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Carbon Monoxide as Biosignature?

Biosignature gases are those that can alert us to the possibility of life on a planet around another star. We’re moving into the era of biosignature observation by studying the atmospheres of such planets through instruments like the James Webb Space Telescope, and the effort to catalog the combinations of atmospheric gases that point to life is intense and ongoing.

One gas has turned out to be controversial. It’s carbon monoxide, which in some quarters has been considered to be the opposite of a biosignature, a clear sign, if detected in sufficient abundance, that a planet is not inhabited. Edward Schwieterman (UC-Riverside) begs to disagree, and a team led by Schwieterman has produced its modeling of biosphere and atmosphere chemistry to focus on living planets that nonetheless support carbon monoxide at levels we should be able to detect. The work appears in the Astrophysical Journal.

Interestingly, the paper harks back to our own planet’s deep past. We don’t expect to see high levels of carbon monoxide on life-giving planets because the gas is so quickly destroyed by chemical reactions on our oxygen-rich world. Go back several billion years, however, and Earth was a place whose oceans carried an abundance of microbial life, with an atmosphere that was at the same time all but devoid of oxygen, all under a surface lit by a much dimmer Sun.

The researchers found through their modeling that a world like this could support carbon monoxide levels of about 100 parts per million, several orders of magnitude higher than the traces we find of the gas in the atmosphere today. To co-author Timothy Lyons (UC-Riverside), that result carries a clear signal:

“That means we could expect high carbon monoxide abundances in the atmospheres of inhabited but oxygen-poor exoplanets orbiting stars like our own sun. This is a perfect example of our team’s mission to use the Earth’s past as a guide in the search for life elsewhere in the universe.”

Useful information indeed, but the findings go beyond worlds around G-class stars to include possible habitable planets around M-dwarf stars like Proxima Centauri, already known to host an Earth-sized planet in its habitable zone. Here the team found that a living world rich in oxygen could support high carbon monoxide levels from hundreds of ppm to several percent.

Image: A rocky planet orbiting Proxima Centauri might sustain liquid water (artist’s depiction). Credit: NASA, ESA, G. Bacon (STSc).

Astrophysical context is all, even if microbial biospheres with high levels of carbon monoxide, as Schwieterman says, “would certainly not be good places for human or animal life as we know it on Earth.” Photochemistry around such stars tells the tale. From the paper:

…the sequence of reactions that ultimately result in tropospheric OH is mediated by NUV radiation that is substantially less plentiful from M-stars such as Proxima Centauri because of lower photosphere blackbody temperatures, with the result that OH production is much less favored and consequently the sinks for CO and CH4 are much less efficient…

which leads to this:

Our results introduce some important caveats to previous suggestions for interpreting CO in planetary atmospheres, particularly for planets orbiting M dwarfs. For example, our demonstration that high CO may be achieved on inhabited planets means that while simultaneously high CO2 and CH4 with little CO is still a compelling biosignature, ambiguous scenarios with high levels of all three gases also exist, and these are mostly relevant for transit transmission observations of habitable zone planets orbiting M dwarf stars. Arguments regarding threshold CH4 levels that are incompatible with abiotic CH4 outgassing rates are in principle still valid (Krissansen-Totton et al. 2018b), but the proposal that CO provides a check on abiotic versus biological origins of CH4 is weakened by our results given likely near-future capabilities.

The ongoing examination of biosignatures is critical, for we could be looking at atmospheric analysis within a few short years. The challenge will be not only to hunt for reliable biosignatures but to avoid overlooking potentially habitable worlds. This paper indicates that two types of living world, one similar to our early Earth, the other around red dwarf stars, are able to support both life and the ready accumulation of carbon monoxide. The reminder that the photochemistry around M-dwarf stars allows substantial buildups of CO while supporting an inhabited world helps to clarify our range of interpretations.

Image: Carbon monoxide features prominently in oxygen-rich atmospheres in the habitable zone of a red dwarf star like Proxima Centauri. Credit: Schwieterman et al.

Ten years from now, will we have identified unmistakable signs of life around another star? I seriously doubt it. My guess is that many of our early efforts will turn up results that are ambiguous enough to allow for a range of conclusions. Thus the need to continue cataloging alternative biosignature gases and to fine-tune the spectral capabilities of the instruments we will use to survey these atmospheres. Also in play, according to the paper, are “…searching for biogenic seasonality” and “advanced methods for calculating atmosphere-surface disequilibria.”

The paper is Schwieterman et al., “Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System,” Astrophysical Journal Vol. 874, No. 1 (15 March 2019). Abstract / full text.


