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Biosignatures: The Oxygen Question

Just how useful is oxygen as a biosignature? It’s a question we’ve examined before, always with the cautionary note that there are non-biological mechanisms for producing oxygen which could make any detected biosignature ambiguous. But let’s go deeper into this, thanks to a new paper on ‘oxygen false positives’ out of the University of California at Santa Cruz. The paper, produced by lead author Joshua Krissansen-Totton and team, offers scenarios that can place an oxygen detection in the broader context that would distinguish any such find as biological.

Let’s begin with the fact that in addition to its obvious interest because of Earth’s history, photosynthesis involving oxygen requires the likely ubiquitous carbon dioxide and water we would expect on habitable zone planets. Helpfully, oxygen should be readily detectable on exoplanets because of its absorption features, which are prominent not only in visible light but in the near infrared and thermal infrared, if we include ozone. Space-based missions as well as ground-based Extremely Large Telescopes should be able to find oxygen signatures.

I found the authors’ discussion of M-dwarfs fascinating. We have to weigh our strategies with these small stars in mind because they are the ones for which atmospheric spectroscopy will first become available for habitable zone rocky planets. Already we’re in deep water, because the oxygen we might find on such worlds could have complex origins. From the paper:

…several features of M‐dwarfs make them susceptible to non‐biological oxygen accumulation. In particular, the extended pre‐main sequence of late M‐dwarfs could yield habitable zone terrestrial planets with hundreds or thousands of bar O2 from XUV‐driven hydrogen loss (Luger & Barnes, 2015). At least some of this oxygen will likely dissolve in a surface magma ocean and be sequestered in the mantle, but retaining oxygen‐rich atmospheres is still possible, especially for highly irradiated terrestrial planets…

Not only does this throw a spanner into the works for biosignature detection, but it could act as an active deterrent to the emergence of life by making prebiotic chemistry impossible. All this is under active study, as the paper’s numerous citations make clear, with the authors adding that “photochemical runaways yielding O2‐CO rich atmospheres remain a strong possibility for late M dwarfs.” These older red dwarfs tend to be the ones of higher astrobiological interest given that younger stars in this category are given to higher amounts of flare activity.

All of this points to the problems of oxygen as a biosignature and the need to examine how non-biological oxygen can accumulate on the planets we’re interested in, and this extends to planets around F-, G- and K-class stars as well, although the problem here seems highly dependent on the initial inventory of volatile elements, as the authors make clear. It’s also clear we have a great deal to learn about oxygen production via non-biological methods like hydrogen escape and water photodissociation, all reviewed in this crisp and clearly written paper. The interplay between atmosphere and geochemistry is the study’s central point:

The robustness of oxygen biosignatures rests on the assumption that for temperate planets with effective cold traps, small abiotic oxygen source fluxes from H escape will be overwhelmed by geological sinks. To test this assumption, it is necessary to model the redox [oxidation-reduction] evolution of terrestrial planet from formation onwards. This is because planetary redox evolution depends on both the initial state of the atmosphere and mantle after the magma ocean has solidified, and on the subsequent internal evolution and atmospheric state. Interior evolution dictates crustal production rates and outgassing fluxes, which determine the efficiency of geologic sinks of oxygen.

The authors use a model of planetary development that includes a wide range of initial volatile elements in varying abundance, taking rocky worlds all the way from their original formation up through eras of geochemical cycling lasting billions of years. The goal is to produce scenarios in which a lifeless planet around various stellar types could evolve with atmospheric oxygen. Context is all, meaning we have to know what other molecules beyond oxygen are available, and the range of outcomes is wide indeed. For a given scenario, distinguishing between false positives and genuine biosignatures is the key, and the paper explores the various options.

Image: By varying the initial inventory of volatile elements in a model of the geochemical evolution of rocky planets, researchers obtained a wide range of outcomes, including several scenarios in which a lifeless rocky planet around a sun-like star could evolve to have oxygen in its atmosphere. Credit: J. Krissansen-Totton).

The photodissociation referred to above occurs as ultraviolet light from the star breaks water molecules into hydrogen and oxygen in the upper atmosphere, with the lighter hydrogen escaping into space and the oxygen remaining as a potentially deceptive biosignature. But the paper also examines how oxygen can be removed from an atmosphere, through outgassing of carbon dioxide and hydrogen, which will react with oxygen. The weathering of rock also affects oxygen levels, all factors that need to be included in this model of geochemical evolution.

