Spectroscopic study of perchlorates and other oxygen chlorides in a Martian environmental chamber

- Nine oxygen chlorides were analyzed using Raman and NIR in a Martian chamber.
- Raman ν1 peak is the best for the identification of oxygen chloride species in mixtures.
- The Raman water peaks of hydrous perchlorates downshift when temperature decreases.
- The rate of NaClO4 ⋅ H2O dehydration was quantified by in-situ NIR spectroscopy.

We report a study where the molecular spectral features of nine anhydrous and hydrous oxygen chlorides were analyzed both under Mars atmospheric pressure and temperature conditions in a Planetary Environment and Analysis Chamber (PEACh) and, for comparison, under ambient laboratory conditions. The goal is to understand the effect of Mars environmental conditions (mainly temperature T and CO2 pressure P) on their spectral features as determined by both Raman and NIR spectroscopy. These results will be used for in situ simultaneous identification of the ClO4− and other intermediate oxygen chloride products generated during a dynamic electrostatic discharge (ESD) experiment. We have three major findings from the first phase of this study: (1) the ν1 Raman peak position is the most sensitive parameter for identifying the cation speciation in perchlorates (e.g., Na, Mg, Ca), the hydration state of magnesium perchlorate (e.g., Mg(ClO4)2 ⋅ xH2O, x=0,2,4,6), and the degree of oxidation of sodium oxygen chlorides (e.g., NaClOy, y=1,2,3,4); (2) ν1 Raman peak positions of most tested hydrous and anhydrous oxygen chlorides show no detectable changes within the tested T and P ranges relevant to the environmental conditions at Mars surface and shallow subsurface, but water Raman peaks of the hydrated salts change following T decreases; (3) under the P & T conditions relevant to current surface and shallow subsurface at Mars mid-latitude regions, both Mg(ClO4)2 ⋅ 6H2O and Ca(ClO4)2 ⋅ 2H2O are stable against dehydration, while NaClO4 ⋅ H2O dehydrates, with a dehydration rate that is a function of T which was quantified by in situ NIR spectroscopy. These results are useful for the interpretations of the data from current orbital remote sensing (Vis-NIR spectra) and from future landed missions (Raman spectra). Furthermore, we have designed a set of systematic ESD experiments to be conducted in PEACh for studying the pathways and the rates of oxygen chloride generation from chloride salts, as a potential mechanism to form oxygen chlorides during Martian dust storm. The results of the current study will be used for in situ simultaneous identification of the ClO4− and other intermediate oxygen chloride products generated during a dynamic ESD experiment.