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Fluid dynamics of planetary cores: formation, heterogeneous convection and rotational dynamics

Periodic Reporting for period 4 - FLUDYCO (Fluid dynamics of planetary cores: formation, heterogeneous convection and rotational dynamics)

Reporting period: 2021-01-01 to 2021-12-31

Understanding the flows in planetary cores from their formation to their current dynamics is a tremendous interdisciplinary challenge. Beyond the challenge in fundamental fluid dynamics to understand these extraordinary flows involving turbulence, rotation and buoyancy at typical scales well beyond our day-to-day experience, a global knowledge of the involved processes is fundamental to a better understanding of the initial state of planets, of their thermal and orbital evolution, and of magnetic field generation, all key ingredients for habitability.

The purpose of the FLUDYCO project is to go beyond the state-of-the-art in tackling three barriers at the current frontier of knowledge. It combines groundbreaking laboratory experiments, complementary pioneering numerical simulations, and fruitful collaborations with leaders in various fields of planetary sciences. Improving on the latest advances in the field, we have addressed the fluid dynamics of iron fragmentation during the later stages of planetary accretion, in order to produce innovative, dynamically reliable models of planet formation. Considering the latest published data for Earth, we have investigated the flows driven in a stratified layer at the top of a liquid core and their influence on the global convective dynamics and related dynamo. Finally, building upon the recent emergence of alternative models for core dynamics, we have quantitatively examined the non-linear saturation and turbulent state of the flows driven by libration, as well as the shape and intensity of the corresponding dynamo.

In the context of an international competition, the originality of this work comes from its multi-method and interdisciplinary character.
Our research activity over the course of the project has led to the publication of 35 articles in top-level journals in physics, mechanics, and planetary sciences. Those papers have reported significant contributions in the 3 scientific tasks initially planned. Major achievements include:
• task 1 - core formation: the first quantification of the turbulent exchanges taking place between the iron drops and the magma ocean during planetary formation by accretion, combining dedicated numerical simulations and innovative laboratory experiments (e.g. Qaddah et al. PEPI 2020, Wacheul & Le Bars PRF 2017). Those experiments have been awarded the 2016 Video APS/DFD Gallery of Fluid Motion Award.
• task 2 - core convection: the first characterization of the wave field emitted in a stratified layer within a planetary core by the underlying convection, using tridimensional numerical simulations (Bouffard et al. GJInt 2021).
• task 3 - core rotation: the first numerical and experimental realization of an inertial wave turbulence driven by harmonic forcing (Le Reun et al. PRL 2017, JFM 2019), which fundamentally changes our view of core turbulence, hence of planetary dynamo.

Beyond fulfilling initially planned tasked, this project has also allowed to explore new horizons following scientific breakthroughs as well newly published data and observations:
• our research in task 1 has been extended towards the closely related question of the dynamics of iron snow within planetary cores, and its relation to planetary magnetic field. We have performed the first experimental study of a falling, dissolving particle in a stratified medium (Huguet et al. PRF 2020).
• beyond our study of mixed stratified / convective dynamics in planetary cores in task 2, we have tackled the same dynamics for planetary atmosphere. Our numerical / theoretical work has led to the first demonstration of the emergence of a long-term, large-scale, oscillating flow in a stratified layer adjacent to a turbulent one, generalizing the concept of QBO to planetary cores (Couston et al. PRL 2018, Léard et al. PRL 2020).
• our research in task 2 has also open new research on the dynamics of Jupiter, with innovative experimental models of its jets, vortices, and their interactions (e.g. Lemasquerier et al. Nature Physics 2020, JFM 2021). This work has also been awarded the 2019 APS/DFD Milton van Dyke Award for a poster.

All team members have significantly taken part to the dissemination of our results towards scientific communities ranging from fluid mechanics to planetology (participation to and organization of conferences, publications), as well as towards a larger public: beyond the setting of a website, our yearly participation to the "Fête de la Science", and several press releases and journal articles, a one-week summer school has been organized in Udine (Italy) and 4 outreach movies have been produced to explain our research activities.
Our activities combining laboratory experiments and numerical simulations have led to progress beyond the state of the art on the three tasks initially planned. Our results will also stimulate further research after the end of the project, including
• task 1 - core formation: the development of new parameterisation schemes for thermochemical exchanges between iron and silicate during planet formation, to better describe their initial state.
• task 2 - core convection: the exploration of the various dynamics and specific dynamos generated in the presence of a stratified layer atop a fast rotating, convecting core.
• task 3 - core rotation: the systematic study of the dynamo driven by wave turbulence in planetary cores.
All team members will keep taking part to the dissemination of our results towards scientific communities over the coming year.
3D numerical simulation of a mixed convective / stably stratified fluid (task 2)
Emergence of a large-scale oscillating flow in a stratified layer adjacent to a turbulent one
Laboratory model of Jupiter bands
Expected wave turbulence dynamics in planetary cores (task 3)
Dynamics of a blob of metal liquid falling into a viscous ambient fluid (task 1)