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Abstract

Top-down solidification has been suggested in the liquid cores of small planets, moons, and large asteroids. Iron snow is then thought to exist, consisting of the crystallization of free iron crystals at the top of these cores and of their settling in a stably stratified ambient, until they remelt in a hotter, deeper region. This inward crystallization and associated buoyancy flux may sustain dynamo action by convection below the remelting depth. However, thermal evolution models are up-to-now oversimplified, assuming a constant-in-time and homogeneous-in-space buoyancy flux at the bottom of the snow zone. Moreover, recent works on the nucleation of solid crystals from the liquid iron alloys in planetary cores have challenged the canonical view of spontaneous crystallization, due to the large supercooling required in metals. Here, we have carried out an analog experiment with the same ingredients — crystallization, sedimentation, melting — as the iron snow. We show that the buoyancy flux can be heterogeneous in time and space, with intense snow events separated by quiescent periods where the snow zone supercools. The sedimentation rate and the diffusive cooling rate govern the timescale of intense and quiet periods, respectively. In our experiments, a wide range of crystal sizes exists, with large crystals overshooting the convection region and challenging the hypotheses underlying the existing evolution models. The spatiotemporal variability of the energy source obviously impacts the shape and intensity of the generated magnetic field, providing alternative explanations for the observed, surprising features of Mercury's and Ganymede's magnetic fields.