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Calculating Life’s Possibilities on Titan

With surface temperatures around -180° C, Titan presents problems for astrobiology, even if its seasonal rainfall, lakes and seas, and nitrogen-rich atmosphere bear similarities to Earth. Specifically, what kind of cell membrane can form and function in an environment this cold? Five years ago, researchers at Cornell used molecular simulations to screen for the possibilities, suggesting a membrane the scientists called an azotosome, which would be made out of the nitrogen, carbon and hydrogen molecules known to exist in Titan’s seas.

The azotosome was a useful construct because the phospholipid bilayer membranes giving rise to liposomes on Earth need an analog that can survive Titan’s conditions, a methane-based membrane that can form in cryogenic temperatures. And the Cornell work suggested that azotosomes would create a similar flexibility to cell membranes found on Earth. Titan’s seas of methane and ethane, then, might offer us the chance for a novel form of life to emerge.

Now we have new work out of Chalmers University of Technology in Gothenburg, Sweden that raises serious doubts about whether azotosomes could develop on Titan. The Cornell work examined the liquid organic compound acrylonitrile, found in Titan’s atmosphere, and built the azotosome idea around it, but the Swedish team’s calculations show that azotosomes are unlikely to be able to self-assemble in Titan’s conditions, for the acrylonitrile would crystalize into its molecular ice.

Martin Rahm (Department of Chemistry and Chemical Engineering, Chalmers University of Technology) is co-author of the paper:

“Titan is a fascinating place to test our understanding of the limits of prebiotic chemistry – the chemistry that precedes life. What chemical, or possibly biological, structures might form, given enough time under such different conditions? The suggestion of azotosomes was a really interesting proposal for an alternative to cell membranes as we understand them. But our new research paper shows that, unfortunately, although the structure could indeed tolerate the extremes of Titan, it would not form in the first place.”

This is interesting work, and not only because we are on track to launch Dragonfly in 2026, a mission to investigate the surface and sample different locations around the moon in an assessment of prebiotic chemistry. What we’re seeing is the emergence of computational astrobiology, the necessary follow-on to studies like the predictive work of 2015. The idea is to model the properties and formation routes of the materials proposed as supporting possible biological processes. In this case, we learn that the azotosome structure that looked so promising is not thermodynamically feasible.

But this work hardly eliminates the possibility of life on Titan. What if, the authors speculate, the cell structure itself is not critical? From the paper:

…on Titan, any hypothetical life-bearing macromolecule or crucial machinery of a life form will exist in the solid state and never risk destruction by dissolution. The question is then whether these biomolecules would benefit from a cell membrane. Already rendered immobile by the low temperature, biological macromolecules on Titan would need to rely on the diffusion of small energetic molecules, such as H2, C2H2, or HCN, to reach them in order for growth or replication to ensue. Transport of these molecules might proceed in the atmosphere or through the surrounding methane/ethane environment. A membrane would likely hinder this beneficial diffusion. Similarly, a membrane would likely hinder necessary removal of waste products of metabolism, such as methane and nitrogen, in the opposite direction.

Image: Researchers looking for life on Titan, Saturn’s largest moon, used quantum mechanical calculations to investigate the viability of azotosomes, a potential form of cell membrane. Credit: NASA / Yen Strandqvist / Chalmers.

At this stage, as the authors note, the limits of prebiotic chemistry and biology on Titan will have to stay in the realm of speculation, but computations like these can inform the choice of sites for Dragonfly as it explores the moon, helping us to match the reality on the ground with theory.

The paper is Sandström & Rahm, “Can polarity-inverted membranes self-assemble on Titan?” Science Advances Vol. 6, No. 4 (24 January 2020). Full text. The 2015 paper on azotosomes is Stevenson, Lunine & Clancy, “Membrane alternatives in worlds without oxygen: Creation of an azotosome,” Science Advances Vol. 1, No. 1 (27 February 2015), e1400067 (full text).

