Titan's atmosphere oddity consistent with methane-based life

By John Timmer Ars Technica

Something strange is afoot in the atmosphere of Saturn's moon Titan, according to data sent back from the Cassini mission. Data returned from a spectrometer on Cassini indicates that there's a large flux of hydrogen in the moon's atmosphere, with the gas forming in the upper atmosphere and being removed from the atmosphere at Titan's surface. We don't currently know what process is ensuring its removal, but the amounts of hydrogen being taken out of the atmosphere are consistent with an earlier proposal of methane-based life.

Titan's atmosphere is rich in hydrocarbon compounds, and chemical changes in the upper atmosphere are driven by the arrival of ultraviolet light from the sun. One of the expected results of the UV exposure is the liberation of molecular hydrogen from methane via a process that produces more complex hydrocarbons. With little oxygen to react with, the molecular hydrogen should remain stable. Some of it will escape into space, but a new paper indicates that a substantial amount of that hydrogen migrates down through the atmosphere towards Titan's surface.

Since it's not accumulating there, some chemical process must be removing it from the atmosphere; right now, we don't know what that process is, and, as NASA's own news piece on the topic notes, the first option for scientists is to consider simple chemistry.

However, the abstract of the paper notes that this level of hydrogen consumption is consistent with an earlier prediction of methanogenic life. In short, the life would get its energy by "burning" the hydrogen with a carbon source instead of oxygen, releasing methane (CH₄) in the process. The source of the carbon is where a second paper (not yet online) comes in. Models of Titan's upper atmosphere suggest that significant amounts of acetylene should be produced by the reactions there, and this would provide an excellent source of carbon to any hypothetical metabolisms. The surprise of the second paper is that there's very little acetylene to be found on Titan's surface.

Two chemical enigmas certainly don't constitute life, and the authors of the latter paper provide a variety of ways to account for the acetylene shortage that don't involve an organism. It's also important to remember that there won't be anything resembling liquid water on the surface of Titan, so anything alive there would have to be living in a methane/ethane soup (not to mention at temperatures nearing -200°C).

Scientists are a cautious bunch, and it's likely that these results will remain in limbo for a while. The discovery of plumes of methane in the atmosphere of Mars was another chemical enigma that might be evidence for life. It's been about a year and a half since their announcement and nobody has come up with a satisfying explanation for their presence (at least as far as I'm aware), but the scientific community is nowhere close to ready to call that conclusive evidence for life.