Cassini finds oxygen-carbon dioxide atmosphere on Saturn's moon Rhea

Nov 26, 2010 by Lin Edwards report
Saturn's moon Rhea. Image: NASA

(PhysOrg.com) -- In a flyby past Rhea, Saturn's second-largest moon, NASA's spacecraft Cassini has revealed the presence of a thin atmosphere of 70 percent oxygen and 30 percent carbon dioxide, which is apparently sustained by chemical decomposition of the moon’s ice-covered surface.

Scientists from the US, UK and Germany analyzing the data from Cassini say the presence of an oxidizing atmosphere is consistent with remote observations by the Hubble telescope and the Galileo probe of Jupiter’s icy moons Europa and Ganymede, but this is the first time the presence of oxygen has been detected directly on any moon or planet.

Rhea is a mere 1,500 kilometers in diameter and is always covered with a thick layer of water ice. The average surface temperature is estimated to be -180°C. The newly discovered oxygen and atmosphere is only 100 kilometers thick, and is so thin that if it were at Earth-like pressures and temperatures, the entire atmosphere would fit snugly into a medium-sized building.

The oxygen is believed to be formed when water molecules are split by energetic particles in a process called radiolysis. The oxygen is then ejected into the atmosphere and is captured by Rhea’s gravity. The data suggests about 130 grams of oxygen per second is produced on Rhea. The carbon dioxide may originate in dry ice trapped inside the moon, or it could be from carbon-rich meteorites hitting the surface and split apart by charged particles in a similar way to the water ice. Another possibility is it could be escaping from the planet’s interior.

The moon is around 527,000 kilometers from Saturn and orbits within the magnetic field. It is radiation from this magnetosphere that is believed to cause the chemical breakdown of the ice on the surface and create the atmosphere.

Leader of the research team, Ben Teolis, from the Space Science & Engineering Division of the Southwest Research Institute at San Antonio, Texas, said the finding implies that oxygen atmospheres associated with irradiated icy moons may be common throughout the universe if the moon has sufficient mass to hold an atmosphere. He said the findings may also help scientists understand how and where oxygen is likely to exist, which will aid in the planning of future unmanned and manned space missions.

Cassini has been orbiting Saturn and its moons since 2004 and flew over Rhea at an altitude of 97 kilometers in March this year. Of ’s moons only Rhea and Titan have enough mass to hold an atmosphere, but Titan’s is mostly nitrogen and methane, with only minute amounts of and carbon dioxide.

The findings were published online on 25 November in the journal Science

Explore further: China to send orbiter to moon and back

More information: Cassini Finds an Oxygen–Carbon Dioxide Atmosphere at Saturn’s Icy Moon Rhea, Science, Published Online 25 November 2010. DOI:10.1126/science.1198366

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User comments : 10

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kevinrtrs
1.2 / 5 (18) Nov 26, 2010
It would be interesting to know just how fast it's losing it's atmosphere compared to how fast it's atmosphere is sustained by the oxygen generation.

This would raise the question of why it still has an atmosphere after 4.5 billion years?
antialias_physorg
5 / 5 (9) Nov 26, 2010
[qThis would raise the question of why it still has an atmosphere after 4.5 billion years?
Why? Rhea certainly hasn't been around for the entire time. and even if it has - what makes you think it has always had the same kind of atmosphere?
Skeptic_Heretic
5 / 5 (9) Nov 26, 2010
Why? Rhea certainly hasn't been around for the entire time. and even if it has - what makes you think it has always had the same kind of atmosphere?
Because Kevin thinks the universe and everything in it is less than 6000 years old.
that_guy
4.4 / 5 (7) Nov 26, 2010
It would be interesting to know just how fast it's losing it's atmosphere compared to how fast it's atmosphere is sustained by the oxygen generation.

This would raise the question of why it still has an atmosphere after 4.5 billion years?


You shouldn't go to this site if you wonder why it still has an atmosphere after 4.5 billion years. The article states the atmosphere is constantly being generated. Otherwise there would be no atmosphere. zero stars for you.
Adam
not rated yet Nov 26, 2010
Interesting that the O2/CO2 mix is sluggish enough to stick around Rhea. It's not exactly the deepest gravity well.
tobyjug3
not rated yet Nov 26, 2010
Hi, could someone answer a question from a non scientist please?
If the oxygen is from the splitting of water molecules- where is the double amount of hydrogen?
Adam
5 / 5 (7) Nov 26, 2010
Hi, could someone answer a question from a non scientist please?
If the oxygen is from the splitting of water molecules- where is the double amount of hydrogen?


Hydrogen is broken off the water as single atoms. These are much lighter than oxygen or carbon dioxide molecules and very quickly escape the vicinity of Rhea. They never stay long enough to form an atmosphere.
gopher65
4.4 / 5 (7) Nov 26, 2010
tobyjug3: Hydrogen escapes very easily. Even relatively large planets like Earth and Venus can't hang on to atomic hydrogen or (more likely) hydrogen gas. On a small moon like Rhea hydrogen just goes *poof* and flies off into space before you can blink.
Ulg
5 / 5 (2) Nov 26, 2010
Hi, could someone answer a question from a non scientist please?
If the oxygen is from the splitting of water molecules- where is the double amount of hydrogen?


Hydrogen has great difficulty staying in Earths atmosphere as well- it quickly works its way into the upper atmosphere and ends up leaving our atmosphere adrift in space. Perhaps would not be the case if had less nitrogen and everything our atmosphere. While our gravity is much greater (close to 40x) we are also exposed to much more solar wind so finding the balance is tricky to having an atmosphere that would maintain hydrogen
tobyjug3
5 / 5 (3) Nov 27, 2010
Thank you for your answers. It makes sennse now
B-))