2:30 PM - 2:45 PM
[PPS02-03] Mechanism of suppressing crater growth in the regolith layer covering bedrock: Effects of impact-induced vibration
Keywords:crater, impact-induced seismic shaking
The surface and internal structures of asteroids are essential information for understanding their origin and evolution, and these characteristics are noticeable for their consideration. One is a central pit on the crater floor. It was observed at the floor of SCI crater on Ryugu, indicating the presence of a cohesive layer beneath the regolith layer. Thus, morphology of impact crater might be a clue to infer the surface structure on solid body. However, few cratering experiments has been done on layered target, and the crater scaling law for such targets remains unclear. The other is impact-induced seismic shaking. These seismic waves propagate and fluidize regolith, resulting in modification of asteroidal surface. The modeled impact-induced seismic motion on a layered asteroid (Richardson Jr. et al., 2005) suggested that regolith on bedrock was easily fluidized. Thus, differences in the inner structure such as layered structure might make differences in the degree of asteroidal resurfacing. However, seismic waves propagating through a regolith-covered bedrock remain unsolved.
To address this, we conducted impact cratering experiments on two-layered targets, consisting of a bedrock with varying strength (26.9 kPa~6.7 MPa) covered by a granular layer (dry sand). We investigated the effects of the thickness of the regolith layer and bedrock strength on the cratering processes. We also observed seismic waves propagating through the target on both the outer surface and the bedrock to reveal the effect of bedrock on the attenuation rate of the acceleration. We made impact experiments in wide impact velocity ranges (3.3 m/s~2 km/s). To observe seismic shaking, we measured the acceleration and the displacement on the bedrock or the outermost surface of target, using accelerometers and a 1D laser displacement meter, respectively. As the result, we observed the transition in the crater type from bowl-shaped to concentric through flat-bottomed with the decrease in granular layer thickness (0~50 mm) under the same impact conditions. The crater size on the granular layer (flat-bottomed or concentric crater) was smaller than that of the bowl-shaped crater, and the size of the flat floor and the pit increased as the granular layer thickness decreased and only the size of pit depended on the bedrock strength. The normalized pit volume, πv, decreased with the increase in the granular layer thickness normalized by the projectile length, T/Lp, although the reduction rate of pit volume didn’t depend on the bedrock strength. As for impact-induced seismic shaking, in the case of the layered target, we observed acceleration waveforms with a frequency intermediate between these excited in the homogeneous sand target and the homogeneous bedrock target. In addition, the maximum acceleration of vibration propagating through the homogeneous bedrock target was approximately 40 times larger than that of homogeneous sand target. We observed the vertical displacement waveform of the bedrock and granular layer. All of them displaced downward just after the impact, then the particle layer vibrated at a higher frequency than that of the bedrock. It implies that the vibration of the bedrock excites the vibration of the particle layer. From these results, we discussed the attenuation of excavation flow in layered targets. We applied the Z model and assumed that the excavation flow velocity at the outermost part of the transient crater of the homogeneous sand target was equal to that of the layered target and that of a flat floor. It was found that when concentric and flat-bottomed craters were formed, the excavation flow attenuated greater than that in case of homogeneous sand targets. This might to be because the vibration of bedrock fluidized the granular layer, as seen in numerical simulations using iSALE shock physics code.
Acknowledgments: We thank the developers of iSALE, including G. Collins, K. Wünnemann, B. Ivanov, J. Melosh, and D. Elbeshausen.
To address this, we conducted impact cratering experiments on two-layered targets, consisting of a bedrock with varying strength (26.9 kPa~6.7 MPa) covered by a granular layer (dry sand). We investigated the effects of the thickness of the regolith layer and bedrock strength on the cratering processes. We also observed seismic waves propagating through the target on both the outer surface and the bedrock to reveal the effect of bedrock on the attenuation rate of the acceleration. We made impact experiments in wide impact velocity ranges (3.3 m/s~2 km/s). To observe seismic shaking, we measured the acceleration and the displacement on the bedrock or the outermost surface of target, using accelerometers and a 1D laser displacement meter, respectively. As the result, we observed the transition in the crater type from bowl-shaped to concentric through flat-bottomed with the decrease in granular layer thickness (0~50 mm) under the same impact conditions. The crater size on the granular layer (flat-bottomed or concentric crater) was smaller than that of the bowl-shaped crater, and the size of the flat floor and the pit increased as the granular layer thickness decreased and only the size of pit depended on the bedrock strength. The normalized pit volume, πv, decreased with the increase in the granular layer thickness normalized by the projectile length, T/Lp, although the reduction rate of pit volume didn’t depend on the bedrock strength. As for impact-induced seismic shaking, in the case of the layered target, we observed acceleration waveforms with a frequency intermediate between these excited in the homogeneous sand target and the homogeneous bedrock target. In addition, the maximum acceleration of vibration propagating through the homogeneous bedrock target was approximately 40 times larger than that of homogeneous sand target. We observed the vertical displacement waveform of the bedrock and granular layer. All of them displaced downward just after the impact, then the particle layer vibrated at a higher frequency than that of the bedrock. It implies that the vibration of the bedrock excites the vibration of the particle layer. From these results, we discussed the attenuation of excavation flow in layered targets. We applied the Z model and assumed that the excavation flow velocity at the outermost part of the transient crater of the homogeneous sand target was equal to that of the layered target and that of a flat floor. It was found that when concentric and flat-bottomed craters were formed, the excavation flow attenuated greater than that in case of homogeneous sand targets. This might to be because the vibration of bedrock fluidized the granular layer, as seen in numerical simulations using iSALE shock physics code.
Acknowledgments: We thank the developers of iSALE, including G. Collins, K. Wünnemann, B. Ivanov, J. Melosh, and D. Elbeshausen.