Oxygen Discovered on Saturn’s Dione –Joins Rhea and Saturn’s Main Rings in Having an Oxygen-Rich Exosphere



Dione, one of Saturn’s 62 moons, has a weak exosphere which includes molecules of oxygen,  joining Rhea and the main rings in Saturn's system in having an oxygen rich exosphere, as well as Jupiter’s moons Ganymede, Europa and Callisto – the target for ESA's proposed JUICE (JUpiter ICy moons Explorer) mission for launch in 2022 –according to new findings from the Cassini-Huygens mission. 

The international mission made the discovery using combined data from one of Cassini’s instruments, called CAPS (Cassini Plasma Spectrometer.  The moons of Saturn  ranging from tiny moonlets less than 1 kilometre across, to the enormous Titan, which is larger than the planet Mercury. Saturn has 62 moons with confirmed orbits, fifty-three of which have names, and only thirteen of which have diameters larger than 50 kilometres.

“It now looks like oxygen production is a universal process wherever an icy moon is bathed in a strong trapped radiation and plasma environment," said Andrew Coates of the University College London Mullard Space Science Laboratory. “Energetic particles hit the icy surface, the hydrogen is lost and molecular oxygen remains as an exosphere. We now know that this happens at Saturn's moons as well as Jupiter's – and it may well occur in extrasolar planetary systems too.”

Cassini flew by Dione on 7 April 2010. During that flyby, CAPS detected molecular oxygen ions near the moon's icy surface, due to bombardment by particles trapped in Saturn's strong magnetic field.
A team of scientists, led by Robert Tokar at the Los Alamos National Laboratory in the US, used the measurements to estimate the density of the molecular oxygen ions to be in the range of 0.01 to 0.09 ions per cubic centimetre. These molecular oxygen ions are produced when neutral molecules are ionized; the measurements confirm that a neutral exosphere surrounds Dione.

Electron measurements from UCL's electron spectrometer (ELS), part of CAPS, played a key role in reaching the conclusion of an exosphere, data from ELS showed the plasma wake due to Dione and characterized the changes in Saturn's magnetosphere during the flyby. 

“Dione's exosphere is very thin – compared to Earth's atmosphere the density is about a million billionth. The exciting thing is that there is oxygen – and the oxygen may be being recycled via the surface,” added Geraint Jones of the UCL Mullard Space Science Laboratory.

Image at top of page shows heat emitted from the interior of Saturn (red) shows up in this false-color image of Saturn, made from data taken in 2008 by Cassini's visual and infrared mapping spectrometer. 

The image of Dione below indicates that the white wisps are composed of deep ice cliffs dropping hundreds of meters. The cliffs may indicate that Dione has undergone some sort of tectonic surface displacements in its past. The bright ice-cliffs run across some of Dione's many craters, indicating that the process that created them occurred later than the impacts that created those craters. Dione is made of mostly water ice but its relatively high density indicates that it contains much rock inside. Giovanni Cassini discovered Dione in 1684.




The Daily Galaxy via University College London

Image Credit: NASA/JPL/ASI/University of Arizona and NASA/JPL/Space Science Institute

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