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Cyclopropenylidene Detected

Using high sensitivity spectroscopic observations from the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have detected a small cyclic molecule called cyclopropenylidene (C3H2) in the atmosphere of Titan, the largest moon of Saturn.

This composite image shows an infrared view of Titan. In this image blue represents wavelengths centered at 1.3 microns, green represents 2.0 microns, and red represents 5.0 microns. A view at visible wavelengths would show only Titan’s hazy atmosphere; the near-infrared wavelengths in this image allow Cassini’s vision to penetrate the haze and reveal the moon’s surface. The view looks toward terrain that is mostly on the Saturn-facing hemisphere of Titan. Image credit: NASA / JPL-Caltech / Space Science Institute.

This composite image shows an infrared view of Titan. In this image blue represents wavelengths centered at 1.3 microns, green represents 2.0 microns, and red represents 5.0 microns. A view at visible wavelengths would show only Titan’s hazy atmosphere; the near-infrared wavelengths in this image allow Cassini’s vision to penetrate the haze and reveal the moon’s surface. The view looks toward terrain that is mostly on the Saturn-facing hemisphere of Titan. Image credit: NASA / JPL-Caltech / Space Science Institute.

“When I realized I was looking at cyclopropenylidene, my first thought was, ‘Well, this is really unexpected,” said Dr. Conor Nixon, a planetary scientist at NASA’s Goddard Space Flight Center.

“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 Dr. Michael Malaska, a planetary scientist at NASA’s Jet Propulsion Laboratory.

“Down here on Earth, it’s not going be something you’re going to encounter.”

“But finding molecules like cyclopropenylidene 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.”

Though astronomers previously found cyclopropenylidene in clouds of gas and dust, such as the Taurus Molecular Cloud, throughout the Milky Way Galaxy, finding it in an atmosphere was a surprise.

That’s because this molecule can react easily with other molecules it comes into contact with and form different species.

Dr. Nixon, Dr. Malaska and their colleagues were able to identify small amounts of cyclopropenylidene at Titan likely because they were looking in the upper layers of the moon’s atmosphere, where there are fewer other gases for cyclopropenylidene to interact with.

“Titan is unique in our Solar System. It has proved to be a treasure trove of new molecules,” Dr. Nixon said.

“We’re trying to figure out if Titan is habitable,” said Dr. Rosaly Lopes, a senior research scientist and Titan expert at NASA’s Jet Propulsion Laboratory.

“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.

“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 Dr. Melissa Trainer, an astrobiologist at NASA’s Goddard Space Flight Center.

“We’ll be looking for bigger molecules than cyclopropenylidene, 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 molecule besides benzene to have been found in Titan’s atmosphere so far.

Although cyclopropenylidene is not known to be used in modern-day biological reactions, cyclic 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 Dr. Alexander Thelen, an astrobiologist at NASA’s Goddard Space Flight Center.

A paper on the findings was published in the Astronomical Journal.

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Conor A. Nixon et al. 2020. Detection of Cyclopropenylidene on Titan with ALMA. AJ 160, 205; doi: 10.3847/1538-3881/abb679

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