Jupiter's moon 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 crustal rifting or tectonic processes. Closer high-resolution photos from "Voyager 2", however, were puzzling: the features lacked topographic relief, and one scientist said they "might have been painted on with a felt marker". Europa is internally active due to tidal heating at a level about one-tenth that of Io. Europa is thought to have a thin crust (less than thick) of water ice, possibly floating on a 50-kilometer-deep (30 mile) ocean.