One of the most exciting things about space chemistry is that it gives us a glimpse of chemistry that is difficult to study — or might not even exist — on Earth. A well known example of this is the chemistry on Saturn's largest moon Titan, which is famous for its lakes of methane. Scientists think that this Saturnian satellite has a hydrocarbon cycle that is much like our water cycle on Earth.
Using data from NASA's Cassini mission, researchers found a mysterious chemical signature in ultraviolet imaging data collected during a flyby of Rhea. They concluded that the most likely contender for this chemical feature is hydrazine, a nitrogen-containing compound typically used in manufacturing on Earth. In fact, hydrazine is one of the compounds used as a propellant for the Cassini spacecraft!
After confirming that Cassini's thrusters were shut off during the flyby of Rhea, the researchers had to consider other possible sources for the hydrazine. On Earth, small amounts of hydrazine are produced naturally by some algae and tobacco plants, but any hydrazine on Rhea wouldn't come from anthropogenic or biological sources. It is also possible that hydrazine could form within the ice on the surface of Rhea, but the moon's thin atmosphere leaves molecules on its surface vulnerable to irradiation that breaks apart the molecules needed to form hydrazine.
The hydrazine could also come from Titan. Scientists don't yet know whether hydrazine could even form on Titan, but the moon's nitrogen-rich atmosphere makes it a promising factory for hydrazine and other similar molecules.
Unfortunately, Saturn and its moons are too far away to further investigate this chemistry any time soon. We might have to wait until NASA's planned Dragonfly mission takes us back Titan again so we can better understand the chemistry there, and perhaps on Rhea too.