What is C3H2? 'Weird' molecule on Saturn moon Titan's atmosphere may help scientists figure out if it's habitable
According to experts, the types of molecules that might be sitting on Titan’s surface could be the same ones that formed the building blocks of life on Earth
Saturn’s largest moon Titan is an extraordinary and exceptional world. Among the solar system’s more than 200 known moons, Titan is the only one with a substantial atmosphere. Researchers have now made an unexpected discovery of a ‘weird’ molecule in Titan’s atmosphere that has never been found in any other atmosphere. Called cyclopropenylidene or C3H2, experts say that this simple carbon-based molecule could be a precursor to more complex compounds that could form or feed possible life on Titan.
"Our work represents the first unambiguous detection of cyclopropenylidene, the second known cyclic molecule in Titan’s atmosphere along with benzene (C6H6) and the first time this molecule has been detected in a planetary atmosphere,” writes the NASA-led team in the study published in Astronomical Journal.
Though scientists have found C3H2 in pockets throughout the galaxy, finding it in an atmosphere was a surprise. The reason: cyclopropenylidene can easily react with other molecules it comes into contact with and form different species. Astronomers have so far found C3H2 only in clouds of gas and dust that float between star systems — in other words, regions too cold and diffuse to facilitate many chemical reactions.
The detection was first made in 2016 using a radio telescope observatory in northern Chile: the Atacama Large Millimeter/submillimeter Array (ALMA). By comparing it to a database of chemical profiles, the team identified the molecule as cyclopropenylidene. The researchers noticed C3H2, which is made of carbon and hydrogen, while sifting through a spectrum of unique light signatures collected by the telescope. These revealed the chemical makeup of Titan’s atmosphere by the energy its molecules emitted or absorbed. “When I realized I was looking at cyclopropenylidene, my first thought was, ‘Well, this is really unexpected,’” says Conor Nixon, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who led the ALMA search.
The largest of Saturn’s 62 moons, Titan is larger than the planet Mercury and is the second-largest moon in our solar system. Jupiter's moon Ganymede is just a little bit larger, by about 2%. Titan’s atmosphere is made mostly of nitrogen, like Earth’s, but with a surface pressure 50% higher than Earth’s. Titan has clouds, rain, rivers, lakes and seas of liquid hydrocarbons like methane and ethane. The largest seas are hundreds of feet deep and hundreds of miles wide.
Dense atmospheres like Titan’s can have a lot of chemical activity and that is a key reason why astronomers are interested in this moon, which is the destination of NASA’s forthcoming Dragonfly mission. Nixon’s team was able to identify small amounts of C3H2 at Titan likely because they were looking in the upper layers of the moon’s atmosphere, where there are fewer other gases for C3H2 to interact with. However, scientists do not yet know why cyclopropenylidene would show up in Titan’s atmosphere but no other atmosphere.
Why is it important?
According to experts, the types of molecules that might be sitting on Titan’s surface could be the same ones that formed the building blocks of life on Earth. Early in its history, 3.8 to 2.5 billion years ago, when methane filled Earth’s air instead of oxygen, conditions here could have been similar to those on Titan today, they suspect.
“We’re trying to figure out if Titan is habitable. So we want to know what compounds from the atmosphere get to the surface, and then, whether that material can get through the ice crust to the ocean below because we think the ocean is where the habitable conditions are,” explains said Rosaly Lopes, a senior research scientist and Titan expert at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California.
Scientists use large and very sensitive Earth-based telescopes to look for the simplest life-related carbon molecules they can find in Titan’s atmosphere. Cyclopropenylidene is the only other ‘cyclic’ or closed-loop molecule besides benzene to have been found in Titan’s atmosphere so far. The study explains that while C3H2 is not known to be used in modern-day biological reactions, closed-loop molecules like it are important because they form the “backbone rings for the nucleobases of DNA", the complex chemical structure that carries the genetic code of life, and RNA, another critical compound for life’s functions. “Benzene was considered to be the smallest unit of complex, ringed hydrocarbon molecules found in any planetary atmosphere. But now, C3H2, with half the carbon atoms of benzene, appears to have taken its place,” the findings state.
Given that it is a rare find, experts are trying to learn more about cyclopropenylidene and how it might interact with gases in Titan’s atmosphere. “We think of Titan as a real-life laboratory where we can see similar chemistry to that of ancient Earth when life was taking hold here. We’ll be looking for bigger molecules than C3H2, but we need to know what’s happening in the atmosphere to understand the chemical reactions that lead complex organic molecules to form and rain down to the surface,” says Melissa Trainer, a NASA Goddard astrobiologist. Trainer is the Dragonfly mission’s deputy principal investigator and lead of an instrument on the Dragonfly rotorcraft that will analyze the composition of Titan’s surface.
Finding molecules like C3H2 is important in seeing the big picture of Titan, emphasizes Michael Malaska, a JPL planetary scientist. “Every little piece and part you can discover can help you put together the huge puzzle of all the things going on there,” explains Malaska.