Water parcels close to their freezing point contract and become heavy on
warming if they are sufficiently fresh, but expand and become buoyant
when salty. We explore the resulting divergent behavior of hydrothermal
plumes in fresh verses salty icy ocean worlds, with particular emphasis
on Enceladus and Europa. Salty oceans develop buoyant plumes which rise
upwards in the water column when energized by localised hydrothermal
vents. Fresh oceans, instead, develop bottom-hugging gravity currents
when heated near the freezing point, because of the anomalous
contraction of fluid parcels on warming. The contrasting dynamics are
highlighted and the implications discussed.