Amazing discovery: Atmosphere of Saturn’s moon Rhea is 70% Oxygen

Its a truly amazing discovery … the Cassini mission, jointly run by NASA and European Space Agency, has enabled the discovery that Rhea, the second largest moon of Saturn, has an atmosphere that is 70 per cent oxygen and 30 per cent carbon dioxide.

This evokes one word … I believe its a scientific term … and that word is … “Wow“.

OK, don’t get too excited, its too thin and too cold for life, so this is about chemistry, not biology. Its also not the first occurrence of oxygen in our solar system, layers containing oxygen had already been detected around the Jovian moons Europa and Ganymede in the 1990s, but only from a distance using NASA’s Hubble Space Telescope. However this is something quite new …

This time, Cassini’s instrument had the chance to “smell” that oxygen, as it flew through it over Rhea’s north pole, just 97 kilometres above the surface, according to the details given on This layer – with an oxygen density probably about 5 trillion times less than on Earth – was “too thin to be remotely detected”, said Ben Teolis of the Southwest Research Institute in San Antonio.

OK, right now I can just hear you screaming the most obvious question, “Where the heck did all that Oxygen come from?” … well …

As the magnetic field rotates around Saturn, particles carried in the field slam into the hemisphere of Rhea that’s facing their flow,” Teolis told BBC News. “They hit that hemisphere and break water molecules on the surface. The atoms are then rearranging themselves to make oxygen molecules, which are sputtered from the surface by additional impacting particles.” That process is likely to be ongoing, with the oxygen molecules created being constantly whipped out into space.

And here is the best quote … this is why its so exciting … its not unique, instead it looks like this is very common …

The discovery on Rhea suggests that many other large, ice-covered bodies throughout the solar system and beyond may harbor thin shells of oxygen-rich air — and, perhaps, complex chemistry, researchers said.”We’ve seen this happening at Jupiter, and now we’ve confirmed it on a Saturn moon,” study lead author Ben Teolis, of the Southwest Research Institute in San Antonio, told “The fact that it’s widespread is very exciting.”

You can read more about this amazing discovery here on …or if you prefer, read the New Scientist Article here …or the article here

However, you can also skip over all the media hype (including mine), and go directly the the peer-reviewed paper that has been published in Science DOI: 10.1126/science.1198366 by clicking here … the abstract for the paper reads …

The flyby measurements of the Cassini spacecraft at Saturn’s moon Rhea reveal a tenuous oxygen–carbon dioxide atmosphere. The atmosphere appears to be sustained by chemical decomposition of the surface water ice under irradiation from Saturn’s magnetospheric plasma. This in situ detection of an oxidizing atmosphere is consistent with remote observations of other icy bodies such as Jupiter’s moons Europa and Ganymede, and suggestive of a reservoir of radiolytic O2 locked within Rhea’s ice. The presence of CO2 suggests radiolysis reactions between surface oxidants and organics, or sputtering and/or outgassing of CO2 endogenic to Rhea’s ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.

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