Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Thursday, June 10, 2010

The (chilled) broth of life

A couple of interesting new results on Titan's atmospheric composition and chemistry have popped up and been quickly swamped by discussions of life on this frigid moon. The story goes; photochemically produced molecules of hydrogen and acetylene have been 'mapped' at different layers of the Titanian atmosphere, including close to, and on, the surface. Intriguingly the hydrogen distribution suggests a 'flow' towards the moon's surface followed by its disappearance. However, there is no obvious way for the gas to be incorporated into surface materials, so it's being transformed or used up by something as it reaches these low altitudes. Similarly, acetylene appears to drop off in abundance and is not present on the surface.

Measurements like these are notoriously tricky, based as they are on remote spectroscopic data from the Cassini mission and require careful consideration of the incompletely understood chemistry of Titan's atmosphere. Indeed, there are basic chemical mechanisms that could be responsible for yanking hydrogen and acetylene out of the atmosphere without putting a trace on the surface. Photochemistry - driven by ultraviolet photons trickling in from the distant Sun - can carry on transforming molecules in the chill 90 Kelvin atmospheric soup. New organic compounds can then just drift to the surface with a different spectroscopic signature. It requires a tough bit of scientific disentangling.

What's got some people excited is an earlier discussion by McKay and Smith in 2005. They pointed out that if some hypothetical organisms existed on Titan then they could derive plenty of chemical energy by reacting hydrogen with acetylene (along with ethane and some other organics). The apparent flow of hydrogen down to Titan's surface would seem to supply plenty of this chemical broth, more than sufficient for a considerable biosphere. It would also seemingly explain the lack of surface acetylene - it's just being gobbled up.

Is this just enthusiasm from a 'pro-life' brigade ? Well, it's certainly premature to suggest that this is a sign of a Titanian biosphere. Just because a chemical route to pulling energy from the environment exists doesn't mean anything is exploiting it. Without liquid water then methane might be the bio-solvent of choice, but that has a lot of challenges - it's a far less versatile solvent than good old H20. Nonetheless, it is pretty amazing that these results point towards exactly the kind of effect you might see if low-temperature, alien, life was slurping around on the shoreline of this enigmatic moon. Clearly Titan is an even more interesting place than we knew.

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