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Abstract

Zonal flows generation is a long-standing issue in rotating fluids, which can be key in the dynamics of planetary fluid layers (e.g. planetary cores). They are indeed believed to play a significant role in angular momentum exchanges between liquid layers and surrounding solid domains (e.g. Roberts & Aurnou, 2012). Moreover, mean flows could be unstable (e.g. Sauret et al., 2014; Favier et al., 2014), which could sustain space-filling turbulence and mixing. Therefore, understanding the formation of mean flows is essential to model the fluid dynamics of many rapidly rotating systems. Zonal flows of planetary liquid cores are usually studied as originating from convection in rotating spheres. In the frame of the ERC THEIA (grant agreement no. 847433), we consider here two alternative origins. First, zonal flows can be generated by orbital forcings (e.g. tides, precession) via nonlinear effects within the Ekman layer (Cébron et al., 2021). These flows survive in the relevant regime of vanishing forcings and viscosity. Their competition with bulk driven zonal flows are then considered for various planets and moons. Second, zonal flows can emerge from double-diffusive convection (DDC) in stratified cores (Monville et al., 2019). Applying our results to the early Earth, we obtain that double diffusion can reduce the critical Rayleigh number by four decades, suggesting that its core was prone to turbulent DDC, with large-scale zonal flows.