 Europa displayed a large number of intersecting linear features in the low @-@ resolution photos from Voyager 1 . At first , scientists believed the features might be deep cracks , caused by <unk> <unk> or <unk> processes . The high @-@ resolution photos from Voyager 2 , taken closer to Jupiter , left scientists puzzled as the features in these photos were almost entirely lacking in topographic relief . This led many to suggest that these cracks might be similar to ice <unk> on Earth , and that Europa might have a liquid water interior . Europa may be internally active due to <unk> heating at a level about one @-@ tenth that of Io , and as a result , the moon is thought to have a thin crust less than 30 kilometers ( 19 mi ) thick of water ice , possibly floating on a 50 @-@ kilometers @-@ deep ( 30 mile ) ocean . 
