Io is a fantastic-looking moon because of its many active (and recently active) volcanoes. No one has ever seen a crater on Io, because the volcanoes are constantly refreshing the surface. Several pictures taken of Io by the Galileo spacecraft just a few months apart looked radically different: a new volcano the size of Montana had just erupted!

The volcanoes are powered by Jupiter's tides, which are the largest in Io than in any other of Jupiter's moons. Galileo has taken images of volcanic plumes that extend more than 60 miles into space.