1:45 PM - 3:15 PM
[PPS03-P16] Experimental Studies on Crater Scaling Law Applicable to Undulating Surfaces and Crater Collapse.
Keywords:crater, asteroid, Ryugu
Impact craters are one of the major geological features on solid bodies such as asteroids and satellites. The crater morphology is affected by the surface of the celestial body. For example, the surface of the rubble-pile asteroids is covered with boulders, resulting in the lack of small craters due to armoring effects. In addition, the cohesion of surface particles affects the cratering efficiency. Furthermore, recent explorations by Hayabusa2 and OSIRIS-REx have revealed that the asteroids Ryugu and Bennu have a huge bulge at the equatorial regions. Large craters are recognized to be concentrated on the bulge of Ryugu, which may show different surface characteristics and/or surface ages from other regions. However, the cratering process on such undulating surfaces is still unknown. In this study, we conduct cratering experiments on a granular target that simulates the undulating topography of an asteroid surface, and construct a scaling law applicable to craters on undulating topography. It may improve the accuracy of estimating the mechanical properties of the celestial surface layer and the surface age.
We prepared targets to simulate the surface topography of asteroids: It was a granular target having the shape of a mountain range. This target was consisted of quartz sand with the diameter of 100 μm and simulated the regolith surface on asteroids. The angle of repose of the quartz sand was ~31°. The inclination θ of the target was set to be 20° and 30°. We also prepared the target having the flat surface (that is, θ= 0°). We changed the distance d for the target, where d was defined as the horizontal distance between the impact point and the summit, and it was changed from 1 to 22 mm. We conducted impact experiments by using a one-stage vertical gas gun at Kobe University. The targets were set in a vacuum chamber and evacuated below 1000 Pa. We used an alumina spherical projectile with the diameter of 3 mm. The impact velocities vi ranged from 63 to 202 m/s at θ = 30°, 3.8 to 89 m/s at θ = 20°, and 76 to 187 m/s at θ = 0°. In order to analyze the crater morphology, we constructed a 3D shape model by using the Metashape software. The length of ellipse in the ridge direction Dma, that in the slope direction Dmi, depth, and the crater volume were measured on the shape model. We observed the impact phenomena by using a high-speed camera.
We found that the crater had an elliptical shape, and the aspect ratio (ratio of the ridge direction length Dma to the slope direction length Dmi) depended on d. The crater volume was also found to depend on d/Dma. The volume was significantly smaller at d/Dma > 0.3 because the transient crater did not cross over the summit and the transient crater was filled by slope direction collapse. At d/Dma < 0.3, the transient crater grew beyond the summit and the crater wall of the ridge direction collapsed toward the crater floor. However, the crater wall of the slope direction collapsed outward of the crater. Then the burial of the crater cavity was limited due to these two mechanisms. In addition, we found that d/Dma controls the aspect ratio (Dma/Dmi) and the depth to diameter ratio (h/Dma). Note that Urashima crater on Ryugu has a small depth-diameter ratio of 0.07. Our experimental results on simulated bulge with the slope angle of 30° indicate that d/Dma = 0.2 for h/Dm=0.07. Since this d/Dma is < 0.3, the shallow crater with h/Dm=0.07 could be mainly caused by collapse of the crater wall of the ridge direction. Furthermore, we found that Dma at θ = 30° was dependent on d/Dma. Thus, we normalized the crater size scaling law for the mountain range target by d/Dma. Comparing it at d/Dma=0 with the crater size scaling law for the flat surface, we found that craters formed on the bulge were always larger than those formed on planes. Therefore, this result may suggest that the surface age of the bulge on Ryugu should be reconsidered.
We prepared targets to simulate the surface topography of asteroids: It was a granular target having the shape of a mountain range. This target was consisted of quartz sand with the diameter of 100 μm and simulated the regolith surface on asteroids. The angle of repose of the quartz sand was ~31°. The inclination θ of the target was set to be 20° and 30°. We also prepared the target having the flat surface (that is, θ= 0°). We changed the distance d for the target, where d was defined as the horizontal distance between the impact point and the summit, and it was changed from 1 to 22 mm. We conducted impact experiments by using a one-stage vertical gas gun at Kobe University. The targets were set in a vacuum chamber and evacuated below 1000 Pa. We used an alumina spherical projectile with the diameter of 3 mm. The impact velocities vi ranged from 63 to 202 m/s at θ = 30°, 3.8 to 89 m/s at θ = 20°, and 76 to 187 m/s at θ = 0°. In order to analyze the crater morphology, we constructed a 3D shape model by using the Metashape software. The length of ellipse in the ridge direction Dma, that in the slope direction Dmi, depth, and the crater volume were measured on the shape model. We observed the impact phenomena by using a high-speed camera.
We found that the crater had an elliptical shape, and the aspect ratio (ratio of the ridge direction length Dma to the slope direction length Dmi) depended on d. The crater volume was also found to depend on d/Dma. The volume was significantly smaller at d/Dma > 0.3 because the transient crater did not cross over the summit and the transient crater was filled by slope direction collapse. At d/Dma < 0.3, the transient crater grew beyond the summit and the crater wall of the ridge direction collapsed toward the crater floor. However, the crater wall of the slope direction collapsed outward of the crater. Then the burial of the crater cavity was limited due to these two mechanisms. In addition, we found that d/Dma controls the aspect ratio (Dma/Dmi) and the depth to diameter ratio (h/Dma). Note that Urashima crater on Ryugu has a small depth-diameter ratio of 0.07. Our experimental results on simulated bulge with the slope angle of 30° indicate that d/Dma = 0.2 for h/Dm=0.07. Since this d/Dma is < 0.3, the shallow crater with h/Dm=0.07 could be mainly caused by collapse of the crater wall of the ridge direction. Furthermore, we found that Dma at θ = 30° was dependent on d/Dma. Thus, we normalized the crater size scaling law for the mountain range target by d/Dma. Comparing it at d/Dma=0 with the crater size scaling law for the flat surface, we found that craters formed on the bulge were always larger than those formed on planes. Therefore, this result may suggest that the surface age of the bulge on Ryugu should be reconsidered.