Io is the fourth largest moon in the Solar System, and one of the best known. It was named after the mystical charecter of the same name from Greek mythology, a priestess of Hera who fell in love with Zeus.
Io is unique in that it contains over 400 active volcanos on its surface, making it the most volcanically active object in the Solar System. Some of the mountains on its surface reach higher than the peak of Mount Everest. Partly due to the lava flows on Io's surface and the various compounds of sulphur it contains, it is coloured in an odd mixture of yellow, red, green and black. It has either a molten iron or iron sulfide core.
Io has been viewed by numerous spacecraft, including the two Discovery crafts and the Galileo craft.