Fluid Dynamics of Deep Interiors in the Outer Solar System

Understanding planetary flows is a tremendous challenge for fluid dynamicists and planetary scientists, because of their extreme regimes, the multiple scales and physical processes involved, and their indirect effect on observational data. The colourful bands of Jupiter are caused by intense east-west winds called zonal jets, whose origin and stability are still poorly understood.

Zonal winds interact with numerous large-scale vortices, including the famous Great Red Spot, and together, jets and vortices belong to an extremely chaotic flow. Building on the discoveries of Galileo and Cassini, Juno has revealed that Jupiter's zonal winds are deep, penetrating over thousands of kilometres into its mantle of liquid hydrogen. In contrast, midlatitude anticyclones are shallower, not exceeding a few hundred kilometres.

Juno also revealed remarkable dynamics at the poles, with polygonal clusters of cyclones, while Saturn exhibits a single polar cyclone with a polygonal jet at the North Pole. These structures raise many fundamental questions: How can we explain their formation, intensity, and size? Why are they so stable?

What factors determine their extent below the cloud level? How do zonal jets and vortices interact with each other? Why are the polar regions of Jupiter and Saturn so different?

Ocean worlds are icy satellites of Jupiter and Saturn thought to harbour global salty oceans beneath their solid surface. One of the primary goals of ESA's Jupiter Icy Moons Explorer (JUICE) and NASA's Europa Clipper missions is to investigate if the Galilean moons Europa and Ganymede have suitable conditions for life. It is crucial to develop accurate models of subsurface ocean circulation for assessing the habitability of these moons as well as their thermal and orbital evolution.

However, it is a formidable challenge as the thick ice cover prevents any direct observations: Which processes drive the ocean's circulation, and what is the effect of rotation? Can lateral temperature contrasts drive a global overturning circulation? What are the properties of heat and material exchanges between the rocky interior and the ice crust?

What would be the impact of the oceanic circulation on observables (gravity, magnetism, rotation, ice thickness)?

New observations and interior models have yet to be matched by self-consistent models of the complex dynamics occurring in the fluid interiors of gas giants and icy moons. Spatial observations are sparse and result from the interaction of multiple physical processes, making it difficult to reach a clear and comprehensive understanding. The goal of the project is to complement measurements by process-oriented modelling and to follow an original and multi-method approach, at the intersection of fundamental fluid mechanics and planetary science.

Our premiss is that key features of the dynamics of gas giants and icy moons can be reproduced in well-controlled laboratory experiment, where physical models can be readily tested. Water will be employed to represent either hydrogen or liquid oceans, and a rotating table will be used to simulate the planet's rotation. The goals of the project are to develop novel experimental analogues of (1) the emergence of zonal winds from waves and their collective behaviour, (2) the vertical structure of zonal winds and their interaction with stratified layers, (3) the complex interaction between zonal winds and vortices at low and high latitudes and (4) horizontal convection in the presence of rotation and its ability to penetrate into a stratified environment.

By combining these fluid mechanics experiments with idealised numerical and theoretical analyses, I will deduce properties of gas giants and ocean worlds inaccessible to direct measurements, but also better understand underlying physical processes which are generic and applicable to other systems such as terrestrial oceans, atmospheres and liquid cores.