By Sean Henahan, Access Excellence

TUCSON- Ganymede, the largest of Jupiter's moons, appears to possess a thin oxygen atmosphere. The finding was announced by the same astronomers who previously discovered oxygen in the atmosphere of a smaller Jovian moon, Europa at a meeting of the American Astronomical Society's Division of Planetary Sciences.

PHOTO: Frost on Ganymede- Water-ice frosts are the likely cause for the brightening seen around the circular rims of these craters located at a high northern latitude (57 degrees) on Jupiter's moon Ganymede in this image taken by NASA's Galileo spacecraft on September 6, 1996.

The team of astronomers used the Hubble telescope's Goddard High Resolution Spectrograph to make ultraviolet observations of Ganymede. They observed the spectrum of Ganymede had the characteristic fingerprint that indicates the presence of oxygen gas. But they were puzzled because the spectrograph had detected two peaks, rather than two as expected. They realized that the wo spikes corresponded with light emitted from two regions near Ganymede's north and south poles, possibly akin to the aurora borealis on Earth. .

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

Ganymede and Europa are both at least partially covered with water ice. The scientists believe that the atmospheric oxygen comes from the icy 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 gas, Hall said.

The atmosphere on Ganymede is likely to be as thin as the gas previously detected on Europa, that is, very thin. It is comparable in pressure to Earth's atmosphere at an altitude of several hundred kilometers, roughly as high as the space shuttle orbits.

"I want to emphasize that all of the results that we have seen related to oxygen do not require nor imply the presence of life," Hall said. This is in contrast to the oxygen in Earth's atmosphere, which is generated by biological activity. "In fact, the surfaces of these moons, as far as we can tell, are completely inhospitable to any life form that we can imagine."

In a related presentation, researchers from the California Institute of Technology offered a theory to account for the presence of oxygen and ozone on Ganymede, as well as the mechanism by which their concentrations are maintained.

According to Dr. Yuk Yung , the oxygen and ozone are remnants of the primordial water that became part of Ganymede when the solar system was formed 4.5 billion years ago. Yung, professor of planetary science at Caltech, theorizes that the water ice on Ganymede has since time immemorial been attacked by two sources: ultraviolet light from the sun, and ions thrown off by the volcanic activity of the sister Jovian satellite Io. Both sources of disturbance have the effect of blasting the water molecules apart. Once the hydrogen and oxygen of the water are separated, the lighter, energetic hydrogen ions blast away from the light gravity of Ganymede into outer space, while the heavier oxygen molecules settle back onto the surface.

Yung also offered a theory to account for the relatively high concentration of molecular oxygen and ozone on Ganymede's surface. While each is present at about one-thousandth its concentration on Earth, both are more prevalent than on any other body in the solar system except Mars, which by coincidence has roughly the same concentrations.

"So far, Earth, Mars, and Ganymede are the only bodies in the solar system with ozone," said Yung.

Yung's theory of oxygen and ozone concentrations on Ganymede assumes the presence of a vast structure of tiny surface cracks in the ice where the frozen oxygen and ozone can reside. To test the hypothesis that such cracks indeed exist, Yung's colleague, Ming-Taun Leu of Caltech's Jet Propulsion Laboratory, devised an experiment in which the conditions of Ganymede could be simulated.

The results showed that such cracks could indeed occur in the ice matrix, and that even a very thin surface coating of ice could harbor the high concentrations of oxygen and ozone seen on Ganymede. As for the destruction of the water molecules, Yung explains that the manner in which the molecules are blown apart can account for the ratio of oxygen to ozone, as well as the net amount of oxygen atoms on the surface.

Light energy from the sun, for example, tends to split a molecule of oxygen (O2) into two oxygen atoms, and a molecule of ozone (O3) into a molecule of oxygen and a single oxygen atom. An oxygen ion that finds its way from Io's volcanoes to Ganymede, however, tends to turn a molecule of oxygen it hits into a molecule of ozone. Or, if it hits an existing molecule of ozone, the particle from Io tends to combine with the three atoms to make two molecules of O2. By this scenario, the energy expended and consumed in these reactions accounts for the amount of oxygen as compared to the amount of ozone.

Yung says that his work is very basic in nature and has little to do with the present amount of oxygen on Earth, because our own planet is dependent on living processes for most of its oxygen. Nonetheless, he says that the study of Ganymede can perhaps lead to answers about how oxygen might have arisen on Earth and Mars before life began.

In another presentation, British scientists reported 'two striking finds' about Jupiter: less water vapor than expected on the basis of existing ideas about Jupiter's atmosphere and the absence of a predicted dense water cloud. The amount of water vapor had been predicted from theories of solar system formation, which assign Jupiter the same mix of chemical elements as found in the Sun. However, the issue remains unresolved, as another team suggested that the Galileo probe had dropped into a relatively dry area of atmosphere.