June 25, 2019
Ryan Whitwam

Titan’s Lakes May Have ‘Bathtub Rings’ of Bizarre Crystals

Saturn’s moon Titan has a lot in common with Earth, but it’s also extremely alien. It’s one of the only objects in the solar system aside from Earth with a thick atmosphere, a toxic organonitrogen haze. It has permanent bodies of liquid on the surface like Earth, but they’re liquid hydrocarbon instead of water. That doesn’t explain the perplexing “bathtub rings” around Titan’s seas, though. New experiments on Earth suggest those rings could come from crystalized acetylene and butane

Titan’s thick atmosphere makes it difficult to survey the surface, but the Cassini probe spent a lot of time spying on Titan’s largest moon. Scientists took particular interest in Titan’s hydrocarbon lakes. These compounds would be gasses on Earth, but temperatures on Titan can reach -290 degrees Fahrenheit  (-179 degrees Celsius). Some lakes near the equator show ring-like shapes on the dry regions surrounding the lakes. 

Researchers from NASA’s Jet Propulsion Laboratory and the California Institute of Technology attempted to simulate Titan’s atmosphere on Earth to assess what those rings might be. The team started with a custom-built cryostat, a container that can cool samples to 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. Next, they added methane, ethane, and other hydrocarbons that exist in trace amounts on Titan. 

The chamber of simulated Titan produced a few interesting compounds. For example, benzene rings with ethane molecules inside as a co-crystal. There were also co-crystals composed of acetylene and butane. On Earth, these are gases used in welding and cooking. These crystals are probably much more common on Titan, based on what we know about its atmospheric composition. 

As hydrocarbon seas evaporate on Titan, the acetylene and butane can drop out in a crystalline form. That could explain the rings seen around seas on the moon. The researchers liken the process to the formation of salt crystals as seas evaporate on Earth. 

Confirming this research with observations will be a challenge. The Cassini mission ended recently, so there are no probes to look at Titan up close. Titan’s thick atmosphere would interfere anyway, so landing on the surface is the best course of action. NASA doesn’t have any firm plans to visit Titan, but there are some proposals in the New Frontiers program that could send a flying drone to the moon. It could be passed over for other missions, though.

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