Like Earth, Mercury is one of four terrestrial planets. It is known as a terrestrial planet because it has a solid ground surface made up of rock and metal. This surface makes Mercury's density very similar to that of the Earth and denser than gas planets, according to Universe Today.
Mercury is distinct in that its core consists of solid or liquid iron. Its mantle is made of hot rock, and its crust is a thin layer of rock. Because of its hot mantle, scientists believe it may have other types of vents to release gases. Mercury does not have any active volcanoes, nor does it have any moons.