Do hypervelocity impacts on carbonaceous asteroids cause significant volatile loss?

Carbonaceous asteroids, which are minor bodies but enriched in water and organics, have been extensively explored by HAYABUSA2 and OSIRIS-REx recently because they are thought to be the major carrier of volatiles to the inner planets in our solar system. The recent explorations revealed that two carbonaceous asteroids, Ryugu and Bennu, and their parent bodies suffered impact bombardments with a variety of impact energies. It has been hypothesized that Ryugu has lost a fraction of the volatiles due to some sorts of heating events [Kitazato et al., 2019]. In contrast, Bennu apparently did not suffer such a volatile loss [Hamilton et al., 2019]. Shock recovery experiments with metal containers performed in the 1980s suggested that chondritic meteorites could easily lose their volatiles during hypervelocity impacts [Tyburczy et al., 1986], leading to different impact histories being inferred to explain the difference in volatile contents. In this study, we revisited the total gas production during impact devolatilization of carbonaceous chondrite analog [Britt et al., 2019], hereafter referred to as CI simulant, with a two-stage light gas gun. We applied a new experimental technique for gas guns, which is referred to as the “two-valve method” [Kurosawa et al., 2019], to minimize the chemical contamination from the gun operation. The two-valve method allows us to investigate impact devolatilization in a fully open system where is the same geometry of natural impact phenomena. 

  Hypervelocity impact experiments were performed with a two-stage light gas gun placed at Planetary Exploration Research Center of Chiba Institute of Technology, Japan. An Al₂O₃ projectile with a diameter of 2 mm accelerated to two different impact velocities v_imp = 3.7 km/s and 5.8 km s^-1. Hereafter, we refer the two velocities as “the low v_imp” and “the high v_imp”, respectively. Helium gas was used to accelerate the projectile instead of frequently-used hydrogen gas to exclude the possibility that a trace amount of the gas for projectile acceleration causes the chemical reduction. A quadrupole mass spectrometer (QMS) was used to measure the composition and amounts of impact-generated gases.

The experimental results are summarized as follows: (1) The total gas production was limited only to a few wt.% of the projectile mass even at the high v_imp, (2) the most abundant product was CO₂ in all the shots, (3) the chemical composition, including CO/CO₂ ratio and H₂/CO ratio, did not depend on impact velocity, (4) the impact-generated vapor was depleted in sulfur with respect to the elemental composition of the CI simulant.

Here, we discuss about the controlling mechanism of impact devolatilization. Since the carbon source in the CI simulant is only crushed coal grains, which is mixed into the CI simulant as an analog for insoluble organic matters, the detected C-bearing gases, which are CO and CO₂, must be produced due to oxidation of the organics in the CI simulant. Thus, the molar ratio of CO to CO₂ is simply determined by the oxygen fugacity, which strongly depends on temperature, in the region where the devolatilization occurs. We could estimate the temperature of the devolatilization region by comparing the oxygen fugacity for the elemental composition of the CI chondrites [Schaefer and Fegley, 2017], suggesting that the temperature is 1,200–1,650 K at both high and low v_imp. We also conducted shock physics modelling with the iSALE shock physics code [Amsden et al., 1980; Ivanov et al., 1997; Wünnemann et al., 2006; Collins et al., 2016]. We estimated post-shock residual temperature field after pressure release with the ANEOS serpentine by taking the endothermic decomposition of hydrous minerals and organics in the CI simulant into account. We found that the temperature in the iSALE was much lower than the temperature of the devolatilization region inferred from the CO/CO₂ ratio. The large gap in the temperature between the experiment and the shock physics modelling indicates that a local energy concentration may be caused by velocity shear between the different grains with a large contrast in shock impedance. The above hypothesis about local heating is consistent with the low efficiency of impact devolatilization. Our experiment, shock physics modelling, and thermodynamic consideration suggest that hypervelocity impacts are not responsible for significant volatile loss from the parent body of Ryugu or from Ryugu itself [Kurosawa et al., Under review].

 

Acknowledgements: This work was supported by ISAS/JAXA as a collaborative program with the Hypervelocity Impact Facility. We appreciate the developers of iSALE, including G. Collins, K. Wünnemann, B. Ivanov, J. Melosh, and D. Elbeshausen. We also thank Tom Davison for the development of the pySALEPlot.

 

Key references:

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Collins, G. S., Elbeshausen, D., Davison, T. M., Wünnemann, K., Ivanov, B. A., and Melosh, H. J. iSALE-Dellen manual, Figshare, https://doi.org/10.6084/m9.figshare.3473690.v2 (2016).

Hamilton, V. E. et al. Evidence for widespread hydrated minerals on asteroid (101955) Bennu. Nature Astronomy 3, 332–340 (2019).

Ivanov, B. A., Deniem, D. & Neukum G. Implementation of dynamic strength models into 2-D hydrocodes: Applications for atmospheric breakup and impact cratering. Int. J. Impact Eng. 20, 411–430 (1997).

Kitazato, K. et al. The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy, Science 364, 272–275 (2019).

Kurosawa, K., Moriwaki, R., Komatsu, G., Okamoto, T., Sakuma, H., Yabuta, H. & Matsui, T. Shock vaporization/devolatilization of evaporitic minerals, halite and gypsum, in an open system investigated by a two-stage light gas gun. Geophysical Research Letters 46, 7258–7267 (2019).

Schaefer, L., & Fegley, B. Jr. Redox states of initial atmospheres outgassed on rocky planets and planetesimals. The Astrophysical Journal 843, 120 (2017).

Tyburczy, J. A., Frisch, B., and Ahrens, T. J. Shock-induced volatile loss from a carbonaceous chondrite: implications for planetary accretion. Earth and Planetary Science Letters 80, 201–207 (1986).