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

Periodic Reporting for period 3 - FLUDYCO (Fluid dynamics of planetary cores: formation, heterogeneous convection and rotational dynamics)

Reporting period: 2019-07-01 to 2020-12-31

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 present 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 address 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 investigate 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 quantitatively examine 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 during the first 30 months of the project has led to the publication of 11 articles (plus 1 submitted and 3 in preparation) in top-level journals, with significant contributions in the 3 scientific tasks described in the DoA. Major achievements include:
• task 1 - core formation: the first description and quantification of the turbulent exchanges taking place during core formation using innovative laboratory experiments, that have been awarded the Gallery of Fluid Motion video prize at the American Physical Society, Division of Fluid dynamics congress in 2016 (Wacheul & Le Bars PRF 2017).
• task 2 - core convection: 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).
• task 3 - core rotation: the first numerical realization of an inertial wave turbulence (Le Reun et al. PRL 2017), which fundamentally changes our view of core rotating turbulence. This work has played a large role in the awarding to B. Favier of the Young Scientist Award of the European Turbulence Conference 2017 (Stockholm, August 21-24).
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 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 will be pursued on the three tasks, following and extending the first results obtained. Additionally, during the next period, efforts will be put on
• task 1 - core formation: the development of new numerical simulations quantifying the thermochemical exchanges at the scale of one drop, and the setting of a new experiment focusing on the iron snow dynamics and its relation to planetary magnetic field.
• task 2 - core convection: the setting of a new experimental set-up, dedicated to the study of the interaction between turbulence and a stably stratified layer in fast rotation.
• task 3 - core rotation: the systematic numerical study of the dynamo driven by libration in planetary cores, and more generally by any mechanical forcing, in the relevant regime of inertial wave turbulence that we have revealed.
All team members will keep taking part to the dissemination of our results towards scientific communities as well as towards a larger public.
Emergence of a large-scale oscillating flow in a stratified layer adjacent to a turbulent one
Expected turbulence dynamics in planetary cores, as determined from local numerical simulations
Dynamics of a blob of metal liquid falling into a viscous ambient fluid
3D numerical simulation of a mixed convective / stably stratified fluid