Mercury is the planet closest to the Sun, and experiences scorching temperatures by day. But it has virtually no atmosphere to trap the heat, so is freezing at night. It is a rocky planet, just over a third the diameter of Earth. Its surface is covered with craters, which make it look similar to parts of the Moon. These craters were formed when the planet was bombarded with meteorites long ago.

The craters that cover almost all of the surface of Mercury are generally shallower than those on the Moon. They vary in size from a few metres to hundreds of kilometres across. In between, there are relatively smooth lava-covered plains crossed by cliffs and ridges.