10:00 AM - 10:15 AM
[PPS06-17] High velocity impact experiments on porous ice aggregates simulating cometary nuclei: Estimation of energy partition from projectile kinetic energy to post shock heat
Keywords:comet, crater, post shock heat, impact melt, porous icy body
Introduction: Recent spacecraft explorations and ground-based observations have revealed that cometary nuclei have very low density, such as they are highly porous icy bodies (e.g., the bulk porosity of 67P/CG is estimated to be 72-74%) (Pätzold et al., 2016). When a small body collides on such porous icy bodies at high velocities, the high-velocity collision deposits a large amount of heat around the crater cavity due to the strong dissipation of the impact energy associated with the rapid attenuation of shock pressure (Kraus et al., 2010). This deposited energy is called post-shock heat and it is one of the most important heat sources on porous icy bodies. The post shock heating would form a temporal water pond below the impact crater, and aqueous alteration and organic chemical reactions might be promoted in the water pool. Particularly, the formation environment of organic matters on cometary nuclei has not been well known yet, and the chemical reaction in a temporal water pond on comets caused by the impact heating is expected to be one of the possible formation environments. There are several numerical simulations related to the post-shock heat and molten ice mass around the impact craters on icy planetesimals (e.g., Kraus et al., 2011), but laboratory experiments have not been studied yet. In this study, we conducted high-velocity impact experiments on porous icy aggregates simulating icy planetesimals and measured post-shock temperatures directly around the impact crater. Then, we examined the effect of target porosity on the post-shock heating.
Methods: We prepared cylindrical porous ice targets with the porosity of 40, 50, and 60% by compacting ice grains (< 710 mm). They were sintered in a freezer (-20ºC). 3–5 thermocouples were embedded in the target at different distances from the impact point to measure the temperature. We conducted impact experiments by using a two-stage light gas gun at Kobe Univ. installed in a cold room at -15°C. The aluminum sphere with the diameter of 2 mm was used as a projectile. The impact velocity was 4.2 km/s for 40% and 60% targets and 3.0–5.8 km/s for 50% targets. We observed cratering processes by using a high-speed camera. The temperatures measured by thermocouples were recorded by a data logger for 5 minutes after the impact. After the impact, we measured the molten ice mass by sieving large ice grains (> 710 mm).
Results & Discussion: The crater diameter increased with increase of the target porosity. The π-scaling law for the crater radius in strength regime was applied to our results (Housen & Holsapple, 2011) and normalized crater size πR was found to be smaller than that of non-porous ice. The temperature rise decreased exponentially with the increase of the distance from the center of crater cavity, and the temperatures near the crater wall rose up beyond 0 ºC. This means that the ice grains near the crater wall were molten. We used the maximum temperature rise ΔTmax of each thermocouple to represent the temperature distribution changing with the distance from the crater center. Furthermore, ΔTmax depended on the target porosity and impact velocity; particularly, it was higher as the target porosity is higher at the same distance from the center of crater cavity. However, this systematic change of ΔTmax were well scaled by using normalized distance, x/Rpitmax (the x is the distance from the center of the crater cavity and, the Rpitmax is the maximum radius of the crater cavity), and the relationship was obtained to be ΔTmax=4.7(x/Rpitmax)-2.6 at x >1.2.
The ΔTmax distribution should be reproduced by using simple heat conduction model. We assumed that the post shock heat deposited at a thin layer on the final crater wall conducted outside the crater. The post shock heat was obtained as the energy conversion efficiency depended on the target porosities: It was 0.09 for 40% target, 0.32 for 50% target, and 0.51 for 60% target, respectively.
Methods: We prepared cylindrical porous ice targets with the porosity of 40, 50, and 60% by compacting ice grains (< 710 mm). They were sintered in a freezer (-20ºC). 3–5 thermocouples were embedded in the target at different distances from the impact point to measure the temperature. We conducted impact experiments by using a two-stage light gas gun at Kobe Univ. installed in a cold room at -15°C. The aluminum sphere with the diameter of 2 mm was used as a projectile. The impact velocity was 4.2 km/s for 40% and 60% targets and 3.0–5.8 km/s for 50% targets. We observed cratering processes by using a high-speed camera. The temperatures measured by thermocouples were recorded by a data logger for 5 minutes after the impact. After the impact, we measured the molten ice mass by sieving large ice grains (> 710 mm).
Results & Discussion: The crater diameter increased with increase of the target porosity. The π-scaling law for the crater radius in strength regime was applied to our results (Housen & Holsapple, 2011) and normalized crater size πR was found to be smaller than that of non-porous ice. The temperature rise decreased exponentially with the increase of the distance from the center of crater cavity, and the temperatures near the crater wall rose up beyond 0 ºC. This means that the ice grains near the crater wall were molten. We used the maximum temperature rise ΔTmax of each thermocouple to represent the temperature distribution changing with the distance from the crater center. Furthermore, ΔTmax depended on the target porosity and impact velocity; particularly, it was higher as the target porosity is higher at the same distance from the center of crater cavity. However, this systematic change of ΔTmax were well scaled by using normalized distance, x/Rpitmax (the x is the distance from the center of the crater cavity and, the Rpitmax is the maximum radius of the crater cavity), and the relationship was obtained to be ΔTmax=4.7(x/Rpitmax)-2.6 at x >1.2.
The ΔTmax distribution should be reproduced by using simple heat conduction model. We assumed that the post shock heat deposited at a thin layer on the final crater wall conducted outside the crater. The post shock heat was obtained as the energy conversion efficiency depended on the target porosities: It was 0.09 for 40% target, 0.32 for 50% target, and 0.51 for 60% target, respectively.