Europa (Credit:NASA)

Europa is unusual because it has very few impact craters. Because there must have been many asteroid collisions since Jupiter and Europa formed, this means that the moon's surface must be fairly new.

Upon closer inspection, we find that Europa is covered with a thick layer of ice. The ice has 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.

Because of this, we think there is a good chance of finding liquid water below the ice. And as life on Earth cannot exist without water, some scientists think that simple forms of life may just exist in the deep oceans of Europa.