One of the planet Mercury's longstanding mysteries involves its weak magnetic field. In short, it shouldn't have one, at least based on its small size. Any molten iron in its core – needed to generate the field – would have cooled long ago. Now, a team of researchers suggest that "iron snow" in the interior could help drive the planet's dynamo. Planetary scientists have suggested that if part of the core is mixed with sulfur, the combination would allow the iron to remain molten at temperatures cooler than needed for molten iron alone.
How would that produce a magnetic field? A team from the University of Illinois at Urbana-Champaign tried to find out. The researchers subjected iron-sulfur mixtures to the high pressures and temperatures of a research anvil – a viselike device that can reproduce the kinds of pressures found deep inside planets. They squeezed and heated each sample to predetermined levels, then quenched them quickly to "freeze" the state of the material within each sample. They cut the samples open and took a look. They found that as the combination cooled, iron atoms formed cubelike "flakes." The team says it suspects that as the molten material in Mercury's upper core cools, similar flakes congeal and begin to sink. In turn, lighter molten sulfur rises. This convection sets up the magnetic field, the team suggests. The team, led by geology professor Jie Li, suggests that Mercury's core may have two distinct regions of this iron "snowfall." The results appeared in the April issue of the journal Geophysical Research Letters.