Comments on this entry are closed.

  • don wilkins March 20, 2019, 14:24

    Niven strikes again!

    In his tales from the Drago Tavern an ancient Chirpsithra returns from circumnavigating the Galaxy. On the alien’s second visit to Earth, the visitor remarks that the planet a billion years ago was populated by an intelligent species capable of great art and philosophy but unfortunately bound to a single world. They lived in a carbon dioxide atmosphere but were eradicated when the atmosphere began to change to an oxygen surplus.

    • Michael March 20, 2019, 15:35

      Can’t be very intelligent then.

    • Robin Datta March 20, 2019, 19:09

      AFAIK the oxidation of carbon compounds produces by far more energy than their anaerobic breakdown: if I recall correctly, 23 ATP molecules are generated for each molecule of glucose oxidised to CO₂ while only 4 molecules of ATP are generated for each molecule of glucose broken down to lactic acid. Since ATP is the energy producing unit in eukaryotes, the former process is a lot more effective.

      Neural tissue and brains have very substantial energy demands, which would be difficult to satisfy by anaerobic metabolism. If we presume that neural tissue and brains or some functional equivalent thereof are a substrate for intelligence, one would need a recyclable/renewable alternative for oxygen to produce sufficient energy for intelligence in the absence of free oxygen.

    • Robin Datta March 20, 2019, 19:40
  • Alex Tolley March 20, 2019, 15:47

    For example, our demonstration that high CO may be achieved on inhabited planets means that while simultaneously high CO2 and CH4 with little CO is still a compelling biosignature, ambiguous scenarios with high levels of all three gases also exist,(…)[emphasis mine]

    My understanding was that high CO levels could be due to cometary impact. Therefore low CO levels were needed for a CO2-CH4 Archaean biosignature. Suggesting that high CO2 could be due to biology just confirms the ambiguity and needs to be eliminated to ensure as little ambiguity as possible.

    I think we may need to get past the universal biosignature phase and accept that we may need to tailor them for different types of worlds and their parent stars. Modeling, finding possible living targets with as many orthogonal approaches as possible, with final confirmation if needed when we can directly image worlds where the ambiguity remains, whether with telescopes (most likely) or probes (far more distant in time).

  • Michael C. Fidler March 20, 2019, 20:26

    Two ways that may give a larger spectral field to work with:

    1. UV flares from brown dwarfs to thru M dwarfs and higher, would be a part of the spectrum that can cause both emission and absorption spectra and fluorescence of gases in the atmosphere of exoplanets. After the star flares there will be a delay in the UV rays reaching the planets and a doppler shifts in the their spectrum. That should give a readable spectrum for many gases that are not in the lower IR range and because the flares are so bright and short lived they should be easy to identify and separate from the original flaring. This would also be good for brown dwarfs since they also have habitable planets and large UV flares.

    Searching for Exosatellites Orbiting L and T Dwarfs: Connecting Planet Formation to Moon Formation and Finding New Temperate Worlds.


    2. Active stars would be creating auroras from their solar storms that impacts exoplanets with magnetic fields. The close in planets and large magnetic fields associated with super earths would have similar timing and doppler shifts of spectral lines as from flares from the Emission and Absorption Spectra and fluorescents of the atmosphere. When our Sun is near its solar maximum the auroras are intense and frequent and should be easy to identify from interstellar distances. The aurora’s will be delayed between the magnetic reconnection on the sun and the delay of the impact of EM radiation and energized particle reaching the north and south magnetic poles of earth’s magnetic field.

    • Harry R Ray March 21, 2019, 10:04

      Since ANY detection of SPECIFIC MOLECULES in the atmospheres predominently rocky(i.e EITHER Earth analogs OR waterworlds)potentially habitable exoplanets by JWST will probably not occur until 2023 0r 2024(with the subsequent papers not published until 2024 or 2025)this may present an opportunity to SCOOP JWST with EXISTING telescopes. BEST CASE SCENARIO: A multi-messenger program could be implemented to monitor flaring activity of TRAPPIST-1 JUST BEFORE potentially habitable planets in the system occur, to make sure that these transits ARE observed, even if monitoring was not PREVIOUSLY SCHEDULED(as for Proxima b, its phase curve is currently being CONTINUOUSLY MONITORED by Sphere/Espresso, so, hopefully we could get superflare activity DURING a few of these monitoring sessions. Since the duration of this campaign is planned to be three years, a paper could come out as early as 2022!). However, the most intriguing possibility lies with Super Earth LHS 1140b! One follow-up study with the Spitzer Space Telescope refined the parameters of the planet and found it to be much larger, but still with a density large enough to still make it a predominantly rocky planet(but TRENDING to waterworld instead of Earth analog), and discovering a non-habitable near Earth-sized planet in a ~3 day orbit. However, a DIFFERENT SST study to search for TTV’s and exomoons was conducted PRIOR to the above mentioned study, but the paper HAS NEVER BEEN PUBLISHED! Could it be that a CO-like spectra was observed at say, a 3 sigma level of confidence, but the authors feared publishing the results due to the, AT THAT TIME, extremely UNLIKELIHOOD that so much CO could exist in LHS 1140b’s atmosphere to make it a viable detection! CAVAET: This is PURE SPECULATION at this point, but WHO KNOWS?!

