This intriguing image shows two of Jupiter's moons in orbit around the giant planet. Io is on the left just above the great red spot. Europa is visible on the right. This image was taken by the Voyager 1 spacecraft as it few past the Jovian system in 1979. Even from this distance the remarkable differences in the two moons is plainly visible. Each of Jupiter's largest moons has a unique look ranging from molten and volcanic to icy and frozen.