The recent confirmation of the existence of a molecule called Acrylonitrile (also known as Vinyl Cyanide) in the atmosphere of Titan has me thinking about the possibilities of alternative biochemistries. This confirmation (following up on previous data from several years earlier) is interesting because Acrylonitrile has been shown in a computer model done in 2015 to self-assemble into sheets and vesicles when dissolved in liquid methane similar to how lipids form membranes in liquid water. These structures are dubbed “Azotosomes”, similar to the term Liposome for the lipid bi-layer structure that makes up the cell membranes of terrestrial life. While definitely not a sure sign of alien life, it does raise some very important questions: Can life, or something similar, arise in conditions vastly different than that of Earth? Could there be aliens swimming in the methane seas of Titan, perhaps?
Before trying to tackle this question, let’s review some facts about Titan.
Titan: Nature’s Petrochemical Lab
Titan is the largest moon of Saturn. It is unique among any of the moons in the Solar System in that it has a complex, substantial atmosphere which is chemically active and dynamic, with clouds, haze, and even rain. Titan’s atmosphere is composed almost entirely of Nitrogen, with a few percent methane and ethane and trace amounts of other hydrocarbons. The thick orange haze is a result of ultraviolet light from the sun breaking down methane and other molecules and reforming them into complex, long-chain hydrocarbons. In effect, the whole moon is shrouded in a world-wrapping blanket of natural smog.
Titan is also absolutely frigid. With an average surface temperature of -180 C°, it is far too cold to support any kind of life we know. Titan’s crust and surface are made up mainly of water ice instead of rock, and at such cryogenic temperatures, it is materially similar to granite in terms of hardness.
Titan is also the only world in the solar system other than Earth that has stable bodies of liquid on its surface. As it is far too cold for water to ever be a liquid, Titan’s seas, lakes, and rivers are instead filled with liquid methane and ethane. Vast seas larger than Earth’s Great Lakes in North America dominate the northern polar region. Titan’s lakes are fed by a methane cycle analogous to the water cycle on Earth; methane evaporates from the surface, condenses into clouds, falls back down as methane rain, and carves out channels and rivers as it flows back to the lakes and seas to evaporate again.
In addition to the methane rain, the complex photochemistry in Titan’s haze layers produces a chemical precipitate called tholin, which is a tar-like mix of various hydrocarbons. This tholin probably coats the ground like a thick mud in some places. Additionally, the equatorial regions are home to vast and gigantic dune fields. Instead of silicate sand, these dunes are made up of hydrocarbon grit mixed with grains of water ice eroded from the hillsides.
As alien as it is, superficially Titan is quite Earth-like. In fact, the chemistry happening in the atmosphere and on the surface is in some ways very similar to what was thought to have happened on early pre-biotic Earth. So could the precursors to life be present on Titan? Perhaps. But if anything is actually alive here, it would have to be very, very different than anything we know. First, with no liquid water, any Titanian life would need something else to act as a solvent for its biomolecules. Water is such a good solvent mainly because it is a polar molecule; the oxygen atom is highly electronegative compared to the two hydrogen atoms bonded to it, and this gives it a slight charge imbalance. Methane, on the other hand, is non-polar, having a more balanced charge distribution. This makes it a poor solvent and as such, the list of compounds soluble in methane is far shorter than for water.
So we have a molecule that can spontaneously form membrane-like structures that would work well for partitioning their interiors off from the surroundings, one of the first basic requirements for a living cell. And it should work in cryogenic liquid methane. Do we know of any molecules or compounds that could carry out pre-biological or even fully-blown biological reactions inside of it? After all, there’s little point in speculating about alternative biochemistries if there is no actual biochemistry to be had.
Noted astrobiologist Chris McKay suggests that one possible chemical pathway life on Titan could exploit is to react naturally-occurring acetylene with Hydrogen, which could serve as an energy source to power exotic metabolisms (unfortunately the paper is behind a paywall, but the abstract is available for free). This reaction would also release methane as a waste product, helping replenish the chemical as it is destroyed by solar UV radiation in the upper atmosphere. If such a process was happening on or near the surface of Titan, we’d expect to see a sudden drop in the concentration of Hydrogen near the surface, as well as far less acetylene than would be expected.
Curiously enough, in 2010, Johns Hopskin University research Darrell Strobel found just such evidence. While analyzing the concentration of Hydrogen in different layers of Titan’s atmosphere, Strobel discovered that Hydrogen was very abundant in the upper atmosphere, but rather depleted further down, driving a downward flow towards the surface. Most interestingly, however, was the fact that the Hydrogen seemed to just disappear near the surface. Another paper published that same year observed far less acetylene than expected near the surface as well.
