If you’re looking for liquid water on Titan, prepare to go deep, perhaps as much as 100 kilometers below the Saturnian moon’s crust, which is itself made of ice. When it comes to exoplanets, we always talk about the habitable zone as a place where liquid water could exist on the surface. Titan clearly fails that test. But is it a place where life could exist anyway?
A new paper gets us into this interesting topic by suggesting that prebiotic chemistry — and possibly even biochemistry — could take place on Titan. The work of Martin Rahm and Jonathan Lunine, working with colleagues David Usher and David Shalloway (all at Cornell University), the study sees Titan as a ‘natural laboratory’ for exploring non-terrestrial prebiotic chemistry given the presence of liquid hydrocarbons and the lack of liquid surface water.
Image: An image of Titan’s surface, as taken by the European Space Agency’s Huygens probe as it plunged through the moon’s thick, orange-brown atmosphere on Jan. 14, 2005. Today, Cornell scientists have chemical evidence that suggests prebiotic conditions may exist there. Credit: ESA/NASA/JPL-Caltech/Univ. of Arizona.
We are in the realm of prebiotic chemistry in an exceedingly alien environment. Says Rahm:
“We are used to our own conditions here on Earth. Our scientific experience is at room temperature and ambient conditions. Titan is a completely different beast. So if we think in biological terms, we’re probably going to be at a dead end.”
Titan’s rivers, lakes and seas have captivated us since the Huygens probe gave us our first glimpse beneath the clouds, and we know that they are filled with liquid methane and ethane flowing beneath a dense atmosphere of nitrogen and methane. Chemistry here is driven by solar photons that produce hydrocarbons and nitrogen-bearing organics. The Cassini mission has shown us that hydrogen cyanide (HCN) is the most abundant nitrogen-bearing product resulting from this atmospheric chemistry, condensing into aerosols that, upon reaching the surface, are evidently transformed into other molecules and polymers (chemical compounds where molecules are bonded together in long, repeating chains).
One of the polymers that may emerge from hydrogen cyanide’s reactions with other molecules is polyimine (pronounced poly-ee-meen), which is flexible even under the low temperatures found on Titan. The paper analyzes the properties of polyimine (pI) and finds that even in these conditions, it can absorb solar energy and become a factor in possible life:
Regardless of the specific chemistry involved, life requires polymorphic molecules that combine flexibility with the ability to form the organized metastable structures needed for function, adaptation, and evolution. This, almost certainly, requires extended molecules capable of intermolecular and intramolecular hydrogen bonding, but such bonds need not involve oxygen; nitrogen is a potential surrogate.
The polymorphism of polyimine compounds could be the key to prebiotic chemistry in the cryogenic conditions of Titan. The authors’ work on pI shows that complex, ordered structures can emerge. Moreover, polyimine is found to be able to absorb a wide range of photons through a relatively transparent ‘window’ in Titan’s atmosphere, thus having a source of energy at its disposal to catalyze prebiotic chemistry even without the presence of water.
But the larger seas that are so striking in Cassini imagery are not necessarily where it would first emerge. These are mostly methane, with significant amounts of ethane and nitrogen. Hydrogen cyanide is mostly insoluble in such mixtures, making it unlikely that polymers would form there. The paper argues that we’re more likely to find hydrogen cyanide undergoing reactions in tidal pools near the shores of the seas and lakes, where the environment is dynamic because of tidal effects (Titan’s orbit is non-circular), and hemispherical variations in sunlight:
Seasonal emptying and refilling of the smaller liquid “lakes,” such as Ontario Lacus in the Southern Hemisphere, and longer-timescale variations in sea levels associated with Saturn’s orbital variations, may allow cycling of these materials between liquid and dry (shoreline) environments, where they would be well positioned to undergo further chemistry.
We learn that possible prerequisites for life do exist on Titan, but what lies ahead is a deeper understanding of how this chemistry evolves. In terms of possible future missions, remote sensing here gives way to the need for direct chemical analysis on the surface, doubtless through a lander sent to shoreline areas of the lakes and seas. As the paper notes, “…only future exploratory missions to Titan can test the hypothesis that natural chemical systems evolve chemical complexity in almost any circumstance.”