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{ 26 comments… add one }
  • Robert Flores March 4, 2020, 14:14

    I have wondered myself if Titans cold chemistry makes reactions too
    unlikely to occur. I think it very unlikely that life could spontaneously
    self assemble on the surface seas of Titan. Such an event requires a volume of chemical reactions that is probably out reach, even assuming 4 billion years-or-so of Titans existence.
    On the other hand, I think Carbonaceous Type Asteroid Strikes are more likely a source to bootstrap any Titan life. I am talking about a large strike that creates an Oasis and takes millions of years to cool and solidify, it might give time to evolve a viable “naked” molecular life.
    At any rate, I think the group is correct that due to it’s cold nature
    life in the Titan “seas” wont need a membrane since the chemistry of
    Methane and Ethane is fairly benign compared to H2O, and w/0
    Free Oxygen, Fluorine Sodium, Clorine and other reactants leads to a tamer chemical soup.
    I can imagine a succesfull living Titan organisms having DNA analogue that instead of using a double helix, it would use more of a toroid(flatened curved inward) shape to contain and extract nutrients via electro-chemical effects. The outer facing side being non-reacting.
    Frankly I would be more concerned about such life being able to do chemical reactions for the equivalent of life sustaining cycles of simple Earth bacteria. There are mildly reactive elements that would need to be present in the Titan Surface Seas.

    • Zanstel March 10, 2020, 4:27

      We can’t discard that Titan had developed life on a potential subsurface oceans and later, that life, migrate to the surface.

      Even the organics could be the byproduct of life in the same way that oxygen is a biological byproduct on Earth

  • Alexander Tolley March 4, 2020, 16:37

    However, on Titan, any hypothetical life-bearing macromolecule or
    crucial machinery of a life form will exist in the solid-state and never
    risk destruction by dissolution (10). The question is then whether
    these biomolecules would benefit from a cell membrane. Already
    rendered immobile by the low temperature, biological macromolecules
    on Titan would need to rely on the diffusion of small energetic molecules,
    such as H2, C2H2, or HCN, to reach them in order for growth
    or replication to ensue. Transport of these molecules might proceed
    in the atmosphere or through the surrounding methane/ethane environment.

    Solid molecules would not make any sense for biology as we know it. Almost all molecular biology requires that molecules can change shape, from rotating around covalent bonds to conforming a linear amino acid string to a functional 3D protein, to opening and closing DNA helixes for transcription and replication, and a host of other mechanisms. In other words, biology is not a set of solid shapes, nor can these objects be fixed to some surface and function as life just by moving small molecules onto and off them. When cells are frozen to effectively vitrify them, life processes stop. Once stopped, some biological molecules can even be preserved for millions of years as recent studies have indicated.

    On Earth, life only survives in a very narrow range of temperatures. The lower the temperature, the slower biology operates. Deep ocean muds that remain at near-constant 4C have such slow processes that growth and replication of microbes can take centuries, rather than minutes. Whatever the biological molecules on Titan, reactions will go very slowly, which implies that Darwinian evolution will also proceed with extreme slowness. That was one reason why it was thought that Titan might just possibly be in a prebiotic state.

    For me, the extracted sentences from the paper make no sense at all. Any exotic chemistry would be just that, chemistry, not biochemistry.

    • Michael Fidler March 6, 2020, 9:31

      What about cosmic rays as a power source or the warmer environment beneath the surface. Life could evolve from these condition into something like a worm or sea slug with its own energy supply.

      • Alexander Tolley March 6, 2020, 11:39

        I know of no life forms that can harvest energetic particles. If they emitted light on impact, then organisms could harvest that. However, that would apply on Earth too. Do we see cosmic rays penetrating the oceans and emitting light? On teh contrary, the energy of such particles is inimical to chemical structures. Any information storage (e.g. DNA) analogs would be constantly disrupted by such particles.

        Cosmic rays do not penetrate deeply. They are largely blocked by our atmosphere, and a few meters of water or rock will eliminate the exposure. This is one reason that colonists on the Moon and Mars are expected to live underground or at least meters of regolith on surface structures to avoid both galactic cosmic rays (GCR) and solar radiation from charged particles.

        If it were possible that GCRs could power some sort of life, I would like to see some theoretical explanations or modeling to show how that could be achieved.

  • Mike Serfas March 4, 2020, 20:14

    I doubt that cell membranes are truly how life began. To be effective, a cell needs many specific channels to let things in and out; those channels imply some prior evolution. I would think that naked ribozymes, the branched structures from splicing intermediates, and coenzymes made with exotic nucleotides like nicotinamide and flavin would be much closer to our origin of life. By the same principle, I would want to see more about options for catalytic activity and genetic coding before developing any strong opinion about what could have happened on Titan.

