"We dont see them more often than we do," said mission member John Spencer, of the Lowell Observatory in Flagstaff, Ariz. "It was exciting to see it right during the Galileo flyby."

Previously, similar eruptions had been detected at only very low resolutions from Earth-based telescopes.

In fact, NASAs Infrared Telescope Facility on Mauna Kea in Hawaii imaged the eruption less than a day before Galileo swung by the moon. In those images, the moon appears as a yellowish blob, with a small pimple where Tvashtar is erupting near Ios north pole.

"We didnt believe we had a chance of catching it," said Laszlo Keszthelyi, a member of the imaging team based at the University of Arizona in Tucson.

Galileo scientist Alfred McEwen, also of the University of Arizona, pegged the odds of imaging such an eruption up close at one in 500. Normally, it would take months of preparation to target such a small area.

The first image mosaic of the giant caldera was released Friday at the fall meeting of the American Geophysical Union in San Francisco.

Tvashtars lavas were so hot estimates run from 1,300 to 2,400 degrees Fahrenheit (1,000 to 1,000 Kelvin) that it overwhelmed Galileos camera, causing it to "bleed" electrons, which appear as splotchy white areas on the image.

"These lavas are very hot, probably hotter than any lavas to have erupted on Earth for a few billion years," said Galileo Project Scientist Torrence Johnson, of NASAs Jet Propulsion Laboratory. JPL, in Pasadena, Calif., manages the Galileo mission.

An interpretative drawing of Tvashtars caldera shows a thin area of molten lava perhaps 14 miles (24 kilometers) long.

Scientists propose that the erupting lavas are likely ultra-mafic, or magnesium-rich in nature. No one has seen similar large-scale lava fountain eruptions on Earth since Laki erupted in Iceland in 1783-84, scientists said.

"(Galileo) is giving us a chance to see something that happens only very rarely on Earth," Keszthelyi said.

The Nov. 25 Io flyby concluded Galileos extended mission to Jupiter. Johnson said, however, there was an agreement in principle to extend the mission through 2000, which includes a planned Jupiter flyby in December of that year by the Cassini spacecraft while en route to Saturn.

The one-year extension would cost anywhere from $8 million to $10 million, most of which would be met with surplus from the current extended mission, NASA Spokesman Doug Isbell said.

Galileo is on track to fly by the moon Europa in early January and Io again a month later.

With 100 volcanoes, Io is the most volcanically active body in our solar system. Its tremendous volcanism is caused by its proximity to Jupiter, which causes tidal flexing in the moon.

In fact, the gravitational tug of Jupiter on Io is so great it causes the moons rock to rise and fall more than 300 feet (100 meters) during each of its 42-hour days, McEwen said. The accompanying friction produced by the flexing melts portions of the moon, which scientists said Friday could include a magma ocean.