The findings seem to support the idea that moons of Jupiter and Saturn could hold extraterrestrial life.
When NASA's Cassini spacecraft buzzed Dione on April 7, 2010, the probe "sniffed" oxygen around the moon—but you couldn't want to do the same. For one thing, oxygen is so thin at the surface that conditions roughly match what you'd encounter 300 miles (483 kilometers) above Earth, researchers say.
Previously, NASA scientists had detected oxygen around Dione via "fingerprints" the moon had left behind as it traversed Saturn's magnetic field. The new study, though, is the first direct observation of oxygen around Dione.
On Earth atmospheric oxygen is largely a byproduct of photosynthesis by, for example, bacteria, plankton, and trees.
But around barren Dione the gossamer oxygen atmosphere is most likely a product of space particles and surface ice, the study says.
As light particles from the sun or energetic particles from Saturn's intense magnetic field hit the moon, they apparently free oxygen and hydrogen molecules within the ice.
"Saturn has a pretty intense magnetic field with a lot of ionized stuff spinning around really fast," said study co-author Robert Tokar.
When those charged particles slam "into the surface of the moon, it sputters things off the ice and produces things like water vapor and oxygen," said Tokar, a space scientist at Los Alamos National Laboratory in New Mexico.
(Related: "Pluto Has 'Upside Down' Atmosphere.")
Dione Not Alone, and We May Not Be Either
This oxygen-making process could be a common one in Saturn's neighborhood, and therefore might up the ante for extraterrestrial life in the solar system—and beyond.
(Related: "Potentially Habitable Planets Are Common, Study Says.")
Cassini data analyzed in 2010, for example, found oxygen around Saturn's moon Rhea, also probably due to the ice-breakdown process. The process is also thought to be occurring around Saturn's icy main rings, which are bombarded by solar light particles, and at Jupiter moons, including Ganymede, Callisto, and Europa, Tokar said.
Dione itself isn't a candidate for extraterrestrial life. With an average temperature of -121 Fahrenheit (-186 Celsius), the moon isn't known to have any liquid water, a prerequisite for life as we know it.
But the more scientists learn about this oxygen-producing process, the better they'll be able to focus the search for alien life elsewhere.
"If you talk about someplace like Jupiter's Europa—where oxygen produced this way could possibly come into contact with a subsurface ocean, where there was carbon—then you could have microbial life," Tokar explained. (See "Could Jupiter Moon Harbor Fish-Size Life?")
The oxygen around Europa wouldn't have been produced by life-forms, he said, "but it certainly could be used for biological processes." In most animals, for example, oxygen is key to turning nutrients into energy.
"And this process that we've identified is going on probably not only in our solar system but in extrasolar planetary systems as well," Tokar said.
"Anywhere there is ice and light or charged particles bombarding that ice, you can create oxygen by this process."
The Dione-oxygen research was published February 9 in the journal Geophysical Research Letters.