Mercury: A Cratered Inferno . ... Credit Copyright: 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 . ... NASA Official: Jay Norris. ...