NASA’s Cassini spacecraft has discovered molecular oxygen ions around Saturn’s icy moon Dione for the first time, an indication that Dione has a thin, netural atmosphere, or an exosphere, the space agency confirmed.
NASA said the oxygen ions are quite sparse, however, with just one for every 0.67 cubic inches of space, or about 2,550 per cubic foot.
And, at the Dione surface, this atmosphere would only be as dense as planet Earth’s atmosphere at 480km above the surface, said NASA, which is also terming this faint atmosphere an exosphere.
The journal Geophysical Research Letters recently covered the Dione discovery.
"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 who is based at Los Alamos National Laboratory in New Mexico.
"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," said Tokar, who was the lead author on the paper.
O2 derived from either solar photons or energetic particles – NASA
He said that Dione’s oxygen appears to derive from either solar photons or energetic particles from space bombarding the moon’s water ice surface and freeing oxygen molecules.
However, Tokar said scientists would now be looking for other processes, including geological ones, which could also explain the oxygen.
"Scientists are now digging through Cassini data on Dione to look at this moon in more detail," confirmed Amanda Hendrix, Cassini deputy project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. Hendrix was not directly involved in the study.
Solar system bodies, including planet Earth, Venus, Mars and Saturn’s largest moon Titan, have atmospheres. NASA said such atmospheres on these planets tend to be typically much denser than what has been found around Dione.
Back in 2010, however, Cassini scientists also detected a thin exosphere around Saturn’s moon Rhea, which they have confirmed as being similar to Dione.
NASA said the density of oxygen at the surfaces of Dione and Rhea is around 5trn times less dense than that at the Earth’s surface.
According to Tokar, scientists had suspected that molecular oxygen would exist at Dione because NASA’s Hubble Space Telescope detected ozone.
But, he said, they didn’t know for sure until Cassini was in a position to measure ionised molecular oxygen on its second fly-by of Dione on 7 April 2010, using the Cassini plasma spectrometer.
On that particular fly-by, Toker said the spacecraft flew within about 503km of the moon’s surface.
Now the Cassini team is also checking out data from Cassini’s ion and neutral mass spectrometer, following a close fly-by on 12 December 2011.