Oscillatory thermal-inertial layer formation in the outer molecular regions of gas giants

The composition and interior structure of giant planets are still a mystery. Data from the space missions Cassini and Juno suggest that the traditional three-layer model, comprising an inner ice and rock core, an intermediate metallic hydrogen dynamo region, and an outer molecular hydrogen envelope, is too simplistic. Instead, models with multiple layers and sublayers throughout the interior better match observations. These layers can be either stably stratified or chemically and/or thermally convecting. Understanding how these layers form is important for constraining and constructing accurate interior models. Here, I use direct numerical simulations (DNS) of rotating thermal convection in a cylindrical domain to mimic the outer molecular envelope in the polar region of a gas giant. Crucially, I employ a kinematic viscosity ν and thermal diffusivity κ that vary with either temperature or height, similar to their dependencies within gas giants. Thus, the Prandtl number P r = ν / κ varies locally throughout the fluid layer, permitting oscillatory thermal-inertial convection if P r < 1 . I show that distinct layers with different mixing properties appear naturally in this system. This finding challenges the common belief that pure thermal convection always produces homogeneously mixed single layers and that only double-diffusive convection is able to create sublayers by staircase formation. Instead, the results reveal that thermal convection with realistic non-constant material properties provides an alternative route to the generation of layers and sublayers inside the molecular envelope. This highlights the importance of considering different possible mechanisms to explain the distribution of heavy elements inside gas giants and to gain deeper insights into their composition.