Mercury is the smallest of the eight planets in the Solar System, with a mean radius of ~ 2440 km, a mean distance to the Sun of ~ 58 million km and an orbital period of ~ 88 Earth days. Mercury is a terrestrial planet with a surface characterized by impact craters and basaltic lava plains, similar to Earth’s Moon. Because it is the closest planet to the Sun and has almost no atmosphere, Mercury experiences the largest variations in surface temperatures of all planets in the Solar System.
Information source: NASA
Image: Mercury as seen from MESSENGER. Source: NASA/John Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.