The effect of Europa and Enceladus analog seawater composition on isotopic measurements of volatile CO2

Science goals for icy ocean worlds missions include characterizing the chemical composition of the surface and interior using remote sensing and in situ techniques. The next class of flight mass spectrometers for these missions will obtain compositional identifications and isotope ratio measurements of volatiles evolved from the ice surface (exosphere) and plumes. These mass spectra will be combined with data from other flight instruments to infer the composition of the interior from volatiles. To ensure accurate interpretation of these measurements, it is critical to verify whether the fundamental assumption that volatiles observed in icy ocean world exospheres or plumes will be a direct reflection of the sub-ice ocean. The present study evaluates whether isotopologues from an initial CO2 gas fractionate by interacting with seawater (brine) of varying salt composition and concentration. δ13CCO2 are affected by the pH of the brine and subsequent speciation of {CO2}, where {CO2} represents the combination of CO2, H2CO3, HCO3-, and CO32-. {CO2} for low pH brines hypothesized for Europa will preferentially speciate as CO2. Analyzed δ13CCO2 for low pH brines are within error of the original δ13CCO2, demonstrating that volatile CO2 in a low pH system will be a direct reflection of the original CO2. However, Europa’s radiative environment and rapid depressurization due to plume ejection may impose fractionation effects. In contrast, high pH systems relevant to Enceladus or a more alkaline Europa are expected to form all carbonate species, while favoring speciation as HCO3-. High pH brines in these experiments include both an original CO2 gas and an isotopically distinct HCO3- (from NaHCO3 salt). These alkaline experiments demonstrate that δ13CCO2 values are highly variable, and depend on the concentration of the dominant carbonate species, (Na)HCO3-. Mass balance estimates indicate that measured δ13CCO2 is a thermodynamically predictable mixture of both carbon sources, suggesting that measurements of δ13C at Enceladus would directly reflect of the sources of CO2 and carbonate buffering in the ocean. δ18O measurements for CO2 interacting with KCl and MgCl2 follow established models for δ18O-ionic strength. CO2 interacting with MgSO4, Na2SO4, and NaCl demonstrate an offset from established δ18O-ionic strength models depending on the concentration of initial CO2. These results suggest that current predictive models for δ18O in brines need to be resolved for changing concentrations of CO2.