Data from GCA article "Hydrothermal experiments reveal methanol, ammonia, and methylamines as tracers of pH and temperature with potential application to Enceladus, Europa, and other icy ocean worlds"

Abstract: Icy ocean worlds in our solar system are prime targets in the search for extraterrestrial life. Enceladus and Europa are icy moons with evidence of global subsurface oceans in contact with rocky interiors, which may provide habitable environments for chemotrophic life. However, the interaction between water and rock and any changes in fluid chemistry are poorly constrained because the icy shells of these moons inhibit direct sampling of subsurface aqueous environments. Characterization of subseafloor processes may depend on chemical signatures in materials that are transported to accessible sampling locations near the surfaces of these icy ocean worlds. Here we present a theoretical framework where organic compounds likely to be present at icy ocean worlds can be utilized as tracers of geochemistry if they approach metastable equilibrium abundances, which we test with laboratory experiments. We use thermodynamic calculations of aqueous solutions to demonstrate that abundances of methanol, ammonia, and methylamines (compounds selected based on Enceladus’ plume data) should approach steady state ratios based on reversible substitution reactions that are systematically controlled by pH and temperature. We test these calculations by performing laboratory hydrothermal experiments ranging in temperature (175-325℃) and pH (~3.5-7.0), demonstrating that chemical concentration measurements alone can be used in predictive models to accurately constrain the different conditions across experimental systems. As examples, with experiments conducted at 250℃, we demonstrate that the chemical tracers in a pH ~3.5 solution predict a pH ≤ ~5.0 and a temperature of ~225-250℃, while the tracers in a solution at pH ~7.0 predict a pH of ~6.0-7.5 and a temperature of ~200-275℃. Notably, all of our high-temperature experiments are cooled to room temperature prior to analysis, indicating that the hydrothermal signal (i.e., the ratio of chemical species) is preserved due to the slowing of reaction kinetics at lower temperatures, which may be analogous to hydrothermal fluids that vent into icy oceans. Similarly, we also performed a partial-cooling experiment that shows a tracer signal produced at 325℃ remains unaltered for a year after the experiment was cooled to 175℃. We compare experimental results from gold reaction vessels to experiments previously conducted in glass vessels to assess the effects of experimental container type and find only minor differences in product distributions over time. Overall, our findings provide additional motivation to target small organic molecules for analysis on future missions to icy ocean worlds in order to constrain subsurface geochemical parameters critical to habitability.