11:00 AM - 1:00 PM
[PPS07-P19] Experimental study of impact ejecta flying from Mars to Phobos using serpentinite and ice blocks as targets
Keywords:high speed ejecta, serpentinite, Phobos
Introduction : MMX mission with sample return from the Martian moon Phobos is expected to provide information about Mars from the investigation of the surface material of Phobos. It is believed that materials derived from Mars exist in Martian moon’s surface (e.g.[1]). The relationship between the size and velocity of the ejecta produced by impact on Mars is important to constrain the estimation of the ejecta amount escaping from Mars and reaching the moon. In laboratory experiments, the information of the size-velocity relationship of ejecta with velocity larger than 1 km/s is limited, and it is difficult to derive information from the high-speed camera images about the size of the ejecta with velocity larger than 4 km/s, the velocity required to reach Phobos from Mars. Therefore, a new method was used to investigate the size-velocity relationship of ejecta by detecting the time of impact of the ejecta on the secondary target using high-speed cameras [2]. In the same method as the previous study, we change the primary target to serpentinite and ice blocks from the basalt blocks used in the previous study. The aim is to obtain knowledge that can be applied for various materials on the surface of Mars.
Experimental method: A two-stage light-gas gun in the Institute of Space and Astronautical Science was used to perform impact experiments with serpentinite and ice block targets. An aluminum sphere with a diameter of 3 mm was used as the projectile, and the impact velocity was about 7 km/s. The projectile impacted vertically with the target. Polycarbonate plates or glass plates (70 mm long and 150 mm wide) were placed as secondary targets at the position 300 mm and 100 mm from the serpentinite and ice blocks, respectively. We defined θ as the angle between the projectile trajectory and normal direction of the surface of the secondary target. When the primary target was serpentinite, the range of θ of the entire secondary target was from 15 °to 40 °. The glass plates were used for the ice primary targets and polycarbonate plates were used for the serpentinite targets. The polycarbonate plate was used to prevent mutual overlap of craters. The timings of the impact of the projectile with the primary target and the impact of the ejecta with the secondary target were captured by the high-speed cameras at a frame rate of 105 fps or- higher. The speed of the ejecta was derived from the time of flight and the distance to the secondary target, and the ejecta size was estimated from the diameter of the crater formed on the secondary target using the π scaling law, where ejecta shape was assumed as sphere. We traced the center of the contour of the crater, measured the crater area, and calculated the crater diameter assuming that the crater is circuler.
Results: Even when the primary target was serpentinite, ejecta with velocities exceeding 4 km/s were found as in the previous study [2]. As a result of investigating the ejecta velocity and size, the faster the ejecta velocity was, the smaller the ejecta size was. The tendency was similar to that of the previous study [2]. Also, when the primary target was serpentinite, at the same ejection velocity, the maximum size of ejecta was similar to that of basalt [2]. In the case of ice block, the ejection velocity observed at the secondary target ranged from 200 m/s to 4 km/s, and only a few µm–size craters were found on the secondary target. The-µm-size crater has not been identified on high-speed camera images, but assuming that these craters were formed at impact velocity of 200 m/s to 4 km/s, the ejecta size would be 1/100 of that of serpentinite.
In addition, two types of craters on the secondary target were observed, one was circular and the other was elongated when the primary target was serpentinite. When the ejected angle θ was small (about 20 °), the crater shape was circular and the diameter was several tens to several hundreds of µm. When θ was large (about 35 °), the crater shape was elongated and the diameter was several mm. The craters found on the ice block target were circuler.
We will re-evaluate the above results, taking into account the effect of the ejecta shape on the crater diameter and the associated effect on the scaling law.
Acknowledgments
This research was supported by the Hypervelocity Impact facility at ISAS/JAXA.
References
[1] Ramsley, K.R., Head III, J.W., 2013. Planetary and Space Science 87, 115-129.
[2] Nomura, K., Nakamura A., Hasegawa S., 2021. JpGU2021 Online, May 30-June 6, 2021. (Poster)
Experimental method: A two-stage light-gas gun in the Institute of Space and Astronautical Science was used to perform impact experiments with serpentinite and ice block targets. An aluminum sphere with a diameter of 3 mm was used as the projectile, and the impact velocity was about 7 km/s. The projectile impacted vertically with the target. Polycarbonate plates or glass plates (70 mm long and 150 mm wide) were placed as secondary targets at the position 300 mm and 100 mm from the serpentinite and ice blocks, respectively. We defined θ as the angle between the projectile trajectory and normal direction of the surface of the secondary target. When the primary target was serpentinite, the range of θ of the entire secondary target was from 15 °to 40 °. The glass plates were used for the ice primary targets and polycarbonate plates were used for the serpentinite targets. The polycarbonate plate was used to prevent mutual overlap of craters. The timings of the impact of the projectile with the primary target and the impact of the ejecta with the secondary target were captured by the high-speed cameras at a frame rate of 105 fps or- higher. The speed of the ejecta was derived from the time of flight and the distance to the secondary target, and the ejecta size was estimated from the diameter of the crater formed on the secondary target using the π scaling law, where ejecta shape was assumed as sphere. We traced the center of the contour of the crater, measured the crater area, and calculated the crater diameter assuming that the crater is circuler.
Results: Even when the primary target was serpentinite, ejecta with velocities exceeding 4 km/s were found as in the previous study [2]. As a result of investigating the ejecta velocity and size, the faster the ejecta velocity was, the smaller the ejecta size was. The tendency was similar to that of the previous study [2]. Also, when the primary target was serpentinite, at the same ejection velocity, the maximum size of ejecta was similar to that of basalt [2]. In the case of ice block, the ejection velocity observed at the secondary target ranged from 200 m/s to 4 km/s, and only a few µm–size craters were found on the secondary target. The-µm-size crater has not been identified on high-speed camera images, but assuming that these craters were formed at impact velocity of 200 m/s to 4 km/s, the ejecta size would be 1/100 of that of serpentinite.
In addition, two types of craters on the secondary target were observed, one was circular and the other was elongated when the primary target was serpentinite. When the ejected angle θ was small (about 20 °), the crater shape was circular and the diameter was several tens to several hundreds of µm. When θ was large (about 35 °), the crater shape was elongated and the diameter was several mm. The craters found on the ice block target were circuler.
We will re-evaluate the above results, taking into account the effect of the ejecta shape on the crater diameter and the associated effect on the scaling law.
Acknowledgments
This research was supported by the Hypervelocity Impact facility at ISAS/JAXA.
References
[1] Ramsley, K.R., Head III, J.W., 2013. Planetary and Space Science 87, 115-129.
[2] Nomura, K., Nakamura A., Hasegawa S., 2021. JpGU2021 Online, May 30-June 6, 2021. (Poster)