9:45 AM - 10:00 AM
[PPS03-04] Dehydration of serpentinite impactor and preservation of released water in crater
Keywords:crater, water, asteroid
Introduction
Recent spectroscopic observations have detected molecular water and/or hydroxyl on the surfaces of differentiated asteroids [e.g., 1,2]. Some of these hydrated materials are thought to have been delivered by impacts of hydrated impactors. In this study, we conducted hypervelocity impact experiments simulating collisions of hydrated impactors to investigate how the dehydration of the impactor and the behavior of the water released depend on impact velocity and impact angle.
Experiments
We performed impact experiments with velocities of 3-7 km/s using a two-stage light-gas gun at the Institute of Space and Astronautical Science (ISAS). The projectiles were made of serpentinite, which contains hydroxyl, and dunite as an anhydrous material used for comparison, while the targets were steel cubes. Impact angles were 30°, 60°, or 90° from the target surface.
The craters formed by the impact experiments were analyzed using a Fourier transform infrared spectrometer (FT-IR) in the near-infrared region under reduced pressure. Samples were irradiated at an angle of 30 degrees from the normal direction of the crater floor, and the reflectance spectra were measured in the normal direction. Microscopic Raman spectra were also measured from the craters and projectiles.
Results
The reflectance spectra inside the craters formed by vertical impacts showed a broad absorption feature in the 3 µm band, where hydroxyl and molecular water absorption overlap, regardless of the projectile material. In contrast, no absorption was observed at any impact velocity near 1.4 µm, where only hydroxyl exhibits absorption. For impacts with pressures below approximately 100 GPa, the absorption at the 3 µm and at the 6 µm band, the latter of which is attributed solely to molecular water, was more pronounced than in craters formed by dunite projectiles. This suggests that the absorption at 3 µm is primarily due to molecular water, with only a small amount of hydroxyl remaining inside the craters. A previous study [3] conducted oblique impact experiments of serpentine onto pumice also demonstrated that molecular water was predominant over hydroxyl within the formed glass. This is thought to be the result of water vapor being trapped as it solidifies into glass. The melt inside the craters in this study may have captured water vapor. Additionally, in oblique impacts, absorption at 2.7 µm due to hydroxyl was observed. This suggests that the dehydration of the serpentinite projectile was suppressed under oblique impact conditions.
The Raman spectra of craters formed by vertical impacts at approximately 3 km/s detected serpentine with a disordered crystal structure. In contrast, no serpentine was detected in craters formed by higher-velocity impacts. This suggests that ~40 GPa is the upper pressure limit for serpentine to survive. This finding is consistent with a previous study [4], which performed shock recovery experiments on chondrites.
Assuming that rocky and metallic asteroids are composed of basalt and iron, respectively, we compared asteroid collisions in the asteroid belt with the results of this study. Based on the velocity distribution of asteroid collisions in the asteroid belt [5], the typical impact velocity range is estimated to be 2.3–7.0 km/s. In the case of a vertical collision with a serpentinite impactor, water vapor released upon impact can be trapped in the melt, regardless of the asteroids’ composition. If molecular water trapped in the melt is present on the surfaces of asteroids, it could be detected through absorption bands at 3 µm and 6 µm in spectroscopic observations.
Acknowledgements
This work was supported by the Hypervelocity Impact Facility, ISAS, JAXA.
Reference
[1] A. Arredondo et al. (2024) Planet. Sci. J., 5, 37.
[2] D. Takir et al. (2017), Astron. J., 153, 31.
[3] R. T. Daly and P. H. Schultz (2018) Sci. Adv., 4, eaar2632.
[4] W. F. Bottke et al. (1994) Icarus, 107, 255-268.
[5] N. Tomioka et al. (2007) Meteorit. Planet. Sci.,42, 19-30.
Recent spectroscopic observations have detected molecular water and/or hydroxyl on the surfaces of differentiated asteroids [e.g., 1,2]. Some of these hydrated materials are thought to have been delivered by impacts of hydrated impactors. In this study, we conducted hypervelocity impact experiments simulating collisions of hydrated impactors to investigate how the dehydration of the impactor and the behavior of the water released depend on impact velocity and impact angle.
Experiments
We performed impact experiments with velocities of 3-7 km/s using a two-stage light-gas gun at the Institute of Space and Astronautical Science (ISAS). The projectiles were made of serpentinite, which contains hydroxyl, and dunite as an anhydrous material used for comparison, while the targets were steel cubes. Impact angles were 30°, 60°, or 90° from the target surface.
The craters formed by the impact experiments were analyzed using a Fourier transform infrared spectrometer (FT-IR) in the near-infrared region under reduced pressure. Samples were irradiated at an angle of 30 degrees from the normal direction of the crater floor, and the reflectance spectra were measured in the normal direction. Microscopic Raman spectra were also measured from the craters and projectiles.
Results
The reflectance spectra inside the craters formed by vertical impacts showed a broad absorption feature in the 3 µm band, where hydroxyl and molecular water absorption overlap, regardless of the projectile material. In contrast, no absorption was observed at any impact velocity near 1.4 µm, where only hydroxyl exhibits absorption. For impacts with pressures below approximately 100 GPa, the absorption at the 3 µm and at the 6 µm band, the latter of which is attributed solely to molecular water, was more pronounced than in craters formed by dunite projectiles. This suggests that the absorption at 3 µm is primarily due to molecular water, with only a small amount of hydroxyl remaining inside the craters. A previous study [3] conducted oblique impact experiments of serpentine onto pumice also demonstrated that molecular water was predominant over hydroxyl within the formed glass. This is thought to be the result of water vapor being trapped as it solidifies into glass. The melt inside the craters in this study may have captured water vapor. Additionally, in oblique impacts, absorption at 2.7 µm due to hydroxyl was observed. This suggests that the dehydration of the serpentinite projectile was suppressed under oblique impact conditions.
The Raman spectra of craters formed by vertical impacts at approximately 3 km/s detected serpentine with a disordered crystal structure. In contrast, no serpentine was detected in craters formed by higher-velocity impacts. This suggests that ~40 GPa is the upper pressure limit for serpentine to survive. This finding is consistent with a previous study [4], which performed shock recovery experiments on chondrites.
Assuming that rocky and metallic asteroids are composed of basalt and iron, respectively, we compared asteroid collisions in the asteroid belt with the results of this study. Based on the velocity distribution of asteroid collisions in the asteroid belt [5], the typical impact velocity range is estimated to be 2.3–7.0 km/s. In the case of a vertical collision with a serpentinite impactor, water vapor released upon impact can be trapped in the melt, regardless of the asteroids’ composition. If molecular water trapped in the melt is present on the surfaces of asteroids, it could be detected through absorption bands at 3 µm and 6 µm in spectroscopic observations.
Acknowledgements
This work was supported by the Hypervelocity Impact Facility, ISAS, JAXA.
Reference
[1] A. Arredondo et al. (2024) Planet. Sci. J., 5, 37.
[2] D. Takir et al. (2017), Astron. J., 153, 31.
[3] R. T. Daly and P. H. Schultz (2018) Sci. Adv., 4, eaar2632.
[4] W. F. Bottke et al. (1994) Icarus, 107, 255-268.
[5] N. Tomioka et al. (2007) Meteorit. Planet. Sci.,42, 19-30.