The model is given weight when we see that it can reproduce the evolution of the atmosphere both on the Earth and on Venus. Using it, then, we can explore possibilities. We can imagine a planet with more water than Earth, one whose deep oceans preclude weathering of rock that would remove oxygen. Conversely, on still molten young worlds with only a small inventory of water, the magma surface can solidify quickly, with water remaining in the atmosphere. Oxygen remains behind as hydrogen in the upper atmosphere escapes. Says Krissansen-Totton:

“The typical sequence is that the magma surface solidifies simultaneously with water condensing out into oceans on the surface. On Earth, once water condensed on the surface, escape rates were low. But if you retain a steam atmosphere after the molten surface has solidified, there’s a window of about a million years when oxygen can build up because there are high water concentrations in the upper atmosphere and no molten surface to consume the oxygen produced by hydrogen escape.”

Another scenario involves high amounts of carbon dioxide in relation to water, resulting in a runaway greenhouse. Here again, as the paper notes, we’ve got an oxygen problem:

The lack of liquid surface water precludes CO2‐drawdown via silicate weathering… Reactions between supercritical water and silicates will be severely kinetically limited by sluggish solid state diffusion, and are therefore assumed to be negligible (Zolotov et al., 1997). Consequently, a dense CO2 atmosphere and supercritical surface temperature persist indefinitely… despite the planet residing in the habitable zone. Moreover, there is sufficient steam in the atmosphere to ensure diffusion‐limited hydrogen escape provides an appreciable source flux of oxygen…

Its focus on the geochemical and thermal evolution of a planet in the habitable zone, emphasizing interactions between crust and atmosphere, make this a noteworthy addition to the ongoing attempt to understand biosignatures. We may well get a biosignature detection involving oxygen relatively quickly once we have the tools in place to delve into rocky worlds in the habitable zone. The effort to sort out its meaning will take considerable time.

I’l stand by a previous prediction: Initial euphoria will quickly wear off as we consider how deeply ambiguous any biosignature detection is going to be. I think we’ll be seeing plenty of interesting hints, but it will be many years before we can say with certainty that we have found life around another star.

The paper is Krissansen-Totton et al., “Oxygen False Positives on Habitable Zone Planets Around Sun‐Like Stars,” Vol. 2, Issue 2 (June 2021). Full text.


Comments on this entry are closed.

  • Alex Tolley April 14, 2021, 13:39

    Corroborating evidence will likely be the way we decide that biosignatures increase teh probability that they are true positives of life. A world with O2, other gases in non-equilibrium, like CH4, a chlorophyll “red edge” and lastly a visual image of teh planet with clear evidence of plant life on the surface.\

    Regarding spectroscopy, I was intrigued by the talk on high-resolution spectroscopy that seemed to offer far better detection of specific gases than conventional methods. I hope to read more about this technique. Ashley?

  • Alex Tolley April 14, 2021, 16:07

    These false positives for biological oxygen could be distinguished from inhabited planets using other contextual clues, but their existence implies next‐generation telescopes need to be capable of characterizing planetary environments and searching for multiple lines of evidence for life, not merely oxygen.

    Who is still relying on oxygen alone as a biosignature? I thought that we had long known that O2 alone was an ambiguous biosignature and that other biosignatures were needed to add context.

    These challenges to M‐dwarf habitability and potential ambiguities with oxygen biosignatures motivates the need to characterize habitable zone planets around F, G, and K‐type stars…

    IOW, the ease of spectroscopic analysis of M_dwarf planets is confounded by their relative ease of have abiotic O2 in their atmospheres. Therefore, shift attention to F,G,&K stars as O2 in teh atmosphere is far less likely to be abiotic.

    From Figure 2 caption:

    Earth’s atmosphere after 4.5 Ga is always anoxic (c) because outgassing and crustal hydration sinks overwhelm oxygen production via photolysis and diffusion‐limited hydrogen escape

    On Earth, even after biotic O2 was generated by photosynthesis, the O2 sinks overwhelmed the sources until the sinks were depleted and teh Great Oxidation Event occurred.

    Because of the focus on finding an Earth II that looks like a present day Earth, we need to remind ourselves that the Earth of today, with continents clothed in plant and animal life, with a rich O2 atmosphere is rather recent in Earth’s history. Most of Earth’s history was a planet with an anoxic atmosphere, with unicellular organisms as the only life form, most of which metabolized anaerobically. If life exists elsewhere in the galaxy, then most of these worlds will not have O2-rich atmospheres, but will still be living.

