Titan, the moon of Saturn, surprised scientists. Astronomers have discovered in its atmosphere cyclopropenylidene (C3H2), an extremely rare carbon-based molecule that is so reactive that it can only exist on Earth in a laboratory setting.
In fact, it is so rare that it has never been found in the atmosphere, in the solar system, or anywhere else before. The only other place where the molecule can remain stable is in the cold void of interstellar space. But on its basis, the emergence of more complex organic molecules is possible, which may one day lead to life.
“We think of Titan as a real laboratory, where we see chemistry similar to what was on ancient Earth when life was born here,” said astrobiologist Melissa Trainer of NASA's Goddard Space Flight Center.
“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 complex organic molecules.”
Cyclopropenylidene, which even NASA researchers describe as a 'very strange little molecule', cannot last long in atmospheric conditions because it reacts very quickly with molecules to form other compounds.
Once the reaction occurs, the cyclopropenylidene no longer exists. In interstellar space, any gas or dust is very cold and diffuse, which means weak interactions, and cyclopropenylidene can exist.
Titan is very different from interstellar space. With hydrocarbon lakes, hydrocarbon clouds and a predominantly nitrogen atmosphere with little methane. The atmosphere is four times thicker than that of the Earth (which is also dominated by nitrogen). Scientists think there is a huge ocean of salt water beneath the surface.
In 2016, a team of scientists 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 titanium's atmosphere in search of organic molecules.
The signature of the strange molecule was found in the tenuous upper atmosphere, high above the surface. Comparing it to a database of chemical profiles, the team identified the molecule as cyclopropenylidene. It is likely that the thinness of the atmosphere at this altitude contributes to the survival of the molecule, but why it appeared on Titan and not on any other planet remains 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 turned out to be a treasure trove of new molecules. '
Cyclopropenylidene is of particular interest because it is known as a ring molecule; its three carbon atoms are linked in a ring (in 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 such molecular rings.
“Their cyclical nature opens up an additional branch of chemistry that allows the creation of biologically important molecules,” said astrobiologist Alexander Thelen of NASA's Goddard Space Flight Center.
The smaller the molecule, the more potential it has – it is expected that reactions involving smaller molecules with fewer bonds will occur faster than reactions involving larger and more complex molecules. This means that reactions involving small molecules will lead to more varied results.
Titanium is already a hive of organic chemical activity. Nitrogen and methane break down in sunlight, causing a cascade of chemical reactions. Whether these reactions can lead to life is a question that scientists are eager to answer.
“We're trying to figure out if Titan is habitable,” said geologist Rosalie Lopez of NASA's Jet Propulsion Laboratory. “So we want to know what compounds from the atmosphere are getting to the surface, and then if this material can get through the ice crust into the ocean below, because we think the ocean is where the habitable conditions are.”
Finding out which compounds are present in the atmosphere is a very important step in this research process. Cyclopropenylidene, an extremely rare molecule, could be a key element in Titan's chemical life.
The research is published in The Astronomical Journal.
Sources: Photo: NASA / JPL-Caltech / University of Arizona / University of Idaho
