Rhea is an icy moon in orbit around Saturn, second in size only to Titan. The low density of this planetary body suggests that it is composed of about 25% rock and 75% water ice. However, the surface temperature of Rhea ranges from a chilly -220 to -170 Celsius (-365 to 275 Fahrenheit), so this water ice is unlikely to melt under even the most optimistic conditions. The heavily cratered surface indicates that Rhea is relatively geologically inactive, although measurements by NASA’s Cassini orbiter have discovered some evidence that tectonic activity may have formed cliffs or other features early in Rhea’s history.
Rhea also holds another second to Titan, as it also hosts a planetary atmosphere. The atmosphere on Rhea is not nearly as dense as that on Titan, though, and the first signs of this wispy air were only observed recently by the Cassini orbiter. Best classified as an exosphere, Rhea’s atmosphere does not behave like any normal gas because the molecules are so sparsely distributed that they rarely collide with one another. A handful of molecules from Rhea’s exosphere were even captured and analyzed by Cassini to reveal the presence of oxygen and carbon dioxide in the thin atmosphere! This was the first time that a spacecraft has directly captured molecules from an oxygenated atmosphere (outside of Earth, of course).
The source of this oxygen on Rhea seems linked to changes in the local magnetic field. Specifically, the magnetic field of Saturn changes in strength over the surface of Rhea due to rotation, which causes charged particles to fall to the surface. These particles bombard the surface ices and decompose the surface to release oxygen into the “air”. These oxygen molecules are still gravitationally bound to Rhea, so they waft upward and contribute to a thin layer of atmosphere. The source of the carbon dioxide is still uncertain, although it may indicate the presence of other materials trapped within the ices.
Earth also has an exosphere, albeit for a much different reason than Rhea. On Earth, the atmosphere consists of many layers, with the troposphere containing over 75% of the mass and most of the weather (this is where we live). The exosphere on Earth is created by the lightest gaseous species--mostly hydrogen--as they diffuse into space. In a sense, the exosphere is the transition region between Earth’s atmosphere and space. Rhea, on the other hand, has no other atmospheric layers, and so its exosphere is a thin molecular shell distinguishable from the near-vacuum of space.
Could the oxygen on Rhea someday help human explorers? Maybe. It is unlikely that we could release enough oxygen to terraform Rhea and make the atmosphere breathable for humans, but the surface ices could provide a treasure trove of rare goods--water, oxygen, and carbon dioxide--for explorers or nearby colonists. And some features of Rhea’s exosphere may in fact be common among icy moons in orbit around giant planets. Perhaps future explorers will often find themselves retreating to wintery worlds for a holiday!