Tenuous Oxygen Atmosphere Found Around Saturn’s Moon Rhea

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A few years ago, astronomers thought they found wispy rings around Saturn’s moon Rhea. Although the possibility of rings around this icy moon was later nixed, astronomers knew there was still something around Rhea that was causing a strange, symmetrical structure in the charged-particle environment around Saturn’s second-largest moon. Now, new observations have shown something else around Rhea that was completely unexpected: an oxygen atmosphere. In March of this year, the Cassini spacecraft made a close flyby of Rhea and recorded data showing a thin atmosphere made up of oxygen and carbon dioxide.

The source of the oxygen is not really a surprise: Rhea’s density of 1.233 times that of liquid water suggests that Rhea is three quarters ice and one quarter rock. The moon’s tenuous atmosphere is maintained by the ongoing chemical decomposition of ice water on the moon’s surface by irradiation from Saturn’s magnetosphere.

Oxygen has also recently been detected in the atmospheres of two of Jupiter’s moons, Europa and Ganymede. Since oxygen is a main component of the atmosphere surrounding Saturn’s rings, astronomers think there could be similar atmospheres around other icy moons that orbit inside Saturn’s magnetosphere.

“The new results suggest that active, complex chemistry involving oxygen may be quite common throughout the solar system and even our universe,” said lead author Ben Teolis, a Cassini team scientist based at Southwest Research Institute in San Antonio. “Such chemistry could be a prerequisite for life. All evidence from Cassini indicates that Rhea is too cold and devoid of the liquid water necessary for life as we know it.”

Of course, there’s always the possibility of life as we don’t know it.

And, there must be some sort of organics on the moon – meaning carbon compounds. The source of the carbon dioxide in Rhea’s atmosphere is not yet known, but its presence suggests that radiolysis reactions between oxidants and organics are ongoing at the moon’s surface.

As far as any of these new findings having a relation to the ruled-out hypothesis of rings around Rhea, Teolis told Universe Today there is still much about Rhea’s environment that is yet to determined. “The electron depletion is currently unexplained,” Teolis said in an email. The sharp, symmetrical drop in electrons detected around Rhea was the initial finding behind the ring theory. “Our current thinking is that it may be related to the ionization of the atmosphere, perhaps in conjunction with electrostatic charging of Rhea’s surface, but I do not have a definitive answer at this point. The atmosphere – magnetosphere interaction is a complex problem, and will take some time to sort out. But for the first time at an icy moon, the Cassini findings give us an in situ observational window onto this interaction, understanding of which is still highly theoretical. We’re working on it.”

This latest data came from Cassini’s ion and neutral mass spectrometer and the Cassini plasma spectrometer during flybys on Nov. 26, 2005, Aug. 30, 2007, and March 2, 2010. The ion and neutral mass spectrometer saw peak densities of oxygen of around 50 billion molecules per cubic meter (1 billion molecules per cubic foot). It detected peak densities of carbon dioxide of around 20 billion molecules per cubic meter (about 600 million molecules per cubic foot).

The plasma spectrometer saw clear signatures of flowing streams of positive and negative ions, with masses that corresponded to ions of oxygen and carbon dioxide.

The scientists said the oxygen appears to rise to an atmosphere when Saturn’s magnetic field rotates over Rhea. Energetic particles trapped in the planet’s magnetic field pepper the moon’s water-ice surface. They cause chemical reactions that decompose the surface and release oxygen.

Releasing oxygen through surface irradiation could help generate conditions favorable for life at an icy body other than Rhea that has liquid water under the surface, Teolis said. If the oxygen and carbon dioxide from the surface could somehow get transported down to a sub-surface ocean, that would provide a much more hospitable environment for more complex compounds and life to form.

The scientists are unsure how the carbon dioxide is released. It could be the result of “dry ice” trapped from the primordial solar nebula, as is the case with comets, or it may be due to similar irradiation processes operating on the organic molecules trapped in the water ice of Rhea. The carbon dioxide could also come from carbon-rich materials deposited by tiny meteors that bombarded Rhea’s surface.

“Rhea is turning out to be much more interesting than we had imagined,” said Linda Spilker, Cassini project scientist at JPL. “The Cassini finding highlights the rich diversity of Saturn’s moons and gives us clues on how they formed and evolved.”

This research appears in the November 25, 2010 issue of Science Express.

Sources: Science, JPL, email exchange with Teolis