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Methane as Biosignature: A Conceptual Framework

A living world around another star will not be an easy catch, no matter how sophisticated the coming generation of space- and ground-based telescopes turns out to be. It’s one thing to develop the tools to begin probing an exoplanet atmosphere, but quite another to be able to say with any degree of confidence that the result we see is the result of biology. When we do begin picking up an interesting gas like methane, we’ll need to evaluate the finding against other atmospheric constituents, and the arguments will fly about non-biological sources for what might be a biosignature.

This is going to begin playing out as the James Webb Space Telescope turns its eye on exoplanets, and methane is the one potential sign of life that should be within its range. We know that oxygen, ozone, methane and carbon dioxide are produced through biological activity on Earth, and we also know that each can be produced in the absence of life. The simultaneous presence of such gases is what would intrigue us most, but the opening round of biosignature detection will be methane. Here I’ll quote Maggie Thompson, who is a graduate student in astronomy and astrophysics at UC Santa Cruz and lead author of a new study on methane in exoplanet atmospheres:

“Oxygen is often talked about as one of the best biosignatures, but it’s probably going to be hard to detect with JWST. We wanted to provide a framework for interpreting observations, so if we see a rocky planet with methane, we know what other observations are needed for it to be a persuasive biosignature.”

The problem of interpretation is huge, given how numerous are the sources of methane. The study Thompson is discussing has just appeared in Proceedings of the National Academy of Sciences, addressing a wide range of phenomena from volcanic activity, hydrothermal vents, tectonic subduction zones to asteroid or comet impacts. There are many ways to produce methane, but because it is unstable in an atmosphere and easily destroyed by photochemical reactions, it needs to be replenished to remain at high levels. Thus the authors look for clues as to how that replenishment works and how to distinguish these processes from signs of life.

Image: Methane in a planet’s atmosphere may be a sign of life if non-biological sources can be ruled out. This illustration summarizes the known abiotic sources of methane on Earth, including outgassing from volcanoes, reactions in settings such as mid-ocean ridges, hydrothermal vents, and subduction zones, and impacts from asteroids and comets. Credit: © 2022 Elena Hartley.

Current methods for studying exoplanet atmospheres rely upon analyzing the light of the host star during a transit as it passes through the planet’s atmosphere, the latter absorbing some of the starlight to offer clues to its composition. To do this well, we need relatively quiet stars with little flare activity. M-dwarfs are great targets for this kind of work because of their small size, so that the transit depth of a rocky planet in the habitable zone will be relatively large and the signal stronger. It’s also useful that small red stars represent as much as 80% of all stars in the galaxy (for a deep dive into this question, see Alex Tolley’s Red Dwarfs: Their Impact on Biosignatures).

Context will be the key in the hunt for biosignatures, with false positives a persistent danger. Outgassing volcanoes should add not only methane but also carbon monoxide to the atmosphere, while biological activity should consume carbon monoxide. The authors argue that it would be difficult for non-biological processes to produce an atmosphere rich in both methane and carbon dioxide with little carbon monoxide.

Thus a small, rocky world in the habitable zone will need to be evaluated in terms of its geochemistry and its geological processes, not to mention its interactions with its host star. Find atmospheric methane here and it is more likely to be an indication of life if the atmosphere also shows carbon dioxide and the methane is more abundant than carbon monoxide and the planet is not extremely rich in water. The paper is an attempt to build a framework for distinguishing not just false positives, but identifying real biosignatures that may be easy to overlook.

Making the process even more complex is the fact that the scope of abiotic methane production on a planetary scale is not fully understood. Even so, the authors argue that while various abiotic mechanisms can replenish methane, it is hard to produce a methane flux comparable to Earth’s biogenic flux without creating clues that signal a false positive. Here we’re at the heart of things; let me quote the paper:

…we investigated whether planets with very reduced mantles and crusts can generate large methane fluxes via magmatic outgassing and assessed the existing literature on low-temperature water-rock and metamorphic reactions, and, where possible, determined their maximum global abiotic methane fluxes. In every case, abiotic processes cannot easily produce atmospheres rich in both CH4 and CO2 with negligible CO due to the strong redox disequilibrium between CO2 and CH4 and the fact that CO is expected to be readily consumed by life. We also explored whether habitable-zone exoplanets that have large volatile inventories like Titan could have long lifetimes of atmospheric methane. We found that, for Earth-mass planets with water mass fractions that are less than ∼1 % of the planet’s mass, the lifetime of atmospheric methane is less than ∼10 Myrs, and observational tools can likely distinguish planets with larger water mass fractions from those with terrestrial densities.

