Observations taken by NASA's Galileo spacecraft five months apart reveal a new dark spot the size of Arizona on Jupiter's moon Io, indicating that dramatic volcanic activity occurred during that time.

"This is the largest surface change on Io observed by Galileo during its entire two-year tour of the Jovian system," said Galileo imaging team member Dr. Alfred McEwen, a research scientist at the University of Arizona in Tucson.

The visible change took place during the five months between Galileo's seventh and tenth orbits of Jupiter. The change is manifested as a dark spot about 249 miles in diameter, surrounding a volcanic center named Pillan Patera, which is named after the South American god of thunder, fire and volcanoes. Dark features at the center of the deposits may be new lava flows.

These changes appear in images taken by the Solid State Imaging system aboard Galileo, with marked differences between the pictures taken on April 4, 1997 and September 19, 1997. In June of 1997 an active plume was observed over Pillan by Galileo and the Hubble Space Telescope with a height of 75 miles, and both Galileo and ground-based astronomers observed an intense hot spot.

"Most of the volcanic plume deposits on Io show up as white, yellow or red due to sulfur compounds. However, this new deposit is gray, which tells us it has a different composition, possibly richer in silicates than the other regions," McEwen explained. "While scientists knew that silicate volcanism existed on Io from high temperatures, this may provide clues as to the composition of the silicates, which in turn tells us about Io's evolution."

"Io is probably primarily composed of silicates, which is the type of volcanic rock found on Earth, " McEwen added, "but the extreme volcanism of Io may have led to the creation of silicate compositions that are unusual on Earth."

The Io images showing the changes in Pillan Patera also reveal alterations in the plume deposit of Pele, the large red oval southwest of Pillan, which may indicate that both plumes were active at the same time and interacted with one another. A dark region southwest of Pele, which appears similar to the Pillan deposits, has been present since the Voyager flybys in 1979.

Io is the most volcanically active body in the Solar System. Scientists hope to learn more about the fiery satellite when Galileo continues its studies over the next two years, during a mission extension known as the Galileo Europa Mission. The extended mission will include eight additional encounters of Europa, four of Callisto, and two close Io flybys in late 1999, depending on spacecraft health. Galileo will pass very close to Pillan Patera in the first of the two Io flybys, so high- resolution images can be acquired over a small portion of this area.

Galileo was launched in 1989 and entered orbit around Jupiter on Dec. 7, 1995. The final satellite encounter of its two-year primary mission will occur on Thursday, Nov. 6, 1997 at 3:32 p.m. EST, when the spacecraft swoops over Europa at an altitude of 1,269 miles.

"The Galileo Orbiter is performing flawlessly and all 11 of its sophisticated science instruments and the radio science investigations are still providing excellent data," said Galileo Project Manager Bill O'Neil of NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. "A great bounty of Jupiter system science has been obtained and the continuing study of these data will surely add many important discoveries. While not all of the original objectives could be met due to the antenna failure, I believe that the overall science return from Galileo will easily exceed what was envisioned at project inception 20 years ago, because our team of scientists and engineers has done such a superb job of capturing the most important observations."

The Galileo mission is managed by JPL for NASA's Office of Space Science, Washington, DC. JPL is an operating division of California Institute of Technology, Pasadena, CA.

Images of Io and other data received from Galileo are posted on the Galileo home page on the World Wide Web at URL:

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