'Hot breath' of Jupiter's moon

Infra-red image (left) of Jupiter's moon Io (right)

By BBC News Online science editor Dr David Whitehouse

The American space agency's Galileo spacecraft has turned up a surprise at Jupiter's moon Io: the tallest volcanic plume ever seen, and it came from a previously unknown volcano.

Close-up observations of Io made by Galileo several months earlier saw a different volcano lofting a giant plume into space - but Galileo saw no sign of the current plume.

The plume rises 500 km high

Adding to the surprise, for the first time a Galileo instrument has caught particles freshly released from an eruption, giving scientists a direct sample of Io material to analyse.

"This was totally unexpected," said Dr Louis Frank of the University of Iowa, US. "We've had wonderful images and other remote sensing of the volcanoes on Io before, but we've never caught the hot breath from one of them until now."

Io is the innermost of Jupiter's four largest moons and the most volcanically active world in the Solar System. Galileo, in orbit about Jupiter since December 1995, has been transmitting to Earth new pictures and data from its flight over Io's north pole in early August.

According to Dr Eilene Theilig of the agency's (Nasa) Jet Propulsion Laboratory in California, "Io just keeps amazing everyone. Now we're eager to see what will be happening there when Galileo flies near Io's south pole in two weeks' time".

'Two great volcanoes'

Galileo scientists had expected that the early August flyby might take the spacecraft right through gases rising from a volcano named Tvashtar, which is situated near Io's north pole.

Tvashtar had been lofting a high plume when last seen seven months earlier by both Galileo and the then passing Cassini spacecraft.

However, the Tvashtar plume has not been found in images from the August flyby. Researchers were startled to find, instead, that a previously unknown volcano just 600 kilometres (370 miles) from Tvashtar was spewing a grand plume as Galileo passed.

"Galileo flew between two great volcanoes," volcanologist Dr Rosaly Lopes said. "The plume we knew about might have settled down before we got there, but this new one sprang up suddenly."

The latest plume appears as a back-lit bulge above Io's surface in two newly released images. A third, new image shows a white ring of material from the plume that has fallen back to the moon's surface, painting a circle around the source of the eruption. A fourth shows another new, large plume deposit near Io's north pole.

"After not seeing any active plumes at all in Io's high-latitude regions during the first five years of Galileo's tour, we've now seen two this year," said Dr Alfred McEwen of the University of Arizona, Tucson.

Close encounter

The new plume rises at least 500 km (more than 300 miles) above the surface, an estimated 10% higher than the tallest ever seen before on Io.

Galileo is on course to fly close to Io again at 0123 GMT on 16 October. This time its trajectory will take it over Io's south pole, which may provide a look at details of another new hot spot near there identified from infra-red mapping data this year.

Galileo will get its sixth and final encounter with Io in January 2002. During its mission, it has also flown 27 close approaches to Jupiter's other three large moons, Europa, Ganymede and Callisto.

Since it began orbiting Jupiter, Galileo has survived more than three times the radiation exposure it was designed to withstand. It is still in good overall health, but performance of some instruments has been down-graded.