The hazy methane-red surface of Titan. NASA/JPL
Even before the wildly successful Cassini-Huygens mission to Saturn and its lunar neighborhood, scientists have looked at the largest moon, Titan, studied it with telescopes and other instruments, noted its methane-rich atmosphere, its extreme cold (around 90 degrees Kelvin, -183C or -290F), and wondered if somehow in its vast collection of organic materials – life of some kind might exist. Almost certainly it would not be life as we Earthlings know it. Our life is water based. On Titan water is frozen so solid, it’s more like iron. No, this would have to be life using other basic materials – hydrogen perhaps, or acetylene, and methane.
Scientists are not immune from having imagination (far from it!), but to give scientific imagination shape and form, what does many a modern astrobiologist do? They make a model, a computer representation of whatever it is they imagine. In this case, they imagine life on Titan and try to model what it would be like, given the materials and conditions that are known to exist on that moon. Once the models are constructed – part math, part computer programming, and part art – they then make predictions. “Thus and so will be found to be true on Titan….” And then they wait for information (data) to confirm or refute their predictions. Some of that information has coming streaming back to Earth from the Cassini mission and its many flybys of Titan (and to an extent from the Huygens probe that landed on Titan’s surface).
What two recent papers find in the Cassini data is interesting because it is consistent with some models of possible life on Titan. They do not say there is life on Titan.
One paper in the journal Icarus [Science Direct, Molecular hydrogen in Titan’s atmosphere] deals primarily with hydrogen. Hydrogen is created in the Titanian atmosphere by ultraviolet sunlight (weak though it is) breaking down methane and acetylene. Models predicted that hydrogen molecules would be evenly distributed in Titan’s atmosphere. According to the Cassini data, they’re not. Apparently about as much hydrogen flows to the surface of Titan as escapes from the atmosphere…and the surface hydrogen molecules disappear. What happens to the hydrogen?
Another paper, about to appear in the Journal of Geophysical Research, studied the distribution of hydrocarbons on the surface of Titan and discovered that there is no acetylene. The models expected acetylene, also produced in the atmosphere, would drift down and coat the surface of Titan. What happened to the acetylene?
Both reports say, in so many words, they don’t know what happens to either the acetylene or the hydrogen. However, (and howevers are interesting to scientists) these findings are consistent with the possibility that some form of life may be consuming them. In a chemical sense, hydrogen could play the role of a biological oxidizer – an accelerator of chemical processes. Acetylene could be a primitive energy source for a methane-based form of life. There are, after all, organisms on Earth that live in a methane environment…
Now, stop here and heed the words of Mark Allen, principal investigator with the NASA Astrobiological Institute:
“Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed,” Allen said. “We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results – for example, reactions involving mineral catalysts.”
That there is organic chemistry happening on Titan is not in doubt, in fact, it appears to be chemically a very dynamic place – despite the super-cold conditions. Remember that in chemistry organic does not equal biological. There are probably enzymes and other substances at work (some of which have been detected but not yet identified) that could produce the organic results such as conversion of acetylene without being even remotely ‘alive.’
As is so often the case in science, more data is needed. Since the Cassini probe is scheduled to make additional passes at Titan, there will be the opportunity to have its instruments look for things that might shed light on the organic chemistry of Titan. Short of finding some kind of methane-based life, it’s that organic chemistry, which is most interesting. Things are happening on Titan (and in its atmosphere), which will keep astrochemists and astrobiologists busy for many careers.