Europa has a moderate atmosphere which is not as dense as Ganymede, but because it has quite a large proportion of water vapor in its atmosphere (which is a strong greenhouse gas), in addition to its proximity to the Sun, it is, on average, 15˚F warmer than Ganymedian tempratures if not warmer with average of 28˚C/83˚F. It is also composed primarily Nitrogen, Oxygen and some other gases.
Europa is entirely covered by water, save for a single mountain-top, Mt. Argus. The underwater landscape, however, is stunning. Thought to be strictly flat, radar confirmed beautiful underwater areas, along with local lore of such landscapes.
Europa was once a moon of Jupiter. Roughly 4.34 Billion years ago, its orbit greatly affected by the pull of Ganymede, it stopped orbiting Jupiter, becoming its own planet. The accumulation of water allowed for life to begin. Roughly 100,000 years ago, Europa was first observed and recorded by Humans on Ganymede. It was given the name 'Europa' by the Ancient Erebs.