Titan, Saturn's largest moon, does not harbor alien life as far as anyone knows, but the prospects for extraterrestrial biology there are about as good there as anywhere else in the solar system.

Numerous promising compounds based on hydrogen, nitrogen and carbon—some of the key constituents of terrestrial biological molecules such as amino acids—have been detected in Titan's atmosphere. Now a new study shows how readily Titan's thick atmosphere produces those chemicals, which might precipitate down to the moon's surface.

Hiroshi Imanaka of the University of Arizona and the SETI Institute in Mountain View, Calif., and Mark A. Smith of the University of Arizona report the results of their laboratory re-creation of Titan's atmospheric chemistry in a study published online this week in Proceedings of the National Academy of Sciences.

The researchers filled a chamber at Lawrence Berkeley National Laboratory's Advanced Light Source with a simulated Titan atmosphere—mostly nitrogen with a dash of methane—and bombarded the mixture with ultraviolet radiation. What they found was that a substantial quantity of nitrogen gas (N2) in the atmosphere is split by photodissociation, then incorporated into reactive compounds such as HCCN and finally into stable organic macromolecules.

"Organic aerosols in the upper atmosphere of Titan might be a hidden [nitrogen] sink," the researchers report, "which eventually accumulate on the surface of Titan with chemical potential for prebiotic evolution."

Of course, it is a long way from nitrogenated organic compounds hundreds of kilometers above Titan to any kind of life down in the methane lakes on the moon's surface. But as with a recent pair of much-publicized studies showing chemical processes consistent with but not indicative of life on Titan, it's at least a step in the right direction.

False-color photo of Titan's atmospheric haze: NASA/JPL/Space Science Institute