Jupiter's Galilean moons. - stock photo

This montage shows the best views of Jupiter's four large and diverse Galilean satellites as seen by the Long Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft during its flyby of Jupiter in late February 2007. The four moons are, from left to right: Io, Europa, Ganymede and Callisto. The images have been scaled to represent true relative sizes of the four moons and are arranged in their order from Jupiter. Io, 2,260 miles in diameter, is notable for its active volcanism, which New Horizons has studied extensively. Europa, 1,938 miles in diameter, has a smooth, icy surface that likely conceals an ocean of liquid water. Ganymede, 3,268 miles in diameter, has a dirty ice surface cut by fractures and peppered by impact craters. Callisto, 2,995 miles in diameter, has a heavily cratered surface.