Mercury is a terrestrial planet, meaning it has a solid, rocky surface primarily composed of silicate rocks and metals. It is the smallest planet in the Solar System and is closest to the Sun

. Mercury's interior is dominated by a very large iron-rich core, which makes up about 70% of the planet's total mass and occupies roughly 57% of its volume. This core is partly molten and is surrounded by a relatively thin silicate mantle and crust, about 350 to 420 kilometers thick. This large metallic core gives Mercury a very high density, second only to Earth among the planets

. The surface of Mercury is heavily cratered and resembles that of Earth's Moon, but it lacks large dark lava plains (maria) seen on the Moon. Mercury has a very thin, trace atmosphere and no moons

. In summary, Mercury is a small, dense, rocky planet with a massive iron core and a thin silicate outer shell, classifying it as a terrestrial planet with unique structural characteristics in the Solar System.