Ocean stratification impedes particulate transport to the plumes of Enceladus

Abstract Water-vapour plumes erupting from Enceladus’ south pole provide a window into the properties of its subsurface ocean, a prime target in the search for life. However, the extent to which plume material represents conditions at Enceladus’ depths is unclear, because of its unknown ocean stratification, which may impede the transport of matter to the ocean top. Previous studies have found conflicting stratification regimes using differing parameter choices and model physics. Here, we build a comprehensive view of Enceladus’ ocean stratification and bottom-to-top transport timescale, across plausible ranges of salinity and tidally- and librationally-induced mixing, accounting for non-linearities in the equation of state for water, geothermal heating and ice-ocean freshwater exchanges. We use theoretical models verified with global ocean numerical simulations. We show that, under a steady state assumption for the ice shell, which requires melting at the poles, there is no parameter choice permitting an unstratified ocean from top to bottom there. As a result, potential hydrothermal products take at minimum 100s of years to reach the plumes. This suggests that either timescales of several months, inferred from Cassini observations, are incorrect, perhaps biased by alternative particulate transport mechanisms, or that Enceladus’ ice shell is not in a quasi-equilibrated state.