ERC Project THEIA

THEIA (2020–2026), led by David Cébron at ISTerre, combined theory, numerical simulations and laboratory experiments to investigate planetary core dynamics. This gallery connects published results with the rotating-fluid apparatus built in Grenoble.

The THEIA and mini-THEIA experiments were conceived and designed jointly by David Cébron and Max Solazzo. Max Solazzo, a CNRS engineer at ISTerre, carried out their computer-aided design (CAD), mechanical construction and assembly.

Planetary dynamos: a magnetic field emerging from a stratified fluid

Could a fluid that is initially stable to ordinary convection nevertheless generate a planetary magnetic field? Simulations by Paul Pružina, Nathanaël Schaeffer and David Cébron show that semi-convection can sustain a dynamo. In some regimes, a convecting interior develops beneath a persistent stably stratified layer. The deeper motions generate the magnetic field, while flows in the overlying layer modify its external structure, favouring strongly dipolar and nearly axisymmetric fields.

The illustration combines Saturn with a numerical visualisation. The simulated fields have features reminiscent of Saturn’s magnetism; the image is not a reconstruction of Saturn’s observed internal structure.

Graphical abstract accompanying Pružina, Schaeffer & Cébron, Semi-convection in rotating spherical shells: flows, layers and dynamos, Journal of Fluid Mechanics 1044, A1 (2026). The paper was accepted during THEIA and published online after the grant ended.

Topography, inertial waves and pressure forces

Flow over relief excites inertial waves. Their pressure fluctuations exert forces on the sloping boundary, transferring momentum between the rotating fluid and the solid. The illustration brings together an idealised periodic topography and vertical sections of the calculated pressure field in three wave regimes.

The study was led at ETH Zürich by Vadim Giraud and Jérôme Noir, with David Cébron, Jérémie Vidal and Rémy Monville. The periodic relief shown here is a model geometry; the experimental photographs below show physical apparatus used to investigate related rotating-flow questions.

Figures 1 and 3, adapted from Giraud, Noir, Cébron, Vidal & Monville, Topographic drag in rapidly rotating fluids, Journal of Fluid Mechanics 1040, A14 (2026), CC BY 4.0.

THEIA experiments: the one-metre tank on WEBER

The intermediate THEIA tank, approximately one metre in diameter, was mounted on the WEBER rotating platform at ISTerre. It was used to develop rotation control, calibration and acquisition procedures, and for spin-up experiments over smooth and rough bottoms.

The daylight view shows the transparent tank, its supporting structure and the rotating platform.

The same apparatus under laser illumination. Together, these views show the mechanical installation and the optical arrangement used to observe the fluid.

Laser diagnostics and flow measurements

Particle image velocimetry (PIV) measures fluid velocities in a plane from the displacement of illuminated tracer particles between successive images.

A closer view of the laser illumination inside the one-metre tank. This is a photograph of the experimental illumination, not a processed velocity field.

Baptiste Penlou developed PIV acquisition and analysis procedures and spin-up protocols in this tank, following earlier experimental work, including Samuel Payen’s contribution in 2025. The smooth- and rough-bottom PIV measurements remained unpublished at the end of the ERC action; these photographs do not constitute quantitative validation of a drag law.

mini-THEIA: the 50 cm experiment

Mini-THEIA, approximately 50 cm across, provided a compact testbed for developing measurement procedures. The dark, irregular surface visible at the bottom of the first photograph is the experimental topography: a physical rough bottom, distinct from the idealised periodic relief in the theoretical illustration above.

The second photograph shows optical imaging around mini-THEIA under laser illumination.

Mechanical design and assembly

Joint experimental conception and design: David Cébron and Max Solazzo.

CAD, mechanical construction and assembly: Max Solazzo.

The assembled apparatus shown here documents this engineering contribution: turning the experimental concepts into rotating-fluid devices used by the researchers. The Experiments page explains the measurement methods and distinguishes apparatus development from the acquisition and scientific analysis of flow data.

People and funding

David Cébron led ERC THEIA and brought together researchers, engineers and collaborators working across theoretical and numerical fluid dynamics, planetary magnetism and laboratory experiments. The team page describes individual contributions; the Publications page gives the scientific references and full authorship.

This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 847433).

Explore the project results | Read more about the experiments | Explore the numerical models.