Mercury is the innermost and smallest planet in the Solar System. It orbits the Sun once every 87.97 Earth days, completing three rotations about its axis for every two orbits. Mercury is similar in appearance to the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. Unlike the Moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth. It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about −183 °C to 427 °C. New observations from a spacecraft orbiting Mercury have revealed that the tiny, pockmarked planet harbors a highly unusual interior — and the craft’s glimpse of Mercury’s surface topography suggests the planet has had a very dynamic history.