Charles Barth, a senior researcher at the Laboratory for Atmospheric and Space Physics and a member of the CU science team that designed and built the ultraviolet spectrometer flying on Galileo, said hydrogen atoms escaping from Callisto implies the Mercury-sized moon has oxygen locked up in its ice and rocks. In 1996 the CU Galileo team detected evidence of oxygen on the surface of Callisto's neighboring moon, Ganymede.

On Ganymede, the UV spectrometer data from Galileo indicated hydrogen atoms were being knocked off the icy surface by charged particles emanating from Jupiter's plasma torus, a massive, doughnut-shaped ring surrounding the planet, said Barth. Because hydrogen atoms are lighter than oxygen atoms, the hydrogen floated out of the atmosphere and into space, leaving the oxygen behind.

But on Callisto, the furthest of the four large moons from Jupiter, it appears that sunlight striking its rock-hard ice is the primary mechanism for separating the hydrogen and oxygen atoms, he said.

Callisto, roughly 3,000 miles in diameter, is the most heavily cratered moon in the solar system. Callisto is the third largest moon in the solar system behind Ganymede and Titan, the dominant moon of Saturn.

"Because it is further away from Jupiter, Callisto does not interact as strongly as Ganymede with the charged particles in the planet's atmosphere," said Barth. "Instead, we believe it is the ultraviolet solar radiation that is knocking the hydrogen atoms out of the ice on Callisto."

The surface of Ganymede is thought to contain about 50 percent ice, while the ice on the surface of Callisto is thought to comprise less than 20 percent of the planet's surface, said Barth, a professor in the astrophysical and planetary sciences department and former director of LASP.

The CU team, which also has been monitoring sulfur dioxide emissions from Io during recent Galileo fly-bys, discovered that the gases are the result of both active volcanoes and the sublimination of frost on Io's surface. The analysis of the Io data, led by LASP Research Associate Amanda Hendrix, indicates the volcanic activity on Io is extremely variable.

The researchers, who made 10 observations of Io volcanoes with the UV spectrometer, found that the thickness of Io's sulfur dioxide atmosphere varied with both time and location over the past year, said Hendrix. Any water present on Io probably disappeared billions of years ago when the volcanic activity commenced.

Launched in 1989 aboard the space shuttle Atlantis, the Galileo spacecraft arrived at Jupiter orbit in December 1995. The Galileo spacecraft is managed for NASA by the Jet Propulsion Laboratory of the California Institute of Technology.

Barth and Hendrix presented the CU-Boulder Galileo results at the fall meeting of the American Geophysical Union in San Francisco Dec. 8 to Dec. 12.

Other LASP researchers on the Galileo UV spectrometer science team led by principal investigator Charles Hord include Ian Stewart, Wayne Pryor, Bill McClintock and Karen Simmons. The team also includes scientists from JPL and the University of Arizona.

Data is being sent from the spacecraft to Deep Space Network antennas located in Goldstone, Calif., Madrid, Spain, and Canberra, Australia. Information from the CU spectrometers is sent on to JPL, then forwarded over data lines to LASP's Space Technology Building in the CU Research Park. The incoming information will be analyzed by faculty and students.

While the original mission goals of Galileo have been accomplished, the instruments will continue to gather data, said Barth. Two primary targets in the next two years will be the continuing search for a possible ocean under the icy surface of the moon, Europa, and additional observations of Io's violent volcanoes.

Note: If no author is given, the source is cited instead.

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• Dysnomia (moon of Eris)
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