Thermal convection in subsurface oceans with variable thickness:
Application to Enceladus.
Abstract
Saturn’s moon Enceladus harbours a global subsurface ocean beneath its
icy crust. Tidal dissipation within the moon’s core generates a
substantial amount of heat which leads to ocean convection.
Understanding this transport of mass and heat within the ocean is key to
understand exchange processes between core, ocean and ice shell.
Previous studies of ocean convection assume oceans of constant thickness
and constant superadiabatic temperature gradients between the inner core
and outer ice shell. However, observations indicate ocean thickness
variations of up to ~20 km from equator to pole and
heterogenous tidal dissipation within the core likely results in
latitude-dependent temperature gradients. In this study, we analyse how
heterogeneities affect circulation patterns and heat transfer in an
Enceladan ocean using three-dimensional direct numerical simulations. We
focus on a degree-2 meridional thickness profile with varying degree of
thermal forcing to analyse the different rotating convection regimes and
compare them with those of an ocean of constant thickness. Our results
show that the non-uniform ocean gives rise to a latitudinally variable
Rayleigh number, causing a potential decoupling of rotating convection
regimes experienced locally within the ocean. As in the constant ocean
thickness scenario, different regimes of convection exist, which depend
on the relative influence of rotation. With increasing thermal forcing,
convection moves from being restricted to equatorial regions to filling
the whole fluid domain. Heat transport efficiency, as measured by the
local Nusselt number, is different for the uniform and non-uniform cases
and depends on the convection regime. Nusselt-Rayleigh relations are
similar to those obtained for an ocean of uniform thickness, but the
relevant Rayleigh number depends on the region of the domain that is
convecting.