Tethys

"With the lowest density of any major moon in the solar system, Tethys is likely composed almost entirely of water ice with very little rock. Its surface contains the usual craters, ridges, and cryovolcanoes you might expect from a body of its size and composition, with one notable impact crater being over 400 kilometers wide. To be honest, Tethys isn't a terribly exciting moon. It's a large, unusually pure snowball in space.At least, that used to be true: after today, it will be your home."