Mariner 10 discovered that Mercury has a magnetic field. Until then, scientists didn't think Mercury had a magnetic field. Mercury's magnetic field is very weak. It is 100 times weaker than Earth's.
An electrical current produces a magnetic field. Movement of a metallic liquid will create an electrical current. A planet with an iron core will produce an electrical current as the planet rotates if the core is molten. Mercury was thought to have a solid core. The magnetic field is evidence that perhaps part of the core is molten.
