by Lonnie Shekhtman
NASA’s Goddard Space Flight Center
Greenbelt, Md. — NASA scientists identified a molecule in Titan’s atmosphere that has never been detected in any other atmosphere. In fact, many chemists have probably barely heard of it or know how to pronounce it: cyclopropenylidene, or C3H2. Scientists say that this simple carbon-based molecule may be a precursor to more complex compounds that could form or feed possible life on Titan.
Researchers found C3H2 by using a radio telescope observatory in northern Chile known as the Atacama Large Millimeter/submillimeter Array (ALMA). They 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,’” said Conor Nixon, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who led the ALMA search. His team’s findings were published on October 15 in the Astronomical Journal.
Though scientists have found C3H2 in pockets throughout the galaxy, finding it in an atmosphere was a surprise. That’s because cyclopropenylidene can react easily 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.
But dense atmospheres like Titan’s are hives of chemical activity. That’s a major reason scientists 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. Scientists don’t yet know why cyclopropenylidene would show up in Titan’s atmosphere but no other atmosphere. “Titan is unique in our solar system,” Nixon said. “It has proved to be a treasure trove of new molecules.”
The largest of Saturn’s 62 moons, Titan is an intriguing world that’s in some ways the most similar one to Earth we have found. Unlike any other moon in the solar system — there are more than 200 — Titan has a thick atmosphere that’s four times denser than Earth’s, plus clouds, rain, lakes and rivers, and even a subsurface ocean of salty water.
Titan’s atmosphere is made mostly of nitrogen, like Earth’s, with a hint of methane. When methane and nitrogen molecules break apart under the glare of the Sun, their component atoms unleash a complex web of organic chemistry that has captivated scientists and thrust this moon to the top of the list of the most important targets in NASA’s search for present or past life in the solar system.
“We’re trying to figure out if Titan is habitable,” said Rosaly Lopes, a senior research scientist and Titan expert at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “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.”
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, scientists suspect.
“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,” said 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.
“We’ll be looking for bigger molecules than C3H2,” Trainer said, “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.
Cyclopropenylidene is the only other “cyclic,” or closed-loop, molecule besides benzene to have been found in Titan’s atmosphere so far. Although 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. “The cyclic nature of them opens up this extra branch of chemistry that allows you to build these biologically important molecules,” said Alexander Thelen, a Goddard astrobiologist who worked with Nixon to find C3H2.
Scientists like Thelen and Nixon are using large and highly sensitive Earth-based telescopes to look for the simplest life-related carbon molecules they can find in Titan’s atmosphere. 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.
Nixon’s team used the ALMA observatory to peer at Titan in 2016. They were surprised to find a strange chemical fingerprint, which Nixon identified as cyclopropenylidene by searching through a database of all known molecular light signatures.
To double check that the researchers were actually seeing this unusual compound, Nixon pored through research papers published from analyses of data from NASA’s Cassini spacecraft, which made 127 close flybys of Titan between 2004 and 2017. He wanted to see if an instrument on the spacecraft that sniffed out the chemical compounds around Saturn and Titan could confirm his new result. (The instrument – called a mass spectrometer – picked up hints of many mysterious molecules at Titan that scientists are still trying to identify.) Indeed, Cassini had spotted evidence for an electrically charged version of the same molecule, C3H3+.
Given that it’s a rare find, scientists are trying to learn more about cyclopropenylidene and how it might interact with gases in Titan’s atmosphere.
“It’s a very weird little molecule, so it’s not going be the kind you learn about in high school chemistry or even undergraduate chemistry,” said Michael Malaska, a JPL planetary scientist who worked in the pharmaceutical industry before falling in love with Titan and switching careers to study it. “Down here on Earth, it’s not going be something you’re going to encounter.”
But, Malaska said, finding molecules like C3H2 is really important in seeing the big picture of Titan: “Every little piece and part you can discover can help you put together the huge puzzle of all the things going on there.”