Galileo's close-up view of the Io sodium jet

The Galileo spacecraft has imaged a remarkable atmospheric escape process o ccurring in the atmosphere of Io, Jupiter's volcanic moon. Electrodynamic c onsequences of Io's motion through Jupiter's magnetosphere drive mega-amp c urrents through Io's ionosphere; some of the sodium ions carrying this curr ent are neutralized as they leave the atmosphere. The Galileo images show t hat the resulting fast sodium jet removes similar to 5 x 10(25) atoms sec(- 1) from Io's atmosphere. The spatial profile of the jet shows that the sour ce region is much smaller than Io itself, perhaps confined to volcanically active regions, or to an ionosphere restricted to the denser atmosphere nea r Io's equator.