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An Astrobiological Role for Titan’s Complex Chemistry?

Although Titan is often cited as resembling the early Earth, the differences are striking. Temperature is the most obvious, with an average of 95 Kelvin (-178 degrees Celsius), keeping water at the surface firmly frozen. Our planet was tectonically active in its infancy, roiled not only by widespread volcanism but also asteroid impacts, especially during the period known as the Late Heavy Bombardment some 4.1 to 3.8 billion years ago.

Throw in the fact that the Earth had high concentrations of carbon dioxide — Titan does not — and it’s clear that we can’t make too broad a comparison between the two worlds. What we do have on Titan, however, is an atmosphere that teems with chemical activity, fueled by light from the Sun and the charged particle environment in the moon’s orbit around Saturn. So we do have a chemistry here that is capable of turning simple organics into more complex ones.

Thus the findings from a new study of archival data using the Atacama Large Millimeter/submillimeter Array (ALMA) are instructive. If we can rule out forms of life as we know them, we can’t exclude combinations that could only form in Titan’s frigid incubator. A potential marker of such chemistry is vinyl cyanide (acrylonitrile), which has been identified in the study and points in interesting directions.

Says lead author Maureen Palmer (NASA GSFC): “The presence of vinyl cyanide in an environment with liquid methane suggests the intriguing possibility of chemical processes that are analogous to those important for life on Earth.”

Image: Archival ALMA data have confirmed that molecules of vinyl cyanide reside in the atmosphere of Titan, Saturn’s largest moon. Titan is shown in an optical (atmosphere) infrared (surface) composite from NASA’s Cassini spacecraft. In a liquid methane environment, vinyl cyanide may form membranes. Credit: B. Saxton (NRAO/AUI/NSF); NASA.

An interesting thought, because the three signals Palmer and team found in millimeter wavelength spectra from ALMA observations in 2014 confirm what Cassini had already hinted at, which in turn reinforce laboratory simulations of Titan’s atmosphere. A surface rife with pools of hydrocarbons — and Titan is a place of methane rains, rivers and seas — could allow molecules of vinyl cyanide to link together, forming membranes that resemble the lipid-based cell membranes found on Earth. Titan’s complex organic molecules along with its nitrogen atmosphere and the presence of carbon-based molecules are provocative ingredients.

From the paper:

… lipid membranes, which are common to Earthly organisms, could not exist in cryogenic methane. Cell membrane–like compartments are crucial for the development of life from a sea of prebiotic reactants. These membranes enclose a small volume of solution, where reactants can be concentrated and (pre)biotic reactions can occur with greater frequency than they would in the dilute environment of an entire lake or sea. They also define individual cells as separate from each other, creating the potential for competition and natural selection.

And a nod to recent computational work on this matter:

Recent simulations have investigated some nitrile species for their potential to form flexible membranes in Titan-like conditions. These simulations suggest that vinyl cyanide (C2H3CN; also known as acrylonitrile or propenenitrile) would be the best candidate species for the formation of these hypothesized cell-like membranes, known as “azotosomes”

And now we have vinyl cyanide confirmed on Titan. At Cornell University, Jonathan Lunine worked with Paulette Clancy on a 2015 paper that discussed molecular simulation and its predictions as windows into prebiotic life conditions. Of the ALMA finding, Lunine says this:

“Researchers definitively discovered the molecule, vinyl cyanide (a.k.a. acrylonitrile), that is our best candidate for a ‘protocell’ that might be stable and flexible in liquid methane. This is a step forward in understanding whether Titan’s methane seas might host an exotic form of life. Saturn’s moon, Enceladus is the place to search for life like us, life that depends on — and exists in — liquid water. Titan, on the other hand, is the place to go to seek the outer limits of life — can some exotic type of life begin and evolve in a truly alien environment, that of liquid methane?”

And let’s not forget what else has been demonstrated by this work. What Lunine, Paulette Clancy and James Stevenson modeled in 2015 was exactly the kind of membrane we are now discussing. The finding of vinyl cyanide thus validates the authors’ 2015 paper, pointing to the value of molecular simulations as an adjunct to experimental and observational data.