    While astronomers may be disappointed that the living worlds they discover are still in the unicellular life era, these worlds may be very interesting for eventual colonization by our interstellar descendants. This is because it might well be easier to start a terraforming program with introduced Earth organisms that can get a head start by using the extant biospheres rather than having to recapitulate this era of life. Rather than hacking down alien jungles a la “Avatar”, colonization may be more like building up ecosystems rather than replacing existing ones. [One might hope those Weyland-Yutani “shake-and-bake” terraforming operations operated more like this latter approach.

    • Patient Observer April 15, 2021, 20:15

      Thanks for a clear and comprehensive explanation. Amateurs like me gain a good appreciation of the science in such maters. The same comment applies to other contributors as well!

  • Drew LePage April 14, 2021, 18:50

    Any discussion of biosignatures immediately reminds me of the situation with Mars in the decades leading up to the Space Age. All of the alleged “biosignatures” that supported the then-widely held belief that Mars had primitive lifeforms turned out to be errors in the interpretation of available data or caused by other processes having nothing to do with life.


    • Adam April 17, 2021, 6:59

      One of the truisms of pre-probe planetology was the atmospheres of Mars and Venus would be mostly nitrogen and carbon dioxide a minor component. That idea died by the end of the 1960s.

  • Geoffrey Hillend April 14, 2021, 18:53

    Both redox and oxidation should work on planets in the life belt around all three types of stars F G and K class stars. It was the great oxidation event that removed the oxygen from the atmosphere of Earth and we got a lot of rust on the surface. Without life to replenish it, the result will be that there won’t a lot of oxygen in an exoplanet’s atmosphere. The question is if there is only parts per million will it be detectible. It might be there would be the problem of false positives. I think if they are false positives there we can remove them by including other information like size and mass which might shed some light on how much atmosphere the exoplanet has.

    On a large water world, an ocean might only remove the abiotic oxygen from the lower atmosphere, but not the upper atmosphere. Venus might not be a good example because it moved out of the life belt early and it has lost a lot of oxygen and water vapor from solar wind stripping and ultra violet solar lysis of water. It was in the life belt over four billion years ago, but he Sun became brighter. The larger the star, the faster the main sequence burning, the faster the life belt moves away from the star. This is important since I don’t think there can be a planet with a very large greenhouse and high surface temperature like Venus at the same distance from a G class star as Earth at the time period of 4.5 billion years of age. A large super Earth water at Earth’s distance around a G class star or the life belt of F or K star world with a big atmosphere might have a temperature above the boiling point of water. Due to the vapor pressure, the boiling point of water will be higher than 212 F at one bar.

    What is important to know is if smaller stars like M dwarfs have less of some element than larger stars. They might not so all of the exoplanets get the roughly the same biosignature elements if they are exoplanets of the same size and mass as the Earth or larger.

  • Geoffrey Hillend April 14, 2021, 18:59

    Another question is if the abiotic oxygen or false positive is only in the parts per million in an exoplanet’s atmosphere, will we be able to detect it from Earth? If we can, it might be a problem in ruling it out for the detection of biotic oxygen. If we could detect a lot of nitrogen, that might help. A large greenhouse should be easy to detect as a black body radiator. There is also methane and nitrous oxide.

  • Andrei April 14, 2021, 21:12

    Thank you for bringing this to everyone’s attention.
    The paper is actually a preprint, and set for publication in June.
    I have already read it in full of course, since they published on an idea I repeatedly have to contradict for several decades – that oxygen in a planets atmosphere automatically would indicate the presence of life.
    They even come up with one more scenario, that of the hot steam world also could end up with an oxygen atmosphere. While my small brain only could come up with the iceworld and water world scenarios.
    And having oxygen in the atmosphere would indeed prevent life from getting started, at present we do not know if worlds like this will turn up to be common or rare exceptions. But if they do turn out to be fairly common, I think we got a duty to seed such life. Not only for the purpose of spreading life in the universe, even if the packages we sent will be nothing more than a starting kit, but if we come to our senses both on how we manage Earth, our resources and get out of the cradle it could become a benefit for future settlers as well.
    Here I see a worthy goal for ‘Breakthrough Starshot’, instead of sending a laser sail that will be unable to send back any meaningful science. Why not use such craft to deliver seeding packages? Yes it might require a space based system for providing the breaking laser light since that will have to be done with extreme precision. But any such project need sponsors, and the idea of seeding other planets might give many outside of the space buff community a nice sense of wonder.

