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Io is one of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon, has the highest density of all the moons, and is the driest known object in the Solar System. With over 400 active volcanoes, Io is the most geologically active object in the Solar System. The dark spot just left of the center is the erupting volcano Prometheus. The whitish plains on either side of it are coated with volcanically deposited sulfur dioxide frost, whereas the yellower regions contain a higher proportion of sulfur. The first reported observation of Io was made by Galileo Galilei on 7 January 1610 using a 20x-power, refracting telescope at the University of Padua. The first spacecraft to pass by Io were the twin Pioneer 10 and 11 probes on 3 December 1973 and 2 December 1974 respectively.
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