Experimental Evaporation and Spectral Analysis of Martian Analogue Brines

Abstract:

Brines are predicted to be stable or metastable on Mars, but their existence has not been directly confirmed with orbital or in situ analysis. In this study, we spectrally characterized precipitates from multicomponent martian analogue brines. The starting chemistry of these brines was derived from literature (e.g., Tosca et al. 2011) and modeled to higher concentration using the geochemical model FREZCHEM9.2. We outfitted a vacuum chamber to simulate modern martian conditions (0.006 bar CO₂ and 0° C), and under both martian and terrestrial conditions, evaporated a suite of analogue brines with varying concentrations of SO₄^2-, Cl^-, and CO₃^2- in addition to relevant cations.

Results from this study indicate that while crystalline salts such as halite (NaCl) and sylvite (KCl) commonly form in tandem with amorphous salts such as epsomite (MgSO₄•7H₂O), amorphous salts with high hydration states dominate VNIR spectra, obscuring the presence of anhydrous, spectrally featureless crystalline salts. Though halite and sylvite constitute the majority wt.% of precipitates in these cases, they remain unidentifiable spectrally. Additional results indicate that desiccating conditions of intimately mixed bischofite (MgCl₂•6H₂O) and antarticite (CaCl₂•2H₂O) precipitates fundamentally determine which salt appears spectrally dominant. We conclude that in regions on Mars where highly hydrated salts have been identified, it is possible that additional, anhydrous salts are present but are spectrally indistinguishable. For example, chlorides may be present in or near sulfate assemblages on Mars, but are not spectrally identified. We also conclude that chloride-dominated brines with available Mg²⁺ and Ca²⁺ cations should be considered good contenders for brine-driven RSL formation, given that precipitates from these solutions are unstable, hygroscopic, and amorphous.

We have generated sufficient spectra for a catalogue of multicomponent evaporites with application to Mars, which can be used as a reference for CRISM spectra. We have additionally collected Raman spectra for multicomponent evaporites. These results can aid in the development of deconvolution techniques for salts mixed at a single pixel scale in CRISM spectra, and interpreting Raman and VNIR data from the SuperCam instrument on the Perseverance Rover.