The paper is Rahm et al., “Polymorphism and electronic structure of polyimine and its potential significance for prebiotic chemistry on Titan,” published online by Proceedings of the National Academy of Sciences 4 July 2016 (abstract). Thanks to Phil Tynan for an early tip on this work.
I can certainly see the attraction of looking at polyimines as they show a similar structure to polypeptides, suggesting that they might indeed mimic protein chemistry and reactions (e.g. catalysis). It would indeed be very interesting to see how far this work can go, and what might be worth detecting and searching for on Titan. If we think of just the metabolic side of life, it would be interesting to know what sort of energy flows could be harnessed and controlled by this chemistry. On Earth, a very different type of polymer handles information storage, but is this necessary, and can a different chemistry substitute, however poorly? All fascinating ideas for astrobiologists to ponder, with implications for the search for life on extra-solar planets.
I’m glad that the PR pieces don’t sensationalize this work, and keep it strictly as prebiotic chemistry, rather than hinting that there might be primitive life on Titan (although that hasn’t stopped the consumers hyping the story).
I have never heard of polyimines before; in fact, the interwebs don’t furnish much explanation of what they are. Any definitive sources for this? Thanks!
This is fascinating work, but we do have some notion of the upper limit on the size of possible life on Titan, as the Huygens images showed what looked like a (very very very cold) desert. We didn’t see anything that looked like the equivalent of trees or bushes or elephants, so if there is any life, it must be rather small.
I’d love to see an Enceladus/Titan mission chosen as New Frontiers 4. An informative overview of NASA thinking on Ocean Worlds missions came at the March meeting of the Committee on Astrobiology and Planetary Science (a summary of which can be found here: http://futureplanets.blogspot.de/2016_04_01_archive.html ).
Not a coincidental publication. The Draft Announcement of Oportunity for NF4 is out this month . Any day now. This team have a record of numerous concepts for Enceladus and/or Titan including JET, ELF,the TiME lander and THEO. I have it on reliable authority we can exoect to see orbiter and lander concepts though whether the budget , perhaps with foreign help ( to a maximum of one third ) , can extend to a joint Enceladus /Titan orbiter AND a lander is unclear . Perhaps I’m being unrealistic and greedy but I hope so. JET and the Time lander where $425 million Discovery submissions . New Frontiers is $850 million plus possibly two $30 million incentives for technology demonstration / use with an extra $300 million allowed for a foreign contribution in cash or equipment , so you never know….. Interesting 18 months or so until a decision made in 2018.
Can anyone tell me if there have been results released following the E21 Enceladus flyby pertaining to molecular Hydrogen? Enceladus may have lost some of it’s exobiological allure if Matija Cuk (http://arxiv.org/abs/1603.07071) is correct, but I’ve been waiting for word on that E21 data.
For those interested, here a link to a short paper from the 47th Lunar and Planetary Science Conference in March on the detection of molecular hydrogen at Enceladus as measured by Cassini’s INMS. http://www.hou.usra.edu/meetings/lpsc2016/pdf/2885.pdf
I’m no professional chemist (let alone biologist), but the extreme low temperature must surely put a severe dent in any putative reaction rates?
A novel by Stephen Baxter was set on Pluto, involving some form of life; Greg Benford’s “Sunborn” is a fuller (and well-developed)treatment. If Benford says there’s life on Pluto must be so…
Earth was very cold in prebiotic times so….it sure is worth keeping an eye on. There is no complex life anywhere but Earth in this solar system but we have just begun to look for simple forms and prebiotic activity. It so exciting to have landed on Titan and Juno…let’s hope for suprises
I still hold out hope for intelligent squid-people below the surface of Europa!
More seriously, it is indeed possible that multicellular life of some complexity may have evolved on the various subsurface ocean moons.
That’s been one factor that bio-pessimists have brought up before about Titan. If one presumes that abiogenesis and evolution are governed at their base by chemical reaction rates, the far slower rates on Titan would suggest that much less “prebiotic time” has passed there than somewhere warmer (like Earth). Even if Titan has the chemical potential for life, slow reaction rates may have prevented anything from actually appearing yet.
But that is the beauty of Titan. It is preserving prebiotic chemistry that has long vanished from Earth. It might give us clues on abiogenesis. It is like a giant cold store that may have samples of chemicals that are analogs of those that appeared on the early Earth before life emerged and destroyed them.
