Jupiters Moon Io with plumes and a sunlit crescent at left. Io is the most volcanically active world in the solar system, with hundreds of volcanoes. Voyager?1?flew past Io on 5 March 1979 from a distance of 20,600?kilometres. The images returned during the approach revealed a strange landscape devoid of impact craters with three volcanic eruption plumes (upper left and lower right).