  • Antonio March 5, 2020, 4:09

    Has nobody simulated Titan’s chemistry in the lab? (i.e. not in a computer, but with real methane et al.)

  • Michael Sloboda March 5, 2020, 9:20

    It’s 20 C in Hong Kong and I walked past an upscale apartment building. As always, I peeked through the railing and saw a dozen turtles in a pond. This afternoon they were all motionless, not that they race at 30 C.

    Centauri Dreams on March 4 mentioned Titan at -180 C. I read somewhere that biological processes slow by 50% with a 10 C temperature drop. Lower the temperature from +20 C to -180 C? That’s 20 doublings = a million-fold slowdown. If it takes an amoeba 15 min to divide, its counterpart on Titan takes a leisurely 30 years. In 4 billion years, as much has happened on Titan as happens on earth in 4000 years. Highly advanced lifeforms? Don’t hold your breath for 10 years.

    • Alexander Tolley March 5, 2020, 11:56

      While I agree in principle with your points, any Titanian would have a very different chemistry that would work in the low ambient temperatures. Darwinian evolution would select for the chemistries that would maximally reproduce.

      While I don’t think Titan has organisms anywhere on it, it may have a subsurface biosphere where liquid water exists. It is a longshot however. A Titan sample mission would at least give us some indication if there is life anywhere on or in Titan. But I’m not holding my breath, and I would put money on Titan being sterile.

    • Mike Serfas March 5, 2020, 16:46

      The ratio of stability extends in the other direction as well. There are many compounds that are highly unstable at room temperature, but can be preserved under cryogenic conditions. Titan has some odd constituents like cyanogen and dicyanoacetylene that wouldn’t be very stable on Earth, but I don’t know very much. Would free radical chemistry be possible on Titan that would not be practical for us? Could we identify any explosives or oxidizers that might stand out as potential evidence or components of life?

    • Antonio March 5, 2020, 17:45

      What you are talking about is Arrhenius equation. The reaction rate decreases exponentially with temperature BUT the coefficients are different for each reaction.

  • Harold Shaw March 5, 2020, 17:04

    It is challenging to describe biology as life without including a cellular membrane in the description. Without one, how do we distinguish life as a system, or system of systems, distinct from non-life? Cellular membranes do so much biologic work, can self assemble and are subject to a host of environmental forces that could supply prebiotic metabolism. I struggle to see how biology can emerge without a membrane. What is being described here sounds more like a complex chemical battery or circuitry. An important part of life but alone, without a system defining membrane, unable to make the to transition to life.

    • Alexander Tolley March 5, 2020, 22:46

      I agree. The basic unit of life on Earth is the cell. It is hard to think how all teh necessary life processes can be operated without some “discreteness”.

      There are certainly theories about abiogenesis and pre-biotic life on rock surfaces without membranes. But then to jump to the idea that biology can then operate the same way seems wrong. Even the simplest process like reproduction gets difficult to consider when the unbound chemistry mixes with another one on a nearby substrate. How would selection operate under this scenario? If the chemistry was trapped in a rock pore to create a discrete system, how would reproduction work if other pores were fully occupied, as they must? Chemistries would mix rather than displace.

      • Harold Shaw March 6, 2020, 15:51

        Perhaps the role of a membrane supports the hypothesis that life is the product of fundamental physics. In the presence of energy, matter arranges itself to maximize efficient transformation of that energy into heat. A membrane would provide a discrete open system where that process could reach extreme complexity. A cell would be a lens focusing entropy.

    • Mike Serfas March 6, 2020, 23:56

      Making soap bubbles at half the temperature of liquid nitrogen sounds difficult. Still, I imagine early life forms on Earth might have done without membranes. For example, picture a self-replicating ribozyme with a long poly(A) tail, to which essential resources (such as NTPs and nicotinamide and flavin cofactors) might be appended at 2′ positions.

