The Galileo spacecraft has imaged a remarkable atmospheric escape process occurring in the atmosphere of Io, Jupiter's volcanic moon. Electrodynamic consequences of Io's motion through Jupiter's magnetosphere drive mega-amp currents through Io's ionosphere; some of the sodium ions carrying this current are neutralized as they leave the atmosphere. The Galileo images show that the resulting fast sodium jet removes ~5¿1025 atoms sec-1 from Io's atmosphere. The spatial profile of the jet shows that the source region is much smaller than Io itself, perhaps confined to volcanically active regions, or to an ionosphere restricted to the denser atmosphere near Io's equator. ¿ 1999 American Geophysical Union