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 (daynight) 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.