5:15 PM - 7:15 PM
[PPS06-P16] Experimental study on impact crater formation on porous icy bodies composed of ice-rock mixtures
Keywords:Comets, Cratering, Collisional physics, Impact processes
Introduction
Comet nuclei are small icy bodies that come from the Kuiper Belt and the Oort cloud and are mainly composed of amorphous and crystalline water ice mixed with dust and volatiles. Additionally, it has been revealed that comet nuclei have low densities and would possess high porosity[1]. Thus, comet nuclei are believed to be survivors of icy planetesimals and to hold information about primitive solar system bodies. Additionally, the surface geological features of comet nuclei have also been investigated by planetary explorations, and various depressions were found on the surface. These depressions are believed to be sinkholes caused by jet eruptions or impact craters caused by small body impact[2]. In this study, to determine whether the circular features observed on the surface of the comet nuclei are of impact origin, we conduct impact experiments on mixed samples that include not only porous ice but also rock. Previous studies have mostly conducted experiments using porous ice. However, in this study, we conducted impact experiments using porous targets made of ice-rock mixtures, which have a composition closer to that of an actual cometary nucleus.
The relationship between crater size and impact conditions was analyzed using the Pi-scaling law, PiR=H1Pi41-3a/3PiY-b/2, where, PiR=(pt/mp)1/3R, and PiY=Y/(ptvi2) , and Pi4=pt/o, and H1 and b are constants that depend on the target material. R represents the crater radius, pt is the bulk density of the target, m is the mass of the projectile, vi is the impact velocity, and Y is the strength of the target.
Experimental method
This experiment was conducted at -15 degree in a cold room. The ice-rock mixture target as an analog of the comet surface was a cylindrical sample with a diameter of 100 mm and a height of 60 mm. This target was prepared by mixing ice particles with rock powder, and compressing it using a hydraulic pump. Additionally, montmorillonite powder was used as the rock powder. The porosity of this sample and rock content were set at 40%. All impact experiments were conducted by using a horizontal two-stage light gas gun located at Kobe University, and the projectile was a 2 mm aluminum sphere. The impact velocity was varied in increments of 1 km/s within the range of 1.04 km/s to 6.02 km/s.The target was placed inside an experimental chamber, and the pressure was set to be 200 Pa before conducting the impact experiment. To observe the crater formation process, the impact experiment was recorded using two high-speed cameras in the chamber. Additionally, after the shot, the target was recovered and analyzed to obtain the spall radius, pit radius, and depth. Furthermore, a compressive strength of the target was measured by using a mechanical testing machine. This compressive strength was then used to apply the Pi-scaling law.
Experimental Results
The experimental results showed that the spall radius and pit radius increased with the increase of the kinetic energy of the projectile. The shape of the pit became more spherical as the velocity increased. Furthermore, at high velocities, protrusions caused by projectile fragments were observed. These results are consistent with the shapes observed in porous snow samples [3]. On the other hand, the shape of the spall was circular and independent on the impact velocity. This shape was a more symmetrical circle than porous snow samples. Additionally, when cutting a sample with an impact velocity of 5 km/s, a white layer, presumed to be melt, was observed. We observed the collision with a high-speed camera, and it revealed that pillar-shaped ejecta was released due to the impact. At high velocities, large spall fragments were also observed as ejecta.
Additionally, compression tests determined that the compressive strength of this target was 0.172 MPa. Considering that the compressive strength of a target with a 50% serpentine content and 40% porosity is approximately 0.36 MPa [4], and that of a porous ice target with 40% porosity is around 0.59 MPa[3], the obtained value can be considered consistent.
From this study, the Pi-scaling laws for the pit radius R were determined as shown below. PiR/Pi4-0.067=10-0.77PiY-0.28, b=0.56. This relationship is close to the scaling law for porous snow samples [3],PiR/Pi4-0.067=10-0.43PiY-0.20.
