5:15 PM - 6:45 PM
[PPS07-P15] Experimental study on impact crater formed on icy crust covering subsurface ocean
Keywords:Crater, Impact Experiment, Icy Satellite, Subsurface Ocean
Icy satellites such as Europa, Ganymede, Callisto, and Enceladus are covered with icy crust, and they have various surface terrains and are rich in diversity. Of particular note are icy satellite with subsurface ocean. There are some theories about the origin of Europa's chaotic terrains, and it is still under debate: one is that the internal heat melted the icy crust (Ivanov et al., 2011) and the other is that the impactor penetrated and fractured the thin or weak crust (Cox et al., 2008). In addition, there is the possibility of life in the subsurface ocean so the JUICE project is being conducted by NASA to explore jovian icy satellites. In this way, many questions still remain about icy satellites with subsurface ocean.
Previous studies focused on the surface topography of icy satellites have been conducted using targets simulating icy crust covered with subsurface ocean. Harriss & Burchell (2017) conducted high-velocity impact experiments using ice plates covering with water, sand, and basalt and found that the crater morphology and size changed with the density of the underlying materials and the thickness of ice plate. Cox et al. (2008) conducted high-velocity impact experiments on ice plate covering with water and snow. They investigated the crater diameter, the number of ejected fragments, and the condition of surrounding crater, and discussed the possibility of forming chaotic terrains due to impactor's collisions. However, there are few studies using ice targets covering with liquid layer, and most studies have focused on the topography on ice plate. Apparently, there are no studies focusing on the crater formation processes in subsurface ocean. In this study, we conducted high-velocity impact experiments on targets simulating the icy crust covering with the subsurface ocean, and studied the effects of the ice thickness and the presence of subsurface ocean on the crater formation processes of ice crust and subsurface ocean.
Targets were prepared by setting ice plates with the thickness of 2.5, 5, and 8mm into an acrylic box with the size of 7.4 cm x 7.4 cm x 7.4 cm, and filling the bottom of the ice plate with silicon oil (viscosity of 10 cSt). We also prepared ice plate targets with the thickness of 2.5, 5, 8, and 11mm. All targets were prepared in cold room at -15℃. We conducted impact experiments using a horizontal two-stage light gas gun installed in a cold room (-15℃) at Kobe University. The aluminum projectiles with the diameter of 1 mm were accelerated at the impact velocity of ~1 km/s. We observed impact phenomena using a high-speed camera.
In the case of the ice plate targets, the crater diameter did not change with the ice thickness but the crater depth slightly decreased with the increase of the ice thickness. Comparing the two-layered targets with the ice plate targets, the crater diameter of two-layered targets was about twice smaller than that of the ice plate targets when the ice thickness was same.
Next, we measured the antipodal velocity, i.e., the growing velocity of the tip of the crater hole, and the shock wave velocity generated from the ice plate just after the impact. The antipodal velocity decreased exponentially with increasing ice thickness in the case of the ice plate targets. In the case of two-layered targets, the antipodal velocity was about 10 times smaller than that of the ice plate target. On the other hand, the shock wave velocity of the two-layered targets decreased exponentially with increasing the ice thickness and approached to ~1200 m/s. The ice thickness at which the shock wave velocity becomes nearly constant might be controlled by the penetration depth of the projectile into the ice plate, so it might change with the projectile properties.
Previous studies focused on the surface topography of icy satellites have been conducted using targets simulating icy crust covered with subsurface ocean. Harriss & Burchell (2017) conducted high-velocity impact experiments using ice plates covering with water, sand, and basalt and found that the crater morphology and size changed with the density of the underlying materials and the thickness of ice plate. Cox et al. (2008) conducted high-velocity impact experiments on ice plate covering with water and snow. They investigated the crater diameter, the number of ejected fragments, and the condition of surrounding crater, and discussed the possibility of forming chaotic terrains due to impactor's collisions. However, there are few studies using ice targets covering with liquid layer, and most studies have focused on the topography on ice plate. Apparently, there are no studies focusing on the crater formation processes in subsurface ocean. In this study, we conducted high-velocity impact experiments on targets simulating the icy crust covering with the subsurface ocean, and studied the effects of the ice thickness and the presence of subsurface ocean on the crater formation processes of ice crust and subsurface ocean.
Targets were prepared by setting ice plates with the thickness of 2.5, 5, and 8mm into an acrylic box with the size of 7.4 cm x 7.4 cm x 7.4 cm, and filling the bottom of the ice plate with silicon oil (viscosity of 10 cSt). We also prepared ice plate targets with the thickness of 2.5, 5, 8, and 11mm. All targets were prepared in cold room at -15℃. We conducted impact experiments using a horizontal two-stage light gas gun installed in a cold room (-15℃) at Kobe University. The aluminum projectiles with the diameter of 1 mm were accelerated at the impact velocity of ~1 km/s. We observed impact phenomena using a high-speed camera.
In the case of the ice plate targets, the crater diameter did not change with the ice thickness but the crater depth slightly decreased with the increase of the ice thickness. Comparing the two-layered targets with the ice plate targets, the crater diameter of two-layered targets was about twice smaller than that of the ice plate targets when the ice thickness was same.
Next, we measured the antipodal velocity, i.e., the growing velocity of the tip of the crater hole, and the shock wave velocity generated from the ice plate just after the impact. The antipodal velocity decreased exponentially with increasing ice thickness in the case of the ice plate targets. In the case of two-layered targets, the antipodal velocity was about 10 times smaller than that of the ice plate target. On the other hand, the shock wave velocity of the two-layered targets decreased exponentially with increasing the ice thickness and approached to ~1200 m/s. The ice thickness at which the shock wave velocity becomes nearly constant might be controlled by the penetration depth of the projectile into the ice plate, so it might change with the projectile properties.