Although it is not the first moon in the Solar System to have been discovered with an oxygen atmosphere, tiny Dione, one of Saturn's smaller attendants, is now counted among the few that do. NASA announced Friday that the agency's Cassini spacecraft orbiting Saturn had detected a very thin sheathe of oxygen ions cloaking the diminutive world.
In a statement, Robert Tokar, study leader and Cassini team member at the Los Alamos National Laboratory in New Mexico, noted: "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."
NASA announced in November 2010 that Rhea, Saturn's second largest satellite, was rich in both oxygen and carbon dioxide. That detection was also provided by Cassini. It triggered speculation that exospheres containing oxygen might be fairly prevalent around colder, larger celestial bodies where there is an abundance of surface water ice.
But that did not include a tiny iced moon like Dione...
Tokar stated that the data gathered by Cassini confirmed speculation by scientists who previously suggested that oxygen might be found after NASA's Hubble Space Telescope detected traces of ozone on the small moon.
But Dione's atmosphere does not give it much to brag about. The slight oxygen presence is akin the Earth's atmosphere at 300 miles above the planet, equal to 5 trillion times less density than that found at the Earth's surface.
But it is nonetheless an atmosphere...
Scientists believe the exosphere is the product of photon or energetic particle bombardment of the satellite's iced surface, a process that frees oxygen molecules. They are also studying data to ascertain if there might be other methods by which the atmosphere can be explained, such as through geological processes.
NASA also admitted that the new data from Dione has sparked more interest in the satellite, where before it had not received much notice and was thought to have been too small to maintain enough gravitational pull to hold an atmosphere.
Dione orbits Saturn at roughly the same distance as does the moon relative to the Earth. However, the small icy planetoid is much smaller, measuring a mere 698 miles across. It also circles its parent in far less time than does the moon, revolving around Saturn once every 2.7 days. Interestingly, Dione was first discovered in 1684 by Giovanni Cassini, for whom the Cassini spacecraft is named.
Besides the two Saturnian satellites, Jupiter offers a pair of moons that hold oxygen in their atmospheres as well -- Europa and Ganymede.
But oxygen isn't the only substance of which an atmosphere can form. Several satellites within the Solar System have atmospheres of varying make-up, such as Jupiter's volcanically active Io, which has a tenuous atmosphere of sulfur dioxide, and Saturn's largest attendant, Titan, which sports a coat of dense nitrogen and methane. Neptune's massive companion, Triton, has a composition similar to Titan's.
(photo credit: NASA / Jet Propulsion Laboratory / Space Science Institute, Wikimedia Commons)