Hydrogen Escaping From Ganymede Hints Oxygen Lurking At Surface

December 16, 1996

Atomic hydrogen found escaping from Ganymede implies Jupiter's largest moon has large amounts of oxygen hovering over or locked up in its icy surface, according to University of Colorado observations with NASA's Galileo spacecraft.

Senior Researcher Charles Barth of CU-Boulder's Laboratory for Atmospheric and Space Physics said observations made with a CU ultraviolet spectrometer on Galileo indicate UV radiation is breaking down Ganymede's rock-hard ice into atomic hydrogen and atomic oxygen. Because atomic hydrogen is the lightest atom and Ganymede has a weak gravitational field, the hydrogen escapes while the atomic oxygen stays behind, said Barth.

Barth speculated the atomic oxygen forms molecular oxygen and ozone which may become trapped in the ice or form a very thin atmosphere above the surface of Ganymede. "If this process has been occurring for the past four billion years since Ganymede was formed, then the moon should have as much oxygen on its icy surface as Earth has in its atmosphere," he said.

Galileo's ultraviolet spectrometer was designed and built at CU-Boulder under the direction of LASP Senior Researcher Charles Hord. Barth, a CU science team member on the spectrograph, presented the new results at the fall meeting of the American Geophysical Union in San Francisco Dec. 14 to Dec. 19.

With a diameter of 3,269 miles, Ganymede is the largest moon in the solar system and is about three-quarters the size of Mars. The moon, which has craters, basins and mountains, is believed to be made of rock and water ice. The surface temperatures of Ganymede hover at a chilly minus 186 degrees F.

The CU-Boulder spectrometer was pointed at Ganymede during a June 27 flyby, passing within 519 miles of the moon. The approach was 70 times closer than the approach of the Voyager 2 spacecraft to Ganymede in 1979.

The spectrometer data from the flyby was stored on a tape recorder onboard the spacecraft for about six weeks before being transmitted to Earth for analysis.

While previous observations with the Hubble Space Telescope have indicated there is oxygen at Ganymede, the direct observations of escaping hydrogen by the CU-Boulder team provide a mechanism for the presence of oxygen, Barth said.

A CU-Boulder team discovered atomic hydrogen escaping from Mars and ozone at its surface in 1969 using NASA's Mariner 6 spacecraft, a finding essentially identical to the new Ganymede discoveries, Barth said.

"Mars is an analog to Earth and Ganymede is an analog to Mars," Barth said. "Although the conditions on Ganymede are extremely cold, it would be interesting to see what processes might take place if there was a way to haul Ganymede into the orbit of Mars."

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.

Planetary scientists view the Jupiter system as an "outdoor laboratory" to study the evolution of Earth, said Barth. "Studying the formation of an atmosphere on Ganymede may help us understand how the early atmosphere of Earth evolved."

Other LASP researchers on the Galileo spectrometer science team 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.

-30-

University of Colorado at Boulder

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