Nasa's Cassini spacecraft has "sniffed" molecular oxygen ions around Saturn's icy moon Dione for the first time, confirming the presence of a very tenuous atmosphere. The oxygen ions are quite sparse – one for every 11 cubic centimetres of space or about 90 000 per cubic metre – show that Dione has an extremely thin neutral atmosphere.
At the Dione surface, this atmosphere would only be as dense as Earth's atmosphere 480 kilometres above the surface. The detection of this faint atmosphere, known as an exosphere, is described in a recent issue of the journal Geophysical Research Letters.
"We now know that Dione, in addition to Saturn's rings and the moon Rhea, is a source of oxygen molecules," said Robert Tokar, a Cassini team member based at Los Alamos National Laboratory and the lead author of the paper. "This shows that molecular oxygen is actually common in the Saturn system and reinforces that it can come from a process that doesn't involve life."
Dione's oxygen appears to derive from either solar photons or energetic particles from space bombarding the moon's water ice surface and liberating oxygen molecules, Tokar said. But scientists will be looking for other processes, including geological ones, that could also explain the oxygen.
"Scientists weren't even sure Dione would be big enough to hang on to an exosphere, but this new research shows that Dione is even more interesting than we previously thought," said Amanda Hendrix, Cassini deputy project scientist at Nasa's Jet Propulsion Laboratory who was not directly involved in the study. "Scientists are now digging through Cassini data on Dione to look at this moon in more detail."
Several solid solar system bodies – including Earth, Venus, Mars and Saturn's largest moon Titan – have atmospheres. But they tend to be typically much denser than what has been found around Dione. However, Cassini scientists did detect a thin exosphere around Saturn's moon Rhea in 2010, very similar to Dione. The density of oxygen at the surfaces of Dione and Rhea is around 5 trillion times less dense than that at Earth's surface.
Tokar said scientists suspected molecular oxygen would exist at Dione because Nasa's Hubble Space Telescope detected ozone. But they didn't know for sure until Cassini was able to measure ionised molecular oxygen on its second flyby of Dione on 7 April 2010 with the Cassini plasma spectrometer. On that flyby, the spacecraft flew within about 503 kilometres of the moon's surface.
Cassini scientists are also analysing data from Cassini's ion and neutral mass spectrometer from a very close flyby on 12 December 2011. The ion and neutral mass spectrometer made the detection of Rhea's thin atmosphere, so scientists will be able to compare Cassini data from the two moons and see if there are other molecules in Dione's exosphere.