Mercury is the closest planet to the Sun and the smallest in our Solar System. Named after the Roman messenger god, the planet is not much larger than the Earth's Moon and superficially resembles it with a surface pock-marked by enormous craters.
These craters were caused by meteorites smashing into the planet's surface in the early stages of the Solar System's evolution, some four billion years ago. Although it's only a third the size of Earth, Mercury is almost as dense. Scientists put down to a massive iron core, which is also responsible for Mercury's magnetic field.