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       Detailed analysis of Jupiter's moon Io reveals a colorful, active world full of surprises, according to five reports published in the May 19 issue of Science, and based on new results from NASA's Galileo spacecraft and Hubble Space Telescope.

       The reports describe giant, erupting plumes migrating with lava flows, red and green deposits that change as unstable sulfur compounds condense from huge plumes, and mountains that may split and slide sideways for hundreds of kilometers, or miles.

       Galileo observations of Prometheus reveal a volcanic field similar to Hawaii's volcanoes, but more active and much larger. Prometheus features an 80-kilometer (50-mile) tall plume of gas and particles erupting from near the end of the lava flows, like where Hawaiian flows enter the ocean. This is Io's most consistently active plume. Its size and shape have remained constant since at least 1979, but the plume location wandered about 85 kilometers (53 miles) to the west between 1979 and 1996.

       "The main vent of the volcano didn't move, but the plume did," said Dr. Rosaly Lopes-Gautier of NASA's Jet Propulsion Laboratory, Pasadena, Calif., lead author of one of the reports.

       "This type of behavior has never been seen on Earth," said Dr. Susan Kieffer of Kieffer Science Consulting, Inc., Ontario, Canada, lead author of a Science report. Kieffer and her colleagues suggest that the Prometheus plume is fed when a "snowfield" of sulfur dioxide and/or sulfur vaporizes under the lava flow and material erupts through a rootless conduit in the flow.

       Scientists had speculated that bright red material on Io came from unstable forms of sulfur condensing from sulfur gas. By combining Galileo and Hubble Space Telescope results, scientists have learned more about the role of sulfur in Io's volcanoes. While Galileo carried out the first of three recent Io flybys in October 1999, Hubble scanned Io with its ultraviolet spectrograph to measure the composition of gases escaping from volcanoes. Hubble detected a surprise -- a 350 kilometer (220 mile) high cloud of gaseous sulfur in the plume ejected by the volcano Pele. The sulfur gas is a specific type, with sulfur atoms joined in pairs, that had never before been seen on Io; it is stable only at the very high temperatures found in the throats of Io's volcanoes. When these molecules fall onto Io's frigid surface (about -160 Celsius or -250 Fahrenheit) away from the volcanoes, they probably recombine into larger molecules with three or four sulfur atoms. The latter types of sulfur are red, so the Hubble results explain the 1,200-kilometer (750-mile) wide, red debris ring around Pele.

       "These Hubble findings should help scientists understand the chemistry of Io's interior," said Dr. John Spencer of Lowell Observatory, Flagstaff, Ariz., lead author of two of the Science papers.

       Galileo has found many other smaller, red patches near Io's active volcanoes, where this sulfur conversion process probably also occurs. The red deposits are found near calderas or shield volcanoes where lava first reaches the surface, often distant from plumes like Prometheus where lava flows apparently vaporize surface materials.

       The composition of bright green materials on Io has been puzzling. In some places, it appears that when red material is deposited onto fresh lava flows, especially on caldera floors, it transforms into green material. It is possible that the surfaces are still warm, which accelerates the transformation of the red types of sulfur and the sublimation of sulfur dioxide. Eventually both red and green materials acquire the pale yellow color that is characteristic of ordinary yellow sulfur, made of rings of eight sulfur atoms.

       Although Io is the most volcanically active body in the solar system, the mountains (up to 16 kilometers or 10 miles high) are not volcanoes. They have no volcanic vents or flows; instead, they appear to be giant tilted blocks of crust. Giant depressions on Io are thought to be calderas formed by collapse over empty magma chambers. Unlike Earth's calderas, many Io depressions have very straight margins, sharp corners, and are located next to mountains. In new images of the Hi'iaka Patera depression and adjacent mountains, it looks as though two mountain blocks have split and slid apart by 145 kilometers (90 miles), forming a pull-apart basin like California's Death Valley or Salton Sea. This is surprising because such large-scale lateral movements on Earth are caused by plate tectonics, but there are no indications of a similar process on Io.

       "We consider it more likely that lateral movements may be driven by deep 'mantle plumes' of rising hot rock masses within Io," said Dr. Alfred McEwen of the University of Arizona, Tucson, lead author of one of the papers.

       New images are available at .

       Galileo has been studying Jupiter and its moons for 4-1/2 years. It completed a two-year primary mission in December 1997 and a two-year extended mission in December 1999. Galileo is continuing its studies under yet another extension, the Galileo Millennium Mission. On Sat., May 20, the spacecraft will fly by Jupiter's moon Ganymede, the largest moon in the solar system, for the first time since May 7, 1997. JPL, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.

5-18-2000 JP