Io, just slightly larger than our own Moon, is the innermost of the four Galilean moons of Jupiter and the most volcanically active object in the Solar System due to the relentless tidal forces from its gigantic parent planet.
The Galileo spacecraft took this true-color photo of Io after entering into orbit around Jupiter. Obviously a very young surface free of craters, the result of constant repaving from volcanic activity.
The Galileo mission ended on Sept. 21, 2003 when the spacecraft plunged into Jupiter's crushing atmosphere after 34 orbits around the giant planet.
This is the best composite true-color image of Io from the Voyager 1 flyby.
This is the only good photo of Io returned from the Pioneer 11 flyby at a distance of about 756,000 kilometers (twice the distance to our Moon). A yellow/orange surface color is clearly visible.
Italian scientist Galileo Galilei has discovered what he calls the Medicean stars (in honor of the Medici family) Jupiter I, II, III and IV orbiting the planet Jupiter when he pointed his telescope to the heavens.
Of course photography did not exist in the 1600's so the picture above is a modern day view through a small telescope that closely represents what Galileo would have seen. Jupiter I we now know as Io. This discovery led to the demise of the long held theory that the Earth was at the center of the universe and a new understanding that the planets (including the Earth) orbit the Sun.