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Abstract

In deep fluid layers of planets (liquid cores, gas giants, icy moons' subsurface oceans), the density depends on temperature and chemical composition, which diffuse at very different rates. Double diffusive instabilities can then generate large-scale flows, which have to be studied in global geometry. We have addressed this little-studied problem by first calculating the instability onset in rotating spheres (SINGE eigensolver, Vidal 2015), showing large decreases of the critical Rayleigh numbers (not present in the usual local box studies). Then, we perform direct simulations (XSHELLS code, Schaeffer 2013) in the fingering regime (unstably stratified composition, stable temperature field), showing the emergence of strong zonal flows at a large scale. Considering the early Earth, we show that double diffusion can reduce the critical Rayleigh number by four decades, suggesting that its core was prone to turbulent double-diffusive convection, with large-scale zonal flows. Our fingering results are then extended to the oscillatory double convection regime (or semi-convection), a regime relevant for gas giants. Using the induction equation, we finally study the dynamo capability of these flows to assess their relevance for planetary dynamos (e.g. for gas giants). Funded by ERC THEIA (grant agreement no.847433)