The Cassini-Huygens mission has identified an oxygen-carbon dioxide atmosphere around Saturn's second-biggest moon, Rhea - the first time a spacecraft has captured an oxygen atmosphere from a world other than Earth.
The atmosphere is unbreathable, to say the least, with an oxygen density around five trillion times less than Earth’s. But there's potentially enough, says the Cassini team, to drive complex chemistry.
"The new results suggest that active, complex chemistry involving oxygen may be quite common throughout the solar system and even our universe," said Dr Ben Teolis, a Cassini team scientist based at Southwest Research Institute in San Antonio.
"Such chemistry could be a pre-requisite for life. All evidence from Cassini indicates Rhea is too cold and devoid of the liquid water necessary for life as we know it."
The ion and neutral mass spectrometer 'tasted' peak densities of oxygen of around 50 billion molecules per cubic meter. It also found peak densities of carbon dioxide of around 20 billion molecules per cubic meter, as well as clear signatures of flowing streams of positive and negative ions, with masses that corresponded to ions of oxygen and carbon dioxide.
The team believes the atmosphere is sustained by high energy particles bombarding its icy surface and kicking up atoms, molecules and ions into the atmosphere.
"Rhea's oxygen appears to come from water ice on Rhea's surface when Saturn's magnetic field rotates over the moon and showers it with energetic particles trapped in the magnetic field," said Professor Andrew Coates from University College London.