Oxygen detected around Saturn's moon Dione
Sen—NASA's Cassini spacecraft has detected oxygen around Saturn's icy moon Dione.
The oxygen particles discovered are very few and far between, and the ultra thin atmosphere is known as an exosphere.
The discovery was made by the Cassini spacecraft that collected data on a flyby in April 2010.
The exosphere is about the same density as you find 480 km (300 miles) above the Earth's surface.
Robert Tokar, a Cassini team member and lead author of the paper explaining the findings, said
"We now know that Dione, in addition to Saturn's rings and the moon Rhea, is a source of oxygen molecules. 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."
Cassini scientists detected an exosphere around Saturn's moon Rhea in 2010 and the exosphere around Dione is very similar.
The density of oxygen at the surfaces of Dione and Rhea is around 5 trillion times less dense than that at Earth's surface.
Scientists suspected molecular oxygen would exist at Dione because NASA's Hubble Space Telescope had detected ozone. But the presence of oxygen was not confirmed until Cassini was able to measure ionized molecular oxygen on its second flyby of Dione on 7 April 2010. The Cassini plasma spectrometer was used to detect the oxygen molecules. Its flyby of Dione on 7 April 2010 was as close as 503 km (313 miles) to the moon's surface.
Scientists are also analyzing data collected by Cassini's spectrometer after a close flyby of Dione in 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.
The discovery of oxygen around both Dione and Rhea could mean that oxygen surrounds other icy moons in the outer solar system. Some, such as Enceladus (another moon of Saturn) and Europa (one of Jupiter’s moons), are believed to have sub-surface water, and it seems possible that molecular oxygen could combine with carbon in sub-surface lakes to form the building blocks of life.
Dione, discovered by Giovanni Cassini in 1684, is one of Saturn's larger moons with a diameter of 1,123 km (698 miles).
Left: Dione captured by Cassini in 2005. Image credit: NASA/JPL/Space Science Institute
It takes Dione just 2.7 (Earth) days to orbit Saturn at a distance of 377,400 km (234,000 miles), a very similar orbital distance to our Moon, which orbits Earth at an average of 238,000 miles.
Dione is an icy moon and is phase locked with Saturn so that the same side of the moon always faces its parent (also the same as Earth's Moon). The orbit of Dione interacts with two of Saturn's other moons, Mimas and Enceladus.
Saturn has over 60 moons - a vast range of different worlds from its largest moon Titan, which has a thick atmosphere made up primarily of nitrogren, to the icy geyser moon Enceladus.
Titan was the first moon to be discovered in 1655 by Christiaan Huygens whose name is honoured by the Cassini-Huygens mission and the Huygens probe that landed on Titan in 2005.
Cassini was launched in 1997 and entered Saturn's orbit in 2004. Cassini continues to provide much information and stunning images of Saturn, its rings and its moons.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.
NASA's Jet Propulsion Laboratory, Pasadena, California, a division of the California Institute of Technology, manages the mission for NASA.
The Cassini orbiter was designed, developed and assembled at JPL. The Cassini plasma spectrometer team and the ion and neutral mass spectrometer team are based at Southwest Research Institute, San Antonio.