When the Galileo spacecraft streaked by Jupiter's moon Io last October, it measured the composition of the charged-particle ring that circles the satellite. The results privately puzzled Galileo scientists because they defied expectations. Just when they thought they had an understanding of Io's atmosphere all worked out, Galileo picked up a strong signature of a gas that was thought to be only a minor component in Io's environment.

The finding is surprising scientists because observations of Io's surface, and the volcanic plumes spewing from its fiery mountains have showed a world where sulfur dioxide -- not monoxide -- is the dominant molecule. In addition, when Galileo swept by Io in 1995, plasma detectors showed a torus strong in sulfur dioxide.

But during the October 11 flyby, "We got close to Io and we picked up what we thought would be a minor constituent in the atmosphere as a major constituent," Russell said. "It came early and it came more intensely than on the previous pass by."

This finding, which has been confirmed by other experiments that directly measure the abundance of ions, is important because it may lead scientists to reassess what they know of Io.

"Whenever you find something that you don't expect. That's always an important thing to underline because that indicates that the model that you were working with has something that is not right in it," Russell said. "So that is suggesting to us that maybe this model, where we expect that the main atmosphere of Io is coming from this particular gas, or that the volcanoes are principally giving out [sulfur dioxide], may be wrong."

But not all scientists who work on Io are so quick to turn the accepted picture of the jovian moon on its head. Louis Frank, a professor of physics at the University of Iowa and leader of Galileo's Plasma particle investigation team, said it is still too early to say what the significance is of the abundant sulfur monoxide.

"The first pass through, the geometry of going through there, was a lot different than it was for this October 11 pass," Frank said. "So that -- coupled with the fact that there has been recent volcanic activity -- it may be temporally variable. But I wouldn't pretend to answer a question like that until we look at one or two more passes past Io."

Russell acknowledges that real conclusions must wait for further observations. There are many possible causes for the changes observed between 1995 and 1999, he said.

The change could be related to volcanic eruptions or it may be tied to Io's position in Jupiter's magnetic field, Russell said. Jupiter has the strongest magnetic field of all the planets in the solar system. Extremely energetic charged particles stream through this field, bombarding the satellites that move through it. Depending on Io's position in this field, conditions could change dramatically.

Unlike Earth, which has a magnetic field that is roughly aligned with its axis, Jupiter's magnetic field is tilted at an angle with respect to its rotational axis. The result is that Jupiter's magnetic field acts like that of a bar magnet stuck haphazardly through a spinning baseball. If the ball is spinning on a vertical axis, but the magnet is tipped at an angle, the magnetic field will wobble as the ball neatly spins.

A change in the composition of the torus could be something that happens cyclically every time Io orbits Jupiter, Russell suggested, or during every rotation of the planet's magnetosphere. It will take more observation to figure out this puzzle, he said.

Whatever more passes reveal, the continuing observations are helping scientists develop an understanding of the chemistry that unfolds on Io, said Nick Schneider, a planetary scientist at the University of Colorado at Boulder. Schneider and a post-doctoral researcher, Michael Küppers recently detected chlorine in Io's atmosphere, using a ground-based telescope.

"This is one of my favorite areas of research," Schneider said. "You start out with volcanoes shooting out sulfur dioxide and you end up with these split-apart ions of sulfur and oxygen. And we've all been trying to figure out what steps are most important along the way."

One possibility is that the charged particles in Jupiter's magnetic field slam into the sulfur dioxide molecules and rip them apart, ionizing some molecules, which are then accelerated to high speeds by the moon's own magnetic field.

Alternatively, sulfur dioxide might just slowly float away from Io to form a large cloud that orbits around Jupiter in the moon's path. Over the long term, charged particles in Jupiter's magnetic field could break down these molecules and leave their parts positively or negatively charged. They could then be grabbed up by Io's torus.

Further observations will certainly help understand these processes. "I think when we get it all sorted out, and understand this process of the charge exchange and the acceleration of particles...we will probably have a nice consistent picture both of the glows and of the production of the Io torus," he said.