-dimensional displacement meter.
The crater diameter and depth were almost same, irrespective of the pressure and the impact velocity. On the other hand, there was a clear difference in the shape around the crater. At 0.3 MPa, wavy ridges appeared around the crater concentric to the center of the crater, and a liner structure radiating from the crater rim was also formed on the upper side of the slope. Their structures became more extensive as the impact velocity increased. No such ridges formed at 350 Pa.
At the crater center, the compressed fragments of quartz sand were deposited in a V-shape that spread toward the upper side of the slope. The angle between the V-shape increased with the impact velocity and the ambient pressure.
The shadowgraph camera was able to visualize the shock waves that formed around the projectile and the ejecta in the air. In this study, the shock wave velocity decreased exponentially with the time. In addition, the ablation (evaporation) of the projectile before impact was observed at the impact velocities of 4 and 5 km/s.
The ejecta curtain was nearly symmetrical at 350 Pa. On the other hand, at 0.3 MPa it was asymmetrical, with a larger angle at the bottom of the slope. As the impact velocity increased, the ejecta curtain angle was formed in a direction perpendicular to the slope.
Observation of the ejecta curtain diagonally from the front revealed a hole on the lower slope of the curtain only at 0.3 MPa. This could be the effect of the accelerating gas or the shock wave around the projectile. The differences in the spreading of the ejecta curtain might contribute to the differences in the shape around the crater.
[1] Suzuki et al. (2013), A formation mechanism for concentric ridges in ejecta surrounding impact craters in a layer of fine glass beads, Icarus 225(2013), 298-307
In previous studies, low-velocity impact experiments under atmospheric pressure or high-velocity impact experiments under vacuum have been conducted [1], but high-velocity impact experiments under atmospheric pressure have not been conducted. In this study, we conducted high-velocity cratering experiments at a pressure close to Earth's atmospheric pressure and investigated the structural changes around the crater.
All experiment were performed using two-stage light gas gun at Kobe University. The impact velocities were varied from 2 km/s, 4 km/s, and 5 km/s under ambient pressures of 350 Pa and 0.3 MPa. The projectile were polycarbonate spheres with a 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. All experiments were observed by two high-speed cameras, one using visible light and the other using a shadowgraph method that visualized density differences. The crater diameter and depth were measured using a two-dimensional displacement meter.
The crater diameter and depth were almost same, irrespective of the pressure and the impact velocity. On the other hand, there was a clear difference in the shape around the crater. At 0.3 MPa, wavy ridges appeared around the crater concentric to the center of the crater, and a liner structure radiating from the crater rim was also formed on the upper side of the slope. Their structures became more extensive as the impact velocity increased. No such ridges formed at 350 Pa.
At the crater center, the compressed fragments of quartz sand were deposited in a V-shape that spread toward the upper side of the slope. The angle between the V-shape increased with the impact velocity and the ambient pressure.
The shadowgraph camera was able to visualize the shock waves that formed around the projectile and the ejecta in the air. In this study, the shock wave velocity decreased exponentially with the time. In addition, the ablation (evaporation) of the projectile before impact was observed at the impact velocities of 4 and 5 km/s.
The ejecta curtain was nearly symmetrical at 350 Pa. On the other hand, at 0.3 MPa it was asymmetrical, with a larger angle at the bottom of the slope. As the impact velocity increased, the ejecta curtain angle was formed in a direction perpendicular to the slope.
Observation of the ejecta curtain diagonally from the front revealed a hole on the lower slope of the curtain only at 0.3 MPa. This could be the effect of the accelerating gas or the shock wave around the projectile. The differences in the spreading of the ejecta curtain might contribute to the differences in the shape around the crater.
[1] Suzuki et al. (2013), A formation mechanism for concentric ridges in ejecta surrounding impact craters in a layer of fine glass beads, Icarus 225(2013), 298-307