Crust : Mercury has a thick crust that is composed mostly of silicate rocks. Mercury may have small ice caps at its north and south poles; this ice stays frozen inside deep craters that are shaded from sunlight. Mantle : Beneath the crust it is a mantle (also made of silicate rocks) that is hundreds of kilometers thick. Core : At the center of Mercury is a partly-molten iron core about 2,300 miles (7,500 km) in diameter (almost half of the diameter of Mercury). This core accounts for about 80% of Mercury's mass. This core generates a magnetic field (which is how we know that Mercury has an iron core). Density : Mercury has a density of 5,430 kg/m 3 , slightly less than that of Earth. Mercury is the second-densest planet in the solar system (after Earth) because of its large iron core.