    • Gary Wilson April 15, 2021, 14:57

      A very interesting idea Andrei but a couple of thoughts. To make the enormous expense worthwhile we would have to have an extensive knowledge of the planetary environment not just the presence of a few gases. Also what types of seeds? Obviously extremely hardy ones would be preferred. What are we trying to achieve by seeding planets? Is it in preparation for a human presence? If so we would have to find a number of viable planetary candidates (or moons I suppose) around nearby stars. It sounds very exciting. Perhaps you have come up with the idea for Breakthrough SeedShot? :)

      • Alex Tolley April 15, 2021, 16:22

        While not specifically about seeding biospheres, George Church suggested in Breakthrough Discuss that Starshot probes should carry engineered life to teh target planets. While he was thinking of this life growing communication technology, obviously a far easier proposition would be to seed bacterial and eukaryote spores to seed a world with terrestrial life. This should only be done if the target world was shown by previous inspections to be sterile yet conducive to life.

      • Andrei April 16, 2021, 15:03

        Thank you for your reply Gary.
        Yes we would indeed need to gather a lot of data before we would be able to say that it’s a planet with an oxygen atmosphere that still lack life. So in my small proposal I did assume that the discovery of such worlds have put those under intense study.
        I used the word ‘seed’ with the thought of sowing – but I did not think of actual plant seeds, but rather spores of unicellular organisms.
        In the reply by Alex below, he’s on the right track – though algae and flagellates also could be sent – all depending on what environment the planet got. Or some of each kind, to start a simple ecosystem.
        The packages sent have to be extremely lightweight anyway. This mean that whatever protection is used against radiation, it might be far from perfect.
        With spores the solution would instead to go for the numbers, by having a large number of dormant spores in each seedling capsule.
        Extending the flight time somewhat by going slower could actually be an advantage, to provide a more substantial protective layer, and the sail design would have to be more complex anyway as it also need to break, either by having a dual sail for breaking or a magnetic one – or any other proposed method.
        The delivery mechanism might be made entirely mechanical perhaps releasing the cargo after a certain amount of entry heat. Spores will not suffer from extreme g-forces. So the final step is at least somewhat fault tolerant.
        The goal could indeed be to prepare other worlds for the long term goal of settlement. But even if we do not manage to leave the solar system. I view the idea of spreading life to be a worthwhile goal by itself. Project SeedShot is a great name, I certainly join up if it materialize. =)

        • Ronald June 23, 2021, 19:27

          I have coined the term Terraseeding a few times.

  • Mike Serfas April 14, 2021, 22:30

    The “eternal runaway greenhouse” graphic reminds me of my reservations about “habitable zones”. It seems like the habitable zone depends very much on the planet. For example, Neptune has an abundance of water, but it is (among other problems) too hot for conventional life. Surely an Earth-sized CO2-rich world could be more congenial at some greater distance from its star?

    “Carbon planets” are an oddity that seemed all the rage for a little while, but I haven’t heard so much of late. The fourth most recent Arxiv abstract I found mentioning the term, from 2017, called them an illusion, disputing an alleged detection: https://arxiv.org/abs/1701.00493 The most recent one I found ( https://arxiv.org/abs/2005.03175 ) assumed they are real, but argued that bombardment with water could transform their upper mantle and surface and give them a reducing atmosphere. (SiC + 2H2O -> SiO2 + C +2H2) It would be interesting to hear what someone in a position to know has to say about carbon worlds today!

  • Michael Fidler April 15, 2021, 10:37

    High carbon worlds + water world together, what would it produce? Could the greenhouse keep these worlds warm even out to Jupiter in our system? This might be the most common type of planet especially around M-dwarfs. There is a transition from earth to super earth to mini Neptune’s that we have NO examples of and my out number all other planets in the universe. This is true even around sun like stars, the variety and differences in atmospheres, oceans and geology of these planets may take centuries to figure out because of the initial conditions when they formed and as they evolved. Could the variety be so far as the variety of molecules that form; More than 90 million unique organic and inorganic chemical substances, such as alloys, coordination compounds, minerals, mixtures, polymers and salts, and more than 65 million sequences. Remember, planets form in different interstellar clouds with different varieties of elements depending on the what type of stars exploded to form those clouds and how close those stars where to each other. What of radioactive isotopes and there off spring? The long term evolution of each individual planet may have a much larger variety then what we see in our solar system. Super earths may be the largest mixing vat for organic compounds that it may make the earth look like a desert in comparison. The universe IS stranger then we can imagine. ;-} Godzilla Lives!

