Active Volcanoes feed Io's sulfurous atmosphere

The atmosphere on Jupiter's moon Io is a witches' brew, composed primarily of the sulfurous exhalations of more than 400 volcanoes that dot the surface. Until now, it has been unclear whether volcanoes spewing hot sulfur dioxide are the main contributors to the atmosphere, or whether the main component is the accumulated cold SO2, much of which is frozen on the surface, but in sunlight evaporates or sublimates into the atmosphere.