9:00 AM - 9:15 AM
[PPS06-06] Subsurface Strength of Solid Bodies Estimated from Crater Morphology: Cratering Experiment on Regolith-Covered Bedrock
Keywords:crater, layered structure, subsurface strength
The internal structure of solid bodies is an essential information to infer their physical properties and chemical composition. In addition, this information leads to understand their origin and evolution. In order to study the internal structure of solid bodies, the morphology of flesh impact crater with a central pit, so called a concentric crater, might be an important clue.
Concentric craters are sometimes observed on solid bodies. For example, it was found on the crater floor of the SCI crater, which was formed by Hayabusa 2 spacecraft on the asteroid Ryugu . It indicated that there was a slightly cohesive layer under the cohesionless regolith layer. Therefore, the morphology of impact craters might be a clue to investigate the subsurface structure of solid body. However, few cratering experiments has been done on layered target, and the crater scaling law for such targets remains unclear.
The final goal of our study is to derive an empirical equation to estimate the mechanical strength of subsurface layer for solid bodies from the crater morphology. But the experimental studies related to crater morphology found on layered targets are quite limited. In particular, the effects of the thickness and the strength of each layer on the crater morphology remain unclear.
To address this, we conducted impact cratering experiments on two-layered targets, consisting of a bedrock with varying strength, Y (20.4-364 kPa), covered with dry sand layer. The thickness of sand layer, T, was ranged from 0 to 100 mm. We made impact experiments using vertical two-stage light gas gun at ISAS/JAXA and the impact velocity was 2 km/s. We used a spherical aluminum with the diameter (Lp) of 1 and 2 mm as the projectile. All experiments were observed by two or three high-speed cameras with the recording speed from 3000 to 105 fps.
As a result, we observed the transition in a final crater type from bowl-shaped "B type" to concentric "C type" through flat-bottomed "F type" with the decrease of sand layer thickness normalized by the projectile length, T/Lp. We also found the sand layer thickness, where the transient crater was F type though the final crater was B type; it is called "FB type". The sand layer thickness showing the boundary between F type and C type decreased as the bedrock strength increased. While FB type craters were seen at 8≦T/Lp<15 regardless of the bedrock strength.
As of the C type crater, the crater radius on the sand layer was smaller than those of F type and B type craters and decreased with the decrease of sand layer thickness. This trend was independent on the bedrock strength. Regarding the central pit formed on the bedrock, its radius increased with the decrease of sand layer thickness and the bedrock strength. The ratio of non-dimensional crater radius on the sand layer, πR, to πR of the homogeneous sand layer, πR∞, decreased with the decrease in T/Lp, when C type crater was formed. When F and B type crater was formed, this ratio was almost 1. The ratio of non-dimensional crater radius of the pit, πR, to πR of the homogeneous bedrock layer, πR0, decreased with the increase in T/Lp. The reduction rate of this ratio was less than that obtained in the previous study [1], thus this depended on the outermost layer strength. Using the crater size scaling relationship for the sand layer and the central pit, we derived the empirical equation for the relationship between the ratio of pit crater size to sand crater size and the bedrock strength.
Finally, we applied this empirical equation to a C type crater on the Moon (23.455°E, 0.697°N). We assume that a basaltic projectile impacted on a basaltic bedrock covered by a dry sand-like regolith layer. The projectile radius could be estimated from the radius of the crater formed on the sand layer. The bedrock strength Y was estimated to be 0.92 MPa and this probably indicates the megaregolith layer strength.
[1] Dohi et al. (2012) Icarus, 218, 751-759
Concentric craters are sometimes observed on solid bodies. For example, it was found on the crater floor of the SCI crater, which was formed by Hayabusa 2 spacecraft on the asteroid Ryugu . It indicated that there was a slightly cohesive layer under the cohesionless regolith layer. Therefore, the morphology of impact craters might be a clue to investigate the subsurface structure of solid body. However, few cratering experiments has been done on layered target, and the crater scaling law for such targets remains unclear.
The final goal of our study is to derive an empirical equation to estimate the mechanical strength of subsurface layer for solid bodies from the crater morphology. But the experimental studies related to crater morphology found on layered targets are quite limited. In particular, the effects of the thickness and the strength of each layer on the crater morphology remain unclear.
To address this, we conducted impact cratering experiments on two-layered targets, consisting of a bedrock with varying strength, Y (20.4-364 kPa), covered with dry sand layer. The thickness of sand layer, T, was ranged from 0 to 100 mm. We made impact experiments using vertical two-stage light gas gun at ISAS/JAXA and the impact velocity was 2 km/s. We used a spherical aluminum with the diameter (Lp) of 1 and 2 mm as the projectile. All experiments were observed by two or three high-speed cameras with the recording speed from 3000 to 105 fps.
As a result, we observed the transition in a final crater type from bowl-shaped "B type" to concentric "C type" through flat-bottomed "F type" with the decrease of sand layer thickness normalized by the projectile length, T/Lp. We also found the sand layer thickness, where the transient crater was F type though the final crater was B type; it is called "FB type". The sand layer thickness showing the boundary between F type and C type decreased as the bedrock strength increased. While FB type craters were seen at 8≦T/Lp<15 regardless of the bedrock strength.
As of the C type crater, the crater radius on the sand layer was smaller than those of F type and B type craters and decreased with the decrease of sand layer thickness. This trend was independent on the bedrock strength. Regarding the central pit formed on the bedrock, its radius increased with the decrease of sand layer thickness and the bedrock strength. The ratio of non-dimensional crater radius on the sand layer, πR, to πR of the homogeneous sand layer, πR∞, decreased with the decrease in T/Lp, when C type crater was formed. When F and B type crater was formed, this ratio was almost 1. The ratio of non-dimensional crater radius of the pit, πR, to πR of the homogeneous bedrock layer, πR0, decreased with the increase in T/Lp. The reduction rate of this ratio was less than that obtained in the previous study [1], thus this depended on the outermost layer strength. Using the crater size scaling relationship for the sand layer and the central pit, we derived the empirical equation for the relationship between the ratio of pit crater size to sand crater size and the bedrock strength.
Finally, we applied this empirical equation to a C type crater on the Moon (23.455°E, 0.697°N). We assume that a basaltic projectile impacted on a basaltic bedrock covered by a dry sand-like regolith layer. The projectile radius could be estimated from the radius of the crater formed on the sand layer. The bedrock strength Y was estimated to be 0.92 MPa and this probably indicates the megaregolith layer strength.
[1] Dohi et al. (2012) Icarus, 218, 751-759