Mercury is the closest planet to the Sun. It is smaller than the moons Ganymede and Titan but larger than the planet Pluto. It is heavily cratered, showing impact scars from countless bombardments by meteoroids. Old lava flows and quake faults also mark its crust. Mercury has no atmosphere, but surprisingly, scientists have found evidence for a polar ice cap on this hot little globe. Mercury was explored in three flybys by the Mariner 10 spacecraft in 1974 and 1975.