Compositional Convection in the Deep Interior of Uranus

Abstract

Uranus and Neptune share properties that are distinct from the other giant planets in the solar system, but they are also distinct from one another, particularly in their relative internal heat flux. Not only does Neptune emit about ten times the amount of heat that emitted by Uranus, the relative amount of emitted heat to the energy they absorb from the sun also differs greatly, being comparable at Uranus and the largest of all giant planets at Neptune. As a result, it is questionable whether thermal convection occurs within the interior of Uranus. However, the presence of an intrinsic magnetic field implies that interior fluid motions must exist. Here, we consider compositional convection driven by the release of hydrogen associated with the formation of large organic networks or diamond precipitation in the deep interior. We test this hypotheses using a set of numerical rotating convection models where the convective driving is varied between thermal and compositional sources and is sufficiently vigorous to not be strongly constrained by rotation. In most cases, we find ice-giant-like zonal flows develop, with three bands characterized by a retrograde equatorial jet and prograde jets at higher latitudes. Large-scale circulation cells also develop and lead to heat and mass fluxes that tend to exhibit local maxima along the equatorial plane. This similarity between convective flows driven by thermal and compositional buoyancy therefore predict Uranus and Neptune to have similar interior dynamics despite Uranus' minimal internal heat flow and may thus explain why both ice giants have comparable magnetic fields.