... Credit Copyright: The Galileo Project , JPL , NASA Explanation: Are any volcanoes on Io currently erupting? To help answer this, scientists instructed the robot spacecraft Galileo to take hourly pictures of this moon of Jupiter prior to its most recent encounter. ... The most prominent plume visible is from Prometheus, visible just below Io 's equator. This plume was first photographed in 1979 by Voyager , so Prometheus appears to have been erupting continuously for at least 18 years! ...