Effects of pH on the Raman Spectra of Brines

Abstract

The use of Raman spectroscopy on Mars can help us understand the geochemistry of past or present brines on Mars which could help us assess the overall habitability of Mars. Raman can detect biosignatures from organic matter, look for signs of transient flowing water, analyze fluid inclusions and measure ion concentrations in solution. While Raman can be used to determine the bulk composition of a brine it is essential to understand what other factors may also affect the Raman spectra. We conducted experiments assessing the effects of pH on the Raman spectra of Mars analog brines. Our results show Raman spectra of carbonate, sulfate, and phosphate-bearing solutions vary considerably with pH, while chloride and perchlorate brines recorded only minor or no changes with pH. We found that changes in pH correspond to changes in concentration for various species which in turn affects peak intensity. By plotting peak heights against pH for certain brine types we should be able to determine the relative pH of an unknown brine of similar composition. We also found that shifts in peak position occur as pH changes, likely due to the presence of different species at different pH conditions. Changes in the spectra represent changes in the overall aqueous chemistry of the solution which can affect habitability and biogeochemical processes.