The effect of gas bubbles on nanoparticle transport and ocean circulation on Enceladus

Saturn's moon Enceladus has a global ocean, heated from below due to tidal heat generation in the solid core. Plumes in the south polar region eject materials from the ocean into space through fissures in the ice sheet, making Enceladus's ocean uniquely accessible for sampling. Silica nanoparticles that are believed to have emanated from these plumes suggest relatively fast ocean transport from the sea floor to the surface (on a timescale of less than about one year). However, due to the relatively weak heat flux, small gravity, deep ocean and likely small (or perhaps even locally negative) thermal expansivity, ocean mixing time-scales by convection and diffusion are expected to be much longer. Here, we consider the potential effect of H2 gas bubbles, believed to emanate from hydrothermal vents at the sea floor, on the transport of nanoparticles. We find that bubbles with a radius of millimeters to centimeters (similar to those on Earth) rise from the sea floor to the surface on a time scale of O(10 days). If these bubbles can trap silica nanoparticles (possibly in clathrate shells that can form around the bubbles), this may explain the fast transport of the nanoparticles. Bubbles also provide buoyancy forcing to the ocean and thus can drive fluid plumes. We develop simple bubble plume models, which suggest that the fluid transport in these plumes may also be fast enough to explain the inferred transport timescale of nanoparticles, if the ocean is unstratified. Finally, we consider the effect of bubbles on turbulent mixing in a stratified ocean. We find that bubbles can provide energy for vertical mixing, which in turn may significantly affect the overturning circulation in a stratified ocean. Including the effect of bubbles on vertical mixing in simulations of the circulation in a stratified ocean provides an interesting path for future research.