It has a very thin atmosphere made mainly of Oxygen and is thought to be one place in the Solar System most likely have life on it. This is mainly due to the thought that a huge ocean lies underneath the smooth, icy surface.
The surface is unusual because it has very few impact craters which leads so to think that the moon's surface must be fairly new. Covered with a thick layer of ice, with lots of cracks in it, and these show up as dark lines streaking across the moon's surface in a criss-cross pattern. It is thought that these sorts of cracks can occur when surfaces melt, drift apart a little and then re-freeze.