9:15 AM - 9:30 AM
[PPS06-07] High-Velocity Impact Experiment on Impact Penetration and Cratering on Icy Crust Covering Subsurface Ocean
Keywords:Impact Experiment, Icy Satellite, Crater
It has been recognized that several icy satellites in our solar system may have subsurface ocean (e.g. Europe and Enceladus). This subsurface ocean could provide important environment for chemical evolution of organic materials related to origin of life. The metabolic energy required for life would have to come from reactions between oxidants and reductants. There may be hydrothermal activity on the floor of subsurface ocean producing reductants such as H2. On the other hand, the surface of icy satellites is irradiated by solar winds to produce oxidants such as O2. To achieve this reaction continuously, the surface materials including oxidants should be transported into the subsurface oceans periodically, and the high-velocity impact of small bodies into the icy crust and penetration of icy crust may be one of the key mechanisms for it.
In this study, we conducted high-velocity impact experiments on water ice plate covering simulated subsurface ocean (silicon oil), and we studied the effects of subsurface liquid on the crater formation process and also investigated the boundary condition of penetration through ice plate. The simulant of subsurface ocean was silicon oil because it didn’t freeze at a cold room at the temperature of -15℃. Then, we changed the ice plate thickness covering the simulated subsurface ocean and the impact velocity, and for the comparison, we also conducted impact experiments on an ice plate without silicon oil.
Impact experiments were conducted by using a horizontal type two-stage light-gas gun set at Kobe University. We prepared two-types of targets: one was only ice plate targets without simulated subsurface ocean on its backside (we call it ice plate targets in the text), and another was ice-liquid layered targets. The ice plate target was fixed inside an acrylic box and the four sides of the plate were fitted or it was just fixed on the metallic board. Ice-liquid layered target was also fixed inside the acrylic box, and then the acrylic box was filled with silicon oil. The thickness of ice plate was ranged from about 2.4 to 20.1mm, a projectile was an aluminum sphere with the diameter of 1mm, and the impact velocity was about 1km/s, 2km/s.
It was found in both ice plate and ice-liquid layered target that the foreside spall diameter increased with the increase of the thickness of ice plate and the diameter was almost same in both cases. Also, it was found that the foreside spall diameter increased with the increase of impact velocity for ice-liquid layered target. On the other hand, we found that the backside spall diameter of ice-liquid layered targets was smaller than that of ice plate target at same impact velocity.
The fragment velocity ejected around the antipodal point of the impact point, antipodal velocity was found to decrease with the increase of the ice plate thickness, and it became 3 to 4 times slower by the presence of liquid. We calculated the antipodal pressure from antipodal velocity for the ice plate targets. As a result, we found that the maximum antipodal pressure was 1 GPa in our experiments and that the penetration thickness for ice plate targets and ice-liquid layered targets was about 9mm and about 6mm, respectively at 1km/s, impact velocity. Moreover, the penetration thickness for ice-liquid layered targets was about 9mm at 2km/s, impact velocity.
This difference of the penetration thickness at the same impact velocity may be caused by the difference of backside spall diameter. For the ice plates targets, an impact can excavate deeper thickness because the backside spall diameter for the ice plate targets is larger than that for ice-liquid layered targets. So, the penetration thickness for ice plate targets may become thicker. The spallation at the backside could be suppressed because the reflected tensile wave at the backside was weaken due to the presence of liquid. Therefore, we speculate that the subsurface ocean suppresses the impact penetration on icy crust.
In this study, we conducted high-velocity impact experiments on water ice plate covering simulated subsurface ocean (silicon oil), and we studied the effects of subsurface liquid on the crater formation process and also investigated the boundary condition of penetration through ice plate. The simulant of subsurface ocean was silicon oil because it didn’t freeze at a cold room at the temperature of -15℃. Then, we changed the ice plate thickness covering the simulated subsurface ocean and the impact velocity, and for the comparison, we also conducted impact experiments on an ice plate without silicon oil.
Impact experiments were conducted by using a horizontal type two-stage light-gas gun set at Kobe University. We prepared two-types of targets: one was only ice plate targets without simulated subsurface ocean on its backside (we call it ice plate targets in the text), and another was ice-liquid layered targets. The ice plate target was fixed inside an acrylic box and the four sides of the plate were fitted or it was just fixed on the metallic board. Ice-liquid layered target was also fixed inside the acrylic box, and then the acrylic box was filled with silicon oil. The thickness of ice plate was ranged from about 2.4 to 20.1mm, a projectile was an aluminum sphere with the diameter of 1mm, and the impact velocity was about 1km/s, 2km/s.
It was found in both ice plate and ice-liquid layered target that the foreside spall diameter increased with the increase of the thickness of ice plate and the diameter was almost same in both cases. Also, it was found that the foreside spall diameter increased with the increase of impact velocity for ice-liquid layered target. On the other hand, we found that the backside spall diameter of ice-liquid layered targets was smaller than that of ice plate target at same impact velocity.
The fragment velocity ejected around the antipodal point of the impact point, antipodal velocity was found to decrease with the increase of the ice plate thickness, and it became 3 to 4 times slower by the presence of liquid. We calculated the antipodal pressure from antipodal velocity for the ice plate targets. As a result, we found that the maximum antipodal pressure was 1 GPa in our experiments and that the penetration thickness for ice plate targets and ice-liquid layered targets was about 9mm and about 6mm, respectively at 1km/s, impact velocity. Moreover, the penetration thickness for ice-liquid layered targets was about 9mm at 2km/s, impact velocity.
This difference of the penetration thickness at the same impact velocity may be caused by the difference of backside spall diameter. For the ice plates targets, an impact can excavate deeper thickness because the backside spall diameter for the ice plate targets is larger than that for ice-liquid layered targets. So, the penetration thickness for ice plate targets may become thicker. The spallation at the backside could be suppressed because the reflected tensile wave at the backside was weaken due to the presence of liquid. Therefore, we speculate that the subsurface ocean suppresses the impact penetration on icy crust.