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Farewell look at Jupiter's burning moon

Tohil Mons
Io's Tohil Mons volcano rises up 19,700 feet (6,000 meters).  

By Richard Stenger

(CNN) -- The Galileo spacecraft took the closest pictures yet of Jupiter's moon Io, a final photo shoot that captures molten lakes and crumbling cliffs on the most volcanic body in the solar system.

The ailing probe, preparing for a fatal flight into Jupiter, took the photos when skimming to within about 121 miles (181 kilometers) of Io's south pole in October.

After months of study, NASA scientists released the images in late May. The portraits, which include infrared and optical pictures, reveal 13 previously unknown volcanoes, according to project researchers.

"Io is a weird place. We've known that even before Voyager. And each time Galileo has given us a close look, we get more surprises," said Torrence Johnson, a Galileo scientist with NASA's Jet Propulsion Laboratory in Pasadena, California.

The Galileo finds raise the total of identified volcanoes on Io to 120, most of them spotted by the bus-sized craft, which has swung by Io six times during its nearly seven-year residency in the Jupiter system.

The first direct visual evidence of Io volcanism came from photos taken by the Voyager spacecraft in 1979 and 1980, but astronomers had little idea how active the Jovian moon really was.

Birdseye view of Io's lava lakes, mountain peaks 
Interactive guide to volcanoes on Io 

Io was originally thought to have about a dozen lava pits, Johnson said. Since then, astronomers have been surprised not only by the number but diversity of the volcanoes.

"The volcanoes on Io have displayed an assortment of eruption styles, but recent observations have surprised us with the frequency of both giant plumes and crusted-over lakes of molten lava," said Alfred McEwen, a planetary scientist at the University of Arizona in Tucson.

The eerie landscape displays evidence of Io's twisted tectonics. Molten cauldrons have blasted plumes of sulfur hundreds of miles high, leaving trails of debris on the surface extending hundreds of miles.

Most of Io's volcanoes are in flat regions, but a few rare specimens resemble crater-topped volcanic peaks like those on Earth, including one jutting up almost 20,000 feet (6,000 meters).

Galileo made its closest pass in January, approaching to within 62 miles (100 kilometers) of Io, the innermost of Jupiter's four major moons.

But passing so close to the dangerous radiation belts of Jupiter, Galileo went into safe mode and could no longer take pictures.

The probe has show increasing signs of wear and tear in the radiation-soaked environment near Jupiter. Its camera remains shuttered and its fuel is nearly exhausted. Last month, its onboard data storage device went on the blink.

The mottled surface of Io, seen on an earlier Galileo flyby
The mottled surface of Io, seen on an earlier Galileo flyby  

Nevertheless, the probe has proved remarkably resilient. It has other working instruments that detect dust, study ultraviolet emissions and take magnetic readings, helping scientists understand the turbulent interactions between Jupiter and its moons.

And in early June, mission scientists coaxed Galileo's tape player back into service.

In November, the $1.4 billion probe will make its last pass over a Jovian satellite. Its flyby over the tiny moon Amalthea will help position it for one final mission.

In September 2003, Galileo will plunge into the crushing atmosphere of Jupiter. The death dive is to ensure the probe does not strike and contaminate the moon Europa, which scientists speculate could harbor microbial life.


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