Life on Titan's world of goo
In the August 8 issue of Science, an international team of scientists has a paper that submits evidence of life in an asphalt lake in Trinidad. Despite having a low water content of 13.5%, it still possesses methane-digesting microbes huddled up in tiny water droplets. One of the authors, Dirk Schulze-Makuch, speculates in an Air & Space Magazine article that the find could have important implications for Saturn’s moon Titan, which is wrapped in chemistries similar to what was found in the lake minus the presence of liquid water.
In fact, its atmosphere is mainly nitrogen, with lakes of liquid methane and ethane on its surface. So, that there are extremophiles living in a world of goo means not all hope is lost for alien life to form on Titan, no matter that such hopes are still too far beyond the ambit of scientific conservatism inspired by how little we know about life’s origins. Nevertheless, the Science paper isn’t the first to demonstrate that life can exist in such extreme conditions similar to those spotted on planetary bodies in the Solar System; in fact, going by previous reports, it isn’t likely to be the last either.
In a 2011 study published in Microbial Biotechnology, South American researchers reported the presence of a fungus, Neosartorya fischeri, that could metabolize asphaltene, “which is considered the most recalcitrant petroleum fraction”. Their work in turn draws from a 1993 study that proved asphalt is susceptible to reacting with certain extracellular enzymes.
However, Schulze-Makuch’s article makes many assumptions. For example, Titan is much colder than Trinidad’s Pitch Lake, a tropical deposition of oil rising up from a tectonic fault at its bed. For another, it is not known if Titan harbors liquid water, which – at least on Earth – is known to decisively encourage the formation of life, just as it did in the lake.
Fortunately – rather, optimistically – astrobiologists have been able to rationalize how life could form on Titan. In 2005, Chris McKay and Heather Smith, both astrobiologists at NASA Ames Research Center, were able to come up with a mechanism by which methanogenic microbes in Titan’s troposphere could be metabolizing acetylene, ethane and some other organic compounds – of which the moon has plenty – to release 54-334 kJ/mol, an amount of energy that similar extremophile critters on Earth have been known to get by on.
They also think it’s possible that the microbes could be catalyzing biochemical reactions despite the low temperature, around -180 degrees Celsius. In either case, their calculations are dependent on the microbes consuming hydrocarbons along with atmospheric hydrogen – an adjustment for convenience. Being a gas with no other sources or sinks in Titan’s atmosphere, any dip in its concentration could be a sign of life, albeit a distant one. McKay had said in a NASA press release in 2010 that “We suggested hydrogen consumption because it’s the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth.”
His and Smith’s hypothesis found some validation in that year – 2010 – when the Cassini space probe found anomalous deficiencies of hydrogen and acetylene, which should be evenly distributed around the moon but weren’t, meaning they were disappearing into somewhere or something, like being consumed. “If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth,” McKay had said.
Just as well, some other astrobiolgists think the cosmic rays bombarding Titan’s atmosphere could be transforming acetylene into more complex hydrocarbons, constituting the non-biological explanation that scientists would like to have out of the way first. Even today, this attitude hasn’t changed because the basis of methanogenic life is still very theoretical, a possibility hinged on chemical reactions worked out by supercomputers. Yet, it’s a tantalizing possibility.
In a 2004 paper by Steven Benner, University of Florida, et al, the authors discuss how life could form without liquid water if only a few other conditions are met: a thermodynamic disequilibrium (a natural mechanism to maintain periodically varying temperatures), “temperatures consistent with chemical bonding” and the presence of a solvent system. The paper itself begins by questioning not how life originated but, in deference to its great adaptability, why life on Earth is what it is.
I reproduce a paragraph from it that I find provides a fitting explanation to why the search for life on Titan (and perhaps also Io and Enceladus) is worth keeping up:
The universe of chemical possibilities is huge. For example, the number of different proteins 100 amino acids long, built from combinations of the natural 20 amino acids, is larger than the number of atoms in the cosmos. Life on Earth certainly did not have time to sample all possible sequences to find the best. What exists in modern Terran [i.e. Earth-bound] life must therefore reflect some contingencies, chance events in history that led to one choice over another, whether or not the choice was optimal.
It’s also after 2004 – in 2013, actually – that we also discovered that Titan might be running out of methane soon. Studies conducted since around 2005 showed that the moon’s source of methane could be less from photochemical reactions in its atmosphere and more from subsurface pockets where the gas could have been trapped. According to a NASA statement, “the current load of methane at Titan may have come from some kind of gigantic outburst from the interior eons ago possibly after a huge impact,” and could run out in tens of millions of years, a short span on the geological timescale.
If so, then, if methanogenic life hasn’t already formed but is likely to, it better do so quickly. If our models have an as yet undetected or undetectable flaw, then, as always, time will tell. If, ultimately, life is already present on Solar System’s second-largest moon, then one can only hope it’s as versatile as the world that hosts it.
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References
- Scientists Find Life in a Lake of Oil, Air & Space Magazine. Accessed August 10, 2014.
- Meckenstock, R.U. et al, Water droplets in oil are microhabitats for microbial life. Science, 8 August 2014: 345 (6197), 673-676. doi: 10.1126/science.1252215
- Uribe-Alvarez, C., Ayala, M., Perezgasga, L., Naranjo, L., Urbina, H. and Vazquez-Duhalt, R. (2011), First evidence of mineralization of petroleum asphaltenes by a strain of Neosartorya fischeri. Microbial Biotechnology, 4: 663–672. doi: 10.1111/j.1751-7915.2011.00269.x
- Fedorak, P.M, Semple, K.M., Vazquez-Duhalt, R., Westlake, D.W.S., Chloroperoxidase-mediated modifications of petroporphyrins and asphaltenes. Enzyme and Microbial Technology, Volume 15, Issue 5, May 1993, Pages 429–437. doi: 10.1016/0141-0229(93)90131-K
- Tobie, G., Lunine, J.I. and Sotin, C., Episodic outgassing as the origin of atmospheric methane on Titan. 28 November 2005, Nature 440, 61-64. doi:10.1038/nature04497
- Benner, S.A., Alonso Ricardo, A. and Carrigan, M.A., Is there a common chemical model for life in the universe?. Current Opinion in Chemical Biology, 2004, 8:672–689. doi: 10.1016/j.cbpa.2004.10.003
