John R. Spencer

Extreme volcanism on Io: Latest insights at the end of Galileo era

Galileo has now completed 7 years exploring Jupiter. The spacecraft obtained breathtaking views of the four major satellites, and studied Jupiters clouds and atmospheric composition, rings, small satellites, and magnetic field. It had five successful close flybys and many distant observations of Io. Scientists already knew from Voyager and Earth-based astronomy that Io is by far the most volcanically active object in the solar system. Galileo has given us stunning color panoramas of Ios surface and unprecedented close views of erupting volcanoes (Figure 1) and the largest active flows observed anywhere. Among recent discoveries about Io, perhaps most astonishing since Voyager, is that some lavas possess emission temperatures greater than any lavas erupted on Earth today and possibly since the start of Earths geologic history. The Io science community has identified three alternative interpretations of Ios hottest lavas: (1) ultramafic material similar to komatiite; (2) superheated lava; or (3) an ultra-refractory substance deficient in silica and rich in Ca-Al oxides.