      That said, I think it more likely that a local community of different small ribozymes would exist, and survival of the community would depend on all of them doing their part. Based on the Usami paper (PMID 28952648) in which glyceraldehyde is produced from polymerized formaldehyde on hydroxylapatite, I would speculate that the original “lipid membrane” might have been assembled with phosphate as part of the mineral with glycerol covalently linked to it. As the mineral dissolved, portions of the membrane would tent over sheltered regions with phosphate-bound nucleotides or short RNAs attached to the hydroxylapatite surface. Occasionally, patches of membrane and mineral would be torn clear of the substrate and land somewhere else. These replicative intermediates might eventually have been regularized into cells.

      To me the “individual” and indeed the “species” seem to be concepts that would have evolved on Earth after life had been in operation for some time. Some of the more remarkable aspects of terrestrial individuality – such as animals having millions of cells with exactly the same genetic code, except in the immune system – might never have evolved at all on some planets.

      So on Titan … what if life never had a membrane, and indeed never evolved individuals? Energy storage at membranes is crucial to Earth life, yet there are many other ways, i.e. batteries, to store high-energy electrons. Fluid mosaics are tremendously useful, yet maybe there are other ways to organize macromolecules efficiently. Instead of individuals, maybe “selfish genes” there could have grown in size and sophistication, but remained independent, developing complex regulatory circuitry to make the best alliances with a wide range of recognized working partners? In this scenario, rather than having one version of a gene for each species of organism, the genes would recognize what “organism” they are part of at the moment and fine-tune themselves to fit those requirements. What if it had an ecosystem where soluble chemicals in the ethane ocean are normally part of “algal” structures with “eyes” like our dinoflagellates, but if they saw something they didn’t work well with, they would dissolve into the ocean and nucleate assemblages of eligible macromolecules to form a suitable “fish” to consume the interloper?

      Yes, this all sounds preposterously far-fetched, but so does our own orderly biology of genetic codes, ATP-generating turbines (F-ATPase), microtubule spindles separating chromosomes and motors driving the twisting of flagella. Our biology seems plausible only because we know it exists!

  • Harold Shaw March 6, 2020, 17:42

    I am grateful for Centauri Dreams. It isn’t afraid of reaching for science fiction.

    Could auto-catalyzing RNA or its equivalent provide a system defining barrier through quorum sensing and/or murmuration? What a cell achieves with a collection of different molecules, a RNA colony would achieve with specialized behavior according to where in the colony the strand is located.

    • Alexander Tolley March 6, 2020, 21:00

      Alternatively, what if the basic components are bonded to a single structure? Think of how DNA loops back on itself with attached proteins to enhance transcription. Suppose instead of releasing separate components into a membraneous sac, the proteins analogs stay affixed to the information molecule analog so that discreteness is managed by bonding rather than a loose collection? Metabolites needed for biochemical pathways would not be freed, but rather stick to the main structure with various bond types and only released when no longer needed. RNA analogs would not be released after transcription, but remain attached, offering some catalytic activity. Perhaps weak bonds maintain a high concentration of metabolites and even some components around the core molecule, rather than being in solution and surrounded by a membrane.

      All good SciFi speculation.

  • Harry R Ray March 7, 2020, 10:58

    Speaking of “Calculating Life’s Possibilities…”. Curiosity has detected Thiophenes(think Benzine rings with a Sulfur atom substituted for one of the Carbon atoms and a Hydrogen atom deleted)on Mars. Thiophenes can be produced both biologically or abiologicly, but these Thiophenes were found in sedimentary rock, and on Earth, Thiophenes found there have a greater probability of being produced biologically than not. If Curiosity can detect Sulfur-rich organic compounds of this complexity, it can certainly ALSO detect much simpler Sulfur-rich organic compounds like Mercaptans, the simplest of which(Methyl Mercaptan)is CH3SH. Mercaptans have an even HIGHER probability of being produced biologically than Thiophenes do. Perhaps the mew mantra for Mars should be: Follow the Sulfur.

  • Michael Fidler March 9, 2020, 20:13

    Interesting and very controversial subject:

    Dr. Chandra Wickramasinghe on Coronavirus from space.
    “World’s leading astrobiologist speaks heretical truth about anomalous transmissions as a result of space borne virus”

    If panspermia is common, could life of a different type then on earth be spread to such worlds as Titan and Europa?

