Jovian moon has thin oxygen atmosphere.

Astronomers using the Hubble Space Telescope have found evidence of a thin oxygen atmosphere on Jupiter's moon Ganymede, the largest of the Jovian satellites. The same team of scientists previously had found a tenuous veil of oxygen around another Jupiter moon, Europa. The observations also suggest that Ganymede, like Earth and Jupiter itself, has polar aurorae -- light displays created when charged particles collide with atmospheric gases.

A team of astronomers used the Hubble Telescope's Goddard High Resolution Spectrograph to make ultraviolet observations of Ganymede. The astronomers were excited when they saw the spectrum had the characteristic fingerprint that indicates the presence of oxygen gas, but were puzzled because the spectrograph had detected two peaks where they were expecting to see at most a single spike. They soon realized that the two spikes could be explained by the existence of light emitted from two regions near Ganymede's north and south poles.

"The bright spikes correspond nicely to the poles of Ganymede," explains astronomer Doyle Hall, Johns Hopkins University, Baltimore, Md., who led the team making the oxygen discovery. He calls the data "very tentative evidence for the existence of polar aurorae."

Ganymede and Europa at least partially are covered with water-ice. The scientists believe that the atmospheric oxygen comes from the frigid surfaces, where oxygen atoms are split off from water molecules that are bombarded by charged particles. Exposure to sunlight and meteor impacts also could create some of the...