Iron snow dynamics
Motivation:
Whereas the solidification in the Earth's core starts from its center, it has been suggested that the cores of smaller planets (Mercury, Mars) and moons (the Moon, Ganymede) begin to crystallize from the top of the outer core, i.e. close to the core-mantle boundary. For such inward solidification of liquid outer cores, a scenario of thermal evolution has been proposed via crystallization of free iron crystals so-called ''iron snow''. Iron solid crystals are denser than the surrounding liquid and sink towards the interior of the core. These crystals may dissolve partially during their fall into the outer core. The release of light elements due to the crystallization of pure iron crystals leads to the formation of a stably stratified layer at the top of the outer core.
Iron snow regime of solidification in a lab:
We are developing new sets of experiments to unravel the dynamics of the iron snow in a stably stratified layer. First, we are interested in the behaviour of a reactive particle, i.e. a solidifying or melting crystal, falling through a stratified layer. The goal of this experiment is to investigate the dissolution rate of such a particle, the mixing induced by its fall and the inertial waves produced in the stratified layer after its sink. In a large tank, a stably stratified layer is implemented using salty water. A salty ice cube (with a spherical shape) is released at the top of the tank. The sphere is denser than the surrounding liquid. During its fall, this large particle then melts and carries lighter fluid from above in its wake. Its sink through the stably stratified layer generates internal waves. We explore the effects of the particle size and the strength of stratification.
In this experiment, we perform Particle Image Velocimetry (PIV) with a powerful laser. Two high-speed cameras (50 frames per second (FPS) and 500 FPS) are used to track the dynamics of the experiments.
Later, we will carry out another experiment in order to track the effect of a large number of particles sedimenting through a stratified layer. Using a large tank with a stably stratified layer of salty water, we will track the dynamics of the fluid and the particles sinking through.
Open positions for this project: check out projects listed here under "Task 1: the fluid dynamics of core formation"
A reactive solid sphere is falling through a stably stratified layer. The sphere entrains its surrounding fluid during its sink.