Mercury is the smallest planet in the solar system, the closest planet to the Sun, and it orbits the sun once every 88 days. Its surface has extensive mare-like plains and is heavily cratered, indicating that the planet has been geologically inactive for billions of years. Mercury is composed by a large iron core with a mantle and crust of silicate material. Surface temperatures range from 90 to 700 Kelvin (-180 to 430 degrees Celsius). Mercury is on average 57,909,176 km away from the sun.