For one thing, the 932-mile-wide (1,500-kilometer-wide), ice-covered moon is more than 932 million miles (1.5 billion kilometers) from Earth. For another, the average surface temperature is -292 degrees Fahrenheit (-180 degrees Celsius).
And at less than 62 miles (100 kilometers) thick, the newfound oxygen layer is so thin that, at Earthlike temperatures and pressure, Rhea's entire atmosphere would fit in a single midsize building.
Still, the discovery implies that worlds with oxygen-filled air may not be so unusual in the cosmos. (Related: "Potentially Habitable Planets Are Common, Study Says.")
At about 327,000 miles (527,000 kilometers) from Saturn, Rhea orbits inside the planet's magnetic field. Rhea's oxygen atmosphere is believed to be maintained by the ongoing chemical breakdown of water ice on the moon's surface, driven by radiation from Saturn's magnetosphere.
The Hubble Space Telescope and NASA's Galileo probe found in 1995 that a similar process creates tenuous oxygen atmospheres on Jupiter's ice moons Europa and Ganymede.
"The major implication of this finding at Rhea is that oxygen atmospheres at icy moons, until now only detected at Europa and Ganymede, may in fact be commonplace around those irradiated icy moons throughout the universe with sufficient mass to hold an atmosphere," said study leader Ben Teolis of the Southwest Research Institute in San Antonio, Texas.
Knowing where and how oxygen exists in the universe may in turn help scientists plan future robotic and manned missions.
NASA's Cassini spacecraft, which has been orbiting the Saturnian system since 2004, detected the oxygen atmosphere around Rhea during a close flyby of the icy moon in March.
Cassini's data show that molecular oxygen (O2) forms inside the moon's surface ice when water molecules (H2O) are split by energetic ions, a process known as radiolysis. The oxygen then gets ejected from the surface ice and captured by Rhea's gravity to form the atmosphere.
(Related: "Pluto Has 'Upside Down' Atmosphere.")
"A loose analogy might be carbon dioxide dissolved, or trapped, in a carbonated beverage, except here we are not talking about liquid water but rather frozen ice at extremely low temperatures," Teolis said.
The amount of oxygen gas produced per second across Rhea's surface weighs about 0.3 pound (130 grams), the study team reports in this week's issue of the journal Science. At room temperature and Earth's atmospheric pressure, Teolis estimates, this amount of oxygen would occupy about 3.5 cubic feet (0.1 cubic meter).
That means Rhea's entire atmosphere, under Earthly conditions, would fill a cube about 70 feet (22 meters) long on each side.
Oxygen and Carbon: Signs of Underground Life?
Cassini also identified the distinctive chemical fingerprint of carbon dioxide in Rhea's atmosphere, indicating the presence of carbon on the moon's surface.
The combination of carbon and oxygen holds implications for finding possible life on ice moons, such as Europa, thought to harbor subsurface liquid oceans. (Related: "Oxygen-Free Animals Discovered—A First.")
"You would expect a very small amount of gas [around an ice moon], but the fact that there is enough to be measurable is what is surprising and indicates that the energetic processes that must be occurring are more widespread than previously thought," said Robert Carlson, a researcher at NASA's Jet Propulsion Laboratory in Pasadena, California.
"Specifically this indicates that the surface is oxidizing, meaning that you can make alcohols and organic acids if carbon is in the surface materials," said Carlson, who wasn't involved in the study.
"There are also ideas that the oxidants, like O2, could be subducting into an ocean below the surface and could be an energy source for any life in these possible oceans."
(See "Could Jupiter Moon Harbor Fish-Size Life?")
Ice Moons May Be Future Reservoirs
The new study may have scientists looking with fresh eyes at Rhea-like moons around other gas giant planets.
"This shows how prevalent high-energy chemistry is on icy satellites, not constrained to just Europa," Carlson said. "I expect there will be more Cassini flyby measurements and that many telescopes will be pointed to the icy satellites now, along with a lot of supporting laboratory work."
In the short term, it's possible the discovery on Rhea will help inform plans for robotic probes, such as a proposed mission to Europa currently being considered by NASA. (Find out more about a potential mission to Europa on the National Geographic Channel.)
"The discovery of Rhea's atmosphere is extremely fortuitous, as it will allow us to anticipate what we might expect to find at Jupiter's moons and design the spacecraft instruments accordingly," study leader Teolis said.
And in the longer term, Teolis added, frozen reservoirs of oxygen on moons such as Rhea may one day become pivotal for deep-space exploration involving human missions.
"In some very distant—and highly speculative—future," he said, "one can imagine that the ices on these moons might be heated or melted to extract oxygen and carbon dioxide, both of which are necessities for the survival of plant and animal life."