A Crucial Ingredient for Life on Saturn's Largest Moon

Titan, Saturn's largest moon, has a thick atmosphere teeming with organic compounds.
Titan, Saturn's largest moon, has a thick atmosphere teeming with organic compounds.NASA / JPL-Caltech / Space Science Institute

When Voyager 1 flew by Saturn and its moons in 1980, it found a thick, orange-brown atmosphere around Titan, the largest moon of the group. The spacecraft’s imaging instruments couldn’t penetrate the atmosphere, but its spectrometers detected a multitude of molecules that are also found on Earth: mostly nitrogen, but also hints of hydrogen, as well as complex organic compounds like ethane, propane, carbon dioxide, carbon monoxide, and many others. The Huygens probe later found lakes of liquid methane on Titan in 2005, and the Cassini spacecraft eventually expanded the list to include propylene, a chemical used in household plastics on Earth, and hydrogen cyanide, an extremely poisonous compound.

Titan, in short, is an incredibly rich world of organic chemistry—and the list is still growing. NASA scientists report they have detected a molecule in the moon’s atmosphere that potentially could form cell-like membranes, according to new research published Friday in Science Advances.

Using data from ALMA, a network of radio telescopes in northern Chile, the scientists found vinyl cyanide, a hydrocarbon compound, in significant amounts in the dense atmosphere. The detection confirms previous measurements by Cassini, which found only hints of the molecule.

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Cell membranes as we know them on Earth—thin, flexible layers made of fatty molecules called lipids—could not survive on Titan. Titan’s atmosphere is mostly made of nitrogen and only traces of oxygen, which actually come from the plumes of icy particles ejected by its neighbor, Enceladus. Titan’s liquid lakes and seas are composed of methane and ethane, and its surface temperature hovers around -290 degrees Fahrenheit.

But vinyl cyanide has certain properties that may allow it to create cell-like membranes, perhaps even in Titan’s environment. Laboratory simulations of the conditions in Titan’s lakes show that vinyl cyanide, compared with other organic molecules, would form the most stable membranes.

The NASA scientists say haze particles in Titan’s upper atmosphere could transport vinyl cyanide down to the surface, where it may interact with the methane and ethane in bodies of water. These reactions could give rise to cell membranes, the key ingredient for the emergence of life—or at least a petri dish in which life can form.

Cell membranes create an enclosed space where reactions can occur more easily and frequently than they would in a big environment with no clear boundaries. Carl Sagan once called Titan a “laboratory for prebiological organic chemistry.” The presence of cell-like membranes would create infinite numbers of tiny laboratories. The NASA scientists say their estimates of vinyl cyanide in Titan’s atmosphere suggest there may be enough molecules dissolved in Ligeia Mare, the moon’s second-largest body of liquid, to build up to 10 million membranes per cubic centimeter.

Titan’s atmospheric chemistry resembles that of an early Earth, which makes the moon an exciting target in the field of astrobiology. It’s serving up a soup chock-full of the some of the ingredients necessary to spark life, like nitrogen, carbon, and hydrogen. “They’re fundamental to life as we know it,” says Sarah Hörst, a planetary scientist at Johns Hopkins University who studies Titan. “They may be fundamental to life as we don’t know it, if only because they’re the most abundant elements in the universe.”

To further investigate the role of vinyl cyanide on Titan, scientists will need to conduct more laboratory simulations, study membrane formation in methane-rich environments, and keep combing through Cassini data until the spacecraft ends its mission and burns up in Saturn’s atmosphere this fall. For now, its discovery serves as a reminder of the moon’s complicated atmosphere—and also of the rare existence of the atmosphere itself. Titan is the only moon in the solar system with a cloudy, planet-like atmosphere, where methane clouds release methane rains that feed methane lakes, in a process similar to the water cycle on Earth.

“I suspect that some people in the community will be like, great, another organic molecule in Titan’s atmosphere—surprise, surprise,” Hörst said. “The thing that people really don’t understand is that this chemistry is uniquely complicated. There really just aren’t other places in the solar system where the atmosphere is doing chemistry that is this complex.”