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).
C.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