Presentation Information
[PPS05-P04]Experimental study of heterogeneous particulate targets: Effects of particle size and burial depth on ejecta processes during crater formation
*Hatsune Okawa1, Masahiko Arakawa1, Minami Yasui1, Sunao Hasegawa2, Hiroya Sako1, Haruna Toyoshima1, Reia Kakinoki1, Anzu Ikoma1, Ohashi Takuma1 (1.Department of Planetology, Graduate School of Science, Kobe University, 2.Japan Aerospace Exploration Agency)
Keywords:
impact crater,ejecta,rubble-pile asteroids
The behavior of ejecta released during impact crater formation provides crucial insights into surface evolution processes, including material transport on planetary bodies. On small bodies with extremely low escape velocities, even slight differences in ejection velocity determine whether ejecta escape or re-impact the surface. Because of the weak gravity, ballistic trajectories can extend over large distances, resulting in widespread surface transport of ejecta. Previous experimental studies of ejecta behavior have primarily used homogeneous fine-grained targets. However, spacecraft observations of asteroids such as Ryugu and Bennu have revealed that rubble-pile bodies are covered not only with fine regolith but also with boulders ranging from decimeter to meter scale, exhibiting a wide size distribution. Therefore, the particle-size dependence of ejecta velocity and angle is a key factor controlling surface evolution on small bodies. Nevertheless, systematic experimental studies on the ejecta behavior of relatively large particles embedded in heterogeneous targets remain limited, particularly regarding how their motion differs from that of the surrounding fine-grained material.
In this study, we focus on particle size heterogeneity characteristic of rubble-pile bodies and conducted impact experiments using particulate targets in which large particles were embedded within a fine-grained matrix. Fine-grained targets were prepared by filling a container with 100 µm glass beads. Colored glass beads with diameters of 5 mm and 10 mm were embedded at controlled depth as tracer particles. The burial depth was varied to evaluate the influence of initial position on ejecta behavior. A projectile was launched at 4 km/s using a vertical two-stage light-gas gun at ISAS/JAXA. The crater formation and ejecta process were recorded using synchronized high-speed cameras.
From the recorded images, we reconstructed the three-dimensional trajectories of the tracer particles and determined the initial positions, ejection positions, velocities, and angles. We analyzed the relationships between particle size, burial depth, and ejection conditions. The results showed that burial depth has little influence on ejection velocity, whereas it significantly affects ejection angle: Deeper burial results in smaller ejection angles. This suggests that ejection velocity is primarily governed by energy transfer during the early stage of impact, while ejection angle is more strongly influenced by internal stress propagation and interparticle interactions within the target.
These results indicate that not only fine particles but also relatively large boulders can be ejected during impact events on rubble-pile asteroids, and that their ejection directions and eventual deposition locations are influenced by the internal structure of the target. The present study provides experimental constraints on the ejecta behavior of heterogeneous particulate targets and improves our understanding of how particle size heterogeneity influences boulder transport and distribution on small-body surfaces.
In this study, we focus on particle size heterogeneity characteristic of rubble-pile bodies and conducted impact experiments using particulate targets in which large particles were embedded within a fine-grained matrix. Fine-grained targets were prepared by filling a container with 100 µm glass beads. Colored glass beads with diameters of 5 mm and 10 mm were embedded at controlled depth as tracer particles. The burial depth was varied to evaluate the influence of initial position on ejecta behavior. A projectile was launched at 4 km/s using a vertical two-stage light-gas gun at ISAS/JAXA. The crater formation and ejecta process were recorded using synchronized high-speed cameras.
From the recorded images, we reconstructed the three-dimensional trajectories of the tracer particles and determined the initial positions, ejection positions, velocities, and angles. We analyzed the relationships between particle size, burial depth, and ejection conditions. The results showed that burial depth has little influence on ejection velocity, whereas it significantly affects ejection angle: Deeper burial results in smaller ejection angles. This suggests that ejection velocity is primarily governed by energy transfer during the early stage of impact, while ejection angle is more strongly influenced by internal stress propagation and interparticle interactions within the target.
These results indicate that not only fine particles but also relatively large boulders can be ejected during impact events on rubble-pile asteroids, and that their ejection directions and eventual deposition locations are influenced by the internal structure of the target. The present study provides experimental constraints on the ejecta behavior of heterogeneous particulate targets and improves our understanding of how particle size heterogeneity influences boulder transport and distribution on small-body surfaces.