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Animation of a flight past Io, one of the Galilean moons of Jupiter. The gas giant planet is seen at the start of the clip. Io is the third-largest moon of Jupiter, and the fourth-largest moon in the Solar System. It is the closest of the major moons to Jupiter. Its proximity to the massive planet means that it is subjected to powerful tidal forces, which heat its interior and make it the most geologically-active body in the Solar System. It has more than 400 active volcanoes, and its surface is covered in young deposits of colourful sulfur, frozen sulfur dioxide and silicate minerals. Along with the Ganymede, Callisto and Europa, Io was discovered by Galileo Galilei in January 1610