Presentation Information
[PPS04-P06]Experimental study on impact crater formation on porous icy bodies composed of ice-rock mixtures
*Kotomi Noguchi1, Masahiko Arakawa1, Minami Yasui1, Haruna Toyoshima1, Ryosuke Kikukawa1 (1.Kobe university)
Keywords:
Comets,Cratering,ice-rock mixtures,porous target,Impact processes
Introduction
In this study, we conducted impact crater formation experiments using comet nucleus analog target to reproduce collision processes between comets and other planetary bodies.
Comet nuclei are considered primitive bodies formed in the outer region of the protoplanetary disk during the early stages of solar system formation. Previous missions targeting comets revealed the composition of comets, effects of solar wind on the surface[1], and the presence of numerous craters on cometary surfaces[2]. Furthermore, future exploration missions may reveal impact craters, from which impact histories and surface ages can be inferred. However, applying lunar crater chronology requires an understanding of crater formation under comet-specific conditions. Therefore, we performed crater formation experiments using comet nucleus analog target.
Comet nuclei are known to have low bulk densities [3], to contain abundant volatile-rich ice [4], and to have dust mass fractions of approximately 33–63 wt.% [5]. To simulate these properties, we prepared porous samples composed of ice mixed with rock powder and investigated crater formation processes on cometary surfaces. Impact experiments were conducted to examine the dependence of crater size on impact velocity, and the results were analyzed using scaling laws. In addition, since target strength is expected to depend on rock content [6], the influence of rock mass fraction on crater morphology was examined.
Experimental Method
All experiments and analyses were conducted in a cold room maintained at -15℃. Cylindrical ice–rock mixture targets (100mm in diameter and 60mm in height) were prepared by uniformly mixing ice particles (<710μm) with montmorillonite powder, packing the mixture into an acrylic container, and compressing it using a hydraulic press. The porosity was fixed at 40%, and targets with rock mass fractions of 30 wt.% and 40 wt.% were prepared.
Impact experiments were conducted by using a horizontal two-stage light gas gun located at Kobe University. A 2-mm diameter aluminum sphere was used as the projectile, and impact velocities ranged from 1 to 6 km/s. The impact process was recorded using two high-speed cameras positioned at the side and the top of the chamber, and was further observed from an oblique front angle using an infrared high-speed camera. After impact, the spall radius, pit radius, and crater depth were measured, and the internal structure was examined using X-ray computed tomography.
Additional cylindrical samples (20 mm in diameter and 30 mm in height) with rock mass fractions of 30–60 wt.% were prepared to measure compressive strength. Uniaxial compression tests were conducted by using a unconfined compression apparatus.
Experimental Results
The experiments showed that a hemispherical pit formed directly beneath the impact point, surrounded by a shallow and wide circular spall region. Both the pit and the spall radii increased as power-law functions of the projectile kinetic energy. With increasing impact velocity, the pit shape evolved from an elongated cavity to a more spherical form. The pit diameter for 30 wt.% targets was always smaller than that for 40 wt.% targets at all the impact velocities. At velocities above 4 km/s, projectile fragments produced deep, protruding sub-pits at the crater floor. These features are consistent with those observed in porous pure-ice targets. The spall region exhibited a more symmetric circular shape than that of porous pure-ice targets, and its morphology showed little dependence on impact velocity or rock mass fraction.
Compression tests indicated that the compressive strength of the porous ice–rock mixture decreased with increasing rock mass fraction (φ). Including the compressive strength of pure ice, an empirical relationship describing the dependence of compressive strength on rock fraction was obtained as Yc=1.7(1-φ)2.8 MPa. Assuming that pit growth is controlled by compressive strength, a crater size scaling law for the pit was derived using the measured strength values:
πR/π4-0.067=10-0.62･πY-0.28. By incorporating the dependence of compressive strength on rock mass fraction, crater sizes formed in the 30 wt% and 40 wt% samples can be scaled consistently.
