Divergent Behavior of Hydrothermal Plumes in Fresh Versus Salty Icy Ocean Worlds

Water parcels close to their freezing point contract and become heavy on warming if they are sufficiently fresh (salinity less than 22g kg⁻¹ for earth's ocean), but expand and become buoyant when salty (salinity greater than 22g kg⁻¹). We explore the resulting divergent behavior of hydrothermal plumes in fresh versus salty icy ocean worlds, with particular emphasis on Europa and Enceladus. Large, salty, putative Europa-like oceans, develop buoyant plumes which rise upwards in the water column when energized by localized hydrothermal vents. Instead, small, fresher, putative Enceladus-like oceans, can develop bottom-hugging gravity currents when heated near the freezing point, due to the anomalous contraction of fluid parcels on warming. Such a bottom-filling regime would most likely be a transient stage in the evolution of an icy moon over geological time. The contrasting dynamics are highlighted and rationalized in terms of key non-dimensional numbers with a focus on the ability of ocean to carry bio-markers from the hydrothermal activity at the bottom to the ice shell at the top. Finally, the implications of our study for prioritizing future missions to icy moons are discussed. An advantage of a mission to a large icy moon (e.g., Europa), rather than a smaller target (e.g., Enceladus), is that a larger moon's ocean would likely support buoyant convection, which could bring signatures of seafloor venting to the outer ice-shell regardless of that ocean's salinity. For smaller icy moons, the nature of convection would hinge on its assumed salinity.