The twin questions of how and where life could begin from prebiotic chemistry are pretty big ones. Indeed, despite the now famous Miller-Urey experiment creating amino acids by zapping simple chemicals with lightning bolts, we’re not much closer to a proper answer for that question. A good place to look for answers though, is Titan. Out on Titan, methane acts like water does on Earth, and the mountains are made from ice and not silicates. Despite these outlandish differences, many believe that the two planets were very similar when they first formed. The only difference essentially is that Earth was simmered on a moderate heat for 4 billion years, while Titan was left in the deep freeze. Even so, some interesting things might have been forming in its atmosphere since then. Things like nucleobases.
Instead of using electrical discharges like Miller and Urey, this latest study concerns photochemistry (a subject rather close to my heart). With it’s thick atmosphere of hydrocarbon broth, Titan is like a photochemical laboratory for planetary scientists. We know within reasonable doubt that it’s distinctive orange haze is coloured by hydrocarbons, not entirely unlike the photochemical smogs that hang over cities like Los Angeles.
So then, you have the basic premise of an investigation so simple, it’s a wonder no one’s already done it. As Titan has spent it’s entire 4.5 billion year* life being bathed in ultraviolet and soft x-rays from the Sun, could anything prebiotic have formed there? Titan’s atmosphere is composed mostly of N2 and CH4, both of which can be easily torn apart by certain frequencies of ultraviolet and x-ray radiation. The resulting molecular fragments are free to recombine into whichever form is most stable. Forms such as adenine.
As has been noted in other papers I’ve read previously, adenine is thermodynamically stable. In fact, it’s the thermodynamic sink for anything with the empirical formula of C5H5N5. With an atmosphere full of carbon, nitrogen and hydrogen, you’d expect, logically, that if Titan’s atmospheric chemistry is driven by sunlight, at least some adenine should form.
Well, so did Pilling et al in this paper. Subsequently, they recreated an analog of Titan’s atmosphere in the lab to try and see what would happen if you hit it with energetic radiation. Their results were very interesting. The experiment yielded a complex mixture of organic molecules, including both aromatic and aliphatic nitriles. This formed a thick tholin** (the reddish tarry stuff that litters many icy moons in the outer solar system). The authors note that the layers of Titan tholin are expected to be over ten metres deep in places. This tholin has been slowly falling onto Titan’s surface for billions of years. Evidently, it could be rich in all sorts of prebiotic chemicals too!
So what about adenine? Well interestingly, they didn’t find much using infrared spectroscopy (a standard tool in chemistry, and one of the few we have when looking through telescopes. The only spectral lines they found were weak, making the identification tenuous at best. They didn’t detect any adenine until they took the sample and put it into some slightly more hardcore analytical chemistry devices. Mass Spectra and NMR showed the fingerprints of adenine… Although there’s always a chance that they were just picking up adenine precursors. Perhaps the molecule itself would take longer to form in any significant quantities. Also, they note that no amino acids were created. Which is interesting, but by no means implies that Titan is free of amino acids…
So it seems, an environment like Titan’s could readily form adenine — one of the nucleobases of DNA and a molecule widely found in biology. Usually in things with lovely 3 letter abbreviations, like ATP, DPN, FAD and suchlike.
You have to wonder what other interesting things Titan’s haze might be hiding. Furthermore, with all of the complex chemistry there, you have to wonder about a point the authors make in their discussion. In a few billion years, when the Sun exhausts it’s hydrogen and swells into a red giant, Titan might have a chance to host Earth-like life. Who knows…?
*Actually, while the author’s state the figure of 4.5 gigayears, it’s probably closer to 4 gigayears (closer to the estimate of Earth’s age). The Sun is roughly 4.5 gigayears old, and it’s quite likely that Titan would have formed later.
**It’s also worth noting that while the authors refer to tholin as a “polymer”, this isn’t accurate. “Oligomer” would be a better term to use.
Pilling, S., Andrade, D., Neto, A., Rittner, R., & Naves de Brito, A. (2009). DNA Nucleobase Synthesis at Titan Atmosphere Analog by Soft X-rays The Journal of Physical Chemistry A DOI: 10.1021/jp902824v