Something strange is happening on Titan

New findings on Saturn’s hydrocarbon-shrouded moon Titan reveal anomalies that although are likely explained by chemical processes, still leave the room open for the possibility of life.

Titan, with fellow moon Tethys in the background. Image: NASA/JPL/SSI.

If correct then the two findings – a depletion of hydrogen and the apparent absence of acetylene at the surface– point to some surprising activity on Titan, which is the ringed planet’s largest moon and is covered in lakes of liquid methane. Molecular hydrogen is the third most common molecular species in Titan’s atmosphere, and in 1980 and 1981 the Voyager spacecraft measured its molar fraction to be 0.001 in the lower atmosphere (a mole is the mass of a substance with the same number of particles as 12 grams of carbon-12, and the molar fraction is the ratio of the number of moles of one substance compared to other substances). However, measurements of the upper atmosphere by NASA’s Cassini spacecraft find a molar fraction of 0.004. The fact that the molar fraction is four times higher in the upper atmosphere, 1,000 kilometre high, is odd because hydrogen is the lightest of all elements and therefore easily escapes into space.

To attempt to solve this puzzle, Professor Darrell Strobel of Johns Hopkins University modelled the flow of hydrogen in Titan’s atmosphere as a computer simulation, which showed that the downward flow of hydrogen from the upper atmosphere to the surface should be 10,000 trillion trillion molecules per second. The lower atmosphere should therefore have much more molecular hydrogen, and Strobel’s computer models, the results of which are published in the journal Icarus, beg the question ‘where does it all go?’

There are two possibilities. Because Titan’s hydrogen comes from molecules of methane being split into carbon and hydrogen atoms by ultraviolet light from the Sun, it is possible that they could then recombine at the surface. However, at temperatures of –179 degrees Celsius, any chemical reactions would proceed very slowly and, with a lifetime of 80,000 years in the atmosphere, the hydrogen should build-up to high levels unless there was a catalyst to speed this reaction along.

“The best guess for a chemical process would be some metal, such as iron particles in a mineral, acting as a catalyst,” Strobel tells Astronomy Now. The discovery of a catalysing mineral on Titan would be a surprising find, but not nearly as revelatory as the second possibility for explaining the hydrogen depletion.

In 2005, Heather Smith and Chris Mckay of NASA Ames Research Center published a speculative paper about the possibility of primitive microbial life on Titan, which would be based around liquid methane rather than water. If such life existed and was consuming hydrogen the same way we do oxygen and plants carbon dioxide, they suggested that it would then, “have a measurable effect on the hydrogen mixing ratio in the troposphere [lower atmosphere],” which to all intents and purposes is what Strobel’s computer model indicates. However, in a statement written by McKay in response to these findings (here) he points out that life is the least likely possibility, while Mark Allen, the Principal Investigator of the Titan team at NASA’s Astrobiology Institute, also remains skeptical.

“Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed,” he says. “It is more likely that a chemical process, without biology, can explain these results.”

However, a second report published in the Journal of Geophysical Research by Dr Roger Clark of the US Geological Survey provides further tantalising and yet inconclusive clues that something strange is happening on Titan. Using data from Cassini’s Visual and Infrared Mapping Spectrometer, Clark reports the absence of the hydrocarbon molecule acetylene on the surface, backing up the non-discovery of the substance by the Huygens landing craft in 2005. According to McKay, acetylene is another compound that would be consumed for chemical energy by Titan lifeforms (which he calls methanogens because of their dependence on liquid methane). However, Mark Allen points out that cosmic rays arriving from space could transform acetylene in the atmosphere into more complex organic molecules, thus hiding its existence.

Ethane is another hydrocarbon molecule that has been long-known to be unusually depleted on Titan’s surface. “The evidence for less ethane and less acetylene than expected seems clear an incontrovertible,” writes McKay. The question of whether chemical or biological processes are to blame is still an open one, but without further evidence it would seem that the safe money is going on the non-biological explanation. One thing is for sure: even after all that we have discovered in six years of fly-bys by the Cassini spacecraft, Titan is still proving to be a moon of mystery.