2:30 PM - 2:45 PM
[MIS19-04] High-velocity oblique impacts on wet surface : the effect of surface water content on temperature increase in cratering and crater morphology
Keywords:Impact experiment, Wet sand, Crater, Impact plume, habitability, Martian crater
Introduction Recently on Mars, seasonal black streak patterns (RSL) have been observed on Martian slopes or crater walls that appear to be caused by melting and seeping of subsurface ice. Such event involving temperature increases near the surface region as seasonal changes can affect the possibility of temporary existence of liquid water on Martian surface which has a near-triple-point environment for water. Asteroid impact, as one of the widely occurring events with temperature increase in Martian surface layer, raises the temperature near impact point to more than 2000°C by compressive heating due to the impact pressure, resulting in projectile vaporization and impact plume from volatiles in target. Furthermore, the velocity difference in target materials after the shock wave propagation causes friction, and entropy increases due to the frictional heat, so that heat remains inside the target as “post shock heat” even after the shock pressure is released. When targets contain ice or liquid water, this post shock heat could be used to generate or heat liquid water inside the targets, or for evaporation. However, research on temperature changes associated with crater formation in wet surfaces is still undeveloped. This is because it is difficult to numerically simulate the complex phenomena of impact heating with frictional heat, and the simulation results tend to strongly depend on the equation of state of the material and physical properties such as compressibility of the porosity, so that experimental data is vital for improving numerical models. On the other hand, experimental studies on this field is also poor because it is difficult to perform high-velocity impact experiments under considerable atmospheric pressure as water can exist as liquid state in laboratories.
In this study, we conducted high-velocity impact experiments on wet sand surface under near-triple-point conditions for water, and investigated not only crater morphology but also the temperature increase associated with cratering in target surface, subsurface, and surrounding atmosphere.
Methods A horizontal two-stage light gas gun at Kobe University or ISAS was used for the impact experiments. As projectiles, polycarbonate spheres (with diameter 4.7 mm) were impacted at 2 km/s-4 km/s onto wet sand target. They were mixtures of quartz sand (500 µm in grain diameter) and water in various proportions (0-13 wt.%). We observed all experiments by a high-speed infrared camera (3000 fps) to study the temperature changes in the target surface and ejecta. In addition, thermocouples were placed at five points just above the impact point at different altitudes from the surface (1cm, 5cm, 10cm, 15cm, and 20cm) to measure the temperature change in surrounding atmosphere at each altitude. Here, the used thermocouples were K-type with linear diameter of 127 µm with sampling rate of 10 ms.
Results The temperature of the target material inside the crater increased to over 250°C, and most of it was ejected in the direction of impact as hot ejecta. On the other hand, low temperature ejecta below 15°C excavated from the crater basement were ejected in all directions except in the direction of impact. These temperatures were lower with lower water content, with the highest ejecta temperature 350°C for target’s water content 0 wt.% and 250°C for 12 wt.%. However, the cooling rate of ejecta was slower when the target’s water content was higher, with the cooling rate for the 0 wt.% target below 100°C at 0.4 s after impact, but for the 12 wt.% target over 100°C even at 1.5 s after impact. Besides, the cooling curve for the 12 wt.% target could not be explained by cooling by thermal conduction. This may be due to the fact that at the 0 wt.% target, sand particles are ejected individually and the ratio of surface area of ejecta exposed to the atmosphere to the volume of ejecta is large, resulting in high radiation efficiency, while at the 12 wt.% target, sand particles adhere each other and form ejecta clumps, resulting in a relatively small ratio of surface area of clumps exposed to the atmosphere to the volume of ejecta clumps, resulting in low radiation efficiency. Furthermore, the temperature increase of the surrounding atmosphere associated with crater formation was consistent with the ejecta cooling curve, suggesting that the hot and cold ejecta exposed to the atmosphere may play roles as heat and cooling sources for the atmosphere surrounding the crater. We will discuss the dependence of this temperature increase on water content at the presentation.
In this study, we conducted high-velocity impact experiments on wet sand surface under near-triple-point conditions for water, and investigated not only crater morphology but also the temperature increase associated with cratering in target surface, subsurface, and surrounding atmosphere.
Methods A horizontal two-stage light gas gun at Kobe University or ISAS was used for the impact experiments. As projectiles, polycarbonate spheres (with diameter 4.7 mm) were impacted at 2 km/s-4 km/s onto wet sand target. They were mixtures of quartz sand (500 µm in grain diameter) and water in various proportions (0-13 wt.%). We observed all experiments by a high-speed infrared camera (3000 fps) to study the temperature changes in the target surface and ejecta. In addition, thermocouples were placed at five points just above the impact point at different altitudes from the surface (1cm, 5cm, 10cm, 15cm, and 20cm) to measure the temperature change in surrounding atmosphere at each altitude. Here, the used thermocouples were K-type with linear diameter of 127 µm with sampling rate of 10 ms.
Results The temperature of the target material inside the crater increased to over 250°C, and most of it was ejected in the direction of impact as hot ejecta. On the other hand, low temperature ejecta below 15°C excavated from the crater basement were ejected in all directions except in the direction of impact. These temperatures were lower with lower water content, with the highest ejecta temperature 350°C for target’s water content 0 wt.% and 250°C for 12 wt.%. However, the cooling rate of ejecta was slower when the target’s water content was higher, with the cooling rate for the 0 wt.% target below 100°C at 0.4 s after impact, but for the 12 wt.% target over 100°C even at 1.5 s after impact. Besides, the cooling curve for the 12 wt.% target could not be explained by cooling by thermal conduction. This may be due to the fact that at the 0 wt.% target, sand particles are ejected individually and the ratio of surface area of ejecta exposed to the atmosphere to the volume of ejecta is large, resulting in high radiation efficiency, while at the 12 wt.% target, sand particles adhere each other and form ejecta clumps, resulting in a relatively small ratio of surface area of clumps exposed to the atmosphere to the volume of ejecta clumps, resulting in low radiation efficiency. Furthermore, the temperature increase of the surrounding atmosphere associated with crater formation was consistent with the ejecta cooling curve, suggesting that the hot and cold ejecta exposed to the atmosphere may play roles as heat and cooling sources for the atmosphere surrounding the crater. We will discuss the dependence of this temperature increase on water content at the presentation.