Saturn's moon, Titan, has a lot in common with Earth, but it's also extremely strange. It is one of the only objects in the solar system, other than Earth, with a thick atmosphere, a toxic mist of organonitrogen. It has permanent bodies of liquid on the surface like Earth, but they are liquid hydrocarbons instead of water. That does not explain the disconcerting "bathtub rings" in Titan's seas, though. New experiments on Earth suggest that these rings could come from crystallized acetylene and butane.
Titan's thick atmosphere makes it difficult to lift the surface, but the Cassini spacecraft spends a lot of time spying on Titan's largest moon. Scientists were particularly interested in Titan's hydrocarbon lakes. These compounds would be gases on Earth, but temperatures on Titan could reach -179 degrees Fahrenheit (-179 degrees Celsius). Some lakes near the equator show ring-like shapes in the dry regions around the lakes.
Researchers at NASA's Jet Propulsion Laboratory and the California Institute of Technology attempted to simulate Titan's atmosphere on Earth to assess what these rings could be. The team started with a custom cryostat, a container that can cool samples at very low temperatures. Titan's atmosphere is almost all nitrogen, so the researchers started with liquid nitrogen and allowed the chamber to warm slightly until it became a gas. They then added methane, ethane and other hydrocarbons that exist in small amounts on Titan.
The simulated Titan chamber produced some interesting compounds. For example, benzene rings with ethane molecules inside a co-crystal. There were also co-crystals composed of acetylene and butane. On Earth, these are gases used in welding and baking. These crystals are probably much more common on Titan, based on what we know about their atmospheric composition.
As the seas of hydrocarbons evaporate on Titan, acetylene and butane may break off in a crystalline form. That could explain the rings seen around the seas on the moon. The researchers compared the process to the formation of salt crystals as the seas evaporate on Earth.
Confirming this research with observations will be a challenge. The Cassini mission has recently ended, so there are no probes to see Titan up close. The thick atmosphere of the Titan would interfere in any way, so landing on the surface is the best course of action. NASA has no firm plans to visit Titan, but there are some proposals in the New Frontiers program that can send a flying drone to the moon. It could be passed on to other missions, though.