Let’s also recall that when searching for biosignatures, terms like ‘Earth-like’ are easy to misuse. Today’s atmosphere is a mix of nitrogen, oxygen and carbon dioxide, but we know that over geological time, the atmosphere has changed profoundly. The early Earth would have been shrouded in hydrogen and helium, with volcanic eruptions producing carbon dioxide, water vapor and sulfur. The oxygenation event that occurred two and a half billion years ago brought oxygen levels up. We thus have to remember where a given exoplanet may be in its process of development as we evaluate it.

So by all means let’s hope we one day find something like a simultaneous detection of oxygen and methane, two gases that ought not to co-exist unless there were a sustaining process (life) to keep them present. An out of equilibrium chemistry is intriguing, because life wants to throw chemical stability out of whack. And by all means let’s accelerate our work in the direction of biosignature analysis to root out those false positives. We begin with methane because that is what JWST can most readily detect.

And as to the question of ambiguity in life detection, JWST is not likely able to detect atmospheric oxygen and ozone, nor will it be a reliable source on water vapor, so its ability to make the call on habitability is limited. Going forward, the authors think that if the instrument detects significant methane and carbon dioxide and can constrain the ratio of carbon monoxide to methane, this will serve as a motivator for future instruments like ground-based Extremely Large Telescopes to follow up these observations. It will take observational tools in combination to nail down methane as a biosignature, but the ELTs should be well placed to take the next step forward.

The paper is Thompson et al., “The case and context for atmospheric methane as an exoplanet biosignature,” Proceedings of the National Academy of Sciences 119 (14) (March 30, 2022). Abstract. See also Krissansen-Totton et al., “Understanding planetary context to enable life detection on exoplanets and test the Copernican principle,” Nature Astronomy 6 (2022), 189-198 (abstract).

{ 13 comments… add one }
  • Geoffrey Hillend March 31, 2022, 15:50

    Even if the JWST does not detect biosignature, I have suspected for a couple of years there would not be any false positives because of the reasons already written here in this paper, the JWST will still hopefully teach us much about nearby exoplanet atmospheres. The JWST still can effectively use transmission spectroscopy and I wonder what the range limit of that is in light years.

  • Alex Tolley March 31, 2022, 16:11

    Relatively high mixing ratios of CH4 and CO2, coupled with low CO is what we expect of a planet like the Archaean Earth before oxygenic photosynthesis was able to oxygenate the atmosphere. IIRC, CO is also a component of comets, so a low atmospheric concentration of CO to CH4 & CO2 rules out cometary impacts as the methane source in the atmosphere.

    As we can expect that a rich oxygen atmosphere arrived relatively late in Earth’s history (O2 was oxidizing the oceans and rocks, before the excess was available to accumulate in the atmosphere), a pre-oxygenated atmosphere is likely the lowest hanging fruit to detect on an exoplanet, both from a number of targets and ease of detection stance.

    I was just watching the documentary: “The Hunt for Planet B” which starts with a 2013 Congressional Committee meeting about astrobiology (and I think the funding of the JWST) where Sara Seager, answers to a question about how long before we can get a biosignature, states that 5-10 years away would be her guess. What with the delays to the JWST, we are just about at that decade boundary, so I do hope that we get some early clues soon. The documentary focussed on Trappist-1e as the best target for the JWST to detect a biosignature. So maybe that may be the first targert for JWST and just maybe the first tentative biosignature that will require follow up to confirm.