    Cause of Cambrian Explosion – Terrestrial or Cosmic?
    “We review the salient evidence consistent with or predicted by the Hoyle-Wickramasinghe (H-W) thesis of Cometary (Cosmic) Biology. Much of this physical and biological evidence is multifactorial. One particular focus are the recent studies which date the emergence of the complex retroviruses of vertebrate lines at or just before the Cambrian Explosion of ∼500 Ma. Such viruses are known to be plausibly associated with major evolutionary genomic processes. We believe this coincidence is not fortuitous but is consistent with a key prediction of H-W theory whereby major extinction-diversification evolutionary boundaries coincide with virus-bearing cometary-bolide bombardment events. A second focus is the remarkable evolution of intelligent complexity (Cephalopods) culminating in the emergence of the Octopus. A third focus concerns the micro-organism fossil evidence contained within meteorites as well as the detection in the upper atmosphere of apparent incoming life-bearing particles from space. In our view the totality of the multifactorial data and critical analyses assembled by Fred Hoyle, Chandra Wickramasinghe and their many colleagues since the 1960s leads to a very plausible conclusion – life may have been seeded here on Earth by life-bearing comets as soon as conditions on Earth allowed it to flourish (about or just before 4.1 Billion years ago); and living organisms such as space-resistant and space-hardy bacteria, viruses, more complex eukaryotic cells, fertilised ova and seeds have been continuously delivered ever since to Earth so being one important driver of further terrestrial evolution which has resulted in considerable genetic diversity and which has led to the emergence of mankind.”
    https://www.sciencedirect.com/science/article/pii/S0079610718300798

    • Steven Ward March 9, 2020, 22:06

      In late October 2019, interstellar comet Borisov passed through the elliptic of our solar system, did earth pass through the dust?

      And now we have the corona virus, hmmm…

      I am not being serious here, but it’s fun to think about. Thanks for the link

  • Michael Fidler March 9, 2020, 21:52

    The link for:
    Dr. Chandra Wickramasinghe on Coronavirus from space.

    https://cosmictusk.com/coronavirus-from-space-wickramasinghe/

  • Michael Fidler March 11, 2020, 22:30

    Steven, that would seem to make more sense since interstellar comets would bring in viruses from outside our solar system. They also seem to be fainter and less able to be identified so harder to trace back to pandemics. What I found interesting is that viruses helped with the evolution of life and possibly control it. Below is the research paper.

    Comments on the Origin and Spread of the 2019 Coronavirus.

    “We propose that the new coronavirus which first appeared in the Hubei province of China was probably linked to the arrival of a pure culture of the virus contained in cometary debris that was dispersed over a localised area of the planet namely China. The sighting of a fireball some 2000 kilometers north of Wuhan on 11 October 2019
    followed shortly after with the first recorded cases in Hubei is suggestive of a causal link. Gene sequencing data of the virus that show little or no genetic variations between isolates, combined with available epidemiological data point to the predominance of a transmission process directly from an “infected” environment, with personto-person transmission playing a comparatively weaker secondary role. The facts relating to this epidemic are discussed and placed in the context of other pandemics that have been recorded throughout history.

    https://www.academia.edu/42041228/Comments_on_the_Origin_and_Spread_of_the_2019_Coronavirus?auto=download

    • Mike Serfas March 14, 2020, 7:50

      SARS and SARS-NCov-2 are two strains of coronavirus that have evolved to enter human cells via angiotensin converting enzyme 2 (ACE2). This interaction is specific: for example, SARS will not bind to the rat ACE2 receptor, and researchers can swap in certain parts of the human receptor to make them interact: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1142572/

      The consequence of this is that when if you find an RNA from space (the SARS genome … wrapped in a protein capsule??) that has the right sequence to make a spike protein that can bind to and enter a human cell via a specific receptor, and then another RNA from space (SARS-NCoV-2) is genetically different but does the same thing to human receptors, there are two conclusions: either (a) aliens are amusing themselves with low-intensity biological warfare, or (b) there aren’t really coronaviruses coming from space, especially considering that they evolve perfectly well on Earth. I strongly favor the latter alternative.

    • Andrei March 15, 2020, 17:08

      Horribly incorrect, and I have even reason to ask what is the purpose of suggesting anything like that.
      Very similar viruses were studied a decade ago, and those came from the Horseshoe bat. That paper or comments are nothing but another conspiracy hypothesis thinly wrapped in scientific lingua!

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