  Previous studies that have considered the ocean circulation on Enceladus have
generally assumed the salinity to be Earth-like. However, according to
observations and geochemical constraints, the salinity of Enceladus' ocean is
likely to be lower, and importantly, it is probably low enough to reverse the
sign of thermal expansivity. We investigate the ocean circulation and
stratification of Enceladus' ocean using a combination of theoretical arguments
and simulations using the MITgcm. We find that, if the salinity is high, the
whole ocean is unstratified, and convection dominates the entire ocean.
However, if the salinity is low enough, there exists a stratified layer at the
surface of the ocean. Such a layer can suppress the vertical flux of heat and
tracers, thereby affecting the heat flux to the ice shell and leading to a
vertical tracer mixing time scale in the stratified layer of at least hundreds
of years. This time scale is inconsistent with a previous estimate of vertical
ocean mixing of several years, based on the size of detected silica
nanoparticles in the plumes, leading us to conclude that either the salinity of
Enceldus' ocean is higher than previously suggested or the interpretation of
silica nanoparticle observations has to be reconsidered.