  • torque_xtr March 31, 2022, 18:24

    Almost no single chemical could be a robust biosignature by itself, but much higher confidence cound be reached by evaluating some kind of atmospheric “disequilibrium measure”. Oxygen or methane alone tells almost nothing, but CH4 and O2 simultaneously is another deal. A detection of 4 or 5 mutually incompatible chemicals – even much more so. I think of something like molar-specific energy liberated by bringing the atmospheric mixture to equilibrium (reacting CH4 with O2, in this example), multiplied by factorial of the number of mutually incompatible species. Or maybe, more precisely, some combinatorial formula which reflects the total number of reactive combinations and also skyrockets with the increase of the first number. It may be useful for technosignature search, too – adding several CFC’s and other complex pollutants to the already disequilibrated mix will result in this “interest measure” increasing by several orders of magnitude, beyond anything achievable on a habitable planet lacking a technosphere.

    • Ron S. April 1, 2022, 9:05

      What I wonder is whether alternative, though speculative metabolisms could reduce the confidence of a positive biosignature signal even with a multi-dimensional signal as you suggest.

      • Alex Tolley April 1, 2022, 14:54

        The idea of life maintaining a disequilibrium of molecular species in their biosphere is unlikely to be different for other types of biological life. The molecular species may differ, but not their disequilibrium.

        What would be missed is if artificial “life”, i.e. machines were common in the galaxy, using non-biological metabolisms. The atmosphere may be at equilibrium (or even non-existent) but the machines/robots could form a rich “machine-sphere” that we would not detect using atmospheric spectra. Consider, could we even detect such robots on the planets or moons in our system if we did not have the resolution to detect them, and like most animal life on Earth, they did not build large structures or even clump together in “herds”. Science fiction stories, and our cultural assumptions about technological intelligence, assume that any such ETI will eventually build “megastructures”. and that applies to the machine civilization too. But that may be an assumption that may be wrong.

        Having said that, I do hope that life proves sufficiently common that we will detect their signs, even if their biology and metabolisms are very different from terrestrial life.

      • Alex Tolley April 1, 2022, 15:43

        On 2nd thought, the atmosphere gas disequilibrium assumes that this is the route that biology uses on Earth. If, so example life was able to use silicon (I don’t think it can) then metabolisms that use SiO2 instead of CO2 would metabolize rock which would never be in a gaseous state. Similarly, if life only reduced SO2 to S, there would be no evidence of sulfur in any atmosphere.

        On Earth, Archaean methanogens exploited the energetics of CH4 metabolism, excreting methane in excess of geologic CH4 production. But there are also bacteria that feed on CH4 and oxidize it. If the 2 communities live in the same structure as different biofilm layers, then the CH4 might not escape to the atmosphere at all, and this disequilibrium would be largely absent, and only reflected in the geologic CH4 production (as it may be on Mars).

        An imaginary (and improbable) biology might reduce SO2 to H2S, or pure S, or SiO2 to (SiH4)n or Si, while oxidizing other elements, like Fe to Fe oxides. Apart from the removal of SO2 for sulfur-based metabolism, there would be no other obvious gas changes to show atmospheric gas disequilibria. If life was restricted to the lithosphere, there might be no obvious signs of life at all. [It is still possible this is the case on Mars, where extant life is only in the crust at a depth where the heat maintains liquid H2O.]

        But since the question we want to know first is whether there is life elsewhere, then detecting “life as we know it” would be sufficient to answer that question (and the first to find that life will get the honors and recognition). After that, detecting truly exotic life would be a very interesting project. As life is so diverse and most of the interesting work is working with samples and ecologies, this will likely have to wait for interstellar ships, whether robotic or crewed. I don’t see much progress being possible with purely remote detection, although ultra-high-resolution telescopes would provide some interesting observations from a naturalist’s POV.

        • NS April 3, 2022, 21:40

          “…wait for interstellar ships…”

          Maybe we would be better off prioritizing missions to (say) ocean worlds within our own solar system? And thoroughly investigating Earth itself for alternate biochemistries? If we wait for interstellar ships capable of addressing those questions, we’re almost certainly talking centuries…

          • Alex Tolley April 4, 2022, 10:01

            It isn’t an “either/or” issue. I assume that we will explore our own system first. However, suppose we find life in Mars or even the icy moons’ subsurface oceans, and in both cases, their biology is very similar, if not identical in core metabolic proteins, genetic code, etc. The suspicion will be that panspermia within the solar system was involved, rather than separate abiogeneses.

