Presentation Information
[PPS04-P05]Experimental study on the impact strength of homogeneous porous ice–rock mixtures
*Sora Sueda1, Masahiko Arakawa1, Minami Yasui1, Hatsune Okawa1, Tatsuya Sakimura1, Kotomi Noguchi1 (1.Kobe University)
Keywords:
Icy bodies,Collisional physics,Impact processes,Ice–rock mixtures,Porous targets
Small icy bodies are considered to be primordial objects formed in the outer region of the early solar nebula and are primarily composed of water ice and rocky materials. Their internal structure and porosity depend on their thermal evolution stage. Small icy bodies that have not experienced sufficient thermal evolution remain non-cohesive and unsintered, retaining a porous and homogeneous internal structure. In contrast, those that have experienced significant thermal evolution may develop layered structures through sintering, partial melting, and gravity-driven differentiation.
During planet formation, numerous planetesimals were thought to grow via collisional disruption and re-accumulation, leading to the formation of rubble pile bodies. Therefore, the present icy bodies may have formed through collisional disruption and accretion of fragments at various stages of thermal evolution. Laboratory impact experiments using icy body analogue are thus essential for understanding the collisional evolution of icy bodies.
In this study, we conducted a series of impact experiments on spherical porous ice-rock mixture targets that simulated un-melted icy bodies with homogeneous internal structures. The effect of rock mixing on the impact strength was quantitatively evaluated. The impact strength, Q*, is defined as the energy density Q at which the largest fragment mass becomes half of the initial target mass.
Nakamura et al. (2024) reported a clear velocity dependence of the impact strength for porous ice targets. In this study, we analyzed the velocity dependence of Q* using scaling relationships and compared the results with those for porous ice targets to clarify the effect of rock mixing.
Target preparation and analyses were conducted in a cold room at Kobe University maintained at -15 °C. Spherical targets with a diameter of 60 mm were prepared as icy body analogue.
The targets were prepared by uniformly mixing ice grains (<710 μm) and montmorillonite powder at a rock mass fraction of 30 wt.%. The mixture was packed into a spherical mold and compressed. After compaction, the samples were sintered in a freezer for 2–3 days. Based on measurements of mass and diameter immediately before the experiments, the porosity of the targets was determined to be 44.6%.
Impact experiments were conducted using horizontal two-stage light gas guns at Kobe University and ISAS/JAXA.
At Kobe University, polycarbonate spherical projectiles with diameters of 2 mm and 4.7 mm were used. Impact velocities were systematically varied from 0.97 to 6.17 km/s. The projectile impacted at the center of the target in a vacuum chamber. The fragmentation process was recorded using two high-speed cameras (105 fps) positioned perpendicular to the projectile trajectory, providing top and side views.
At ISAS, polycarbonate spherical projectiles with a diameter of 7 mm were used at an impact velocity of approximately 1.2 km/s. In addition to high-speed imaging (5 x 105 fps), flash X-ray radiography was employed. Four imaging plates were installed in the chamber, and internal fragmentation processes were visualized by varying the timing of X-ray exposure.
Flash X-ray images revealed that, at all impact velocities, the projectile penetrated deeply into the target before cracks propagated throughout the entire body.
From the relationship between the normalized largest fragment mass (ml/Mt) and the energy density Q, the impact strength was estimated to be Q*≈136 J kg-1(where ml and Mt denote the largest fragment mass and the initial target mass, respectively).
However, data near an impact velocity of 1 km s-1 systematically deviated from the general trend, suggesting a velocity dependence on ml/Mt in the low-velocity regime. Following the analytical method of Nakamura et al. (2024), reanalysis using a single power-law scaling successfully fitted all data. This approach enabled estimation of Q* at arbitrary impact velocities.
In the velocity range of 1-6 km s-1, comparison with porous ice targets reported by Nakamura et al. (2024) showed that ice-rock mixture targets (30 wt.% rock) exhibit approximately 30% lower catastrophic disruption strength. This indicates that the incorporation of rocky material reduces the impact strength of porous icy targets. Furthermore, the largest fragment mass decreases more rapidly with increasing energy density.
During planet formation, numerous planetesimals were thought to grow via collisional disruption and re-accumulation, leading to the formation of rubble pile bodies. Therefore, the present icy bodies may have formed through collisional disruption and accretion of fragments at various stages of thermal evolution. Laboratory impact experiments using icy body analogue are thus essential for understanding the collisional evolution of icy bodies.
In this study, we conducted a series of impact experiments on spherical porous ice-rock mixture targets that simulated un-melted icy bodies with homogeneous internal structures. The effect of rock mixing on the impact strength was quantitatively evaluated. The impact strength, Q*, is defined as the energy density Q at which the largest fragment mass becomes half of the initial target mass.
Nakamura et al. (2024) reported a clear velocity dependence of the impact strength for porous ice targets. In this study, we analyzed the velocity dependence of Q* using scaling relationships and compared the results with those for porous ice targets to clarify the effect of rock mixing.
Target preparation and analyses were conducted in a cold room at Kobe University maintained at -15 °C. Spherical targets with a diameter of 60 mm were prepared as icy body analogue.
The targets were prepared by uniformly mixing ice grains (<710 μm) and montmorillonite powder at a rock mass fraction of 30 wt.%. The mixture was packed into a spherical mold and compressed. After compaction, the samples were sintered in a freezer for 2–3 days. Based on measurements of mass and diameter immediately before the experiments, the porosity of the targets was determined to be 44.6%.
Impact experiments were conducted using horizontal two-stage light gas guns at Kobe University and ISAS/JAXA.
At Kobe University, polycarbonate spherical projectiles with diameters of 2 mm and 4.7 mm were used. Impact velocities were systematically varied from 0.97 to 6.17 km/s. The projectile impacted at the center of the target in a vacuum chamber. The fragmentation process was recorded using two high-speed cameras (105 fps) positioned perpendicular to the projectile trajectory, providing top and side views.
At ISAS, polycarbonate spherical projectiles with a diameter of 7 mm were used at an impact velocity of approximately 1.2 km/s. In addition to high-speed imaging (5 x 105 fps), flash X-ray radiography was employed. Four imaging plates were installed in the chamber, and internal fragmentation processes were visualized by varying the timing of X-ray exposure.
Flash X-ray images revealed that, at all impact velocities, the projectile penetrated deeply into the target before cracks propagated throughout the entire body.
From the relationship between the normalized largest fragment mass (ml/Mt) and the energy density Q, the impact strength was estimated to be Q*≈136 J kg-1(where ml and Mt denote the largest fragment mass and the initial target mass, respectively).
However, data near an impact velocity of 1 km s-1 systematically deviated from the general trend, suggesting a velocity dependence on ml/Mt in the low-velocity regime. Following the analytical method of Nakamura et al. (2024), reanalysis using a single power-law scaling successfully fitted all data. This approach enabled estimation of Q* at arbitrary impact velocities.
In the velocity range of 1-6 km s-1, comparison with porous ice targets reported by Nakamura et al. (2024) showed that ice-rock mixture targets (30 wt.% rock) exhibit approximately 30% lower catastrophic disruption strength. This indicates that the incorporation of rocky material reduces the impact strength of porous icy targets. Furthermore, the largest fragment mass decreases more rapidly with increasing energy density.