Discovered by Galileo Galilei in 1610, Io is Jupiter's closest and third largest moon. It played a significant role in the first measurement of the speed of light, calculated by 17th-century Danish astronomer Ole Rømer. The most geologically active of Jupiter's moons, Io has 30 active volcanoes that are probably energized by the tidal effects of Jupiter's enormous mass. In 2001, the unmanned spacecraft Galileo came within 110 miles (180 km) of Io. What did its images reveal about Io? Discuss (http://forum.thefreedictionary.com/postst68670_Jupiter-s-Io-Observed-by-Spacecraft--i-Galileo--i---2001-.aspx)