Interior structures of Earth, Mars and the Moon, to scale
All of the terrestrial planets have a three-part layered structure. At the center is a metallic, iron-rich core, part of which may be molten. Above the core is a thick middle layer called the mantle, made of silicate rock (composed mostly of silicon, oxygen, iron, and magnesium), making up most of the bulk of the planet. Above the mantle is a relatively thin crust of less-dense rocky material. The crust has more lighter elements (aluminum, sodium, calcium, sulfur) than the mantle.