The next steps? Further laboratory studies of the reactions involving vinyl cyanide. The authors point out that experimental work on membrane formation in cryogenic methane would help us determine just how viable a pathway this might be for astrobiology. We can also use infrared observations of Titan to study the transport of vinyl cyanide to the surface and map its spatial distribution, especially over the moon’s northern seas and lakes.

The paper is Palmer et al., “ALMA Detection and Astrobiological Potential of Vinyl Cyanide on Titan,” Science Advances Vol. 3, No. 7 (28 July 2017) (full text). The 2015 paper on modeling prebiotic environments is Stevenson, Lunine & Clancy, “Membrane alternatives in worlds without oxygen: Creation of an azotosome,” Science Advances Vol. 1, No. 1 (27 February 2015) (full text).

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{ 21 comments… add one }
  • DJ Kaplan August 4, 2017, 12:22

    I’ve got my money placed on Titan; this is where we will find something resembling life.

  • Andrei August 4, 2017, 13:46

    Titan is a chemical factory, this is a good study but remote sensor can only give a peephole of organic chemistry there.
    Alternative life system is possible yes, even if it is not found but only is started we could learn a lot from the possibilities there!
    Yes this moon is different, less available energy, a few important aminoacids cannot be made at all, but the chemical process there could give insights on how life started on Earth also.
    We don’t need to go to another star to have a very interesting world to explore, with submarine, paddleboat, rover and zeppelin.
    Sadly such a complex space mission will not be possible in the near future or a lifetime.

  • Alex Tolley August 4, 2017, 16:33

    I’d like to see what sort of hypothetical metabolism might be possible on Titan. If vesicle compartments can be formed, what sort of compounds do they need to contain and concentrate to create some simple metabolic pathway to extract energy from the environment? In a “metabolism first” abiogenesis, information storage isn’t necessary and we might expect naturally budding vesicles as they “grow”.

    Those vesicles would most likely be found in the hydrocarbon lakes and streams, so it is a pity we don’t have a Titan boat or submarine explorer that could look for such “life” as one of its goals.

    • DJ Kaplan August 7, 2017, 12:20

      Good point, Alex. We might be stuck on defining life as using DNA or an analog for information transfer. Maybe there is life wherein information is not as important as metabolism.

  • Adam Crowl August 4, 2017, 17:06

    Titanian life will be very strange if found, though clearly Huygens landed in a desert or something, since visible azotosome life wasn’t evident. Earth’s largest lifeforms were microbes for ~2 billion years or so, thus we can’t conclude much from that little patch of cryogenic stony ground we’ve already seen.

    • ljk August 7, 2017, 8:54

      Why was that area of Titan chosen for Huygens? Or did they have no real say in where the probe landed on that moon?

      I am suddenly reminded of Mariner 4 imaging the most “boring” regions of Mars and the Galileo atmosphere probe hitting an atypical “dry” area of the Jovian cloud system.

      • DJ Kaplan August 8, 2017, 12:14

        Often, probe landing sites are chosen for their safety to the probe.

  • H. Floyd August 4, 2017, 19:57

    To Titanic observers, Earth’s organic surface would seem like an impossible ecosystem of roiling acids and solvents.

    Are there any further ways we can model proto-organic membranes? Imagine, a ‘periodic table’ of lifelike processes out there, each on distinct chemical and energetic scales. Exobiology could be an unfathomably understated field.

    • DJ Kaplan August 7, 2017, 12:17

      Very good. From the point of view of a hypothetical Titanian, Earth would be a nightmare of solar radiation and oxidation.

  • Geoffrey Hillend August 5, 2017, 17:05

    The problem with azotosome idea is that it only covers the making of a the cell membrane. What about inside the cell? What kind of catabolism and anabolism would liquid methane support in a cell? It’s certainly can’t involve sugar. What kind of proteins in the DNA would such a cell have? Water is also a better solvent than methane. The chemical reactions would also have to be much slower at lower temperatures.

  • Andrew Palfreyman August 5, 2017, 18:59

    Reaction rates at Titan’s temperature must be miniscule. I don’t see what the fuss is about. It’s dead, Jim.

