Presentation Information
[PPS04-P14]Impact Experiment on Penetrated Crater on Icy Crust Covering Subsurface Ocean
*Ryosuke Kikukawa1, Minami Yasui1, Hatsune Okawa1, Masahiko Arakawa1 (1.Kobe University)
Keywords:
Impact Experiment,Icy Satellite,Crater
It has been recognized that several icy satellites in our solar system may have subsurface ocean. This subsurface ocean could provide important environment for chemical evolution of organic materials related to origin of life, so we take notice of these objects as habitable planet.
As you know, there are variety of surface geology on the surface of icy satellites. Previous study proposed that the depth-to-diameter ratios of craters on these icy satellites were affected by subsurface ocean. Also, there is an area named as chaos terrain, where is constructed by many fragments like icebergs floating on the sea. This chaos terrain could be formed by impact penetration through icy crust. Consequently, to search for evidence of a subsurface ocean and its surface morphologies, it is crucial to understand the characteristics of an impact crater formed on an icy crust and the impact conditions that result in the penetration of the icy crust.
In this study, we conducted high-velocity impact experiments on water ice plate covering simulated subsurface ocean, 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 or brine because they didn’t freeze in a cold room at the temperature of -15℃. Then, we changed ice plate thickness, the impact velocity and liquid temperature. For the comparison, we also conducted impact experiments on an ice plate without liquid on the backside.
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 ice plate targets without simulated subsurface ocean on its backside, and another was ice-liquid layered targets. The thickness of ice plate was ranged from 2.4 to 20.1mm. A projectile was an aluminum sphere with the diameter of 1mm, and the impact velocity was 1km/s or 2km/s.
It was found in both ice plate and ice-liquid layered target that the foreside spall diameter was observed to be almost same in both target configurations. Also, it was found that the foreside spall diameter increased with the increase of liquid temperature 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 the same impact velocity.
Furthermore, the thickness of ice plate targets that can be penetrated by a projectile is thicker than that of ice-liquid layered target at the same impact velocity. This difference of the thickness for the ability of the penetration may be caused by the difference of backside spall diameter and depth. Assuming that the shock wave generated by a projectile impact is approximated by a triangular wave, in an ice-plate target, the reflection wave would become a tensile wave with an equal magnitude of the shock wave at an ice-vacuum interface. Then, this tensile wave lead to significant growth of backside spall, resulting in deeper excavation area at the backside. So, the thickness of ice plate target that was penetrated by a projectile became thicker. On the other hand, for an ice–liquid layered target, the presence of a liquid layer behind the ice plate weakens the tensile wave reflected at the boundary between an ice plate and liquid, thereby suppressing backside spallation. So, penetration thickness of ice-liquid layered target became thinner.
When the penetration occurred, we determined the minimum shock pressure at the ice-liquid interface ranged from 86.7 to 126 MPa based on our experiments. However, the penetrable thickness increased as the liquid temperature exceeded -15℃. Thus, considering that the interface between subsurface ocean and icy crust is close to 0℃ in icy satellite, the interface pressure corresponding to the maximum penetrable thickness is expected to be lower than that derived from this study at -15℃.
As you know, there are variety of surface geology on the surface of icy satellites. Previous study proposed that the depth-to-diameter ratios of craters on these icy satellites were affected by subsurface ocean. Also, there is an area named as chaos terrain, where is constructed by many fragments like icebergs floating on the sea. This chaos terrain could be formed by impact penetration through icy crust. Consequently, to search for evidence of a subsurface ocean and its surface morphologies, it is crucial to understand the characteristics of an impact crater formed on an icy crust and the impact conditions that result in the penetration of the icy crust.
In this study, we conducted high-velocity impact experiments on water ice plate covering simulated subsurface ocean, 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 or brine because they didn’t freeze in a cold room at the temperature of -15℃. Then, we changed ice plate thickness, the impact velocity and liquid temperature. For the comparison, we also conducted impact experiments on an ice plate without liquid on the backside.
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 ice plate targets without simulated subsurface ocean on its backside, and another was ice-liquid layered targets. The thickness of ice plate was ranged from 2.4 to 20.1mm. A projectile was an aluminum sphere with the diameter of 1mm, and the impact velocity was 1km/s or 2km/s.
It was found in both ice plate and ice-liquid layered target that the foreside spall diameter was observed to be almost same in both target configurations. Also, it was found that the foreside spall diameter increased with the increase of liquid temperature 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 the same impact velocity.
Furthermore, the thickness of ice plate targets that can be penetrated by a projectile is thicker than that of ice-liquid layered target at the same impact velocity. This difference of the thickness for the ability of the penetration may be caused by the difference of backside spall diameter and depth. Assuming that the shock wave generated by a projectile impact is approximated by a triangular wave, in an ice-plate target, the reflection wave would become a tensile wave with an equal magnitude of the shock wave at an ice-vacuum interface. Then, this tensile wave lead to significant growth of backside spall, resulting in deeper excavation area at the backside. So, the thickness of ice plate target that was penetrated by a projectile became thicker. On the other hand, for an ice–liquid layered target, the presence of a liquid layer behind the ice plate weakens the tensile wave reflected at the boundary between an ice plate and liquid, thereby suppressing backside spallation. So, penetration thickness of ice-liquid layered target became thinner.
When the penetration occurred, we determined the minimum shock pressure at the ice-liquid interface ranged from 86.7 to 126 MPa based on our experiments. However, the penetrable thickness increased as the liquid temperature exceeded -15℃. Thus, considering that the interface between subsurface ocean and icy crust is close to 0℃ in icy satellite, the interface pressure corresponding to the maximum penetrable thickness is expected to be lower than that derived from this study at -15℃.