Mercury, computer artwork. Mercury is the closest planet to the Sun. It is a rocky world covered with meteorite impact craters. Its diameter (4880 kilometres) 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 minus 170 degrees Celsius. This is the hemisphere that was imaged by the Mariner 10 probe in 1974 and 1975.