  • Alex T. March 21, 2019, 4:34

    Short Summary of this article:
    We need more observations that will give us the lot of new scientific data , gained by modern and future advanced astronomic tools, after we will have this new data amount we can try to build different models based on real world statistics. Meanwhile everything releted to “biosignature” are pure speculations .
    “We know well , that we know nothing…”

  • Adam Byrne March 21, 2019, 17:29

    What kind of results would be completely unambiguous?

    • Gareth Wilson March 22, 2019, 0:04


    • Alex Tolley March 22, 2019, 2:08

      High resolution images or video showing recognizable plants and animals, preferably with David Attenborough doing the narration? ;)

      • Michael C. Fidler March 22, 2019, 10:48

        Near Gigapixel Imaging of Exoplanets from the Gravity Lens Regions.


        “We could resolve exoplanets around Proxima B to 450-meter resolution using a one-meter telescope SGL mission.”

        • Alex Tolley March 22, 2019, 12:02

          1/2 kilometer resolution wouldn’t be enough. It might show areas that could be interpreted as forests or fields, but not the nature of them unless spectral analysis confirmed they were living. If life was still at the single cell stage, then at best, spectral analysis might be able to determine life in shallow coastal areas versus open oceans. Without “ground truth” there is likely to be ambiguity.

          • Michael C. Fidler March 22, 2019, 22:23

            Yes, but chemical biosignatures would also be very easy to due at the same resolutions, for forest and fields. The ability to observe at select spectral lines would be feasible because of the brightness of the object at the 450 meter resolution. Basically like looking at the sun in specific freqs:

            This would prove very quickly and easily what type of life and it’s metabolism. Now what would be better is to develop light weight mirrors/laser sails (These are already being developed) that can be propelled at .2 light speed to the designated solar gravity lens points for different exoplanet solar systems. This would be much quicker to do then sending the Breakthrough Starshot probes and give huge amounts of data on the exoplanets over very long time periods. Easier, Safer, Quicker, faster data returned, much longer observations, ETC…
            Send out 1000’s of lightweight telescopic probes to the SGL positions.
            Sounds like a WIN/WIN!

            • Michael C. Fidler March 25, 2019, 10:40

              Here is the problem with Breakthrough Starshot, it is designed to investigate only the nearest exoplanets to our solar system over very long cruise times. The huge advantage in the solar gravity lens (SGL) is if a concentrated effort to develop lightweight systems to collect the photons the return will be for ONE THOUSAND different exosolar systems. That could be as many as 8,000 planets for 1000 spacecraft. With data being returned for years on what these planets are like and receiving it from the spacecraft in 3 to 7 days.

              • Alex Tolley March 25, 2019, 15:05

                There is still the “small problem” of getting each craft to at least the focal point, and maintaining its position wrt to the planet as it orbits its star and the star’s proper motion wrt to our sun. If a small, beamed craft can do this, then good. OTOH, maybe a large space telescope with light collecting mirrors and star shades spaced around a suitable orbit may be another approach.

                • Michael C. Fidler March 26, 2019, 11:50

                  Yes, but those are big problems and slowing it down when reaching the point is a bigger problem. The Breakthrough Starshot has had similar problems but with the dedication of many brilliant scientist they have been looked at and with open minds they have found solutions. Today, I came across an article by a scientist at JPL that gave a very pessimistic assessment that imaging would take a very long time. Then another JPL scientist in a February, 2019 paper says just the opposite, so go figure.

                  Optical properties of the solar gravitational lens in the presence of the solar corona.


                  • Alex Tolley March 26, 2019, 12:29

                    The craft doesn’t really need to slow down as the focal point is really a focal line. The tracking is more of a problem as this is perpendicular to the focal line.

                    If imaging is simpler than initially thought, that is encouraging.

                    However, I would like to see some examination of the competing technologies to image planets.

