The presence of large amounts of sulfur on the surface may help explain the extraordinary nature of the Io volcanoes. One model considered for the satellite postulates that it is covered by a sea of liquid sulfur several kilometers deep, with a crust of solid sulfur and, below the surface, liquid sulfur dioxide. Calculations by Sue Kieffer of the U.S. Geological Survey and others indicate that the expansion of the sulfur dioxide in such a model can explain the observed eruption velocities of up to a kilometer per second. The volcanic plumes on Io appear to be made primarily of sulfur and sulfur dioxide. Both are molten as they emerge from the vent, but they quickly cool as the plume rises 100 or more kilometers into the near vacuum of space. Unlike terrestrial volcanoes, there is almost no gas in the plumes. It requires about half an hour for the fine particles of solidified sulfur and sulfur dioxide snow to fall back to the surface, where they form the colorful rings that mark the major eruptive sites. Almost all the roughly 100 000 tons of material erupted each second by the Io volcanoes snows back to the surface. But apparently a part—perhaps 10 tons per second—escapes from Io and is captured by the Jovian magnetosphere. Another part contributes to an ionosphere—a tenuous atmosphere of electrons and ions—that surrounds Io. The injection of several tons of particles each second into the magnetosphere has dramatic consequences that can be seen even from Earth. [Illustration: Direct evidence of an atmosphere on Io was obtained during the Voyager 1 flyby by the IRIS. In the region near the volcano Loki and its associated “lava lake,” infrared spectra clearly showed the signature of sulfur dioxide gas. It is not known whether this gas was a temporary feature associated with the eruption of Loki or if it might be present on Io more generally. Other evidence, however, points to the sulfur dioxide atmosphere as a transient feature. A small amount of the sulfur dioxide escapes and is broken apart by sunlight to provide the oxygen and sulfur ions observed in the Io torus.] Sulfur dioxide gas cloud Plume -235° F Hot surface areas (45° F) The Io Torus