Simulating space-weathering on Phobos: He2+, Ar7+, and O+ ion irradiation of simulants, minerals, and organics

             Airless bodies experience significant space weathering from solar wind, galactic cosmic rays, and micrometeorites bombardments [1]. However, the biggest Martian moon, Phobos, is peculiar in the Solar System because, due to its proximity with its host planet, its surface is also altered by Martian heavy atmospheric escaping oxygen ions. Because Phobos is tidally locked the sub-Martian hemisphere is strongly affected by these ions and a strong hemispheric dichotomy on Phobos might be expected. To characterize the expected modifications caused by oxygen irradiation and compared it with solar wind irradiation, we investigated in laboratory the spectroscopic, photometric, physical, and chemical modifications of various ion irradiated samples.

            To represent the surface of Phobos, several samples based on their mineralogical composition and/or spectroscopic properties were selected, including two Phobos simulants (UTPS, OPPS) [2,3], olivine, phyllosilicate (saponite), coal (anthracite and DECS-19 from the Penn State Coal Bank), and iron sulfide. This study investigated the spectro-photometric variations induced by space-weathering with spectroscopic measurements ranging from 0.4 to 3.6 µm with different geometry of observations. Additionally, mid-infrared (MIR) reflectance spectra (1.25 – 18 µm) were also obtained to study the modifications of shape and positional shifts of two key MIR features for mineralogical interpretation: the Christiansen feature (CF) and the Reststrahlen band (RB). Modifications of the physical properties were investigated through scanning electron microscopy (SEM) and atomic force microscopy (AFM). Chemical variations were monitored by using Raman spectroscopy with 532 nm and 248.6 nm laser excitations, and energy-dispersive X-ray (EDX) spectroscopy. In addition, we prepared plasma focused ion beam (pFIB) sections and subsequently performed Transmission Electron Microscopy (TEM) analysis to study the microstructural, chemical, and mineralogical changes induces by space-weathering inside the samples.

             We irradiated the various samples with 7 keV O⁺ ions, reaching a maximum fluence of 6.10¹⁵ ions.cm^-2, representing about 10³ years of irradiation on Phobos’ surface. To represent the light and heavy solar wind ions contribution, we irradiated the same samples with 36 keV He²⁺ and 126 keV Ar⁷⁺ ions, respectively. The irradiation experiments were performed using the ARIBE beamline at the large heavy ion national accelerator (GANIL, France), under ultra-high vacuum (P ~ 10^-7–10^-9 mbar) and at ambient temperature.

          We explored the spectroscopic modifications induced by space-weathering, with a specific focus on the evolution of the spectral slope and of some key absorption bands such as the 2.7 µm O-H feature and the 3.4 µm C-H aliphatic and aromatic features. We found no modification of the 2.7 µm feature after O⁺ ion irradiation in saponite and Phobos simulants. However, a small decrease of 10% in the C-H absorption band depths was observed in the DECS-19 sample. This decrease in the C-H feature may be consistent with the partial amorphization observed through Raman spectra associated with this sample. Regarding spectral slope and reflectance level, most of the samples do not exhibit variations, except OPPS for which a darkening and reddening is observed after O⁺ irradiation, and DECS-19 with a slight bluing in the visible. In the MIR, for all samples, no modifications are observed for both CF and RB feature(s). New transmission electron microscopy (TEM) imaging results will also be presented. Unlike O⁺ irradiation, He²⁺ and Ar⁷⁺ irradiations led to significant spectral modifications for most of the samples, in terms of spectral slope and reflectance level, for example with a strong bluing and brightening for UTPS.

            This study shows that oxygen irradiation from Martian atmospheric ions might have a limited effect on the spectroscopic properties due to their low energy, and hence their low projected range in the regolith grains. On the contrary, solar wind ions may significantly alter the surface of Phobos. However, Phobos returned samples from the JAXA Martian Moons eXploration (MMX) mission [4] – which will be launched in autumn 2026 – may exhibit traces of alteration at the submicron scale by these oxygen ions, which may be important to understand the history of the Martian system.

Acknowledgements: The authors acknowledge the Centre National d’Études Spatiales (CNES) for the continuous support, and the Grand Accélérateur National d'Ions Lourds (GANIL, France) for the time allocated on the ARIBE beamline for ion irradiation experiments under proposals P1377_23 and P1402_24.

References: [1] Pieters and Noble (2016), JGR Planets, 121, 10 [2] Wargnier et al. (2024), Icarus, 421, 3 [3] Miyamoto et al. (2021), EPS, 73, 214 [4] Kuramoto et al. (2022), EPS, 74, 12