講演情報
[PPS04-P16]Oblique impact experiments on quartz sand under variable atmospheric pressure: Implications for crater formation processes on Venus
澤 みゆう1、荒川 政彦1、*保井 みなみ1、大川 初音1、豊嶋 遥名1、長谷川 直2 (1.神戸大学大学院理学研究科惑星学専攻、2.宇宙航空研究開発機構宇宙科学研究所)
キーワード：
金星、大気圧、斜め衝突実験、エジェクタカーテン欠損
The morphology of impact craters on bodies with high-density atmospheres, such as Venus, is significantly different from that on the Moon and Mars, which have low-density atmospheres. Observations by Magellan revealed that approximately half of Venusian craters exhibit lobate ejecta, petal-shaped ejecta deposits, and outflows, in which impact melt flows preferentially in a specific direction. These features strongly suggest that ejecta deposition and flow during oblique impacts depend on the azimuth angle. However, systematic physical models describing the interaction among the atmosphere, impactor, and ejecta during oblique impacts remain limited. Therefore, we conducted impact experiments on quart sand while varying the impact angle, impact velocity, and ambient air pressure to clarify the formation processes of impact craters and ejecta deposition mechanisms under high-density atmospheric conditions.
Impact experiments were performed using a one-stage light gas gun, two-stage light gas guns, and a one-stage powder gun at Kobe University and ISAS/JAXA. The impact velocity ranged from 50 m/s to 5 km/s, the impact angle from 20º to 90º, and the ambient air pressure from 40 Pa to 100 kPa. Quartz sand was used as a target material to simulate the Venusian surface. A high-speed camera was used to observe the impact phenomena. In addition, the shadowgraph method was employed to visualize shock waves, and an infrared high-speed camera was used to measure the temperature distribution of the ejecta. These diagnostic methods enabled us to analyze the dynamic behavior of the ejecta curtain and the morphology of the final crater.
When the ambient air pressure approached atmospheric pressure, a bow shock formed in front of the high-speed projectile. We observed adiabatic compression of the atmosphere (up to approximately 270 ºC), followed by a sudden temperature drop due to adiabatic expansion after the projectile passed (down to approximately -14 ºC). Analysis of the ejecta curtain quantitatively demonstrated that the reduction of the curtain base angle occurred earlier during its growth as ambient air pressure increased. Furthermore, high-velocity sand ejected from near the impact point lost momentum due to air resistance and was deposited near the crater rim.
At impact velocities higher than 700 m/s, sand near the impact point was disrupted and compressed, forming solid clusters. The size of these clusters increased with increasing impact velocity and ambient air pressure. In some experiments, the clusters were thermally altered and turned black, suggesting significant heating. The spatial distribution of these clusters showed enhanced downstream deposition as the impact angle decreased and the ambient air pressure increased. Under identical impact conditions, a large defect formed on the upstream side of the ejecta curtain. Additional experiments simulating this ejecta curtain defect indicated that the presence of a continuous shielding surface, which suppresses near-surface ejecta eruption without limiting crater growth, is essential for the formation of the defect.
These results demonstrate that the atmosphere acts not only as a mechanical resistance but also an important physical factor controlling the thermal modification and transport of target materials near the impact point, thereby influencing impact energy distribution on planets with atmospheres.
Impact experiments were performed using a one-stage light gas gun, two-stage light gas guns, and a one-stage powder gun at Kobe University and ISAS/JAXA. The impact velocity ranged from 50 m/s to 5 km/s, the impact angle from 20º to 90º, and the ambient air pressure from 40 Pa to 100 kPa. Quartz sand was used as a target material to simulate the Venusian surface. A high-speed camera was used to observe the impact phenomena. In addition, the shadowgraph method was employed to visualize shock waves, and an infrared high-speed camera was used to measure the temperature distribution of the ejecta. These diagnostic methods enabled us to analyze the dynamic behavior of the ejecta curtain and the morphology of the final crater.
When the ambient air pressure approached atmospheric pressure, a bow shock formed in front of the high-speed projectile. We observed adiabatic compression of the atmosphere (up to approximately 270 ºC), followed by a sudden temperature drop due to adiabatic expansion after the projectile passed (down to approximately -14 ºC). Analysis of the ejecta curtain quantitatively demonstrated that the reduction of the curtain base angle occurred earlier during its growth as ambient air pressure increased. Furthermore, high-velocity sand ejected from near the impact point lost momentum due to air resistance and was deposited near the crater rim.
At impact velocities higher than 700 m/s, sand near the impact point was disrupted and compressed, forming solid clusters. The size of these clusters increased with increasing impact velocity and ambient air pressure. In some experiments, the clusters were thermally altered and turned black, suggesting significant heating. The spatial distribution of these clusters showed enhanced downstream deposition as the impact angle decreased and the ambient air pressure increased. Under identical impact conditions, a large defect formed on the upstream side of the ejecta curtain. Additional experiments simulating this ejecta curtain defect indicated that the presence of a continuous shielding surface, which suppresses near-surface ejecta eruption without limiting crater growth, is essential for the formation of the defect.
These results demonstrate that the atmosphere acts not only as a mechanical resistance but also an important physical factor controlling the thermal modification and transport of target materials near the impact point, thereby influencing impact energy distribution on planets with atmospheres.