    • Alex Tolley April 15, 2021, 16:17

      I thought that an orbit just beyond Mars was the limit as after that CO2 freezes out as snow and no longer acts as a GHG. While not conclusive due to smaller gravitational wells, if CO2 could stay gaseous and warm a world, wouldn’t the icy moons of Jupiter have thing gaseous CO2 atmospheres?

      • Michael Fidler April 15, 2021, 20:43

        The difference is is the super earth gravity well and water worlds. We need to look much closer as to what these worlds will be like, an example is Barnard’s star b and Proxima Centauri c, the two closet super earths. Larger cores and more active volcanically would make for very different worlds then the Jovian moons.

        Barnard’s Star Planet May Not Be Too Cold for Life After All.


        • Alex Tolley April 17, 2021, 12:46

          Are you positing a water world where a thick CO2 GHG atmosphere replaces the ice crust over a liquid ocean warmed by volcanism from the radioactive decay in the mantle (or perhaps gravitational flexing in a close orbit about an ice giant)? I’d like to see some climate modeling to show that is possible.

          [The space.com article only suggests liquid water in subsurface pockets, not liquid surface water.]

          • Michael Fidler April 17, 2021, 15:03

            Yes, and modeling other outcomes in such worlds. Their evolution may have a very large variety of possibilities. Look at this list on nearby exoplanets and you can see the most common objects are superearths. The latest is the superearth with possible rings around it that may be orbiting Alpha Centauri A.


  • Alex Tolley April 15, 2021, 12:13

    And having oxygen in the atmosphere would indeed prevent life from getting started…

    Can we be sure of this?

    We don’t know how life on earth started. We assume the early Earth’s reducing conditions were necessary, but we don’t know whether such conditions are possible in habitat niches when O2 is present. [For example anoxic conditions in the oceans after the Permian extinction].
    We also cannot exclude seeding by panspermia from a nearby living world.

    IOW, I am suggesting this is not a binary condition. There may be worlds where life exists, and where photosynthesis may be contributing to the atmospheric oxygen, while abiotic oxygen generating processes are dominant (unlike Earth where photolysis is insignificant compared to photosynthesis O2 production).

    In the distant future, a world with an O2 atmosphere might be very convenient for colonization by aerobic organisms, including humans.

    • Andrei April 20, 2021, 5:35

      No one was around to see it happen, so we cannot say we’re certain about anything. Yet the more I learned about the complexity of a single living cell, and the genetics. The more amazed I get. Meaning that the test tube in which this set of events happened have to be of immense size = an entire planet. (This is why I tend to think that Enceladus, a mere 500 km might be to small. While I do think it’s a very good idea to look for unicellular life on Europa – which might be a border case with a fair chance.)
      Now how fast and easy DNA and RNA fall apart so easily in our atmosphere today, I know very well that such molecules cannot have developed in the presence of oxygen.
      As for panspermia, I have always avoided that line of thought. Beside the fact that it is distantly related to religious thinking and the ‘hand of god’ – in this case replaced by the idea that ‘aliens’ have been behind panspermia. And since that would mean it have happened elsewhere, the consequence would be that there’s no reason to study the origin of life at all. Or ending up in an endless recursive reasoning of where that life in turned also have come from yet another system or galaxy.
      With panspermia or aliens we end up in one endless cycle of speculation, as a researcher I have always tended to avoid such – else I would never make any progress. And then there’s people who complain that we all are atheists! …there’s a very good reason for that! =)

      • Alex Tolley April 20, 2021, 12:41

        Now how fast and easy DNA and RNA fall apart so easily in our atmosphere today, I know very well that such molecules cannot have developed in the presence of oxygen.

        Perhaps I didn’t make myself clear. What I am saying is don’t treat the early Earth as a platonic, homogenous object. Conditions are patchy. The Earth is highly oxidizing today, but that doesn’t mean there are not places where conditions are reducing. It is in these places where life could start, just as if the whole planet was reducing.

