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Abstract:
The deep interior of planets can be indirectly investigated using rotational motions such as precession or nutations. Turbulent flows driven by precession have also been proposed for generating dynamo in planetary cores (e.g. to explain the early Earth or Moon’s paleofield). In the frame of the ERC THEIA, we focus on bulk turbulent flows that are promising for planetary applications (since they are driven by topographic effects instead of viscosity). To probe the low-viscosity regime of planetary interiors, we investigate precession-driven flows in stress-free ellipsoids using asymptotic analysis and simulations. Contrary to previous expectations, we show that this model could unlock numerical difficulties of no-slip simulations. In a regime relevant to planets, we obtain the flow forced in triaxial cores, exhibiting a second mode resonance. Then, we investigate the transition towards turbulence through the bulk instabilities driven by precession in ellipsoids (the conical-shear instability and the inertial instabilities). We also provide scaling laws for the saturation amplitude of these instabilities, which are relevant for planetary applications but often hampered in experiments or no-slip simulations. Then, we study the dynamo capability of these flows for the Earth and Moon over geological times. Finally, we outline that precession could interact nonlinearly with the nutations, which might be key to interpret the dissipation at the core-mantle boundary observed in the nutation data.
