Io is the most volcanically-active world in the Solar System. But these volcanoes are like nothing seen on Earth. Io's volcanoes are composed of plumes of sulphur that spray the surrounding landscape with lurid chemical colours. The plumes play a role in producing Io's wonderful auroral lights. Showers of electrons striking the moon's thin atmosphere give rise to a blue glow.
Red light comes from oxygen in the atmosphere, and green light from sodium.
Jupiter eclipse The team of scientists reporting in Science analysed auroral images of Io taken by the Galileo spacecraft. The pictures of colourful light emissions on the moon were taken during an eclipse by Jupiter. The pictures help us to understand Io's atmosphere and the processes that generate the emissions. Io's eerie glow dims noticeably when the moon lingers in Jupiter's shadow. The likely explanation, concludes the team, is a partial collapse of the moon's atmosphere.