Exploring Exogenic Sources of Plume H2 at Enceladus

Cassini’s exploration of Enceladus’ plume provided tantalizing glimpses into the chemistry of the moon’s interior ocean. The abundance of H₂ in the plume, for instance, seems to suggest hydrothermal processes operating within Enceladus’ ocean that could support its habitability. But some caution is warranted when drawing conclusions about the ocean composition from the plume, since space weathering can alter plume material as it transits from the source to the spacecraft. We quantitatively evaluate the extent of this modification in terms of the H₂ generated by radiolysis of plume ice grains, using a Monte Carlo model that captures both production and transport. We find that plume photoelectrons are the dominant drivers for radiolysis despite depletion of electron density by nanograin charging. Under nominal conditions, radiolytic H₂ production appears to be insufficient to account for the reported H₂ mixing ratio of ~1%, so the hypothesis of a hydrothermal source still stands. However, we note that our estimates are acutely sensitive to the assumed size distribution of emitted plume grains, which is not well constrained by Cassini observations. We show that, for example, a relatively high power-law exponent of 5 for the grain size distribution, which is consistent with some Cassini-based estimates, makes a radiolytic source for the reported H₂ much more plausible. This demonstrates that further in-situ measurements are needed to support reliable inferences about the Enceladus ocean from observations of the exterior.