[1] Patzold et al. 2016, Nature 530, 63-65
[2] El-Maarry et al. 2015, OSIRIS images. A&A 583, A26
[3] Sasai et al. 2024, Icarus 411 115929
[4] Arakawa et al. 2004, Icarus 170 193-201
Comet nuclei are small icy bodies that come from the Kuiper Belt and the Oort cloud and are mainly composed of amorphous and crystalline water ice mixed with dust and volatiles. Additionally, it has been revealed that comet nuclei have low densities and would possess high porosity[1]. Thus, comet nuclei are believed to be survivors of icy planetesimals and to hold information about primitive solar system bodies. Additionally, the surface geological features of comet nuclei have also been investigated by planetary explorations, and various depressions were found on the surface. These depressions are believed to be sinkholes caused by jet eruptions or impact craters caused by small body impact[2]. In this study, to determine whether the circular features observed on the surface of the comet nuclei are of impact origin, we conduct impact experiments on mixed samples that include not only porous ice but also rock. Previous studies have mostly conducted experiments using porous ice. However, in this study, we conducted impact experiments using porous targets made of ice-rock mixtures, which have a composition closer to that of an actual cometary nucleus.
The relationship between crater size and impact conditions was analyzed using the Pi-scaling law, PiR=H1Pi41-3a/3PiY-b/2, where, PiR=(pt/mp)1/3R, and PiY=Y/(ptvi2) , and Pi4=pt/o, and H1 and b are constants that depend on the target material. R represents the crater radius, pt is the bulk density of the target, m is the mass of the projectile, vi is the impact velocity, and Y is the strength of the target.
Experimental method
This experiment was conducted at -15 degree in a cold room. The ice-rock mixture target as an analog of the comet surface was a cylindrical sample with a diameter of 100 mm and a height of 60 mm. This target was prepared by mixing ice particles with rock powder, and compressing it using a hydraulic pump. Additionally, montmorillonite powder was used as the rock powder. The porosity of this sample and rock content were set at 40%. All impact experiments were conducted by using a horizontal two-stage light gas gun located at Kobe University, and the projectile was a 2 mm aluminum sphere. The impact velocity was varied in increments of 1 km/s within the range of 1.04 km/s to 6.02 km/s.The target was placed inside an experimental chamber, and the pressure was set to be 200 Pa before conducting the impact experiment. To observe the crater formation process, the impact experiment was recorded using two high-speed cameras in the chamber. Additionally, after the shot, the target was recovered and analyzed to obtain the spall radius, pit radius, and depth. Furthermore, a compressive strength of the target was measured by using a mechanical testing machine. This compressive strength was then used to apply the Pi-scaling law.
Experimental Results
The experimental results showed that the spall radius and pit radius increased with the increase of the kinetic energy of the projectile. The shape of the pit became more spherical as the velocity increased. Furthermore, at high velocities, protrusions caused by projectile fragments were observed. These results are consistent with the shapes observed in porous snow samples [3]. On the other hand, the shape of the spall was circular and independent on the impact velocity. This shape was a more symmetrical circle than porous snow samples. Additionally, when cutting a sample with an impact velocity of 5 km/s, a white layer, presumed to be melt, was observed. We observed the collision with a high-speed camera, and it revealed that pillar-shaped ejecta was released due to the impact. At high velocities, large spall fragments were also observed as ejecta.
Additionally, compression tests determined that the compressive strength of this target was 0.172 MPa. Considering that the compressive strength of a target with a 50% serpentine content and 40% porosity is approximately 0.36 MPa [4], and that of a porous ice target with 40% porosity is around 0.59 MPa[3], the obtained value can be considered consistent.
From this study, the Pi-scaling laws for the pit radius R were determined as shown below. PiR/Pi4-0.067=10-0.77PiY-0.28, b=0.56. This relationship is close to the scaling law for porous snow samples [3],PiR/Pi4-0.067=10-0.43PiY-0.20.
[1] Patzold et al. 2016, Nature 530, 63-65
[2] El-Maarry et al. 2015, OSIRIS images. A&A 583, A26
[3] Sasai et al. 2024, Icarus 411 115929
[4] Arakawa et al. 2004, Icarus 170 193-201