This might not matter in a panspermia scenario; The life could have originated elsewhere over a very long period of time. Since cold bodies can hold an atmosphere with much less gravity, (Titan itself demonstrates this.) correspondingly less energetic events could send cryogenic life into space. And the usual power laws suggest bodies comparable to Titan should be much more common than Earthlike bodies.
Regarding reaction rates, the flip side of Titan’s icy slow chemistry was of course explored in Robert L. Forward’s Dragon’s Egg, where strong force “chemistry” on the surface of a neutron star proceeds millions of times faster that terrestrial chemistry, and the organisms that sprout up there evolve correspondingly faster.
“Poly-ee-meen?” That is almost impossible to say aloud with any clarity, unless you self-consciously pause in the middle. Can we change the name? Or at least say it’s okay to pronounce it “poly-eye-meen” or “poly-im-meen” or something?
Hey what about Carl Sagan and Bishun Khare’s tholins?
http://www.planetary.org/blogs/guest-blogs/2015/0722-what-in-the-worlds-are-tholins.html
A paper on the subject:
http://www.baylor.edu/content/services/document.php/124336.pdf
Tholins may not be confined to our Sol system:
http://iopscience.iop.org/article/10.1086/592961/fulltext/
Imines are basically nitro analogues of ketones and aldehydes where the oxygen has been replaced by a nitrogen-alkyl group. You do see them in terrestrial biochemistry but often as intermediates (it’s how Vitamin B6 deaminates amino acids for example). They act as ligands in co-ordination chemistry and in Amadori rearrangements in carbohydrate chemistry. This work is actually quite exciting, as polyimines could perform an autocatalytic role (with the weak insolation you get at the Titanian surface) much like RNA is envisaged to do in the RNA world scenario.
Couple this with the speculation by Stephenson’s team (in Stevenson, Lunine & Clancy, “Membrane alternatives in worlds without oxygen: Creation of an azotosome,” Science Advances 27 February 2015; Vol. 1, No. 1) that acetonitriles could play the role on Titan that phospholipid membranes play on earth (i.e., making cell walls) and Shultze-Makuch’s speculation (in Schulze-Makuch D & Grinspoon DH Biologically Enhanced Energy and Carbon Cycling on Titan? 2005 arxiv.org/pdf/physics/0501068) about possible acetylene and free radical metabolism in the Titanian atmosphere, and we have a possible model for Titanian cryolife.
Andrew Palfreyman and Tulse both raised a valid objection – the Arrhenius rate constant drops by half for every 10 0C the temperature is lowered – but it’s not an insurmountable problem. The chemistries people are proposing here are highly reactive (some of these compounds only exist as short-lived metabolic intermediates on Earth at our ambient temperatures and free radicals are incredibly fleeting) so, yes, they could very well develop over reasonable timescales on cold worlds like Titan and sustain reasonable levels of biological activity (admittedly slow by our standards – but then many high latitude bacteria have slow metabolic rates on Earth.
As for the size of Titanian life – obviously I’m out on a limb here, but the limiting factor on Earth is surface tension and osmotic pressure. In a non-polar fluid these limitations are relaxed – so it’s conceivable you might get large-ish (i.e., naked eye visible) slowly metabolizing unicellular organisms living in a phtoautotrophic ethane-methane biosphere. Or not. We’ll just have to wait and see.
I think the limiting factor is how long has Titan had active organic chemistry.
Is it possible the source of Organic compounds is not native? What if it is
less than 200Mya. Consider an impact on Titan consisting of of carbonaceous chondrite asteroid. a bolide some 10 miles in dia, coming from deep in the oort cloud would have the power to melt the crust, spewing much of it’s carbon cargo skyward, but still melting through the Ice crust, leaving few signs of impact after a few million years.(Refreezing of ice + methane snowfall)
But assuming the Organic chemistry is ancient. Perhaps that “Island” on Ligeria Mare, that disappeared is not an Ice formation. maybe it’s a dense mat “Large unicellular organism” Mr Tynan describes in the prior post. Maybe is was a favored feeding ground, which once used up, caused a diffusion to a less concentrated feeding mode.
Thank you Phil!
I think it’s remarkable that we are essentially creating virtual life-forms based on tiny amounts of scientific data. Time will tell if our hunches prove correct.