Mosaic of images collected by NASA's Galileo spacecraft on November 25 shows a fountain of lava spewing above the surface of Jupiter's moon Io. The active lava was hot enough to cause what the camera team describes as "bleeding" in Galileo's camera, caused when the camera's detector is so overloaded by the brightness of the target that electrons spill down across the detector. This shows up as a white blur in the image. Photo: NASA/JPL

During a recent close flyby of Jupiter's moon Io, NASA's Galileo spacecraft observed a fiery lava fountain shooting more than a mile above the moon's surface.

The images, showing a curtain of lava erupting within a giant volcanic crater, was unveiled Friday during the American Geophysical Union's fall meeting in San Francisco. Galileo took the pictures on Thanksgiving night, November 25.

"We've finally caught a close-up of a massive volcanic eruption in action on Io," said Galileo project scientist Dr. Torrence Johnson of NASA's Jet Propulsion Laboratory, Pasadena, CA. "The erupting lava was so hot and bright, it over-exposed part of the camera picture and left a bright blur in the middle."

These lava fountains were hot enough and tall enough to be observed by the NASA Infrared Telescope atop Mauna Kea, HI. By combining data from this telescope and Galileo observations, scientists have their best chance ever to pin down temperatures of the extremely hot lava on Io.

The images show a region of giant calderas, or crater depressions, in Io's northern latitudes. They came from two of Galileo's onboard instruments -- the camera and near-infrared mapping spectrometer, which observes wavelengths invisible to the unaided eye.

Lava fountains provide the most spectacular volcanic show on Earth, although the fountains found in Hawaii and elsewhere on Earth rarely exceed a few hundred yards in height. Because their appearances are infrequent and brief, it is very difficult to target these events. "Catching these fountains was a one-in-500-chance observation," said Galileo scientist Dr. Alfred McEwen from the University of Arizona in Tucson.

New results from the most powerful volcano in the solar system, Loki, were discussed at the press conference. These include recent observations of Io by infrared telescopes in Hawaii and Wyoming, and two other Galileo instruments, the photopolarimeter radiometer and near-infrared mapping spectrometer. These data show large changes in the output of heat at Loki over time, with huge portions of the lava surface appearing to be of a uniform temperature.

The telescope observations show that Loki began a period of major eruption in early September, and Galileo caught the eruption in full force during its October flyby of Io. While observing Loki's 120-mile (193-kilometer) wide caldera, one Galileo instrument found a sharply defined region that was much hotter than the rest.

"We think the hot region is the site of the eruption that began in September," said Dr. John Spencer of Lowell Observatory, Flagstaff, AZ, a co-investigator for the photopolarimeter radiometer, which maps surface temperatures by measuring heat radiation. "Eventually the new lava may spill out to cover the rest of the caldera."

Image taken by Galileo of Io on November 25 shows some of the curious mountains found there. The Sun is illuminating the scene from the left, and because it is setting, the Sun exaggerates the shadows cast by the mountains. By measuring the lengths of these shadows, Galileo scientists can estimate the height of the mountains. The mountain just left of the middle of the picture is 13,000 feet high and the small peak to the lower left is 5,000 feet high. Photo: NASA/JPL

The Io flybys were challenging and risky, because Io lies in an area of intense radiation from Jupiter's radiation belts, and radiation can harm spacecraft components. In fact, radiation-related problems garbled some of the pictures taken by Galileo during its October 10 Io flyby. Galileo team members thought the images were a lost cause, but engineers at JPL's Measurement Technology Center were able to fix them with the help of LabVIEW software from National Instruments in Austin, TX.

"It would be like watching a scrambled cable signal on television, and then using software to unscramble the signal," Johnson said. "JPL engineers had to break the code that was inadvertently introduced by the radiation near Io."

"They only had one-fourth of the data needed to reconstruct the images," said Dr. Laszlo Keszthelyi, a Galileo research associate at the University of Arizona. "These guys found a way to intelligently guess the missing bits. It seemed to be mathematically impossible, but they pulled it off."