[1] Keller et al. 2015 [2] El-Maarry et al. 2015 [3] Groussin et al. 2019 [4] Eberhardt et al., 1987; Gasc et al. 2017 [5]Marschall et al. 2025 [6]Arakawa et al. 2004
In this study, we conducted impact crater formation experiments using comet nucleus analog target to reproduce collision processes between comets and other planetary bodies.
Comet nuclei are considered primitive bodies formed in the outer region of the protoplanetary disk during the early stages of solar system formation. Previous missions targeting comets revealed the composition of comets, effects of solar wind on the surface[1], and the presence of numerous craters on cometary surfaces[2]. Furthermore, future exploration missions may reveal impact craters, from which impact histories and surface ages can be inferred. However, applying lunar crater chronology requires an understanding of crater formation under comet-specific conditions. Therefore, we performed crater formation experiments using comet nucleus analog target.
Comet nuclei are known to have low bulk densities [3], to contain abundant volatile-rich ice [4], and to have dust mass fractions of approximately 33–63 wt.% [5]. To simulate these properties, we prepared porous samples composed of ice mixed with rock powder and investigated crater formation processes on cometary surfaces. Impact experiments were conducted to examine the dependence of crater size on impact velocity, and the results were analyzed using scaling laws. In addition, since target strength is expected to depend on rock content [6], the influence of rock mass fraction on crater morphology was examined.
Experimental Method
All experiments and analyses were conducted in a cold room maintained at -15℃. Cylindrical ice–rock mixture targets (100mm in diameter and 60mm in height) were prepared by uniformly mixing ice particles (<710μm) with montmorillonite powder, packing the mixture into an acrylic container, and compressing it using a hydraulic press. The porosity was fixed at 40%, and targets with rock mass fractions of 30 wt.% and 40 wt.% were prepared.
Impact experiments were conducted by using a horizontal two-stage light gas gun located at Kobe University. A 2-mm diameter aluminum sphere was used as the projectile, and impact velocities ranged from 1 to 6 km/s. The impact process was recorded using two high-speed cameras positioned at the side and the top of the chamber, and was further observed from an oblique front angle using an infrared high-speed camera. After impact, the spall radius, pit radius, and crater depth were measured, and the internal structure was examined using X-ray computed tomography.
Additional cylindrical samples (20 mm in diameter and 30 mm in height) with rock mass fractions of 30–60 wt.% were prepared to measure compressive strength. Uniaxial compression tests were conducted by using a unconfined compression apparatus.
Experimental Results
The experiments showed that a hemispherical pit formed directly beneath the impact point, surrounded by a shallow and wide circular spall region. Both the pit and the spall radii increased as power-law functions of the projectile kinetic energy. With increasing impact velocity, the pit shape evolved from an elongated cavity to a more spherical form. The pit diameter for 30 wt.% targets was always smaller than that for 40 wt.% targets at all the impact velocities. At velocities above 4 km/s, projectile fragments produced deep, protruding sub-pits at the crater floor. These features are consistent with those observed in porous pure-ice targets. The spall region exhibited a more symmetric circular shape than that of porous pure-ice targets, and its morphology showed little dependence on impact velocity or rock mass fraction.
Compression tests indicated that the compressive strength of the porous ice–rock mixture decreased with increasing rock mass fraction (φ). Including the compressive strength of pure ice, an empirical relationship describing the dependence of compressive strength on rock fraction was obtained as Yc=1.7(1-φ)2.8 MPa. Assuming that pit growth is controlled by compressive strength, a crater size scaling law for the pit was derived using the measured strength values:
πR/π4-0.067=10-0.62･πY-0.28. By incorporating the dependence of compressive strength on rock mass fraction, crater sizes formed in the 30 wt% and 40 wt% samples can be scaled consistently.
[1] Keller et al. 2015 [2] El-Maarry et al. 2015 [3] Groussin et al. 2019 [4] Eberhardt et al., 1987; Gasc et al. 2017 [5]Marschall et al. 2025 [6]Arakawa et al. 2004