            For the sheer diversity of life, both simple and complex, we will want to examine the organisms in close-up. By analogy, suppose we only lived on the Moon and had no way to reach Earth to study the biosphere. The richness of life would be unavailable for us to study. No lunar David Attenborough producing exquisite nature programs and no biologists able to experiment on the biology of Earth’s life.

            My argument is that it will take interstellar ships to enable the examining and cataloging of exoplanet life, a task that will occupy humanity for many millennia.

            Regarding shadow biospheres. Again, no need to stop looking, although I suspect that it does not exist on Earth, at least not in places where the main tree of life exists. If other biologies once existed on Earth, they were displaced by the life we see all around us. They may only exist in refugia. The argument that they are so different that they can coexist with the rest of our biosphere is a weak one, IMO. But, as aI say, keep looking. The cost is trivial, and any discoveries are very interesting.

  • FrankH April 1, 2022, 14:18

    Titan has a chemical imbalance in its atmosphere, so that can’t be taken as a positive biosignature by itself.

    Around an Earth-sized (not a “super Earth”= mini-Neptune) planet in its star’s HZ, then it’s a strong positive.

    Around a HZ mini-Neptune it would be ambiguous.

    • Alex Tolley April 1, 2022, 17:13

      Titan’s atmosphere

      which gases are out of equilibrium? Or are you referring to the H2 excess [near the surface?]?

  • Robin Datta April 2, 2022, 3:18

    Xenobiologic or post-biologic “machine ecosystems” may “fly under the radar” for human conceived, designed and implemented detection systems.

  • ljk May 2, 2022, 10:49

    Evidence of farming on exoplanets should be visible to James Webb Space Telescope

    Industrial-scale agriculture has changed the make up of our atmosphere. So “exofarms” ought to be visible on Earth-like planets orbiting other stars.

    By The Physics arXiv Blog | Published: Wednesday, April 20, 2022



  • ljk May 2, 2022, 17:01

    How the Disappearance of the Dinosaurs Created an Hospitable World for Humans

    Riley Black on the Causes and Consequences of the Great Extinction

    By Riley Black

    April 26, 2022


    To quote:

    Not only would mammals have remained small under an extended regime of non-avian dinosaurs, but the earliest, shrew-like primates might have stayed in tight competition with the dominant marsupials. Our ancestors would have been molded in different ways, and it’s likely, if not certain, that the world would never have been suitable for a mostly hairless, bipedal ape with a big brain and a penchant for remodeling the planet. The mass extinction at the end of the Cretaceous isn’t just the conclusion of the dinosaurs’ story, but a critical turning point in our own. We wouldn’t exist without the obliterating smack of cosmic rock that plowed itself into the ancient Yucatán. Both stories are present in that moment. The rise and the fall are inextricable.

    And here, we often leave the epic tale. The dinosaurs were dominant, even cocky in our prehistoric visions. The largest, strangest, and most ferocious of all inhabited the Late Cretaceous world of soggy swamps and steaming forests. A wayward asteroid suddenly ended their reign, leaving the meek to inherit the Earth. Just as the dinosaurs once benefitted from a mass extinction that allowed them to step out of the shadow of ancient crocodile relatives 201 million years ago, so, too, were our warm-blooded, snuffly little forebears the recipients of good fortune they never earned nor have ever repaid.

    We entirely gloss over the nature of recovery, or what made the difference between the survivors and the dead. We obsess over what we lost—blinded to how, even in the shocking cold that followed the initial heat of annihilation, life was already beginning to reseed and recover. It’s an extension of how we often cope in the wake of our own personal traumas, remembering the wounds as we struggle to see the growth stimulated by terrible events. Resilience has no meaning without disaster. Life’s losses were sharp and deeply felt 66 million years ago, but each fiddlehead struggling for light, each shivering mammal in its burrow, each turtle that plopped off a log into weed-choked waters set the stage for the world as we know it now.

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