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Titan, Saturn’s already rather weird moon, just got a little weirder. Astronomers have detected cyclopropenylidene (C3H2) in its atmosphere, an extremely rare carbon-based molecule that is so reactive that it can only exist on Earth under laboratory conditions.
In fact, it is so rare that it has never been detected before in an atmosphere, in the Solar System or elsewhere. The only other place it can remain stable is the cold void of interstellar space. But it could be a building block for more complex organic molecules that could one day bring to life.
“We think of Titan as a real-life laboratory where we can see chemistry similar to that of ancient Earth when life was taking hold here,” said astrobiologist Melissa Trainer of NASA’s Goddard Space Flight Center, one of the main scientists tasked with investigating the moon in the next Dragonfly mission to be launched in 2027.
“We will be looking for molecules larger than C3H2, but we need to know what is happening in the atmosphere to understand the chemical reactions that lead to the formation and rain of complex organic molecules.”
Cyclopropenylidene – which even NASA researchers describe as a “very strange small molecule” – does not tend to last long in atmospheric conditions, because it reacts very quickly and easily with other molecules, forming other compounds.
Once it does, it’s no longer cyclopropenylidene. In interstellar space, any gas or dust is usually very cold and widespread, which means that the compounds do not interact much and cyclopropenylidene can remain suspended.
Titan is very different from interstellar space. It’s a bit soggy, with lakes of hydrocarbons, clouds of hydrocarbons and a predominantly nitrogen atmosphere, with some methane. The atmosphere is four times denser than the Earth’s atmosphere (also dominated by nitrogen). Below the surface, scientists think there is a huge ocean of salt water.
In 2016, a team led by planetary scientist Conor Nixon of NASA’s Goddard Space Flight Center used the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile to probe the lunar atmosphere for organic molecules.
It was in the tenuous upper atmosphere, high above the surface, where they detected an unknown chemical trace. By comparing it to a database of chemical profiles, the team identified the molecule as cyclopropenylidene. The subtlety of the atmosphere at that altitude is likely to contribute to the molecule’s survival, but why it appears on Titan and no other world is a mystery.
“When I realized I was looking at cyclopropenylidene, my first thought was, ‘Well, this is really unexpected,” Nixon said. “Titan is unique in our Solar System. It has turned out to be a treasure trove of new molecules.”
Cyclopropenylidene is of particular interest because it is what is known as a ring molecule; its three carbon atoms are linked together in a ring (well, a triangle, but the principle is the same). Although cyclopropenylidene itself is not known to play a biological role, the nitrogenous bases of DNA and RNA are based on these molecular rings.
“Their cyclic nature opens up this extra branch of chemistry that allows you to build these biologically important molecules,” said astrobiologist Alexander Thelen of NASA’s Goddard Space Flight Center.
The smaller the molecule, the greater the potential it has: reactions involving smaller molecules with fewer bonds should happen faster than reactions involving larger, more complicated molecules. This means that reactions involving smaller molecules, purely through numbers, should lead to a more diverse range of results.
Previously, benzene (C6H6) was thought to be the smallest hydrocarbon ring molecule found in any atmosphere (including that of Titan). Cyclopropenylidene has beaten.
Titan is already a hive of organic chemical activity. Nitrogen and methane break down in sunlight, triggering a cascade of chemical reactions. Whether these reactions can lead to life is a question scientists are eager to answer.
“We’re trying to figure out if Titan is habitable,” said geologist Rosaly Lopes of NASA’s Jet Propulsion Laboratory. “So we want to know which compounds from the atmosphere make it to the surface, and then, if that material can cross the ice crust to the ocean below, because we think the ocean is where the habitable conditions are.”
Determining which compounds are present in the atmosphere is a very important step in this research process. Cyclopropenylidene may be small and strange, but this extremely rare molecule could be a key piece of the puzzle of Titan’s chemistry. Now we just have to figure out how it fits.
The research was published in The Astronomical Journal.
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