Deep-UV Raman spectroscopy: A novel heuristic method to characterize volcanologically relevant glasses on Mars

Deep ultraviolet Raman spectroscopy is an essential component of the Perseverance rover operating on Mars. Here we propose a proof of concept of deep-UV Raman structural characterization of volcanologically relevant silicate glasses to provide a suitable analytical method of UV Raman spectra collected on Mars. The results show few but substantial spectral differences concerning those obtained by conventional Raman scattering using visible light laser sources. The evolution of the UV Raman spectra between 825 and 1300 cm-1 is confirmed to be more sensitive to the silica network's short-range structure than that below 700 cm-1 when compared to their visible counterpart. We adopted a Gaussian fitting model to parametrize the number of bridging oxygens resulting from the tetrahedral-oxygens stretching vibrations along a sub-alkaline join of volcanic glasses. We used these parameters to empirically provide a relation with the glass silica content. The model was externally validated with glasses having different and known compositions (in particular different amounts of iron, alkali and alumina) extending the validation set also to other magmatic series (i.e. alkaline and shoshonitic). Our approach enables a fast screening of deep-UV Raman spectra collected on Mars to disentangle the glass structure and retrieve its silica content within a given magmatic series. This method is relevant for planetary exploration and applies both to microanalysis of dry or hydrate volcanologically relevant glasses and general technical glasses since deep-UV Raman spectroscopy is particularly effective in reducing the unwished photoluminescence and increasing the signal-to-noise ratio.