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. Its surface is striated by cracks and streaks, 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.
The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath the surface, which could conceivably harbor extraterrestrial life. Numerous science fiction works have been based on this premise.
Europa in "Manuscript Tradition"Edit
The odd oceanic creatures under Europa's icy crust were one of the first two known examples of extraterrestrial life, the other being the microbes of Mars. It was quite some time before any further alien life was discovered.