    • Andrei August 6, 2017, 2:20

      That is correct, Titan is an icebox.
      The chemical reaction we see come from upper atmosphere where the Sun provide energy.
      Organic chemistry but not alive is not important?

      Lower down there might be samples on how life got started.
      If I could decide, I say: All hands let go of what you are doing, and start to build a Titan mission with submarine, boat, rover and zeppelin.
      (No I did not say sample return, even I am realistic – haha!)

    • Michael August 6, 2017, 3:03

      Many of these chemicals are the precursors of life’s components, life as you have stated on Titan would be very slow at those temperatures.

    • Brett Bellmore August 6, 2017, 11:34

      Reaction rates for reactions that happen at a reasonable rate on Earth are minuscule on Titan. Life on Titan would have to rely on reactions that happen extremely fast on Earth, reactants that are too unstable at Earth temperatures to keep around to do chemistry with.

      Such reactants exist. The real question is what life on Titan would EAT; What’s the ultimate energy source? Or rather, penultimate, as the ultimate energy sources would be the same as on Earth: Volcanic heat, solar radiation.

      Presumably one such source would be metastable compounds produced by the action of solar radiation on Titan’s atmosphere. Look for the life in places where such compounds get naturally concentrated, the methane pools around the poles.

      The thing is, you can’t expect a lot of life on Titan; Even if there is life there, the Titan biosphere has to be starved for energy.

  • Ronald August 6, 2017, 0:51

    In agreement with Geoffrey Hillend and Andrew Palfreyman, because of the extremely low temperatures, and lack of liquid water as a solvent, (bio)chemical reaction rates must be very slow, and any evolution as well: as a rule of thumb, chemical reactions slow down by a factor 2-3 for every 10 K lower. This means that comparable chemistry would be at least on the order of a million times slower on Titan than on early Earth. Even any precursor to life wouldn’t even be close to starting.

    • John Freeman August 8, 2017, 8:35

      Very true, but that’s also the point: It’s a snapshot of pre biochemistry, that might perhaps be heading towards soething life like in a very, very slow way. The interest is the same as that directed at prmitive, orgainic rich asteroids, and comets. It doesn’t have to be aluve to be deeply informative about the universe and our place in it – in facr I often find myself thinking that modern space exploration is a bit too focussed on searching for life. The universe is still amazing and worth exploring, even if we truly are alone.

    • Dick Modderkolk August 9, 2017, 14:44

      I don’t believe “comparable chemistry” is what we can expect on Titan. The average temperature is reported to be in the 90s Kelvin. While on Earth it’s 280 something Kelvin. I doubt speed of chemical reactions scale linearly, but wouldn’t temperature be the main factor driving this process?

  • Rob Flores August 6, 2017, 17:15

    There is one compound that could be the backbone
    of analogues to Carbons role in biochemistry on Titan. Silicone.

    In temps above 0 C, Silicone forms Very unstable amino acid like chains. (and it’s only small retinue “amino acids” and like compounds neccesary for life on Earth that can form)

    Two big problems with this (i’m sure there’s more)

    A) there is no evidence of large amount silicone on the surface of Titan. (athough if Titan has cryovolcanoes I supposed it could spewed up from the deep ocean.

    B) At -155C to -160C, it may be too cold for chemical reactions.

    C) I have no idea if any reactions are possible in a liquid methane solvent(aqueous analogue) inside a cell membrane made of acrylonitrile

  • ljk August 7, 2017, 9:02

    Here is a paper from 1997 by some Titan experts on what may happen in terms of life on that moon when Sol becomes a red giant:

    http://www.lpl.arizona.edu/~rlorenz/redgiant.pdf

    Another interesting relevant site:

    http://www.titanexploration.com/lifetitan.htm

    And this one from 2016:

    http://www.exoclimes.com/topics/the-far-future-of-the-solar-system-titan-as-an-ocean-planet/

  • andy August 8, 2017, 5:08

    Our planet was tectonically active in its infancy

    Was it though? Certainly it was geologically active, whether that geology involved plate tectonics is another matter.

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