With a thick atmosphere and icy surface chock-full of organic molecules, Saturn’s largest moon Titan offers a tantalizing glimpse into prebiotic chemistry. An alluring part of its otherworldly landscape is the hydrocarbon lakes composed primarily of methane and ethane, where truly alien chemistry may be transpiring. Little is known about Titan’s surface chemistry, but researchers now report evidence for a new and ubiquitous organic mineral made of acetylene and butane. The mineral likely remains after hydrocarbon lakes dry up, shaping Titan’s geology and landscape (ACS Earth Space Chem. 2019, DOI: 10.1021/acsearthspacechem.9b00275).
Using one of the spectrometers onboard the Cassini spacecraft, scientists peered through Titan’s hazy nitrogen and methane atmosphere to observe the moon’s surface. Infrared spectrometers highlighted bright spots—concentrations of organic molecules that have a set stoichiometry like minerals on Earth—in formations resembling bathtub rings around the hydrocarbon lakes. Scientists infer that these molecules may have fallen from the atmosphere into the lakes and precipitated as solids when the hydrocarbon lakes evaporated.
Work by researchers attempting to understand the chemistry of Titan’s surface suggests that the minerals are likely co-crystals made of acetylene and n-butane, both of which are readily available on Titan. These would not be the first organic crystals research indicates may be present there. But Morgan L. Cable of NASA’s Jet Propulsion Laboratory, a coauthor on this most recent study who helped identify the other co-crystals, says the newest one is significant because it’s probably the most abundant. High amounts of both acetylene and n-butane are known to “snow” down from the atmosphere—which means a crystal made from these components could be prevalent on Titan’s surface and key to understanding the chemistry that occurs there.
The new study aimed to determine if a co-crystal of acetylene and n-butane could form under Titan-relevant conditions and, if so, what kinds of physical properties it might have. The researchers cooled a special sample holder to 90 K to simulate Titan’s surface temperature. They deposited acetylene and n-butane together or sequentially inside, and used a Raman spectrometer to measure the crystals that formed. The two molecules spontaneously made a co-crystal that remained intact up to 190 K—roughly 40° higher than pure acetylene can withstand alone, suggesting the crystals could be stable enough to persist over long periods of time. The researchers also condensed liquid ethane onto the co-crystals to verify they could withstand erosion from rain on Titan.
Their potential prevalence means these co-crystals could play a major role in shaping the landscape over time, says coauthor Robert Hodyss of JPL. They could be carried away by the winds and form dunes or get eaten away by liquid methane and ethane to form plateaus or high-walled rims around the lakes. The crystals could also interact with other molecules on Titan’s surface, affecting erosion, transport, and deposition. Despite the frigid temperatures that would slow these chemical reactions, Hodyss says that many important intermolecular interactions occur that would impact Titan’s geology. “There is actually a lot more chemistry happening on the surface than was previously thought,” he says.
Given interest in the possibility of life on Titan, the researchers also speculate that acetylene-butane reservoirs could serve as food supplies. Certain microbes on Earth eat acetylene, and the same could be true on Titan.
At the moment, the researchers can only infer the existence of the co-crystal on Titan. However, Cable is a coinvestigator on NASA’s Dragonfly mission, which is slated to land on Titan in 2034 and provide further evidence for the presence of these co-crystals on the surface.
According to Jonathan I. Lunine of Cornell University, a former scientist on the Cassini mission who was not affiliated with the study, these co-crystals may reveal new and different chemistry from the reactions taking place in Titan’s atmosphere. These findings, he says, are exciting because we have very little information about the surface chemistry. “It’s natural to wonder if there might be some exotic biochemistry going on,” he adds. “Co-crystals could be a part of that story.”