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[PPS03-P12] Effects of crater collapse and asymmetric ejecta curtains on the celestial surface.
Keywords:crater, asteroid
Impact craters are one of the major geological features on solid bodies such as asteroids and satellites. The morphology of such craters varies depending on the surface conditions: the crater shape formed on a flat surface is a circle, while that formed on a slope is an ellipse. Furthermore, the crater formed on a slope collapsed to deform its shape due to avalanches after the crater formation. In addition, the distribution of ejected material deposited on the surface during the crater formation could affect the surface geology of asteroids. Asteroids and satellites have various topographic features on their surface such as slopes, bulges, and canyons. Recently, Hayabusa2 and OSIRIS-REx have revealed that the asteroid Ryugu and Bennu have a huge bulge at the equatorial region, and the shape of several huge craters found on the equatorial region of Ryugu were affected by the bulge.
The theory on the cratering process has been constructed based on the laboratory impact experiments on a flat surface. This theory is not enough to apply to the undulated surface. Therefore, it is necessary to construct the cratering theory applicable to the undulated surface in order to study the origin of irregular shape of huge craters on Ryugu. In this study, we conducted cratering experiments on mountain range targets composed of granular material simulating the undulated surface of asteroids, and we investigated the effects of the undulated surface on the cratering processes such as ejecta curtain growth, ejecta deposition process, and crater collapse.
To simulate the undulated surface of asteroids, a granular target with a mountain range shape was prepared. The inclination angle θ from the horizontal surface of the target was 20° and 30°, and the horizontal distance d between the summit of the target and the impact point was changed from 0 mm to 35 mm. The cratering experiments were also conducted on a flat surface (θ = 0°). We used a vertical single-stage light gas gun at Kobe University and a vertical two-stage light gas gun at ISAS for the impact experiments. The impact velocities were 41-218 m/s (Kobe University) and 2.0-2.2 km/s (ISAS). In order to analyze the crater shape, a 3D shape model of each crater was constructed from 20 photo images using a software named Metashape. The cratering process was observed by a high speed camera at the frame rate of 10^3-2x10^4 fps.
An asymmetric ejecta curtain was observed during the crater formation when a projectile impacted at 0.09 < d/D < 0.20, where D is the crater diameter in the direction of the ridge. The low-velocity ejecta was deposited around the crater to form the elevated rim and then the transient crater was observed to collapse in the direction of the slope and into the crater interior. The final crater shape was ellipse long in the direction of the ridge. The depth-diameter ratio and the volume of the crater were measured from the 3D shape model, and the ratio decreased with the increase of d/D. This indicats that the elevated rim on the upslope side collapsed into the crater interior, then the crater depth became shallower. The crater volume increased with the increase of d/D, but the volume decreased at d/D > 0.1 due to the collapse of the transient crater.
The theory on the cratering process has been constructed based on the laboratory impact experiments on a flat surface. This theory is not enough to apply to the undulated surface. Therefore, it is necessary to construct the cratering theory applicable to the undulated surface in order to study the origin of irregular shape of huge craters on Ryugu. In this study, we conducted cratering experiments on mountain range targets composed of granular material simulating the undulated surface of asteroids, and we investigated the effects of the undulated surface on the cratering processes such as ejecta curtain growth, ejecta deposition process, and crater collapse.
To simulate the undulated surface of asteroids, a granular target with a mountain range shape was prepared. The inclination angle θ from the horizontal surface of the target was 20° and 30°, and the horizontal distance d between the summit of the target and the impact point was changed from 0 mm to 35 mm. The cratering experiments were also conducted on a flat surface (θ = 0°). We used a vertical single-stage light gas gun at Kobe University and a vertical two-stage light gas gun at ISAS for the impact experiments. The impact velocities were 41-218 m/s (Kobe University) and 2.0-2.2 km/s (ISAS). In order to analyze the crater shape, a 3D shape model of each crater was constructed from 20 photo images using a software named Metashape. The cratering process was observed by a high speed camera at the frame rate of 10^3-2x10^4 fps.
An asymmetric ejecta curtain was observed during the crater formation when a projectile impacted at 0.09 < d/D < 0.20, where D is the crater diameter in the direction of the ridge. The low-velocity ejecta was deposited around the crater to form the elevated rim and then the transient crater was observed to collapse in the direction of the slope and into the crater interior. The final crater shape was ellipse long in the direction of the ridge. The depth-diameter ratio and the volume of the crater were measured from the 3D shape model, and the ratio decreased with the increase of d/D. This indicats that the elevated rim on the upslope side collapsed into the crater interior, then the crater depth became shallower. The crater volume increased with the increase of d/D, but the volume decreased at d/D > 0.1 due to the collapse of the transient crater.