... September 12, 1996 . Mercury: A Cratered Inferno . Credit: Mariner 10 , NASA . Explanation: Mercury's surface looks similar to our Moon's. Each is heavily cratered and made of rock. ... But Mercury is unique in many ways. Mercury is the closest planet to the Sun , orbiting at about 1/3 the radius of the Earth's orbit. As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius to an unbearably hot 400 degrees Celsius . ... About APOD | ...