Influences of Non-Oberbeck–Boussinesq Effects on Tracer Transport in Icy Ocean Worlds

The subsurface oceans on icy satellites are potentially habitable. To understand their habitability, we need to know how tracers with various lifetimes distribute. Convection is the main vehicle for tracer transport, and we expect convection on icy satellites to differ from regular rotating convection, because as pressure increases, water's thermal expansivity can vary by orders of magnitude or even reverse sign near freezing point. Any variation of fluid properties would break the Oberbeck–Boussinesq approximation, leading to non-Oberbeck–Boussinesq (NOB) effects, measured by a coefficient $ϵ$. In this work, we identify two competing impacts of NOB effects on tracer transport. The first promotes overall upward tracer transport at ${ϵ}^2$-order, while the second enhances transport near the bottom source but inhibits transport further up at ${ϵ}^3$-order. In weakly nonlinear regime, the former effect dominates, causing more tracers reaching the ice shell. While in strongly nonlinear regime, the latter effect dominates, reducing tracer concentrations near the ice shell. By varying particle lifetimes, we find that NOB corrections are most pronounced when particle lifetime is comparable to the timescale of upward tracer transport. Additionally, when NOB effects are strong enough to create a stratified layer in the upper part of the ocean, tracer transport into the stratified layer is set by energetics. These effects are expected to prolong the transport timescale of chemical tracers or biosignatures from the seafloor to the ice shell on icy satellites.