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29 July 2017

Molecule found in Titan's atmosphere may form cell-like membranes



The discovery, reported today in Science Advances, was made using the highly sensitive Atacama Large Millimeter Array radio telescope in Chile.

Saturn's largest moon has long been considered an ideal candidate for organic life elsewhere in our galaxy, said the study's lead author, Maureen Palmer from the NASA Goddard Space Flight Center.

"So when there's all these organic molecules along with a possible liquid habitat, it's intriguing from the perspective of looking for weird forms of life," Ms Palmer said.

"In some ways Titan is sort of Earth-like," Ms Palmer said.

The moon's atmosphere is around 95 per cent nitrogen, and there are lakes and seas on its surface. But unlike Earth, these lakes and seas contain liquid methane rather than water.

Cells membranes important for life

Vinyl cyanide (C2H3CN) is an organic molecule used in the manufacture of plastics and synthetic rubbers. While it is toxic to Earth-based life forms, it is ideally suited to form cell-like membranes in the hydrocarbon-rich environment found on Titan.

Cell membranes are thought to be important for the development of life in a liquid environment because they can create a sealed-off chamber where biochemical reactions can take place more easily than in the more dilute open water setting.

Computer simulations have suggested that vinyl cyanide would behave in a similar fashion in Titan's environment, to the phospholipid molecules that make up cell membranes found in life on Earth.

However, this behaviour is yet to be demonstrated in the laboratory setting.

Earlier research using data from the Cassini spacecraft hinted that vinyl cyanide existed on Titan, but more definitive proof was needed.

Fortunately, the Atacama Large Millimetre Array telescope regularly turns towards Titan and uses the moon to calibrate its energy measurements.

When the research team looked at the frequencies of light being emitted from Titan, they saw a peak that corresponded to the exact frequency of light emitted by the vinyl cyanide molecule.

The molecular signature is actually located high up in Titan's atmosphere — around 200 kilometres above the moon's surface — but that doesn't mean it's only found there.

"If we wanted to hypothetically form membranes, it would need to reach the surface, which seems like a plausible thing that would happen," Ms Palmer said.

"Titan has this haze and organic particles are continuously falling out of the atmosphere, and also there's rain which can bring molecules down in the droplets."

Study could shape our understanding of early life on Earth

Because Titan has some things in common with a primitive Earth before our planet's atmosphere became so oxygen rich, research such as this could also shape scientists' understanding of how life first developed on Earth.

"We still don't really know how life got started on Earth, and so studying how membranes or how possibly prebiotic molecules could form on Titan is another angle of attack of the bigger question of how any life could get started," Ms Palmer said.

The next steps are to do even higher resolution studies of Titan's atmosphere to see how the vinyl cyanide molecules are distributed across the moon's surface.

Ms Palmer also hoped the laboratory experiments might give a real-world demonstration of vinyl cyanide forming membrane structures.

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In a different study, which was published Wednesday (July 26) in The Astrophysical Journal Letters, scientists identified some of these negatively charged ions as "carbon chain anions." That's a big deal, because carbon chain anions are viewed as building blocks to more complex species, and may have been involved in the reactions that led to the rise of life on Earth, members of this study team said.