Iron core of Mercury. Much larger proportionally than other planets (about 60% of its mass), making Mercury an Iron planet. Originally thought to have cooled early on, doubts occurred when Mariner 9 showed it had a magnetic field. Proven to be molten in 2007. Significant amount of a lighter element, such as sulfur, must be present in order to prevent its cooling. The magnetic field strength is about 1% that of Earth, so it must not have completely solidified. It has strong north-south assymetry, causing a lop-sided magnetic field.
Confirmed bursts of energetic particles in Mercury's magnetosphere are a continuing product of the interaction of Mercury's magnetic field with the solar wind. Magnetic field stronger in North than South, indicating something odd about its core. This leaves the south polar regions more exposed to solar particles.