Mercury is made mainly of heavy metals, most iron with a rocky crust (mantle). After Earth it is the second most dense planet in our Solar System.
Like the Moon, Mercury has lots of craters but it has no moon of its own. Most of Mercury’s craters were caused by comets and asteroids hitting the planet in the early years of the Solar System being formed. Unlike other planets, Mercury surface is not renewed through volcanic activity so it will always have these ancient craters.
Because Mercury is small and it has little gravity it cannot hold on to an atmosphere. As it is so close to the Sun, the solar winds quickly blow away any gases that are released from Mercury. As there is no atmosphere to help control the temperature there are big differences in day-time and night-time temperatures. The surface that faces the Sun can reach about 427 °C with temperatures on the night-time side drop to almost -173 °C.
The first spacecraft to visit Mercury was Mariner 10 in 1974 and it mapped just under half of the planet’s surface. A second mission, MESSENGER, was launched in 2004. During its mission it mapped the surface of Mercury's northern hemisphere in great detail. The image on the left is the map created from the images. The lowest areas are shown in purple, and the highest areas are shown in red.