Presentation Information
[PPS04-P13]Experimental study on impact strength of icy planetsimals with thermally evolved layered structures
*Tatsuya Sakimura1, Minami Yasui1, Hatsune Okawa1, Reia Kakinoki1, Ohashi Takuma1, Masahiko Arakawa1, Sunao Hasegawa2, Yuya Yamamoto2 (1.Department of Planetology, Graduate School of Science, Kobe University, 2.JAXA(ISAS))
Keywords:
impact fragmentation,layered structures
To clarify the collisional evolution of icy planetesimals in the outer solar system, it is essential to understand their impact strength. Asteroids and comets that experienced catastrophic collisions preserve records of planet formation, and investigating their origin and evolution is therefore crucial for constraining planetary formation processes. In particular, asteroid Ryugu is considered a rubble-pile body formed by the reaccumulation of impact fragments and is rich in organic materials and clay minerals. This suggests that its parent body experienced melting of ice due to radiogenic heating and/or impact heating. Melted water ice likely reacted with silicate minerals to produce hydrous silicates through aqueous alteration, followed by gravitational differentiation that concentrated these materials toward the center of the planetesimal. As a result, the parent body is inferred to have evolved into a differentiated icy planetesimal with a layered structure consisting of a clay-rich core and an icy mantle. Investigating the impact strength of such thermally evolved, layered icy planetesimals and the mass ratio of core and mantle fragments in the rubble-pile bodies is therefore of particular interest.
Previous impact experiments have mainly targeted homogeneous analog materials. In this study, we conducted impact experiments on analog samples representing thermally evolved icy planetesimals with a core–mantle layered structure to examine how internal layering affects impact disruption. The targets were 60-mm-diameter layered spheres with two different core sizes (30 and 40 mm in diameter). To simulate icy planetesimals that experienced melting, the mantle consisted of porous ice with a porosity of 40%, while the core was composed of a clay mixture made of bentonite powder and silicone oil. Impact experiments were carried out using horizontal two-stage light-gas guns at Kobe University and the Institute of Space and Astronautical Science (ISAS/JAXA). Impact velocities ranged from 1 to 7 km-1, and polycarbonate projectiles with diameters of 2, 4.7, and 7 mm were used. All experiments were recorded with a high-speed camera, and experiments conducted at ISAS/JAXA additionally employed flash X-ray imaging to observe the internal structure of the targets.
We also found that the normalized mass of the largest fragment is controlled by both Q and the normalized mantle thickness tm/dp (mantle thickness normalized by projectile diameter). For similar Q, a larger normalized mantle thickness results in a larger normalized largest-fragment mass. This trend suggests that after penetrating the mantle, the projectile velocity decays rapidly, reducing the velocity and consequently suppressing core fragmentation.
To quantify this effect, we first performed impact experiments on clay spheres without mantles to determine the relationship between the normalized largest fragment mass and Q. Assuming that the projectile velocity decays exponentially with normalized penetration distance in the mantle, v = vi exp[−k(tm/dp)], where vi is the initial velocity and k is an experimentally determined constant, we defined an effective specific energy delivered to the core, Qc_eff, by dividing the projectile kinetic energy at the core by the core mass [1]. This leads to
Qc_eff = Qc_0 exp[−k′(tm/dp)]. [1]
When k′ = 1, the relationship between the normalized largest core fragment mass and Qc_eff is well described by a single power-law scaling [2]:
ml_c / Mt_c = 10^(3.269±0.348) Qc_eff^(−1.149±0.103). [2]
Thus, Qc_eff successfully scales not only the degree of core fragmentation in layered targets with varying mantle thicknesses but also the degree of fragmentation of single-material clay targets. Because real planetesimal collisions involve diverse internal structures and impactor sizes, this scaling law, which explicitly incorporates layering and projectile size, provides a more realistic framework for understanding collisional disruption processes.
Regarding mantle fragmentation, we found that the normalized largest mantle fragment mass correlates strongly with the antipodal velocity. Since the antipodal velocity is proportional to the impact-induced stress at that location, the largest mantle fragment mass is closely related to the antipodal stress. Denoting this stress as σLt, the relationship can be expressed by the following power-law scaling [3]:
ml_m / Mt_m = 10^(2.729±0.245) σLt^(−1.256±0.08). [3]
Previous impact experiments have mainly targeted homogeneous analog materials. In this study, we conducted impact experiments on analog samples representing thermally evolved icy planetesimals with a core–mantle layered structure to examine how internal layering affects impact disruption. The targets were 60-mm-diameter layered spheres with two different core sizes (30 and 40 mm in diameter). To simulate icy planetesimals that experienced melting, the mantle consisted of porous ice with a porosity of 40%, while the core was composed of a clay mixture made of bentonite powder and silicone oil. Impact experiments were carried out using horizontal two-stage light-gas guns at Kobe University and the Institute of Space and Astronautical Science (ISAS/JAXA). Impact velocities ranged from 1 to 7 km-1, and polycarbonate projectiles with diameters of 2, 4.7, and 7 mm were used. All experiments were recorded with a high-speed camera, and experiments conducted at ISAS/JAXA additionally employed flash X-ray imaging to observe the internal structure of the targets.
We also found that the normalized mass of the largest fragment is controlled by both Q and the normalized mantle thickness tm/dp (mantle thickness normalized by projectile diameter). For similar Q, a larger normalized mantle thickness results in a larger normalized largest-fragment mass. This trend suggests that after penetrating the mantle, the projectile velocity decays rapidly, reducing the velocity and consequently suppressing core fragmentation.
To quantify this effect, we first performed impact experiments on clay spheres without mantles to determine the relationship between the normalized largest fragment mass and Q. Assuming that the projectile velocity decays exponentially with normalized penetration distance in the mantle, v = vi exp[−k(tm/dp)], where vi is the initial velocity and k is an experimentally determined constant, we defined an effective specific energy delivered to the core, Qc_eff, by dividing the projectile kinetic energy at the core by the core mass [1]. This leads to
Qc_eff = Qc_0 exp[−k′(tm/dp)]. [1]
When k′ = 1, the relationship between the normalized largest core fragment mass and Qc_eff is well described by a single power-law scaling [2]:
ml_c / Mt_c = 10^(3.269±0.348) Qc_eff^(−1.149±0.103). [2]
Thus, Qc_eff successfully scales not only the degree of core fragmentation in layered targets with varying mantle thicknesses but also the degree of fragmentation of single-material clay targets. Because real planetesimal collisions involve diverse internal structures and impactor sizes, this scaling law, which explicitly incorporates layering and projectile size, provides a more realistic framework for understanding collisional disruption processes.
Regarding mantle fragmentation, we found that the normalized largest mantle fragment mass correlates strongly with the antipodal velocity. Since the antipodal velocity is proportional to the impact-induced stress at that location, the largest mantle fragment mass is closely related to the antipodal stress. Denoting this stress as σLt, the relationship can be expressed by the following power-law scaling [3]:
ml_m / Mt_m = 10^(2.729±0.245) σLt^(−1.256±0.08). [3]