A Laboratory Model for Iron Snow in Planetary Cores

Solidification of the cores of small planets and moons is thought to occur in the "iron snow" regime, in which iron crystals form near the core-mantle boundary and fall until re-melting at greater depth. The resulting buoyancy flux may sustain convection and dynamo action. This regime of crystallization is poorly understood. Here we present the first laboratory experiments designed to model iron snow. We find that solidification happens in a cyclic pattern, with intense solidification bursts separated by crystal-free periods. This is explained by the necessity of reaching a finite amount of supercooling to re-initiate crystallization once the crystals formed earlier have migrated away. When scaled to planetary cores, our results suggest that crystallization and the associated buoyancy flux would be strongly heterogeneous in time and space, which eventually impacts the time variability and geometry of the magnetic field. In small planets or moons with iron core, solidification proceeds from the top down, producing solid iron crystals at the top of the core. These crystals then fall until they melt at deeper depth, where the temperature is larger. By analogy with snow in the atmosphere, this regime is called iron snow. It creates motions in the liquid core and provides energy for generating a magnetic field. However, the key aspects of this regime remain largely unknown. Using analog laboratory experiments, we have found that solidification occurs in a cyclic pattern, with periods of intense crystal formation followed by quiet periods with no crystals. This happens because crystallization needs a certain amount of cooling below the solidification temperature to be triggered, during which all crystals have risen and melted. Applied to planetary cores, it means that the iron snow would be heterogeneous in space and time, with intermittent and localized crystals falling. This would affect the shape and strength of the planet's magnetic field. We have carried out an experimental study of the dynamics of iron snowOur experiments present crystallization cycles, with intense solidification bursts separated by quiet periodsThis cyclic pattern is controlled by thermal diffusion and by the amount of supercooling required for crystallization