        While Earth [its oceans] needed to become oxidizing to evolve aerobic life, on a planet that was already mostly oxygenated, aerobic life could evolve at the boundaries of the reducing habitats where oxygen was intruding.

  • Alex Tolley April 15, 2021, 12:28

    OT. In the post “How Planetesimals Are Born” Ron S. made an interesting comment about the origin of one suspected Oort cloud origin comet: Hale-Bopp.

    It got me thinking. If the Oort cloud is not directly observed, but rather inferred by the orbits of long-period comets, and if those comets were actually Kuiper Belt objects gravitationally kicked out so that their aphelions are in the space where the Oort is believed to exist, is it possible that the Oort cloud does not exist?

    The Oort is depicted as a spherical cloud extending from around 2000 AU (well beyond the Kuiper belt) to perhaps a light-year or more. But is this size and shape just a result of the trajectories of the comets? Is there any direct observation of other systems that would confirm such a cloud? Is the Oort cloud shape dictated by the dynamics of gas and dust collapse around star formation?

    Because if it is an illusion, maybe this removes the possible counterfactual of Oort cloud comets in the authors’ model of planetesimal formation.

    • Ron S. April 15, 2021, 15:51

      I’ve wondered the very same thing. I agree that there was gas and small particles (sometimes called “pebbles”) driven out of the inner solar system early in its formation when the sun lit up and the small stuff not already gravitationally bound to large bodies would have been forcefully pushed outward.

      But after that? Once you go beyond the Kuiper belt the particle density would be so low that agglomeration would be rare, and what is that far out is very susceptible to gravitational disruption by occasional passes of stars and other large bodies.

      It leads me wonder whether there is no Oort cloud at all, or at least none that is distinguishable from the ISM itself. The expelled gas and particles for all forming stellar systems would gradually mix and merge. As a significant source of comets? Probably not.

      But as Alex says, off topic.

  • Dave Moore April 15, 2021, 13:49

    This paper has crystalized some ideas that I’ve had floating around for a while. I think we should think in terms of the oxidation state of planets when considering the evolution of life and the occurrence of alien technological civilizations.

    All planets have an oxidation state. Jupiter, for instance, is almost completely reducing as a result of its preponderance of Hydrogen. But smaller planets particularly Earth-sized ones tend to lose Hydrogen over their lifetime and therefor become more oxidizing. Venus is an example of a highly oxidized planet even though it doesn’t have free oxygen in its atmosphere. Most of the constituents in its atmosphere are oxidized gases (co2, H2so4). And it’s lithosphere is probably oxidized as it is assumed that the oxygen from all the lost water has combined with its rocks.

    Mars has a high oxidized surface, full of chlorates that oxidize organic matter on contact, but because its had less geological activity, its lithosphere is probably more reducing than Venus’s.

    Earth started out with a moderately reducing surface that gradually became more oxidizing as time went on. It has been assumed that photosynthetic life was responsible for the oxidation of Earth, but let us look at the effect life had on the oxidation of Earth. What photosynthetic life did was make our atmosphere more oxidizing and through the deposition and burial of organic matter make our lithosphere more reducing. There was no net effect. And if you look at the mass of Oxygen in the atmosphere and the mass of the buried organic matter compared to the mass of Earth it is very much at the margins. And geological overturn evens this out.

    Suppose Earth’s gradual oxygenation is due to hydrogen loss from water vapor which increased the average oxygenation state of the upper lithosphere and surface to a point that allowed photosynthetic life to overwhelm the chemical sinks for Oxygen.

    If we look at a planet for its potential to give rise to complex life, we have to consider its oxidation state: too reducing and while simple life may evolve, there will never be free oxygen to power multicellular life.

    If the planet starts off too oxidizing, then organic molecules are destroyed killing the possibility of life evolving.

    A planet, for complex life to evolve, has therefor to start off as mildly reducing and then reach at some point what I call the oxidation line, which allows for that build up sufficient free oxygen so that high energy life can evolve, which I think is the precursor of intelligent, technological species.

    • Alex Tolley April 15, 2021, 16:28

      Suppose Earth’s gradual oxygenation is due to hydrogen loss from water vapor which increased the average oxygenation state of the upper lithosphere and surface to a point that allowed photosynthetic life to overwhelm the chemical sinks for Oxygen.