      • Harry R Ray March 25, 2019, 11:34

        Alex Tolley: Fungae and algeae imaged by Opportunity “growing” on “blueberry” concretions!?!?! Images appear in the Journal of Astrobiology and Space Science> BE VERY VERY SKEPTICAL HERE! My take: Probably just H2O or CO2 frost, but please check it out very carefully, in case I am wrong. HOWEVER: I CANNOT explain away two(Sol 1145 and Sol 1148) images of the SAME AREA showing the “blueberries” THEMSELVES growing! Go to https://curiosmos.com. Click on “A New Study finds possible evidence of Life on Mars.”

    • Alex T. March 22, 2019, 4:11


      • Adam Byrne March 22, 2019, 14:39

        Haha thanks guys, actually I was referring to chemical biosignatures in terms of total unambiguousness, and should have clarified that but I appreciate the other vivid images!

        • Alex T. March 23, 2019, 3:21

          The problem, that people that present time working on the “biosignatures” question, in reality do not know what they are going to search and find, in this case unambiguous reasult – is nonsense…

  • Michael Fidler March 21, 2019, 23:08

    But the uv flares and the exoplanets position plus accurate timing of transits is the only way that we can tell us if the planet is moving away or toward us. The flare is like a flash bulb going off and the delay is the time the light takes to reach the planet. The Doppler shift is when the planet is at the orbit position that is approaching or receding from earth. Both effects could be used to advantage in separating the spectral lines in from the stars flare. The big plus is that the intense flare would override any other background noise including the spectrum from the star’s normal emissions.

  • Michael C. Fidler March 25, 2019, 12:05

    A New Study Finds Possible ‘Evidence’ of Life on Mars.

    The Scientific Study published in the Journal of Astrobiology and Space Science provides evidence of possible Martian Life: at least fifteen images showed what appeared to be basic forms of life.

    Evidence is reviewed which supports the hypothesis that prokaryotes and eukaryotes may have colonized Mars. One source of Martian life, is Earth. A variety of species remain viable after long term exposure to the radiation intense environment of space, and may survive ejection from Earth following meteor strikes, ejection from the stratosphere and mesosphere via solar winds, and sterilization of Mars-bound spacecraft; whereas simulations studies have shown that prokaryotes, fungi and lichens survive in simulated Martian environments–findings which support the hypothesis life may have been repeatedly transferred from Earth to Mars. Four independent investigators have reported what appears to be fungi and lichens on the Martian surface, whereas a fifth investigator reported what may be cyanobacteria. In another study, a statistically significant majority of 70 experts, after examining Martian specimens photographed by NASA, identified and agreed fungi, basidiomycota (“puffballs”), and lichens may have colonized Mars. Fifteen specimens resembling and identified as “puffballs” were photographed emerging from the ground over a three day period. It is possible these latter specimens are hematite and what appears to be “growth” is due to a strong wind which uncovered these specimens–an explanation which cannot account for before and after photos of what appears to be masses of fungi growing atop and within the Mars rovers. Terrestrial hematite is in part fashioned and cemented together by prokaryotes and fungi, and thus Martian hematite may also be evidence of biology. Three independent research teams have identified sediments on Mars resembling stromatolites and outcroppings having micro meso and macro characteristics typical of terrestrial microbialites constructed by cyanobacteria. Quantitative morphological analysis determined these latter specimens are statistically and physically similar to terrestrial stromatolites. Reports of water, biological residue discovered in Martian meteor ALH84001, the seasonal waning and waxing of atmospheric and ground level Martian methane which on Earth is 90% due to biology and plant growth and decay, and results from the 1976 Mars Viking Labeled Release Experiments indicating biological activity, also support the hypothesis that Mars was, and is, a living planet. Nevertheless, much of the evidence remains circumstantial and unverified, and the possibility of life on Mars remains an open question.


    I just thought they were rocks, wonder if they are any good Sautéed?

  • Michael C. Fidler March 27, 2019, 9:32

    GRAVITY instrument breaks new ground in exoplanet imaging.
    March 27, 2019, ESO

    “Our analysis showed that HR8799e has an atmosphere containing far more carbon monoxide than methane—something not expected from equilibrium chemistry,”

    Read more at: https://phys.org/news/2019-03-gravity-instrument-ground-exoplanet-imaging.html

  • ljk April 3, 2019, 12:10

    Looking for Life on Mars: Viking Experiment Team Member Reflects on Divisive Findings

    Patricia Straat looks back on the Viking lander experiment that aimed to find microbes

    By Clara Moskowitz on April 2, 2019