Presentation Information
[PPS04-P02]Cratering Experiment on Low-Temperature Icy Regolith and Establishment of Scaling Law
*Mizuki Kosaka1, Masahiko Arakawa1, Minami Yasui1, Hatsune Okawa1, Ryosuke Kikukawa1, Kotomi Noguchi1, Tatsuya Sakimura1 (1.Kobe University)
Keywords:
Gravity regime,non-cohesive porous ice,regolith layer
Impact craters are ubiquitous on solid bodies in the solar system and record their formation and surface evolution. Small outer solar system bodies mainly consist of water ice. Their surfaces are covered with an icy regolith formed by reaccumulated ejecta. Because surface temperatures are typically below 150 K, the regolith remains unsintered and non-cohesive. Thus, impact craters on an icy regolith layer may be gravity-dominated when the excavation does not reach the cohesive region beneath the regolith layer.
Although impact experiments have simulated both rocky and icy bodies, most icy-target studies have focused on cohesive materials in the strength-dominated regime. In this study, we prepared unsintered ice granular targets at −80 ºC to simulate icy regolith and conducted free-fall impact experiments using glass bead projectiles to derive the crater size scaling relationship.
In a cold room maintained at -15 ℃, we prepared ice granular targets, conducted impact experiments, and analyzed the results. We crushed and ground cm-sized ice blocks into small particles with grain sizes between 125 to 710 μm for ice granular targets. These particles were cooled in liquid nitrogen filled in an insulated container prior to experiments and placed into a metal container with a diameter of 100 mm and a height of 50 mm. The container was tapped several times during the particle filling process to reduce the pores among the particles, resulting a porosity of 53 %. For comparison, 500 µm quartz sand was also used as a target. Impact experiments on non-cohesive icy targets were performed in a tabletop clean booth using a free-fall method to prevent frost on the cooled target surface. We used glass beads with diameters of 3, 5, 7, 10 and 15 mm as projectiles. Impact velocities ranged from 3.12 to 3.83 m/s. The impact phenomena were recorded using a high-speed camera. After each experiment, the crater profile was measured using a two-dimensional laser profiler.
The angle of repose of ice particles at -80 ℃ was measured by using a fixed funnel method. We cooled the funnel by liquid nitrogen prior to the measurements and carefully poured ice particles into it to prevent them from clogging, allowing the ice particles to accumulate continuously and form a cone on a solid plane. The slope of the accumulated cone collapsed when the slope angle reached a critical angle known as the angle of repose and gradually decreased until it reached the angle of collapse. These values were compared with those of 100 and 500μm quartz sand, as well as 100 μm glass beads. The angle of repose of non-cohesive ice particles was 44°, larger than that of quartz sand (40°) and glass beads (24°). This may be attributed to interlocking among irregularly shaped ice particles.
Impact craters on ice targets show a bowl-like shape with a deposited rim. The crater radius increased with increasing the projectile kinetic energy as a power-law function. Furthermore, the crater radius was almost the same as that of quartz sand at the same kinetic energy. When the 3 mm glass bead was used, it remained on the crater floor without penetrating the target after the impact. For 5 and 7 mm projectiles, they remained on the crater or were half buried in the floor, whereas the 10 and 15 mm projectiles were always half buried. For the sintered snow, a gap appeared between the root of ejecta curtain and the target surface[Arakawa and Yasui, 2011]. In contrast, the ejecta curtain grew continuously from the surface without the gap in our experiments, confirming that our targets were non-cohesive, including the presence of deposited rim. The crater size scaling relationship obtained and the scaling parameter, μ, was obtained to be 0.82, which means the crater formation occurred in a gravity-dominated regime. Furthermore, the non-dimensional crater radius for ice targets was smaller than that for sand targets, likely due to the higher friction coefficient of ice particles.
Although impact experiments have simulated both rocky and icy bodies, most icy-target studies have focused on cohesive materials in the strength-dominated regime. In this study, we prepared unsintered ice granular targets at −80 ºC to simulate icy regolith and conducted free-fall impact experiments using glass bead projectiles to derive the crater size scaling relationship.
In a cold room maintained at -15 ℃, we prepared ice granular targets, conducted impact experiments, and analyzed the results. We crushed and ground cm-sized ice blocks into small particles with grain sizes between 125 to 710 μm for ice granular targets. These particles were cooled in liquid nitrogen filled in an insulated container prior to experiments and placed into a metal container with a diameter of 100 mm and a height of 50 mm. The container was tapped several times during the particle filling process to reduce the pores among the particles, resulting a porosity of 53 %. For comparison, 500 µm quartz sand was also used as a target. Impact experiments on non-cohesive icy targets were performed in a tabletop clean booth using a free-fall method to prevent frost on the cooled target surface. We used glass beads with diameters of 3, 5, 7, 10 and 15 mm as projectiles. Impact velocities ranged from 3.12 to 3.83 m/s. The impact phenomena were recorded using a high-speed camera. After each experiment, the crater profile was measured using a two-dimensional laser profiler.
The angle of repose of ice particles at -80 ℃ was measured by using a fixed funnel method. We cooled the funnel by liquid nitrogen prior to the measurements and carefully poured ice particles into it to prevent them from clogging, allowing the ice particles to accumulate continuously and form a cone on a solid plane. The slope of the accumulated cone collapsed when the slope angle reached a critical angle known as the angle of repose and gradually decreased until it reached the angle of collapse. These values were compared with those of 100 and 500μm quartz sand, as well as 100 μm glass beads. The angle of repose of non-cohesive ice particles was 44°, larger than that of quartz sand (40°) and glass beads (24°). This may be attributed to interlocking among irregularly shaped ice particles.
Impact craters on ice targets show a bowl-like shape with a deposited rim. The crater radius increased with increasing the projectile kinetic energy as a power-law function. Furthermore, the crater radius was almost the same as that of quartz sand at the same kinetic energy. When the 3 mm glass bead was used, it remained on the crater floor without penetrating the target after the impact. For 5 and 7 mm projectiles, they remained on the crater or were half buried in the floor, whereas the 10 and 15 mm projectiles were always half buried. For the sintered snow, a gap appeared between the root of ejecta curtain and the target surface[Arakawa and Yasui, 2011]. In contrast, the ejecta curtain grew continuously from the surface without the gap in our experiments, confirming that our targets were non-cohesive, including the presence of deposited rim. The crater size scaling relationship obtained and the scaling parameter, μ, was obtained to be 0.82, which means the crater formation occurred in a gravity-dominated regime. Furthermore, the non-dimensional crater radius for ice targets was smaller than that for sand targets, likely due to the higher friction coefficient of ice particles.