10:45 AM - 12:15 PM
[PPS07-P05] Investigation of the degree of the dehydration of serpentinite impactor remaining in the crater formed by hypervelocity impact
Keywords:crater, hydrate, impact, asteroids
Introduction
On terrestrial planets, impacts of small bodies such as asteroids and comets released volatiles, which played an important role in the formation of oceans and atmospheres. The transport of materials through collisions between celestial bodies is considered universal in the solar system. For example, spectroscopic observations by the Dawn spacecraft have shown that there are hydrated minerals that impacts may have brought on the surface of the differentiated asteroid Vesta [1]. The material present on the surface may significantly affect spectroscopic observations of asteroids, leading to the estimates that differ from the material that make up the bulk of each asteroid [2].
In this study, we conducted spectroscopic measurements of craters impacted by serpentinite projectiles to estimate the degree of dehydration of the remaining serpentinite material in the craters formed by hypervelocity impacts. We also examined the surface profiles of the craters and constrained the fraction of projectile materials remaining in the craters.
Experiments
We performed impact experiments with velocities of about 3-7 km/s using a two-stage hydrogen gas gun at the Institute of Space and Astronautical Science (ISAS). The projectiles were cylinders of serpentinite and dunite for comparison (3 mm in diameter and 2.3 mm in height), and the targets were steel cubes.
Near-infrared reflectance spectra have been collected from the craters under reduced pressure using a Fourier Transform Infrared Spectrometer (FT-IR). Samples were irradiated at an angle of 30 degree from the normal direction of the crater floor, and the reflectance spectra were measured in the normal direction. The surface profiles of craters were measured using a laser profiler (measurement pitch 50 µm, accuracy 0.1 µm).
Results
Absorption around 2.7 µm was observed in the craters impacted by serpentinite projectiles at 3-6 km/s. This indicates that hydroxyl in the serpentinite projectile was retained in the craters. According to the previous study [3], when a projectile impacted a serpentinite target, it was completely dehydrated on condition that generated shock pressure exceeding 30 GPa. Using the impedance matching method (see [4] for parameters), it was found that hydroxyl was retained even when the shock pressure generated in this experiment exceeded 100 GPa. In a previous study [2] in which a serpentinite projectile was impacted obliquely into a copper target, absorption at 1.4 µm indicating the presence of hydroxyl, was confirmed by spectroscopy, while the shock pressure was estimated to be 79 GPa. This indicates that hydroxyl in serpentinite projectiles is hard to dehydrate even under collisions that generate high pressure.
There was a tendency for absorption by hydroxyl to decrease with increasing shock pressure. However, the present measurement method cannot compare spectroscopic characteristics before and after the impact experiment. The differences in the amount of serpentine contained within individual projectiles may influence the result.
The surface area of the crater was determined from the cubic functions that approximate the shape of the crater's cross-section. Here the crater was assumed to have an axisymmetric shape. Assuming that projectile material adhered to the crater surface with a thickness of 10 µm, it was estimated that about 5% of the projectile material remained in a crater.
Acknowledgements
This research was supported by the Hypervelocity Impact Facility, ISAS, JAXA.
References
[1] M. C. De Sanctis et al. (2013) Meteoritics & Planetary Science, 48, 2166-2184.
[2] G. Libourel et al. (2019) Science Advances, 5, eaav3971.
[3] J. A. Tyburczy et al. (1986) EPSL, 80, 201-207.
[4] T. Katsura et al. (2014) Icarus, 241, 1-12.
[5] R. T. Daly and P. H. Schultz (2018) Meteoritics & Planetary Science, 53, 1364-1390.
On terrestrial planets, impacts of small bodies such as asteroids and comets released volatiles, which played an important role in the formation of oceans and atmospheres. The transport of materials through collisions between celestial bodies is considered universal in the solar system. For example, spectroscopic observations by the Dawn spacecraft have shown that there are hydrated minerals that impacts may have brought on the surface of the differentiated asteroid Vesta [1]. The material present on the surface may significantly affect spectroscopic observations of asteroids, leading to the estimates that differ from the material that make up the bulk of each asteroid [2].
In this study, we conducted spectroscopic measurements of craters impacted by serpentinite projectiles to estimate the degree of dehydration of the remaining serpentinite material in the craters formed by hypervelocity impacts. We also examined the surface profiles of the craters and constrained the fraction of projectile materials remaining in the craters.
Experiments
We performed impact experiments with velocities of about 3-7 km/s using a two-stage hydrogen gas gun at the Institute of Space and Astronautical Science (ISAS). The projectiles were cylinders of serpentinite and dunite for comparison (3 mm in diameter and 2.3 mm in height), and the targets were steel cubes.
Near-infrared reflectance spectra have been collected from the craters under reduced pressure using a Fourier Transform Infrared Spectrometer (FT-IR). Samples were irradiated at an angle of 30 degree from the normal direction of the crater floor, and the reflectance spectra were measured in the normal direction. The surface profiles of craters were measured using a laser profiler (measurement pitch 50 µm, accuracy 0.1 µm).
Results
Absorption around 2.7 µm was observed in the craters impacted by serpentinite projectiles at 3-6 km/s. This indicates that hydroxyl in the serpentinite projectile was retained in the craters. According to the previous study [3], when a projectile impacted a serpentinite target, it was completely dehydrated on condition that generated shock pressure exceeding 30 GPa. Using the impedance matching method (see [4] for parameters), it was found that hydroxyl was retained even when the shock pressure generated in this experiment exceeded 100 GPa. In a previous study [2] in which a serpentinite projectile was impacted obliquely into a copper target, absorption at 1.4 µm indicating the presence of hydroxyl, was confirmed by spectroscopy, while the shock pressure was estimated to be 79 GPa. This indicates that hydroxyl in serpentinite projectiles is hard to dehydrate even under collisions that generate high pressure.
There was a tendency for absorption by hydroxyl to decrease with increasing shock pressure. However, the present measurement method cannot compare spectroscopic characteristics before and after the impact experiment. The differences in the amount of serpentine contained within individual projectiles may influence the result.
The surface area of the crater was determined from the cubic functions that approximate the shape of the crater's cross-section. Here the crater was assumed to have an axisymmetric shape. Assuming that projectile material adhered to the crater surface with a thickness of 10 µm, it was estimated that about 5% of the projectile material remained in a crater.
Acknowledgements
This research was supported by the Hypervelocity Impact Facility, ISAS, JAXA.
References
[1] M. C. De Sanctis et al. (2013) Meteoritics & Planetary Science, 48, 2166-2184.
[2] G. Libourel et al. (2019) Science Advances, 5, eaav3971.
[3] J. A. Tyburczy et al. (1986) EPSL, 80, 201-207.
[4] T. Katsura et al. (2014) Icarus, 241, 1-12.
[5] R. T. Daly and P. H. Schultz (2018) Meteoritics & Planetary Science, 53, 1364-1390.