The Galileo spacecraft has imaged the tallest volcanic plume ever seen, debris hurled into space by a previously unknown volcano on Jupiter's moon Io, NASA researchers announced Thursday.
The spacecraft also captured particles from the eruption, the freshest volcano particles Galileo has obtained.
"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 encounter occurred during an Aug. 6 flyby of Io and the images and data have been downloaded gradually since then.
Io is the innermost of Jupiter's four largest moons and the most volcanically active object in the solar system. Researchers had expected to gather data during the flyby from a different volcano, one known as 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.
But the Tvashtar plume has not been found the images. Researchers were startled to find, instead, that a previously unknown volcano just 370 miles (600 kilometers) 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 Alfred McEwen of the University of Arizona.
The plume appears as a backlit 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 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.
The particles were detected in Galileo's plasma science instrument as the aging spacecraft sped within 194 kilometers (120 miles) of Io's surface. The volcanic material reached the spacecraft no more than a few minutes after rushing out of the source vent on the ground, researchers believe.
The particles are apparently snowflakes made of sulfur-dioxide molecules with as many as 15 to 20 molecules clumped together in each flake. Scientists will study the particle impacts to learn about the temperature and speed of the gas in the plume.
Galileo is due to fly near Io's south pole on Oct. 16.
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