Just as snow crystals form in the upper atmosphere, then fall to lower, warmer elevations and melt, scientists believe a phenomenon called iron snow happens in the molten iron cores of some planetary bodies. Cooling near the core-mantle boundary creates crystals of iron, which melt as they fall deeper into the hot core. This movement may create magnetic fields in some smaller bodies like Mercury and Jupiter's moon Ganymede, but its dynamics are not well known.