The Cassini-Huygens mission has discovered a tenuous atmosphere infused with oxygen and carbon dioxide at Saturn’s moon Rhea – the first time a spacecraft has captured direct evidence of an oxygen atmosphere at a world other than Earth.

The NASA-led international mission made the discovery using combined data from Cassini's instruments, which includes a sensor designed and built at the University College London’s (UCL) Mullard Space Science Laboratory.

Results from the mission reveal that the atmosphere of Rhea, Saturn's second-largest moon at 900 miles (1,500 kilometers) wide, is extremely thin and is sustained by high-energy particles bombarding its icy surface and kicking up atoms, molecules, and ions into the atmosphere.

The density of oxygen is probably about 5 trillion times less dense than it is in Earth's atmosphere. However, the formation of oxygen and carbon dioxide could possibly drive complex chemistry on the surfaces of many icy bodies in the universe.

"The new results suggest that active, complex chemistry involving oxygen may be quite common throughout the solar system and even our universe," said Ben Teolis, a Cassini team scientist based at Southwest Research Institute in San Antonio, Texas. "Such chemistry could be a prerequisite 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 discovery of this tenuous atmosphere provides key information on how radiation can drive chemistry on icy surfaces throughout the universe,” said Geraint Jones from UCL’s Mullard Space Science Lab.

Rhea's tenuous atmosphere makes it unique in the Saturn system. Titan has a thick nitrogen-methane atmosphere, with very little carbon dioxide and oxygen.

UCL's Mullard Space Science Laboratory led the design and building of the electron spectrometer of the Cassini plasma spectrometer (CAPS), which detected negative ions streaming off Rhea's surface in 2005. Another part of CAPS detected positive ions on the opposite side of Rhea in 2005 and 2007. Completing the picture of Rhea's atmosphere, Cassini's ion and neutral mass spectrometer detected neutral particles when Cassini swept within 60 miles (100 km) of the moon's surface in March 2010.

"Our instrument turns out to be a fabulous detector of negative ions as well as electrons,” said Andrew Coates, also from UCL’s Mullard Space Science Laboratory. “We've already found negative ions are important at Titan and Enceladus. And now, tracing back the trajectory of these ions really pinpoints the source of the atmosphere near Rhea's surface."

The ion and neutral mass spectrometer detected peak densities of oxygen of around 1 billion molecules per cubic foot (50 billion molecules per cubic meter). It detected peak densities of carbon dioxide around 600 million molecules per cubic foot (20 billion molecules per cubic meter). The plasma spectrometer also saw clear signatures of flowing streams of positive and negative ions, with masses that corresponded to ions of oxygen and carbon dioxide.

"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 Coates.

The carbon dioxide may be the result of "dry ice" trapped from the primordial solar nebula, similar to the case of comets, or it may be due to similar irradiation processes operating on the organic molecules trapped in the water ice of Rhea. The carbon dioxide could also come from carbon-rich materials deposited by tiny meteors that bombarded Rhea's surface.

The finding is consistent with earlier Cassini results that show Rhea to be a particularly dark-looking moon, sporting some carbon-based coating on its surface.