Titan atmosphere experiment reveals clues to life's origins

University of Arizona scientists have performed laboratory experiments that show how atmospheric nitrogen can be incorporated into organic molecules, an important step in determining models for the formation of life on nitrogen rich bodies such as Earth and Titan.

Titan, enveloped in a thick orange smog, is seen here by the Cassini orbiter in October 2007, with Saturn's rings and small moon Epimetheus in the foreground. Image: NASA/JPL/Space Science Institute.

Saturn's giant moon Titan is the only known planetary-sized body – apart from the Earth – to sport a thick nitrogen rich atmosphere. “Titan is so interesting because its nitrogen-dominated atmosphere and organic chemistry might give us a clue to the origin of life on our Earth,” says Hiroshi Imanaka. “Nitrogen is an essential element of life.”

The nitrogen gas in Titan's atmosphere has to be converted into a more chemically active form first, however, that is capable of driving chemical reactions that form the basis of biological systems. But how complex organic molecules initially become nitrogenated in Titan's atmosphere is a mystery.

In an experiment to simulate what happens when sunlight hits Titan's atmosphere, nitrogen and methane gas was blasted with high-energy UV light. The nitrogen molecules emit blue light when zapped by the UV light. The white in the picture surrounds the pinhole opening that is the source of the UV beam. Image: Hiroshi Imanaka, University of Arizona.

Experiments conducted by Hiroshi Imanaka and Mark Smith using the Advanced Light Source at Lawrence Berkeley National Laboratory’s synchroton have shed some light on the matter. By irradiating a nitrogen-methane gas similar to the composition of Titan's atmosphere with high energy ultraviolet rays to simulate the effects of solar radiation on Titan's atmosphere, as recorded by the Cassini spacecraft, the team created nitrogen-containing organic molecules. Most of the nitrogen moved directly into solid compounds, rather than gaseous ones, whereas previous models predicted the nitrogen would move from gaseous compounds to solid ones in a lengthier stepwise process.

Imanaka and Smith suspect that such compounds are formed in Titan’s thick atmosphere and eventually fall to the moon's surface where they could be exposed to the conditions that enable the evolution of life. The question is, do the smog particles that make up Titan's hazy orange-tinted atmosphere contain nitrogen? Imanaka and Smith say that even if some of the particles are the same nitrogen-containing organic molecules they created in the laboratory, conditions conducive to life will be more likely.

The results of these experiments could help scientists develop specific instruments on future missions that could seek out these nitrogen-rich molecules.