"Mercury is one of the least-understood of the planets in our Solar System. Its distance from the Sun is just over one-third that of the Earth, and it contains a mass just 5½ percent that of Earth....Mercury has a weak magnetic field, about one percent as strong as Earth's… Mercury is so small that most scientists expected its core to have cooled and solidified long ago. Those scientists speculated that the magnetic field seen today may have been "frozen" into the planet when the core cooled."