The space probe Galileo has found evidence of oxygen on one of Jupiter's moons, scientists said yesterday at a meeting of the American Geophysical Union. Galileo, which has been orbiting Jupiter and its moons for two years, detected hydrogen atoms escaping from the surface of Callisto, the most distant of Jupiter's four giant moons. The escaping hydrogen indicates there probably was oxygen in the ice, said Dr Charles Barth, a scientist from Boulder in Colorado.
The oxygen discovered on Callisto could be produced by solar radiation hitting the ice and separating the hydrogen and oxygen atoms that made it up, he said. "Because it is further away from Jupiter, Callisto does not interact as strongly as Ganymede [another Jupiter moon] with the charged particles in the planet's atmosphere. Instead we believe it is the ultraviolet solar radiation that is knocking the hydrogen atoms out of the ice on Callisto." Galileo has also sent back data indicating that sulphur dioxide emissions from a third moon, Io, were caused by intense volcanic activity. Io probably had water at one time but it disappeared billions of years ago when the volcanic activity began, the scientist said.