      The evidence suggests that it wasn’t until photosynthesis was evolved that the oceans became more oxidized as evidenced by the iron band formations. If photolysis was the cause, then shouldn’t these have started to form earlier in the geologic record? Secondly, marine life became dominant O2 sources. This would have happened quickly, whilst photolysis would remain a slow process. Doesn’t the paleoatmosphere data suggest a rapid oxygenation as soon as the O2 sinks were exhausted, indicating the photosynthesis, rather than photolysis was the dominant source then, as it is today?

      • Dave Moore April 16, 2021, 15:46

        Photolytic Oxygen would initially oxidized Methane, H2S and CO in the atmosphere, so initially it wouldn’t have had much effect in the oceans.

        Secondly, the arrival of photosynthetic algae 2.3 billion years ago did not change Earth much. The level of Oxygen in the atmosphere was in the order of a couple of percent and most of the ocean was anoxic, with only a thin layer of oxygenated water on the surface. Then about 600 million years ago the oxygenated area of the ocean, which had fluctuated from just the surface waters to moderate depths, spread to the deep ocean and the level of oxygen in the atmosphere built up to ~15%. No one knows why this happened. Photosynthetic algae were much the same as a billion years before. (This was the Ediacaran when the first macroscopic creatures were evolving, but this could easily be an effect of high oxygen levels, not the cause.)

        This suggests some sort of tipping point was reached, and it is what I was referring to as the Oxygen line. If Earth had more active volcanism or a slightly more reducing Lithosphere then this tipping point may never have been reached and complex, energetic life might never have emerged.

        (This is why I am particularly interested in the Ediacaran. There’s a big mystery at the center of it, and its creatures look very alien.)

        The main point of my post was to get people thinking in terms of a planet’s oxidation levels when considering the evolution of life.

        • Ronald June 23, 2021, 19:25

          So, the composition of the crust and mantle, its ratio of elements, in particular Fe, Mg, Ca, Si, may be of importance here as well the. Interesting idea.

  • torque_xtr April 15, 2021, 13:53

    The water planets can be complicated. If the ocean is deeper than ~200 km, a layer of insulating high pressure ices will form at the bottom. Thermal conductivitiy of ice VII at lower pressures is comparable to that of rocks (http://users.mrl.illinois.edu/cahill/kyushu11.pdf), so there will be basal melting, ice layer viscous convection and abundant “cryovolcanism” at the ice/water boundary, all of which will greatly enhance mixing in both directions. Methane and ammonia comes with water, so in the abscence of stripping, aquaplanets are likely to have thicker N2 atmospheres. Both gases act as oxygen sinks, too.

    I doubt also that deep-water aquaplanets will have suppressed mixing due to pressure-increased astenosphere viscosity. Radiogenic heat needs somewhere to go, so instead of tectonics there will be heat-pipe- or stagnant lid-type volcanism on the ocean floor anyway, with time-averaged levels comparable to Earth.

    Yet there is one firm point – water is abundant and it’s photolysis produces easily escaping H2 and much heavier O2, with rates and efficiencies widely varying and not fully understood. There is abiotic oxygen in the atmosphere of Mars. 1 Pa is almost non-existent by Earth’s standards, but in terms of oxygen fugacities and redox potentials, it is much the same – and greatly different from truly reducing environments like present-day Venus or Archaean Earth.

  • Geoffrey Hillend April 15, 2021, 15:26

    Deep time plays a major role as well as the type of star and exoplanet. An exoplanet very close to the size of our Earth without a Moon might produce life but by 3.9 billion years of age loose some atmosphere which could cause that life to become extinct and what little oxygen was there would be removed by natural processes so the age of the exosolar system is important.

    Also I think is was mentioned in some posts about an earlier Centauri Dreams paper that Earth 4.1 billion years ago might not have any oxygen or not show any trace of oxygen in spectra observed from many light years. I do agree with the idea of a large super Earth water world around an M dwarf might have a false positive of oxygen from photolysis of the x ray flares with a large, thick atmosphere.

  • Patient Observer April 15, 2021, 20:31

    The subject paper indicated that modeling predicted Venus’s present runaway greenhouse condition and (my assumption) it was always that way. Yet researchers suggest that Venus may have been habitable in the geological past. Per Wikipedia, habitability may have persisted to about 700 million years ago. Another article (can’t locate the reference) suggested habitability as recently as 250 million years ago. Either figure opens the possibility of advanced multicellular life. I like to image a world of dinosaurs and tropical jungles or even intelligent life in the making.

