Proton irradiation of olivine and meteorites pellets: an evidence of surface hydroxylation by space weathering on airless bodies

Introduction

The presence of hydroxyl groups on the lunar soil has been reported by four spacecrafts since 2009 [1-4]. Numerous experimental studies have demonstrated that OH bonds can form as a result of proton bombardment on various silicate samples (e.g. [5]). Here we present a new study on proton implantation on S-type asteroid analogs to support the Hayabusa2 mission extent (also called Hayabusa2#) which will fly by asteroid (98943) Torifune in July 2026 [6]. The goal is to investigate whether the formation of OH bonds is a general process that can take place at the surface of most airless bodies, including S-type asteroids, or not. This could have major implications for Solar System history, particularly regarding the origin of water on Earth.

 

Methods

The samples used are a San Carlos olivine and three L-LL type ordinary chondrite meteorites (NWA 7985, Sahara 98222, Bensour), which appear to be good analogs for the surface of (98943) Torifune. The samples were crushed into powders (<45 µm) and pressed into pellets, then heated for 24 hours at 400°C under vacuum to remove most of adsorbed water.

The pellets were placed in the INGMAR vacuum chamber (IAS and IJCLab) [7], which allows monitoring the samples by the Near-InfraRed (0.8 - 5 µm) reflectance spectroscopy during ion implantation, as a simulation of solar wind proton implantation. The irradiation was conducted on the SIDONIE ion separator, part of the MOSAIC platform at IJCLab, under high vacuum (10^-9 Torr). NIR spectra were acquired for each sample using a Fourier Transform spectrometer (Tensor37 Bruker, N₂ purged) at five fluences: 3*10¹⁶, 6*10¹⁶, 1*10¹⁷, 2*10¹⁷, and 3*10¹⁷ H⁺/cm². For comparison, we also recorded the spectrum of the unirradiated samples. The energy used was 20 keV per proton, except for the stage with the highest fluence, where it was 15 keV.

 

Results and discussion

All samples exhibit a broad absorption band around the 3 µm region, which is characteristic of H₂O stretching vibrations. This suggests that the heating process did not completely remove these components and/or that the few minutes required to transfer the samples into the vacuum chamber may have been sufficient to reintroduce a fraction of atmospheric H₂O in them. However, an increase in the band depth at ~2.8 µm is identified in all samples when the spectra of irradiated samples are ratioed to the spectrum of unirradiated samples. This feature is specific to the stretching vibrations of the OH group, which implies that OH bonds were formed during proton implantation and increased in number with increasing fluence. The maximum band depth is reached for Bensour LL-chondrite with a 3.5% increase relative to the unirradiated spectrum; however, the best spectra are obtained for the olivine sample, which exhibits the most significant relative band depths at all fluences below 3*10¹⁷ H⁺/cm².

Some studies have reported a saturation limit where OH bonds can no longer form in the implantation layer [8]. At this point, the destruction of OH bonds is as efficient as their formation, and the band depth stabilizes, which is not observed in this study. This suggests that more OH bonds could form in samples exposed to higher fluence levels.

However, it is possible that the heavy ions in the solar wind break the OH bonds faster than they form, even though H⁺ accounts for ~95% of the ions in the solar wind.

The maximum fluence used in our study would correspond to an exposure time of approximately 1.5-3*10⁷ years at 1 AU from the Sun, considering only protons at 15-20 keV. However, protons in the energy range 1-15 keV are orders of magnitude more abundant. Even considering a lower OH production yield and/or a less efficient detection (NIR photons penetrate much deeper than the penetration depth of solar wind ions), we expect these lower energy ions to contribute to the OH band depth, thus lowering significantly the corresponding timescale in space. For comparison, Nakauchi et al. estimated an exposure time of 10² years for a fluence of 3*10¹⁷ H⁺/cm² [9].

 

Conclusion

According to our preliminary results, OH bonds can be formed on airless bodies, such as (98943) Torifune, by solar-wind proton implantation. Through secondary processes, hydroxyls could also rearrange into H₂O molecules on the surface of these bodies [9]. If the depletion of OH bonds by heavy solar ions is weaker than its formation yield, Hayabusa2# could detect signatures of OH and/or H₂O with the NIRS3 spectrometer during the flyby of (98943) Torifune in July 2026.

 

References

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[3] Pieters, C. M. et al. (2009). Science 326, 568–572

[4] Simon, A. A. et al. (2019). Geophysical Research Letters 46, 6322–6326

[5] Ichimura, A. S. et al. (2012). Earth and Planetary Science Letters 345-348, 90-94

[6] Hirabayashi, M. et al. (2021). Advances in Space Research 68, 1533-1555

[7] Lantz, C. et al. (2017). Icarus 285, 43-57

[8] Schaible, M. J. & Baragiola, R. A. (2014). Journal of Geophysical Research: Planets 119, 2017-2028

[9] Nakauchi Y. et al. (2021). Icarus 355, 114-140