Europa is the fourth largest moon of Jupiter, and the fifteenth largest body in the solar system. It was one of the four moons discovered by Galileo Galilei in 1610. The Jovian system was first visited by the Pioneer 10 spacecraft in 1973, then its sister craft Pioneer 11 a year later. The Voyager 1 and Voyager 2 spacecraft both passed through the Jovian system in 1979, gathering further information. The Galileo probe orbited Jupiter from 1995 to 2003, building up a detailed model of many of the Jovian Moons, including Europa.
The surface of Europa.
The picture that of Europa that emerged from these missions was of a frozen, icy, world, slightly smaller than our moon. However the surface of Europa is not smooth, the surface is dotted with circular areas of broken terrain known as lenticulae ('freckles' or 'spots'), thought to be caused by water breaking through to the surface. Some of these areas show particularly disruption, with sections of what appear to be shattered crust locked in smoother areas of ice, resembling icebergs caught in frozen sea-ice. These areas are referred to as chaoses, or chaotic terrains.
The Conamara Chaos, one of Europa's larger chaotic terrains.
Planetary scientists have taken these chaotic terrains as evidence of a liquid ocean beneath the surface of Europa, kept fluid by heat generated by tidal forces as Europa is periodically stretched then released by the gravity of the other moons. From time to time, it is thought, these tidal forces cause water to leak up to the surface, creating the lenticulae and chaoses. This subsurface ocean has often been considered the most likely site for extra-terrestrial life within the solar system by both scientists and science-fiction writers, on the basis that all life as we understand it needs life to survive.
The 16 November 2011 edition of the journal Nature, contains a paper by a team lead by Britney Schmidt of the Jackson School of Geosciences at the University of Texas at Austin, in which the authors detail a detailed study of both the chaos terrains of Europa, alongside similar areas around subglacial volcanoes on Earth. From this they conclude that the chaos terrains could be generated by large lakes as shallow as 3 km bellow the surface of the moon. Like the proposed ocean they could be kept liquid by tidal forces, and occasionally be caused to breach the surface by the same. Being so close to the surface such lakes would be more temporary structures than the ocean, and would be far less likely to harbor life. The presence of the lakes would not preclude the presence of an ocean as well, but Schmidt et al. have demonstrated that it is not necessary to generate the observed surface of Europa.