Iron Snow Falls Inside the Earth's Outer Core

Earth's outer core is a churning ocean of liquid iron and nickel, roughly 2,200 kilometers thick, at temperatures up to 5,000 degrees Celsius and pressures that compress metal to nearly double its surface density. Under these conditions, something counterintuitive happens at the bottom of that liquid layer: iron crystallizes out of solution.

A 2019 study by researchers at the University of Texas and the University of Illinois used seismic data from earthquakes to probe the structure just above the inner core boundary. They found evidence for a mushy, partially solid layer — iron crystals suspended in liquid metal — that's consistent with what lab experiments predict when iron is subjected to core-like pressures. The crystals, denser than the surrounding liquid, sink toward the inner core and accumulate there. Geologists call it iron snow.

The snow melts as it settles into still-hotter depths, releasing lighter elements that buoy upward. This creates a compositional convection — movement driven by density differences in composition, not just temperature — that may be partly responsible for driving the geodynamo, the process that generates Earth's magnetic field.

At the inner core boundary, the layer of partially solidified slush is estimated to be around 200 to 300 kilometers thick. The inner core itself grows at about a millimeter per year as iron freezes onto it from below, releasing latent heat that also contributes to the convection. Earth's magnetic field, and by extension the shielding of the surface from solar wind, depends in part on this slow geological snowfall.