Could these be a potential smoking gun for aliens? Well, maybe. Strobel himself agreed that it was consistent with McKay’s predictions. However, there could be any number of non-biological explanations for the missing Hydrogen and acetylene, as well as non-biological mechanisms for methane replacement. It could even just be human error. Ultimately, another spacecraft will have to be sent to Titan to follow up on these results. But frustratingly, with Cassini fated to dive into Saturn’s atmosphere this Septemeber, ending its 13-year long mission, no spacecraft will available to study Titan up close in the foreseeable future.
But a few days ago I came across an abstract from a paper published in 2015 about the potential for a group of molecules called polyethers to function as the backbone of a genetic biopolymer in liquid hydrocarbons. Unfortunately, the paper is behind a paywall, but I was able to request a copy from my university library. This is incredibly important since the only naturally occurring genetic biopolymer we know of needs to be dissolved in liquid water to function; DNA. There is one downside, though: the methane seas of Titan are far too cold for polyethers to remain soluble. If there is life on Titan, it’s DNA-equivalent molecule won’t be using polyethers. Perhaps something else on Titan could do the job, but right now, it’s not clear what that could be. It’s possible that Titan might just be too cold for even exotic hydrocarbon life. Does that rule out Titan as a home for life? Not definitively, I think. Even if the surface is too cold even for exotic hydrocarbon biochemistry, Titan is thought to have a subsurface water-ammonia ocean deep beneath its icy crust. That life, however, would be much more difficult to find.
Polyether-based genetic molecules would be well-suited to warmer worlds with a similar environment to Titan, though. “Warm” is a relative term here, of course; 85 to 231 K, or -188 C° to -42 C°. And rather than being dissolved in liquid methane, these polyethers would be most soluble in liquid propane. And while methane and propane are both non-polar molecules and therefore are rather poor solvents, in general, the warmer a substance is the better a solvent it becomes. Thus, a “warm” propane ocean is a better solvent than lakes of cryogenic liquid methane.
The area of the phase diagram over which a substance is liquid plays a huge role in how well that substance can form stable bodies of liquid on the surface of a world. Compared to propane, methane is a liquid over a far smaller range of temperatures and pressures. On Titan, this causes the lakes and seas to be confined to the colder polar regions. Water’s liquid range is also rather broad, which is another reason why it is such a good solvent for biological processes. On a world even marginally warmer than Titan, methane would likely be unable to condense into lakes and seas (although the upper atmosphere could get cold enough for clouds to occasionally form), but propane would still be liquid, and a propane cycle could drive weather patterns similar to Earth and Titan. Azotosomes could still possibly form, and now they will also have interesting molecules to stuff inside themselves. Provided Hydrogen and acetylene are also present, the same hydrocarbon-based metabolism outlined by McKay could also play out inside them. Now all that’s needed is an actual self-replicating, information-carrying molecule that can undergo Darwinian evolution.
So we’ve got a potential backbone, but we still need a nitrogenous-base analog; something to actually encode genetic information with. None are currently known, but that doesn’t mean they don’t exist. There are hundreds of billions of planets in our galaxy, each one is an independent chemical laboratory carrying out world-spanning experiments in parallel. If there are viable alternative biochemistries which don’t need water, they exist somewhere.
Until next time, keep looking up.
References and Additional Reading:
James, S., Lunine, J., & Clancy, P. (2015). Membrane alternatives in worlds without oxygen: Creation of an azotosome. Science Advances, 1. Retrieved from http://advances.sciencemag.org/content/advances/1/1/e1400067.full.pdf
Strobel, D. (2010). Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions. Icarus. Retrieved from https://web.archive.org/web/20120824195338/http://astrobiology.jhu.edu/wp-content/uploads/2010/06/Icarus-2010-Strobel.pdf
McLendon, C., Opalko, F. J., Illangkoon, H. I., & Benner, S. A. (2015). Solubility of Polyethers in Hydrocarbons at Low Temperatures. A Model for Potential Genetic Backbones on Warm Titans. Astrobiology, 15(3), 200-206. doi: 10.1089/ast.2014.1212
Cook, J., & Weselby, C. (2010, June 03). What is Consuming Hydrogen and Acetylene on Titan? Retrieved from https://web.archive.org/web/20110629185640/http://www.jpl.nasa.gov/news/news.cfm?release=2010-190