High Quality Content by WIKIPEDIA articles! Volcanism on Io, a moon of Jupiter, produces lava flows, volcanic pits, and plumes of sulfur and sulfur dioxide hundreds of kilometres high. This volcanic activity was discovered in 1979 by Voyager 1 imaging scientists. Observations of Io by passing spacecraft (the Voyagers, Galileo, Cassini, and New Horizons) and Earth-based astronomers have revealed more than 150 active volcanoes. Up to 400 such volcanoes are predicted to exist based on these observations. Io`s volcanism makes the satellite one of only five known currently volcanically active worlds in the solar system (the other four being Earth, Venus, Saturn`s moon Enceladus, and Neptune`s moon Triton).