Europa, which is slightly smaller than the Earth's Moon, is the innermost of the ice-covered Galilean satellites. Its surface consists of a mixture of predominantly water ice and lesser amounts of finely powdered silicate.
During the far encounter phase of Voyager 1 in March 1979, Europa appeared smooth with little differences in elevation apparent on the surface. Broad zones of dark and light terrain were noted, as well as linear markings stretching a third of the way across the satellite.
Close-up views provided by Voyager 1 and 2 indicate that the light and dark colored lines are the dominant feature on this moon of Jupiter. These structures represent depressed and elevated areas on the satellite, although little differences in elevation have been noted. Only a few possible impact craters have been detected in Voyager images.
Like Io, Europa is subject to strong gravitational attraction — inwards by Jupiter and outwards by Ganymede. However, clear-cut patterns of crustal stretching are not present. A complex interaction between the gravitational forces and the moon's own evolution is most likely responsible for the curvilinear furrows and ridges.
View of Europa from the Galileo spacecraft
Icy ridges on Europa. The prominent ridge cutting across the center of the image is 2.6 km (1.6 mi.) wide and 300 meters (980 ft.) high.
An ocean of water may exist under Europa's icy crust. NASA is planning a mission to Europa to explore the question of whether it could harbor simple life.