The formation of H2O and Si-OH by H2+ irradiation in major minerals of carbonaceous chondrites

Yusuke Nakauchi, Masanao Abe, Makiko Ohtake, Toru Matsumoto, Akira Tsuchiyama, Kohei Kitazato, Keisuke Yasuda, Kohtaku Suzuki, Yoshinori Nakata

In Press, Journal Pre-proof, Available online 3 October 2020



• We report the spectral modification of hydrous silicate minerals due to solar wind implantation by conducting laboratory experiments.
• Among the spectral modifications, we clearly identified the first positive evidence of formation of H2O due to hydrogen irradiation.
• The saturation timescale of this spectral modification is derived to be from 10² to 10³ yr.”

“As the parent bodies of carbonaceous chondrite (CC) meteorites contain valuable information that will help us better understand the formation and evolution of the early solar system, it is vital to search for them. To date, searching for those parent bodies has been conducted by comparing visible to near-infrared reflectance spectra of CCs with those of small bodies. It is known that the reflectance spectra of small bodies and, more widely, airless planetary bodies can be modified via space weathering. However, the effects of irradiated solar wind protons, which account for 95 % of the solar wind, on the spectra of CC-like materials, are still poorly understood. Here we report on the experimental spectral modification of hydrous silicate minerals due to hydrogen irradiation via simulating the implantation of solar wind protons onto parent bodies of CCs. The samples we used for our experiment were serpentine and saponite, which are two major mineral components of CCs. We irradiated the samples with 10 keV H ions to achieve the estimated saturation level (1.3 to 1.7 × 1018 ions cm−2), and measured their reflectance spectra in the wavelength range from 1.5 to 5.0 μm both before and after irradiation. For serpentine, the sample was irradiated step by step, and the reflectance spectra were measured after each irradiation step.

After irradiation, we observed changes in the absorption features at 2.77 μm and 2.85 μm. These changes imply that H2O and Si-OH radicals, respectively, would have been formed by a reaction between the irradiated hydrogen and the hydrous silicate minerals. In this study, spectral modifications due to hydrogen irradiation near 0.7 and 2.3 μm were not observed. This indicates that hydrogen irradiation broke the Si-O-Si bonding within the tetrahedra in hydrous silicate mineral structures, but it did not break the metal-OH bonding of the octahedra. In contrast to previous studies, our results imply that solar wind irradiation should also form H2O on anhydrous silicate minerals. This suggests the possibility of H2O formation on the surfaces of planetary bodies located inside of the snow line. The saturation timescale of spectral modification based on our results is estimated to be from 102 to 103 yr at 1 AU. This timescale is much shorter than that of the micrometeorite bombardment (106 to 107 yr), and it is comparable to the exposure age of small bodies. Therefore, we expect that spectral modification for small body surfaces would be mostly saturated.”