Saturn's second-largest moon, Rhea, has a thin atmosphere of oxygen and carbon dioxide, according to a new study.
The finding provides new insights into the chemical processes that occurred in the solar system, including the Earth, 3.5 billion years ago.
Oxygen has been detected remotely in the atmospheres of moons such as Europa and Ganymede, but this is the first time it has been found "in situ" and near the ringed planet.
Earlier this year, the Cassini spacecraft used a spectrometer to "sniff" the atmosphere as it flew within 97 kilometres of the north pole of Rhea.
Dr. Ben Teolis of the Southwest Research Institute in Texas and colleagues report on the findings Friday in the journal Science.
The researchers believe the oxygen is released by "irradiation from Saturn's magnetospheric plasma" and a large fraction of the oxygen is still locked inside the moon's ice.
But the source of the carbon dioxide remains a mystery.
"Atmospheric CO2 might result from sputtering of primordial CO2 in Rhea's ice, or radiolysis reactions between surface water molecules, radiolytic [splitting of] oxygen and carbonaceous minerals or organics possibly present in the surface ice, and/or deposited by micrometeorite bombardment," they write.
Despite the presence of oxygen in Rhea's atmosphere, it isn't enough for humans. One cubic metre of Rhea's atmosphere contains approximately 50 billion molecules of oxygen — 0.00000001 per cent that found on Earth.
Insights into early Earth
Dr. Marc Norman, a planetary scientist at the Australian National University's Planetary Science Institute in Canberra, says the Cassini result is "a new and very interesting finding."
"It implies that the detection of oxygen is not necessarily an indicator of life," says Norman. "We may have to be a little bit careful using it as definitive proof for alien life."
He says the finding could also provide insights into how oxygen first formed on Earth.
According to Norman, there is strong evidence to support the notion that oxygen existed in the Earth's atmosphere 2.5 billion years ago known as the "great oxygenation event."
"But there have been some intriguing indications, from rock cores drilled from the Pilbara region of Western Australia, that there was more oxygen in the environment much earlier, basically from the start of the rock record 3.5 billion years ago," he says.
But Norman says there is "robust debate" over whether photosynthetic organisms were entirely responsible.
He says one possibility, supported by the Rhea finding, is the Sun's radiation released oxygen from the Earth's ice-covered surface.
"The Sun may have caused some of these locally enriched areas of oxygen that have been detected in early cores," he says. "[But that] is a question that needs to be looked at."