Magentic Field and Inner Core
When Mariner 10 passed by Mercury, it discovered that the magnetic field is 1% as strong as Earth's. The field has an axis of inclination of 7 degrees to Mercury's axis of rotation and forms a magnetosphere around the planet. The source of this magnetic field is not yet understood, however, since Mercury's interior is comprised mainly of a large iron core (about 1800 to 1900 km thick), a percentage of it must be molten for the generation of the field. Another source of the field may be from partially iron rocks that were magnetized by a much stronger magnetic field in Mercury's younger age.
Mercury has a high density of 5.44 g/cm^3, denser than Earth's 5.52 g/cm3. Like on Earth, the high density indicates a high percentage of metal, around 70 percent by weight compared to the other 30 percent by weight silicate. Therefore, the massive amount of metal makes up about 75% of the planet's radius and 42% of the planet's volume.