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South Pole of Io

South Pole of Io
South Pole of Io A flat plain, probably formed by the repetitive eruption of thin, low-viscosity sulfur, covers the south pole of Io. The dark circular features are calderas formed by such eruptions. The calderas seen in this Voyager image are filled with either liquid sulfur at moderate temperatures or solid, chilled deposits of originally high-temperature sulfur. NASA / JPL / USGS / Larry Soderblom