The Cassini spacecraft has detected atmospheric oxygen on Rhea, a rocky, icy moon orbiting Saturn. 950 miles in diameter (less than half that of our moon), the tiny world is covered with water ice, which likely produces free oxygen as it is bombarded with charged particles from the magnetosphere of its parent planet, Saturn. Such oxygen-rich atmospheres have been found on other satellites in the solar system Europa and Ganymede, both orbiting Jupiter, both host atmospheres containing oxygen but its presence in another planetary system suggests that the phenomenon could be common.
Intriguingly, Rhea’s atmosphere also contains carbon dioxide. It’s a more puzzling result; on Earth, carbon dioxide is commonly associated with biochemical processes. Researchers are guessing that Rhea’s CO2 was delivered by meteorites or is the product of ancient reactions in the moon’s now-dormant interior.
Cassini has examined Rhea’s atmosphere on two previous occasions since its launch in 1997, but never found oxygen until the most recent flyby, which took it to within 60 miles of the planet’s surface, close enough to enter the thin, diffuse atmosphere. In January the probe will make a closer approach, skimming the southern pole at an altitude of 47 miles. It’s another example of Cassini‘s extraordinarily long and varied career as a planet-science probe the spacecraft has visited Venus, Jupiter, and Saturn, as well as many of the gas giants’ moon. It has dipped into other atmospheres as well, including that of Saturn’s moon Enceladus and remotely, that of Saturn’s largest moon Titan, when it launched the Huygens probe that landed on the surface of the methane-rich planet in 2004).[via Space.com]