Mercury has a radius of 2439 km (1524 mi), and the metallic iron-nickel core is believed to make up about 75% of this distance. Measurements of the planet's magnetic field made by Mariner 10 as it flew by the planet indicates that this core is likely to be hot and fluid. In contrast with the other terrestrial planets, the rest of the planet is probably made up of a solid rocky layer topped with a thin crust about 100 km thick. The surface of Mercury is covered with a variety of craters formed from the impact of meteorites billions of years ago and other features that reveal information about the evolution of the planet.