Libration, precession, tidal distortions: on the importance of mechanical forcing in the organization of planetary and stellar flows – LIPSTIC
Libration, precession, tidal distortions: on the importance of mechanical forcing in the organization of planetary and stellar flows
It is often assumed that the convective movements are responsible for the fluid flows in planetary envelopes, and in particular for dynamos. However, alternative mechanisms related to disturbances of planet rotation may also come into play. Our goal is to study the importance of three effects present at a global scale, but still largely unknown: forcings by libration, precession, and tidal distortions.
Towards an alternative description of planetary flows and dynamos
The flows in fluid layers of planets, moons and stars, are of primary interest for their internal dynamics and temporal evolution. Indeed, internal flows induce energy dissipation and generate torques, especially significant for turbulent flows that influence the orbital dynamics of stars and planets systems. Internal flows are also directly responsible for magnetic field generation. Finally, stellar and planetary heat fluxes are directly linked to flows in fluid layers. It is thus of fundamental importance to have a global understanding of the organization of fluid motions in all fluid layers of planets and stars.<br />The purpose of the present project is thus to go beyond the present state-of-the-art and to reassess the classical models of internal dynamics of planets and stars, based on convective motions only. It takes place in an international context of increased interest and scientific activity in the view of the latest results from space missions and extrasolar systems observations, that highlight the need to go beyond the standard models in order to understand the variety of planetary and stellar configurations. <br />We aim at systematically quantifying the role of three processes generically present at the planetary or stellar scales, but often neglected when looking at their driving influence in fluid layers: precession, tidal distortion and libration. Indeed, a huge amount of energy is stored in the rotational dynamics of planets and stars: those mechanical forcings could play the role of efficient conveyers that extract this energy and drive large scale and intense fluid motions. These effects have long been neglected, mainly because of a misunderstanding: critiques focused upon establishing the energetic irrelevance of the laminar response to mechanical forcing, rather than considering the fully turbulent case. Our purpose is thus to describe the generic features of the associated turbulent flows, combining experimental and numerical approaches.<br />
Our project is based on an interdisciplinary and multi-methods approach, with a special focus on laboratory experiments, which makes it especially original. It is based primarily on the skills of the team «Rotating and geophysical flows« at IRPHE, specialized in the hydrodynamics and magnetohydrodynamics of rotating fluids. The strength of our group rely on its strong expertise in fundamental fluid mechanics combining theoretical, experimental and numerical approaches, as well as on its renowned expertise in developing generic scaling laws from simplified laboratory models for applications in planets and stars. These skills will be appropriately complemented by collaborations with internationally renowned planetary scientists and astrophysicists. In a highly competitive international environment, this interdisciplinary collaboration is a unique opportunity to open new horizons in fluid instabilities and to make significant advances in our knowledge and understanding of the dynamics of stars and planets, which challenge the standard models.
Our experimental and numerical work at the frontier of fluid mechanics and planetary science, led for the moment two significant results:
1- the description of a new route to turbulence in planetary cores involving an original excitation mechanism of the flow by tides (Sauret et al. 2014).
2- the detailed experimental and numerical description of the mechanisms of instability generated by libration in planetary cores, and the first trends towards a generic description of the associated turbulence (Grannan et al. 2014, Favier et al. 2015 ).
The application of these innovative results to the specific cases of the Earth and Moon are now being studied. Furthermore, our experimental work will continue, focusing on a comprehensive study of precession flows in spheroidal geometry. Our numerical work will focus on describing ever more extreme regimes of turbulence, as well as the magnetohydrodynamics aspects of flows forced by libration.
1. Experimental study of global-scale turbulence in a librating ellipsoid, AM Grannan, M Le Bars, D Cébron, JM Aurnou, Physics of Fluids 26 (12), 126601, 2014 (http://spinlab.ess.ucla.edu/wp-content/uploads/2015/01/Grannan2014.pdf)
2. Tide-driven shear instability in planetary liquid cores, A Sauret, M Le Bars, P Le Gal, Geophysical Research Letters 41 (17), 6078-6083, 2014 (http://www.svi.cnrs-bellevue.fr/spip/asauret/pdf/article/GRL_2014.pdf)
3. Generation and maintenance of bulk turbulence by libration-driven elliptical instability, B Favier, AM Grannan, M Le Bars, JM Aurnou, Physics of Fluids 27 (6), 066601, 2015 (http://scitation.aip.org/content/aip/journal/pof2/27/6/10.1063/1.4922085)
4. Wave field and zonal flow of a librating disk, S Le Dizès, Journal of Fluid Mechanics, in press, 2015
It is a commonly accepted hypothesis that convective motions are responsible for most flows in planetary and stellar fluid layers, and in particular that convective motions are responsible for planetary dynamos, as it is the case on Earth today. However, the validity of the convective dynamo model can be questioned in certain planets, for instance in Ganymede, in Mercury… Besides, even in planets where the dynamo is of convective origin, additional driving mechanisms may significantly modify the organization of fluid motions in their core. The purpose of this ground breaking project is thus to evaluate the importance of three mechanical forcings present at the planetary scale, but still largely unknown from a fluid mechanics point of view: libration, precession, and tidal distortions. Beyond planetary cores, we aim at systematically quantifying their driving influence and consequences in large scale flows in any fluid layer of astrophysical bodies, such as atmospheres of gas giants, subsurface oceans of icy satellites and convective/radiative zones of stars in extrasolar systems.
This project is by its essence interdisciplinary, lying at the frontier between fluid mechanics, planetology and astrophysics. By combining theoretical, numerical and experimental approaches, we will first study at IRPHE the stability and organization of fluid motions generated by these alternative driving mechanisms in simplified models. Then, relying on selected collaborations with renowned specialists in planetology and astrophysics, we will apply our general results to natural systems, taking into account their specificities and complexities, and we will challenge our conclusions with available data coming from the latest spatial missions, observations and measurement campaigns. In a highly competitive international environment, this interdisciplinary collaboration is a unique opportunity to open new horizons in fundamental fluid mechanics as well as in the understanding of internal dynamics and orbital evolutions of planets and stars.
Project coordination
Michael LE BARS (Centre National de la Recherche Sientifique Délégation Provence et Corse _Institut de Recherche sur les Phénomènes Hors Equilibre (IRPHE))
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partner
CNRS DR12 _ IRPHE Centre National de la Recherche Sientifique Délégation Provence et Corse _Institut de Recherche sur les Phénomènes Hors Equilibre (IRPHE)
Help of the ANR 172,640 euros
Beginning and duration of the scientific project:
March 2014
- 36 Months
