Mercury. Artwork of the planet Mercury seen from space. Mercury is the closest planet to the Sun. It is a rocky world covered with meteorite impact craters. Its diameter is less than half that of Earth and it has a surface area that is about the same as that of the Atlantic Ocean. The solar wind blasts atoms from its surface, forming a very thin atmosphere that is constantly lost to space. The surface temperature extremes are the greatest in the solar system, ranging (day-night) from 400 to -170 degrees Celsius. It is thought that Mercury has a large iron core, surrounded by silicate rock layers. The Milky Way is seen in the background.