Layering by Double-Diffusive Convection in the Subsurface Oceans of Europa and Enceladus

Subsurface oceans rich in salts may be prevalent in the icy worlds of the outer solar system. Surface observations have led to various hypotheses for the transport of materials from the seafloor to the surface by hydrothermal plumes, and raise questions about heat transfer mechanisms. Chemical heterogeneity affects the vigor of convection, the forms of plumes, the generation and destruction of stratified or finger structures in the ocean, and thus the transport of heat and materials from the interior to the surface. Here, we investigate the layering phenomenon in a double-diffusive convection system, which can occur when both the temperature and concentration influence the density of the fluid. The persistence of layers may depend on the buoyancy ratio, the Rayleigh number, boundary conditions, and initial conditions, which alter the chemical distribution and thus the balance between thermal and chemical buoyancies. Our simulations suggest that the layering could exist for a longer duration if the buoyancy ratio is raised with boundary conditions that maintain a large concentration difference. When the layers are present, heat and material transport are significantly inhibited through the subsurface ocean from the silicate interior to the base of the icy shell.