Insight into Enceladus’s ocean chemistry, habitability, and past from fractionation studies of the erupting plume

Abstract

The erupting plume of Enceladus provides an ideal opportunity to investigate the chemistry and astrobiological potential of the subsurface ocean. However, the complexities of the eruption process likely result in a plume that is chemically fractionated and distinct from its ocean source. Chemical fractionation in the plume is not well understood, but it has ramifications for both extrapolation from plume measurements to ocean composition, and for long-term changes to the ocean chemistry due to preferential eruptive loss. In this work we (1) numerically model gas fractionation over the course of an Enceladus plume eruption, including gas exsolution from the ocean, (2) use laboratory experiments to constrain and validate our numerical models of gas exsolution, and (3) use models to investigate the long-term effects of plume eruption on Enceladus’s bulk chemistry and constrain the longevity of plume eruption. We find that Enceladus’s ocean is likely gas- and ammonium-rich and moderately alkaline, with free energy for methanogenesis. We also find that terrestrial models and measurements of mass transfer can generally be applied to carbon dioxide exsolution under Enceladus conditions, but may underestimate mass transfer coefficients of insoluble gases. Finally, we constrain overall timescales of plume eruption to 30–300 Myr and find that Enceladus’s early ocean may have been carbon dioxide-rich and acidic, but was more likely ammonium-rich and basic. Our work advances our understanding of this small, dynamic moon and the nature of its ocean-plume connection, and provides tools for the interpretation of future spacecraft measurements at Enceladus.