Ethene-ethanol ratios as potential indicators of hydrothermal activity at Enceladus, Europa, and other icy ocean worlds

Enceladus and Europa are among the best candidates in the solar system to harbor extraterrestrial life. These icy moons host global subsurface oceans and rocky interiors, with heat input from tidal forces that may drive hydrothermal circulation, providing possible habitats for chemotrophic organisms like those found at seafloor hydrothermal vents on Earth. However, kilometers-thick ice shells covering these moons make direct observation of such environments unfeasible, and researchers must instead rely on materials brought to the surface to infer subsurface conditions. We have developed a geothermometer technique to use measurements of ethanol and ethene that escape to the surface as indicators of hydrothermal activity in subsurface environments of icy ocean worlds. In hydrothermal laboratory experiments (250 °C, 40 bar), we show that ethanol undergoes dehydration to form ethene, and that the ratio of ethene to ethanol approaches a constant value consistent with independent predictions from thermodynamic modeling. This suggests that ethene-ethanol ratios measured in natural systems can be used to infer temperatures of the source environments in which they last equilibrated. In the plume of material being ejected from Enceladus' subsurface, molecular mass fragments consistent with the presence of ethanol and ethene were detected, although existing data appear insufficiently precise to constrain their abundances for this geothermometer technique. Future missions to Europa and other icy ocean worlds will be better equipped to identify and quantify organic compounds, and the accuracy of the geothermometer technique demonstrated in the present study suggests that ethanol and ethene should be priority targets for analysis.