Rhea provides clues to the origins of atmosphere

Science this week reports new data from the Cassini probe confirming that Saturn’s moon Rhea has the tenuous beginnings of an atmosphere.  A thin veil of O2 and CO2 gas skirts the moon’s icy surface at one in five trillionth the pressure of Earth’s atmosphere.

Rhea, one of over 60 moons of Saturn, is comprised of 75% ice and 25% rock.

The Cassini probe captured a sample of the atmosphere and used mass spectroscopy to confirm the presence of CO2 and O2. A model produced from this data indicates that O2 was formed at the ice surface by irradiation from solar ultraviolet light and possibly charged particles from deep space. UV rays cause chemical changes in the ice, leading to the spontaneous ejection of O2 gas.

The origin of CO2 in Rhea’s atmosphere is less clear. It is hypothesized that the CO2 is native to Rhea’s ice, or that it formed by interactions between atmospheric O2 and carbonaceous grains on the surface. The authors suggest that carbonaceous grains may have been depsited by micrometeorites from elsewhere in the solar system, possibly even Earth.

O2 and CO2 are believed to have accumulated on Earth during the evolution of life and metabolic processes such as aerobic respiration and photosynthesis. The presence of these gases is considered a criterion for the possible presence of life on a planet. However, the formation of an O2-rich atmosphere on Rhea of entirely photo-driven origin calls this assumption into question. The study provides novel insight into some of the processes that lead to formation of an atmosphere.

A newly released image of Rhea from NASA's Cassini probe


B. D. Teolis et al., Science 330, 1813 (2010); 10.1126/science.1198366.


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