Presentation Information
[PPS03-P08]Experimental study on impact fragmentation of boulders on asteroid surfaces
*Tetsushi Sakurai1, Akiko Nakamura1 (1.Graduate School of Science, Kobe University)
Keywords:
asteroid,boulder,impact
Previous spacecraft observations have revealed that asteroid surfaces are covered by boulders and pebbles. Observations of asteroid Itokawa by Hayabusa showed that its surface is largely covered by meter-scale boulders (Fujiwara et al., 2006). In addition, smooth terrains consisting of particles ranging from millimeters to centimeters in size were observed on Itokawa’s surface (Yano et al., 2006). The surfaces of asteroid Ryugu, Bennu, and Dimorphos are also covered with blocks of various sizes, as revealed by observations by Hayabusa2 (Watanabe et al., 2019), OSIRIS-REx (Lauretta et al., 2019), and DART (Daly et al., 2023), respectively. These rubble-pile asteroids are thought to have formed through the disruption of their parent bodies and the subsequent re-accumulation of fragments.
Impact fragmentation of boulders is one of the essential processes for understanding the surface evolution of rubble-pile asteroids. This process depends not only on the physical properties of the boulders (e.g., porosity and mechanical strength), but also on the surrounding materials. A previous study conducted impact experiments on rock targets (cubic blocks approximately 6 cm on a side) under three conditions: (1) placed on the surface of a granular layer, (2) partially buried, and (3) fully buried within the granular layer composed of silica sand with particle sizes of 0.6–0.8 mm (Durda et al., 2011). The results showed that deeper burial led to reduced fragmentation. Because rubble-pile asteroids consist of boulders and particles of varying sizes, systematic investigations using granular layers with different particle sizes are required to understand how surrounding materials influence the impact fragmentation of boulders.
Therefore, in this study, we conducted impact experiments to investigate the effect of surrounding particle size on boulder fragmentation on asteroid surfaces. In the experiments, 11- mm-diameter glass spheres placed on glass-bead layers with particle sizes of 0.1, 1, 3, and 10 mm were used as targets. The projectiles were 3.2-mm-diameter glass spheres, launched at velocities of 240–270 m/s. The degree of fragmentation, defined as the ratio of the mass of the largest fragment to the original target mass (i.e., the largest fragment mass fraction), was evaluated. The results showed that the largest fragment mass fraction was found to depend on the particle size of the underlying glass-bead layer. The largest fragment mass fraction was higher for the 1-mm and 3-mm glass-bead layers than for the 0.1-mm glass-bead layer. In contrast, the results for the 10-mm bead surface showed a bimodal tendency, with values either smaller or larger than those obtained without a glass-bead layer.
Impact fragmentation of boulders is one of the essential processes for understanding the surface evolution of rubble-pile asteroids. This process depends not only on the physical properties of the boulders (e.g., porosity and mechanical strength), but also on the surrounding materials. A previous study conducted impact experiments on rock targets (cubic blocks approximately 6 cm on a side) under three conditions: (1) placed on the surface of a granular layer, (2) partially buried, and (3) fully buried within the granular layer composed of silica sand with particle sizes of 0.6–0.8 mm (Durda et al., 2011). The results showed that deeper burial led to reduced fragmentation. Because rubble-pile asteroids consist of boulders and particles of varying sizes, systematic investigations using granular layers with different particle sizes are required to understand how surrounding materials influence the impact fragmentation of boulders.
Therefore, in this study, we conducted impact experiments to investigate the effect of surrounding particle size on boulder fragmentation on asteroid surfaces. In the experiments, 11- mm-diameter glass spheres placed on glass-bead layers with particle sizes of 0.1, 1, 3, and 10 mm were used as targets. The projectiles were 3.2-mm-diameter glass spheres, launched at velocities of 240–270 m/s. The degree of fragmentation, defined as the ratio of the mass of the largest fragment to the original target mass (i.e., the largest fragment mass fraction), was evaluated. The results showed that the largest fragment mass fraction was found to depend on the particle size of the underlying glass-bead layer. The largest fragment mass fraction was higher for the 1-mm and 3-mm glass-bead layers than for the 0.1-mm glass-bead layer. In contrast, the results for the 10-mm bead surface showed a bimodal tendency, with values either smaller or larger than those obtained without a glass-bead layer.