Illustration of Galileo at Jupiter

Jupiter's moon Io has pulled a surprise on NASA's Galileo spacecraft, hurling up the tallest volcanic plume ever seen, which arose from a previously unknown volcano.

A different volcano had been lofting a plume seven months earlier, but Galileo saw no sign of that plume during its latest Io flyby in early August.

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 analyze. "This was totally unexpected," said the leader of that experiment, Dr. Louis Frank of the University of Iowa, Iowa City. "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. Galileo smelled the volcano's strong breath and survived."

The Jupiter-orbiting spacecraft has been gradually transmitting to Earth the new pictures and data from its flight over Io's north pole in early August, said Dr. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Io just keeps amazing everyone," she said. "Now we're eager to see what will be happening there when Galileo flies near Io's south pole in two weeks."

Galileo engineers and scientists had anticipated that the Aug. 6 flyby (Aug. 5, Pacific time) might take the spacecraft right through gases rising from a volcano named Tvashtar near Io's north pole. Tvashtar had been lofting a high plume when last seen seven months earlier by both Galileo and the 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 kilometers (370 miles) from Tvashtar was spewing a grand plume as Galileo passed.

"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 Galileo imaging team member Dr. Alfred McEwen of the University of Arizona, Tucson. The latest 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.

The images are available at the University of Arizona at .

The new plume rises at least 500 kilometers (more than 300 miles) above ground, McEwen estimated, nearly 10 percent higher than the tallest ever seen before on Io.

Scientists using Galileo's infrared mapping instrument have pinpointed the site of the eruption as a new hot spot at a location that was not known to be an active volcano, said JPL volcanologist Dr. Rosaly Lopes. It was surprising that the site leapt to such intense activity so abruptly with so little evidence of former volcanism, she said.

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

The particles detected in Galileo's plasma science instrument as the aging spacecraft sped within 194 kilometers (120 miles) of Io's surface likely came from the new hot spot rather than Tvashtar, Frank said. The volcanic material reached the spacecraft no more than a few minutes after rushing out of the source vent on the ground. The particles are apparently snowflakes made of sulfur-dioxide molecules with as many as 15 to 20 molecules clumped together in each flake.

Frank and co-workers will try to wring information from the particle impacts about the temperature and speed of the gas in the plume. "That will get us a step closer to knowing about the newly released material from a volcanic vent," he said.

Galileo is on course to fly about as close to Io again at 0123 Universal Time on Oct. 16 (6:30 p.m. Oct. 15, Pacific time). Its trajectory will take it close to Io's south pole, which may provide a look at details of another new hot spot near there identified from infrared mapping data this year. The polar passes in August and this month were also designed to provide data indicating whether Io generates its own magnetic field, as its sibling moon Ganymede does and Earth does.

Io is the innermost of Jupiter's four largest moons and the most volcanically active world in the solar system. Galileo will get its sixth and final encounter with Io in January 2002. It has also flown 27 close approaches to Jupiter's other three large moons: Europa, Ganymede and Callisto. Since it began orbiting Jupiter in 1995, Galileo has survived more than three times the radiation exposure it was designed to withstand. It is still is good overall health, but performance of some instruments has been degraded.

JPL, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA's Office of Space Science, Washington, D.C. Additional information about the mission is available online at .

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