Presentation Information
[PPS04-P15]Measurement of residual impact temperatures surrounding an impact crater formed in frozen sand layer
*Ohashi Takuma1, Minami Yasui1, Masahiko Arakawa1, Hatsune Okawa1, Haruna Toyoshima1 (1.Kobe University)
Keywords:
crater,impact
Introduction:
In our solar system, collisions among planetary bodies are universal phenomena, and understanding its physical processes is crucial for elucidating the planetary evolution. In the formation of habitable environments, clarifying how icy bodies such as asteroids delivered water through collisions is one of the most crucial challenges in planetary science. Recent sample return missions by Hayabusa2 revealed the evidence of aqueous alteration which is the interaction between liquid water and minerals on the parent body of Ryugu. This research has heightened the scientific interest in the interactions between water and heat. Additionally, observations by HiRISE on the Mars Reconnaissance Orbiter (MRO) have confirmed that extensive permafrost layers exist subsurface on Mars, and impact events on such bodies are thought to form unique geological features. One prime example is the "rampart craters," which exhibits a raised rim with sediment deposits distributed in a petal-like pattern around the crater. While this terrain is believed to involve the melting of permafrost layers due to impact energy, the behavior of residual heat in permafrost and its specific effects on crater morphology are not yet fully understood. Therefore, in this study, we conducted impact experiments targeting permafrost layers and measured the residual impact temperatures around the crater, with the objective of clarifying the thermal effects of impact residual heat on the crater surroundings.
Experiment method:
To simulate permafrost, frozen sand targets were prepared by mixing 100 µm quartz sand with water and freezing the mixture. The wet sand was packed into a cylindrical acrylic container (internal diameter of 140mm, external diameter of 150mm, depth of 60mm), and thermocouples were installed inside the sample using a specialized fixture. By tapping the container filled with the wet sand, target surface was formed flatly and it was frozen at -20℃ in a freezer. Four 0.1mm-diameter Type K thermocouples were installed inside the sample for temperature measurement. In this study, we varied the water content of the target samples from 0% to 20%, thereby changing the porosity from 0% to 40%. For each sample, we recorded temperature changes around the crater both before and after impact using a data logger with a 100Hz sampling interval. Additionally, to simulate the effect of increased gravity on crater growth, we placed an aluminum plate with hole on the surface of frozen sand samples. In this surface-constrained experiment called a fixed surface experiment, fragments exposed to high pressure and temperature which would normally be ejected from a free-surface target were kept on the target. This made it possible to examine the influence of gravity on post-impact residual heat. We used the horizontal two-stage light gas gun at Kobe University to conduct the experiment, and accelerated 2mm diameter aluminum sphere projectile at impact velocities ranging from 2 to 6 km/s.
Result:
For a dry sand target at the impact velocity of 2km/s, we observed the temperature rises of the thermocouples after the impact, and their temperature rises depended on the positions of the thermocouples. The maximum temperature rises gradually diminished away from the impact point, and the peak temperature was reached more slowly for the thermocouples set at far from the impact point. Additionally, when an aluminum plate was placed on the target for the fixed surface experiment, the cratering process was suppressed, producing minimal depth growth. Compared to target with free surface, a crater formed on the fixed surface retained high-temperature materials on the crater wall for an extended duration longer than that on the crater formed on the free surface.
In our solar system, collisions among planetary bodies are universal phenomena, and understanding its physical processes is crucial for elucidating the planetary evolution. In the formation of habitable environments, clarifying how icy bodies such as asteroids delivered water through collisions is one of the most crucial challenges in planetary science. Recent sample return missions by Hayabusa2 revealed the evidence of aqueous alteration which is the interaction between liquid water and minerals on the parent body of Ryugu. This research has heightened the scientific interest in the interactions between water and heat. Additionally, observations by HiRISE on the Mars Reconnaissance Orbiter (MRO) have confirmed that extensive permafrost layers exist subsurface on Mars, and impact events on such bodies are thought to form unique geological features. One prime example is the "rampart craters," which exhibits a raised rim with sediment deposits distributed in a petal-like pattern around the crater. While this terrain is believed to involve the melting of permafrost layers due to impact energy, the behavior of residual heat in permafrost and its specific effects on crater morphology are not yet fully understood. Therefore, in this study, we conducted impact experiments targeting permafrost layers and measured the residual impact temperatures around the crater, with the objective of clarifying the thermal effects of impact residual heat on the crater surroundings.
Experiment method:
To simulate permafrost, frozen sand targets were prepared by mixing 100 µm quartz sand with water and freezing the mixture. The wet sand was packed into a cylindrical acrylic container (internal diameter of 140mm, external diameter of 150mm, depth of 60mm), and thermocouples were installed inside the sample using a specialized fixture. By tapping the container filled with the wet sand, target surface was formed flatly and it was frozen at -20℃ in a freezer. Four 0.1mm-diameter Type K thermocouples were installed inside the sample for temperature measurement. In this study, we varied the water content of the target samples from 0% to 20%, thereby changing the porosity from 0% to 40%. For each sample, we recorded temperature changes around the crater both before and after impact using a data logger with a 100Hz sampling interval. Additionally, to simulate the effect of increased gravity on crater growth, we placed an aluminum plate with hole on the surface of frozen sand samples. In this surface-constrained experiment called a fixed surface experiment, fragments exposed to high pressure and temperature which would normally be ejected from a free-surface target were kept on the target. This made it possible to examine the influence of gravity on post-impact residual heat. We used the horizontal two-stage light gas gun at Kobe University to conduct the experiment, and accelerated 2mm diameter aluminum sphere projectile at impact velocities ranging from 2 to 6 km/s.
Result:
For a dry sand target at the impact velocity of 2km/s, we observed the temperature rises of the thermocouples after the impact, and their temperature rises depended on the positions of the thermocouples. The maximum temperature rises gradually diminished away from the impact point, and the peak temperature was reached more slowly for the thermocouples set at far from the impact point. Additionally, when an aluminum plate was placed on the target for the fixed surface experiment, the cratering process was suppressed, producing minimal depth growth. Compared to target with free surface, a crater formed on the fixed surface retained high-temperature materials on the crater wall for an extended duration longer than that on the crater formed on the free surface.