PROBING AQUEOUS PROCESSES IN SALTY SYSTEMS: USING RAMAN AND ESEM TO CHARACTERIZE GEOFLUIDS AND INVESTIGATE BRINE-MINERAL INTERACTIONS THROUGHOUT THE SOLAR SYSTEM

If we “follow the water” through our Solar System, we also find salts. Abundant salt deposits observed on Mars and Ceres, combined with low freezing points required to maintain liquid water on these planetary surfaces and within subsurface oceans on icy moons, suggest that salty water is the norm throughout the Solar System. Even on Earth--likely the freshest planet-- 97% of surface water is salty. To understand and interpret aqueous processes in these systems, we must consider brine-rock interactions in addition to more dilute water-rock interactions. Indeed, brine-rock interactions can lead to unexpected aqueous chemistries, alteration rates, and mineral assemblages even at very low water-rock ratios, including cation exchange and dissolution reactions in nanoscale films formed through deliquescence.

Using Environmental Scanning Electron Microscopy (ESEM) we observe mineral-brine reactions in situ at planetary analogue P-T conditions, including cation exchange reactions in the absence of bulk liquid water. These experiments further document that calcium sulfate minerals similar to those observed on Mars can form within seconds to minutes along clay-salt interfaces when relative humidity is elevated during temperature cycling. We have also developed new Raman methods to characterize ions and measure their concentrations in near-saturated planetary analogue brines and ice-brine mixtures. This method allows in situ analysis of planetary fluids without direct interaction with the sample, thus preserving planetary protection protocols. In addition, our experiments demonstrate that Raman analyses also effectively observe brine-mineral alteration products that often fall below detection limits of X-ray diffraction measurements, including unexpected phases like korshunovskite [Mg₂Cl(OH)₃•4(H₂O)] observed in pyrite-brine and ferrihydrite-brine experiments. These new Raman and ESEM methods can facilitate novel studies of brines and brine-mineral interactions to better understand the role of salty geofluids in planetary processes and also to inform our understanding of brine-mineral reactions on Earth, including processes critical to developing sustainable energy and water resources.