Effect of oxygen ion irradiation on the visible, infrared, and Raman spectra of Phobos candidate materials: implications on the space weathering by Martian atmosphere-derived ions

Abstract The origin of Martian satellites is still under debate. Nevertheless, such information could give insights into the evolution of the Martian system and planetary formation processes. To address these questions, MMX (Martian Moons eXploration) will conduct spectroscopy (visible, near-infrared, Raman) and sample return from Phobos. When interpreting these spectra, an understanding of the space weathering effect is crucial. Recently, MAVEN discovered that Phobos is bombarded by oxygen ions originating from the present Martian atmosphere. However, the effect of oxygen ion irradiation on the visible, near-infrared (NIR), or Raman spectra of Phobos-like material remains uninvestigated. Therefore, in this study, we performed an exploratory oxygen ion irradiation experiment using a microwave plasma on olivine, basalt, and Phobos simulant and examined its effect on the visible/infrared/Raman spectra to be observed by the MMX mission. The irradiation experiments were conducted for 30 and 60 h. After irradiation, the reflectance of the Phobos simulant in the visible/NIR range increased nearly seven times, from 0.03 to 0.21. Raman analysis revealed the disappearance of the carbon bands, suggesting that the carbon particles contained in the simulant were oxidized by oxygen ions. For olivine, while the visible/NIR spectra show a reddening and brightening trend, suggesting oxidation of iron, the Raman spectra show a decrease in the Mg/(Fe + Mg) ratio from ~93 to ~87%. Basalt showed both reddening and bluing trends depending on the observation region, likely due to heterogeneity in the mineral abundance at observational spot scale. However, in the mid-infrared analysis, a decrease in the Reststrahlen Band was observed, suggestive of amorphization. Amorphization was also suggested by the decrease in Raman peak intensity and an increase in fluorescence background for all samples. These results may suggest that oxygen ion space weathering could cause spectral alterations in a manner different from that known for solar wind or micrometeoroid impact. However, further studies that consider the combined effects of other particles, such as the solar wind and atomic oxygen, are required to fully understand the space weathering environment in the Mars system.