These particular volcanoes were seen erupting by both Voyager 1 and Voyager 2, so it is likely that they were active for at least four months. Io is the innermost and third largest of Jupiter's four main moons, which were first observed by Galileo. It is one of the most volcanically active bodies in the Solar System - Voyager 1 observed 8 volcanic plumes on Io between March and July 1979. The volcanic activity is believed to be due to immense tidal effects caused by Io's proximity to Jupiter and another moon, Europa, heating and melting Io's interior. NASA's two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 2 reached Jupiter in 1979 before flying on to reach Saturn in 1981, Uranus(1986) and Neptune (1989).
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