ISOTOPIC MIXING MODELS OF EUROPA SEAWATER BRINES FROM LABORATORY SIMULATIONS

The major focus of the planned orbiter mission to Europa is to determine the chemical composition of the ice and subsurface ocean and to assess the habitability of Europa. Our laboratory simulations of Europa brines mimic mission-relevant remote sensing techniques by measuring mass spectra and isotopic ratios of volatiles produced from theorized brines. Brines vary in composition from sulfate-rich, chloride-rich, and bicarbonate-rich, and analyses are performed on volatile CO₂ produced from single, double and triple compositions using the upper and lower limits of concentrations suggested by prior modeling studies. Volatiles from simulated brines demonstrate varying degrees of isotopic disequilibrium in both δ¹³C and δ¹⁸O, suggesting a significant and complex isotopic fractionation could be observed in volatiles from Europa. Experiments also vary the concentration of CO₂ from 0% to 2% as a possible contribution from rock-water interactions on Europa. Mixing models generated from these data indicate that the mixing of CO₂ in briny solutions has the most profound effect on the isotope ratio of volatile CO₂, and that the original isotope ratio of CO₂ added to the system is significantly altered by the presence of other carbonate species (e.g. HCO₃^-). Brines that are both sulfate-rich and chloride-rich demonstrate a consistent δ¹³C and δ¹⁸O disequilibrium across varying salt and CO₂ concentrations. In contrast, bicarbonate-rich brines consistently demonstrate equilibrium conditions over a range of CO₂ concentrations and pH. These results suggest that alkaline brines, such as those theorized on Enceladus, may be in equilibrium, but that an acidic system such as Europa may be out of equilibrium. Characterizing the compositions and conditions that promote disequilibrium in Europa’s seawater is essential for understanding the compositional relationships between the seawater, ice, plumes, and volatile off-gassing, as well as evaluating the potential for habitability on Europa.