Mercury is the only inner planet other than the Earth that has a significant magnetic field (220 nT). This field, along with the planet's high density and small size relative to the Earth, indicates that it probably has a molten iron core. The magnetic field has two poles, and is tilted in the same direction as Mercury's axis. Mariner 10 observed a shock wave called a "bow shock" in front of the planet, where the planet's magnetic field meets the solar wind.