Saturn's moon Mimas, imaged by the Cassini spacecraft. A new study shows how such small ice moons could sustain a liquid ocean beneath an icy shell, and how that could give rise to surface features. Credit: NASA/JPL-Caltech/Space Science Institute The outer planets are surrounded by dozens of moons locked in thick shells of ice. Some of these frozen worlds, including Saturn's moon Enceladus, are believed to contain vast oceans of liquid water trapped between their icy crust and rocky interiors.