Totally Tropical Titan

Our solar system is full of mysteries, and it seems the more we look, the more we find, and the less we can explain. Most recently, the puzzles of the martian methane and the beautifully lethal venusian clouds have been joined by a new curiosity – Titan’s tropical lakes. I should hastily point out that tropical in this sense simply means that these lakes are near Titan’s equator. Don’t let the terminology fool you. The word “tropical” always invokes visions of balmy sunsets and warm ocean breezes in us, but you’re unlikely to find those on Titan. With an average surface temperature of around 94 K, the Titanian climate is nearly cold enough to chill oxygen into a liquid. Not that there is any oxygen on Titan. Suffice to say, it’s not exactly like the Caribbean…

All the same, in light of the discovery of these lakes, it seems like a good time to revisit the astrobiology of this smoggy little world. The idea has been considered for some time now of whether Titan might be able to support some kind of life. In fact, the idea was given quite some consideration by one Carl Sagan back in the 1970s. Since then though, ideas have developed that perhaps any prospective Titanian organisms might be methanogenic. The mechanism put forward by McKay and Smith in 2005 is that, in the absence of liquid water, an organism might metabolise molecular hydrogen and acetylene to give methane:

C2H2 + 3H2 ⟶ 2CH4

Life could potentially use a reaction like this to acquire energy comparable with the way methanogenic bacteria work here on Earth. The result would be drastically depleted H2 and acetylene (or ethane, or other larger hydrocarbons) at the surface, and an abundance of CH4. Interestingly enough, in 2010 some detailed calculations implied that there may well be a lack of both hydrogen and acetylene at the surface of Titan, and despite how easily it forms in Titan’s upper atmosphere, the Huygens probe could barely measure the ethane at ground level because it was so scarce. The latest discovery of methane lakes in regions of Titan where rain isn’t normally seen to fall would appear to add some further weight to this idea.

We, of course, shouldn’t get too excited just yet. There are still quite a number of missing pieces to this puzzle. Indeed, other reviews of the data have suggested that such methane-based life may be quite unlikely. All the same Titan’s methane has to be replenished somehow. Exactly as with the methane on Mars, Titan’s methane is destroyed by sunlight hitting the moon’s upper atmosphere. If not replenished, in a few tens of millions of years it would be all gone.

The ground on Titan is seemingly a lot like wet sand, damp with liquid methane. Huygens managed to detect plenty of methane after it landed, despite the instrument it was using being embedded several centimetres into the ground. Indeed, it’s quite likely that Titan is a “muddy” little planet with plenty of wet ground and humid air near its surface. This is coupled with the fact that Titan’s lakes are expected to be ephemeral in nature, appearing and disappearing over time.

All in all, while there’s no way we can definitively state that there is any kind of life on Titan, the evidence so far is compelling, it has to be said. In closing, a final thing to consider is the temperature. Yes, Titan is cold. Very cold. And both chemical and biochemical reactions would occur very slowly there. But remarkably, at least one species of Earth microbe, named colwellia, has been found to be capable of both surviving and metabolising at temperatures which would make Titan seem… well… genuinely tropical. Perhaps the door shouldn’t be fully closed on the idea of methanogenic titanian life just yet…

I have an addendum to this article.

(I’ve seen the Titan lakes story all over, but I think the original tip of the hat goes to Ikenbot).

Images: NOAA/NASA/ESA

This post was chosen as an Editor's Selection for ResearchBlogging.orgC.P. McKay, & H.D. Smith (2005). Possibilities for methanogenic life in liquid methane on the surface of Titan Icarus, 274-276 DOI: 10.1016/j.icarus.2005.05.018

Giuseppe Mitri, Adam P. Showman, Jonathan I. Lunine, & Ralph D. Lorenz (2007). Hydrocarbon lakes on Titan Icarus, 385-394 DOI: 10.1016/j.icarus.2006.09.004

Francois Raulin (2008). Astrobiology and Habitability of Titan Space Sci Rev, 37-48 DOI: 10.1007/s11214-006-9133-7

Darrell F. Strobel (2010). Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions Icarus, 878-886 DOI: 10.1016/j.icarus.2010.03.003

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About invaderxan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
This entry was posted in astrobiology, chemistry and tagged , . Bookmark the permalink.

20 Responses to Totally Tropical Titan

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  8. 0gre says:

    I am still awe-inspired by the fact that we (humans) accomplished a landing on Titan.

  9. Idle Emma says:

    Titan is a fascinating place. It would be wonderful if life was discovered on it. Or anywhere else in the solar system and beyond, I’m not picky.

    • invaderxan says:

      Oh, most certainly. It’s going to happen, I’m quite sure. The big question is, though – where will it be? I’d love to say Venus, but as far as the solar system, I think Europa really is the safest bet. As for outside the solar system? Well… Speculating about that is part of what this blog is for! :)

      • I’m starting to think that, given suitable initial conditions, life might be everywhere: Venus’ upper atmosphere, sub-surface Mars, Titan, outer-system ice moons with sub-surface oceans …

        • invaderxan says:

          I like the way you think! :)

          • It’s starting to look like planets etc. are natural by-products of the formation of stars. Could it be that life are natural by-products of the formation of planets etc. (and who knows, stars etc)?

            Unusual chemical disequilibria possibly indicative of life _have_ been noted in the atmospheres of Venus, Mars and Titan …

            • invaderxan says:

              It’s quite a logical conclusion to draw, and others have said the same before now. The only trouble is that, scientifically speaking, without proof all we have is speculation. Chemical disequillibria exist, certainly, and I’ve written about all three of those examples at great length here in this blog. But the trouble is that all of these have alternative explanations too. They just can’t be said to be unambiguously indicative. Not yet.

              Until someone scrapes lichen off a martian cave wall or scoops microbes from the venusian sky, all we can do is collect evidence and find reasons to keep looking closer. In my opinion, as of right now, that evidence is abundant. I look forward to the day they find conclusive proof…

            • invaderxan says:

              On a related note, that planets are a natural byproduct of star formation is, in terms of logical thought, an obvious assertion to make. Proving that this might be the case, however, has taken somewhat more effort…

  10. Ash says:

    So, life on Titan, when will they know for sure, one way or the other?

    • invaderxan says:

      It’s honestly impossible to say. We’d need to go there and take a closer look below the clouds, and no one’s planning on going back to Saturn in the near future. The Huygens craft only transmitted for about 90 minutes after it landed. If we could send a rover, like the ones on Mars, that would definitely help!

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