Measurements from the Cassini spacecraft, under a close flyby of 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.

Source: Science