Europa, or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter and the sixth-closest to the planet of all the 79 known moons of Jupiter. Thus, it is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei. It was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman god Jupiter). Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and has a water-ice crust and probably an iron-nickel core. It has a very thin atmosphere, composed primarily of oxygen. Cracks and streaks striate its surface, but craters are relatively few. In addition to Earth-bound telescope observations, Europa has been examined by a succession of space probe flybys, the first occurring in the early 1970s. Europa has the smoothest surface of any known solid object in the Solar System. 

Our Solar System

The Solar System is the gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, the dwarf planets, and small Solar System bodies. Of the objects that orbit the Sun indirectly—the natural satellites—two are larger than the smallest planet, Mercury.