Europa is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of the 79 that have been identified by NASA. Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock, has a water-ice crust and what researchers think is an iron-nickel core. It has a very thin atmosphere, composed primarily of oxygen and its smooth surface has led scientists to hypothesise that a water ocean exists beneath the surface, which could conceivably harbour extraterrestrial life.