3:30 PM - 3:45 PM
[PPS07-16] HighI-velocity Oblique Impact May Form a Hot Spring on a Crater Floor of Icy Bodies
Keywords:Impact crater, Post shock heat, Comet, Oblique Impact, Melt pool
Introduction
Analysis of return sample from asteroid Ryugu shows that Ryugu has a very close relationship to CI chondrite, and the parent body of Ryugu might be originated from the region beyond the snow line of the solar nebula based on the discovery of CO2 gas in water enclosed in the return sample [1, 2]. This would suggest that the parent body of Ryugu might be an icy body mainly composed of water ice and volatiles. Furthermore, the return sample analysis found evidence of aqueous alternation related to organic matters and silicate minerals up to 30°C [2, 3]. Thus, the parent body of Ryugu might be an icy body that would be heated to melt water ice and cause thermal alteration between liquid water and other minerals together with organic matters. It is widely accepted that the heat source of the parent body is short-lived radiogenic elements such as 26Al, so far, the parent body of Ryugu [2]. While high-velocity impact among icy bodies is another heat source to melt water ice on the surfaces [4]. This impact left the residual heat that could easily met water ice and cause hot spring on the crater floor. This hot spring on the crater floor may be maintained for some period proportional to the crater size, so that the relationship between the crater size and the impact induced residual heat is quite important to study the contribution of high-velocity impacts to the aqueous alteration of icy bodies. In this study, we firstly try to measure the surface temperature of the crater floor formed on porous ice by using a high-speed infrared camera.
Method
High-velocity impact experiments were conducted by using a two-stage light gas gun set at Kobe University and ISAS/JAXA. The basic set up of the targets and the observation system were the same as those of Sasai et al. (2023) [4]. We prepared porous ice targets and the porosity and the compressive strength of the target were 0.4 and 100kPa, respectively. The projectile of Al sphere with the diameter of 2 mm was impacted on the porous ice target with the initial temperature of -15 °C at the velocity of 3.0 km/s and 4.2km/s. The difference from Sasai et al. (2023) [4] is that the target surface was inclined to the projectile trajectory, so the impact angle was set from 45° to 15 °. Moreover, a high-speed infrared camera observed the crater floor; the camera was taken at the speed of 3000 fps with the exposure time of 20 ms at ISAS and 600 fps with the exposure time of 1 ms at Kobe Univ.
Results & Discussion
Fig.a shows a snapshot taken by the high-speed infrared camera, and Fig.b shows a photo of the crater formed on the recovered target, where the impact velocity is 6 km/s and the impact angle is 45°. Comparing both images, we notice that the hot round area in Fig.a is almost consistent with the crater in Fig.b. We calculated the temperature averaged at the crater floor with time. So, we obtained the average temperature at 0.2 s after the impact for the representative temperature because before 0.2 s there were many fragments hiding the crater floor and they interfered the observation of the crater floor. We also obtained the local maximum temperature, not average, on the crater floor at 0.2 s. Fig.1a shows that the average temperature is 8.1°C and the maximum temperature is 11.9 °C. We analyzed all the results in these experiments with different impact angles and velocities, we then found that the maximum temperature did not depend on both impact angles and impact velocities: They were between 15 °C and -1 °C, and a half of the data was above 10 °C. Furthermore, we discovered hot water area at the temperature of 45 °C before 0.2 s and it was maintained for 0.1 s. This evidence suggests that a warm water layer like a hot spring was formed.
References:[1] Yokoyama, T. et al. (2022) Science, 379(6634), eabn7850. [2] Nakamura, T. et al. (2022) Science, 379(6634), eabn8671. [3] Ito, M. et al. (2022) Nature Astronomy, 6(10), 1163-1171. [4] Sasai, H. et al. (2023) Icarus, 115929.
Analysis of return sample from asteroid Ryugu shows that Ryugu has a very close relationship to CI chondrite, and the parent body of Ryugu might be originated from the region beyond the snow line of the solar nebula based on the discovery of CO2 gas in water enclosed in the return sample [1, 2]. This would suggest that the parent body of Ryugu might be an icy body mainly composed of water ice and volatiles. Furthermore, the return sample analysis found evidence of aqueous alternation related to organic matters and silicate minerals up to 30°C [2, 3]. Thus, the parent body of Ryugu might be an icy body that would be heated to melt water ice and cause thermal alteration between liquid water and other minerals together with organic matters. It is widely accepted that the heat source of the parent body is short-lived radiogenic elements such as 26Al, so far, the parent body of Ryugu [2]. While high-velocity impact among icy bodies is another heat source to melt water ice on the surfaces [4]. This impact left the residual heat that could easily met water ice and cause hot spring on the crater floor. This hot spring on the crater floor may be maintained for some period proportional to the crater size, so that the relationship between the crater size and the impact induced residual heat is quite important to study the contribution of high-velocity impacts to the aqueous alteration of icy bodies. In this study, we firstly try to measure the surface temperature of the crater floor formed on porous ice by using a high-speed infrared camera.
Method
High-velocity impact experiments were conducted by using a two-stage light gas gun set at Kobe University and ISAS/JAXA. The basic set up of the targets and the observation system were the same as those of Sasai et al. (2023) [4]. We prepared porous ice targets and the porosity and the compressive strength of the target were 0.4 and 100kPa, respectively. The projectile of Al sphere with the diameter of 2 mm was impacted on the porous ice target with the initial temperature of -15 °C at the velocity of 3.0 km/s and 4.2km/s. The difference from Sasai et al. (2023) [4] is that the target surface was inclined to the projectile trajectory, so the impact angle was set from 45° to 15 °. Moreover, a high-speed infrared camera observed the crater floor; the camera was taken at the speed of 3000 fps with the exposure time of 20 ms at ISAS and 600 fps with the exposure time of 1 ms at Kobe Univ.
Results & Discussion
Fig.a shows a snapshot taken by the high-speed infrared camera, and Fig.b shows a photo of the crater formed on the recovered target, where the impact velocity is 6 km/s and the impact angle is 45°. Comparing both images, we notice that the hot round area in Fig.a is almost consistent with the crater in Fig.b. We calculated the temperature averaged at the crater floor with time. So, we obtained the average temperature at 0.2 s after the impact for the representative temperature because before 0.2 s there were many fragments hiding the crater floor and they interfered the observation of the crater floor. We also obtained the local maximum temperature, not average, on the crater floor at 0.2 s. Fig.1a shows that the average temperature is 8.1°C and the maximum temperature is 11.9 °C. We analyzed all the results in these experiments with different impact angles and velocities, we then found that the maximum temperature did not depend on both impact angles and impact velocities: They were between 15 °C and -1 °C, and a half of the data was above 10 °C. Furthermore, we discovered hot water area at the temperature of 45 °C before 0.2 s and it was maintained for 0.1 s. This evidence suggests that a warm water layer like a hot spring was formed.
References:[1] Yokoyama, T. et al. (2022) Science, 379(6634), eabn7850. [2] Nakamura, T. et al. (2022) Science, 379(6634), eabn8671. [3] Ito, M. et al. (2022) Nature Astronomy, 6(10), 1163-1171. [4] Sasai, H. et al. (2023) Icarus, 115929.