Measuring Perchlorate and Sulfate in Planetary Brines Using Raman Spectroscopy

Liquid water likely exists on the surface of Mars and below the icy crusts of Europa, Enceladus, and Titan. Pluto, Ceres, and Ganymede also show evidence of liquid water at or near the surface. Quantitative solute analyses would provide critical data needed to understand geochemical conditions throughout our solar system, including potential habitability of planetary bodies. We have developed and tested a Raman spectroscopic method for measuring perchlorate and sulfate in brines and dilute waters that utilizes simple Raman peak height ratios rather than peak area ratios. Ratios of the target anion and OH- bending water peaks yield linear fits with positive slopes and r² values >0.99. Calibration fits for each solute–brine combination and ultrapure water yield similar linear equations, suggesting this method can detect and quantitatively measure solutes in complex aqueous solutions. No sample preparation or physical contact with the sample is required. Therefore, this method can be employed without contaminating the fluid or the spacecraft.