The Galileo spacecraft, currently on an extended mission to explore Jupiter and its many satellites, has returned stunning images of the moons and added volumes to scientific understanding.

But long before Galileo, Voyager 1 provided the first clues that Io, one of Jupiter's four largest moons, was a fiery place.

"Galileo returned far better photographs of higher quality than the flyby Voyagers," Van der Woude admits. "But the first discovery was an eye-opener."

Voyager 1 observed nine volcanic eruptions on Io in 1979. Back then, scientists viewed it as a major discovery that such a small moon could support such tremendous volcanic activity (Io has been accused of looking like a pizza). A few months after the initial discovery, Voyager 2 flew by Io and showed that some of the volcanoes were still erupting, adding to the perplexity.

This Voyager 1 image, taken in March 1979 from about 304,000 miles away (490,000 km), shows a volcano tossing material 100 miles into Io's atmosphere. The brightness of the plume has been computer enhanced by NASA, but the relative color of the plume was preserved.