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Primitive chemistry in planetary atmospheres: From the upper atmosphere down to the surface

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Could life emerge on Titan?

The organic chemistry going on in the atmosphere of Saturn’s largest moon could hold all the ingredients needed for life to emerge, a European project finds.

Space

With a surface temperature of around -200 °C, Titan does not sound like a particularly hospitable place. However, this icy moon orbiting Saturn may host the building blocks of life – and offer a suitable environment for putting them together. Titan, which is twice the size of our Moon, has some striking similarities with Earth: its climate involves a cycle of liquids with clouds and rain, as well as rivers, lakes and seas. It is also the only known moon with a thick atmosphere. The PRIMCHEM (Primitive chemistry in planetary atmospheres: From the upper atmosphere down to the surface project), supported by the European Research Council (ERC), has now found that the chemical interactions taking place in this atmosphere could potentially foster the emergence of life on Titan. Organic compounds, which are found in all living organisms, are molecules generally containing carbon atoms bonded to hydrogen. They can contain a number of other elements called heteroatoms, including nitrogen. Nitrogen forms an important part of the structure of proteins and DNA bases. “Titan’s atmosphere is mainly made up of molecular nitrogen and methane. We have shown that, at high altitudes, nitrogen drives organic growth and produces large molecules,” explains Nathalie Carrasco, PRIMCHEM principal investigator and professor of Geosciences at the University of Versailles Saint Quentin which hosts the project. Titan has an ideal atmosphere for exploring the scenario of an atmospheric origin for nitrogen incorporation into prebiotic molecules, she says: “Upper atmospheric layers are directly exposed to harsh solar UV radiations, triggering extremely efficient chemical reactions.”

Organic potential

The team also found that the large nitrogen-rich molecules produced at high altitudes continue to evolve during their descent in the atmosphere, generating huge organic potential at Titan’s surface. Liquid water, Carrasco explains, is another prerequisite for life. “Titan possesses a deep-water ocean in its interior, and liquid water can be brought up to the surface through cryovolcanism. Our discovery of nitrogen-rich atmospheric molecules interacting with liquid water at Titan’s surface therefore hints at the possibility of the emergence of life on Titan.” The team’s findings were made possible through the Cassini-Huygens space mission, which first revealed the existence of large organic molecules on Titan. To unravel their chemical composition, the PRIMCHEM team developed an experimental platform to recreate the conditions on Titan in the lab – harsh UV irradiation, low pressure and temperatures of down to -200 °C. The chemical processes triggered were then observed in situ using analytical devices connected to the reactors.

A glimpse of the early Earth

The project’s work could also help to improve our understanding of how life may have arisen on our own planet: the pressure at its surface, and the chemical composition of its atmosphere, make Titan the best-known observable analogue of the early Earth. “Our results suggest that the chemistry in the upper atmosphere of the early Earth fostered the incorporation of nitrogen in prebiotic molecules, before they interacted with the primitive oceans,” Carrasco notes. PRIMCHEM’s groundbreaking discoveries will directly inform the work of DragonFly, NASA’s mission to Titan which will launch in 2027 to explore how far pre-life chemistry may have progressed there and advance our knowledge of the emergence of life on Earth and beyond.

Keywords

PRIMCHEM, Titan, moon, atmosphere, organic chemistry, prebiotic, early Earth, DragonFly mission, life

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