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       New observations by NASA's Galileo spacecraft reveal dozens of volcanic vents on Jupiter's fiery moon Io where lava sizzles hotter than any surface temperatures recorded on any planetary body in our solar system. Temperatures this high are not known to have occurred on Earth for billions of years. At one such volcanic vent, known as Pillan Patera, two of Galileo's instruments have indicated the lava temperature may be 2,000 Kelvin (3,140 degrees Fahrenheit). These results are reported in the July 3 issue of the journal Science.

       "The most likely explanation for these very high temperatures is that the eruptions contain magnesium-rich silicates," said Dr. Alfred McEwen of the University of Arizona, Tucson, AZ, a member of Galileo's solid state imaging camera team. "We've tentatively identified magnesium-rich orthopyroxene in lava flows around these hot spots. This leads us to conclude that silicate volcanism is taking place with lava compositions expected to melt at a very high temperature. We must now think of Io's volcanoes in terms of the type of very high-temperature silicate volcanism which was found on Earth during its early days, and which we suspect occurred also on Venus and Mars."

       The new findings by the Galileo camera and the spacecraft's near infrared mapping spectrometer have updated scientists' information on Io's volcanic processes. Previously, Io observations made by the Voyager spacecraft in 1979 put the highest temperature estimates at about 650 Kelvin (710 degrees Fahrenheit). This led many scientists to believe that Io's volcanic activity was caused by low-temperature sulfur volcanism. In 1986, ground-based telescope observations increased the temperature estimates to above 900 Kelvin (1,160 degrees Fahrenheit), which suggested that silicate volcanism was occurring at least occasionally, just as it does on Earth today. In 1996 and 1997, Galileo identified 30 locations with temperatures higher than 700 Kelvin (800 degrees Fahrenheit).

       "This new data indicate that high-temperature eruptions on Io are a basic and common part of its active volcanic processes," said Dr. Torrence Johnson of JPL, Galileo project scientist. Johnson led the group that found the high temperature eruption in 1986. He is also a member of the near infrared mapping spectrometer team. "Io's current volcanic activity may have a lot in common with ancient volcanic processes on Earth and other planets. Since the geologic record from those times is very sparse, it's quite exciting to be able to study this type of volcanism going on today."

       "This discovery of high-temperature silicate volcanism provides us with an extremely important clue to understanding the geophysical processes within Io," McEwen explained. Io is heated by periodic tides as it orbits Jupiter, along with the other Galilean satellites (Europa, Ganymede and Callisto).

       Armed with this new information, scientists also hope to learn more about the composition of Io's crust. "Io's extreme volcanic activity is expected to result in a low-density crust rich in silica, sodium and potassium," said McEwen. "However, the high-temperature volcanism suggests that the crust may be composed of heavier lavas."

       Galileo's solid state imaging camera observed Io during 11 eclipses in 5 orbits, when Io was in Jupiter's shadow, and sunlight was blocked so the camera could better see the glowing volcanic vents. Io's hot spots were also studied by the spacecraft's near infrared mapping spectrometer during 11 orbits, mostly when Io was not in eclipse. The camera provides high spatial resolution to image the hottest features and map color variations, while the spectrometer can observe at many wavelengths and is sensitive to a wider temperature range. Thus, the combination of both instruments provides a powerful means to study Io's volcanism. The camera and spectrometer together have discovered a total of 41 hot spots on Io.

       Scientists hope to gather more detailed information about Io with two planned close flybys in late 1999, as long as the Galileo spacecraft remains healthy. Galileo has been orbiting Jupiter and its four largest moons, including Io, for 2-1/2 years. It is currently in the midst of an extended journey, known as the Galileo Europa Mission, with eight flybys of Europa and four of Callisto, in addition to the Io flybys.

       Galileo Europa Mission is managed by JPL, a division of California Institute of Technology, Pasadena, CA.

       Additional information and images taken by the Galileo spacecraft are available on the Internet at http://www.jpl.nasa.gov/galileo . Images are also available at http://photojournal.jpl.nasa.gov .

7/2/98 JP