Wikipedia - Featured Article - Europa
Europa (pronounced /jʊˈroʊpə/ yew-ROE-pə listen (help·info); or as Greek Ευρώπη) is the sixth-nearest and fourth-largest moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei (and, some say, independently by Simon Marius), and named after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete. It is the smallest of the four Galilean moons.
At just over 3,000 kilometres (1,900 mi) in diameter, Europa is slightly smaller than Earth's Moon and is the sixth-largest moon in the Solar System. Though by a wide margin the least massive of the Galilean satellites, its mass nonetheless significantly exceeds the combined mass of all moons in the Solar System smaller than itself. It is primarily made of silicate rock and likely has an iron core. It has a tenuous atmosphere composed primarily of molecular oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This young surface is striated by cracks and streaks, while craters are relatively infrequent. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extraterrestrial life. Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity.
Although by 2007 only fly-by missions have visited the moon, the intriguing characteristics of Europa have led to several ambitious exploration proposals. The Galileo mission provided the bulk of current data on Europa, while the Jupiter Icy Moons Orbiter, cancelled in 2005, would have targeted Europa, Ganymede and Callisto. Conjecture on extraterrestrial life has ensured a high profile for the moon and has led to steady lobbying for future missions.