    Alas, the atmospheric traces of phosphine in the venereal atmosphere may not actual exist per my favorite science explainer – Sabine Hossenfelder. Arghh!

  • ljk April 16, 2021, 10:36

    Nothing like grabbing a chunk of potential biosignature and getting it to Earth for proper examination, as with Mars…


  • Alex Tolley April 18, 2021, 19:27

    To further add ambiguity. This paper concerns hidden life
    on a world with a true, life-generated O2 biosignature.

    On a hypothetical planet where surface
    vegetation never developed, or surface
    conditions are more extreme than on presentday
    Earth, or a high impact rate favours
    cryptic photosynthesis, or where the complete
    surface is ocean-covered, a substantial amount
    of O2 production could occur within cryptic
    photosynthetic communities. On such a planet
    a surface biosignature such as the Vegetation
    Red Edge would never develop. The reflection
    surface spectrum of such extrasolar worlds
    would be dominated by standard mineral and
    water reflections. Prior to the rise of oxygen in
    the Earth’s atmosphere and the formation of
    an ozone shield, the initial production of
    oxygen would have been accomplished by a
    photosynthetic biota protected from UV
    radiation that was essentially cryptic.
    Marine cryptic photosynthesis would have
    been most important as net carbon burial
    would likely have been more effective in
    the oceans than on land, and therefore
    would have made a greater contribution to
    atmospheric oxygenation. From this
    standpoint, the Earth during the onset of
    the Great Oxygenation Event ~2.4 Ga ago
    until the appearance of a red edge ~0.45
    Ga ago may have exhibited a cryptic
    photosynthesis signature.

    Cockell, Kaltenegger, Raven. “Cryptic photosynthesis – extrasolar planetary oxygen without a
    surface biological signature”, https://arxiv.org/abs/0809.3990

  • ljk May 5, 2021, 15:00

    Book Review: A Zoologist Imagines What Alien Life Might Look Like

    In “The Zoologist’s Guide to the Galaxy,” Arik Kershenbaum speculates about the universal lessons of evolution on Earth.


    To quote:

    Kershenbaum has studied wolves in Yellowstone National Park, dolphins in the Red Sea, and small mammals called hyraxes in Israel, and the crux of his argument revolves around his experience as an evolutionary biologist. If we can understand how life evolves here on Earth, we can then ask pertinent questions about how and why creatures on other planets might develop in a certain way. After all, Kershenbaum points out, the laws of physics are constant throughout the universe, so we can view Earth as an “evolutionary testing ground” for realistic solutions to life’s problems.

    Much of his book is organized around a series of chapters probing at different aspects of animal life on earth. Kershenbaum walks readers through chapters on movement, communication, intelligence, sociality, information, and language, describing why each of these tenets of life evolved, how they evolved, how they present or don’t present in humans and other animals, and what we can take from our understanding to postulate what aliens might be like. For example, in his chapter on sociality, he explores the costs and benefits of the development of complex societies on earth, showing how cooperation forms when it’s evolutionarily advantageous, and extrapolating a theory that as long as relatedness exists on alien planets, kin selection will drive at least some cooperation in those societies. In other words, if it works for us here, it’ll likely work on alien planets. “Teatime with our alien neighbors may be possible after all,” he tells us.

    After all, says Kershenbaum, aliens might be telekinetic, or all-knowing, or little green men with big heads, but why? Some outcomes are simply not likely, like a hyper-intelligent alien floating through the universe and philosophizing for no reason. Others are quite likely: For example, if a neutrally buoyant alien must move through fluid, then it follows that that alien will evolve fins or some other means of stabilizing itself. Other possibilities raise intriguing questions, fit for a sociologist, about what life might be like in other worlds: For example, could a planet support two linguistic species without one enslaving the other?

    Readers might raise an eyebrow at the premise of this book. After all, can we really use what we know about evolution on Earth to extrapolate to the vast unknowable universe? But Kershenbaum cleverly anticipates these potential criticisms. He acknowledges that people might disagree with his assumptions; all he asks is for readers to take away some conclusions about what alien life might be like, based on educated guesses.

    Kershenbaum is also quick to second-guess himself or to present alternate conclusions to his theories. For example, some scientists believe that mathematical principles could act as a universal language for communication with alien species — but Kershenbaum also points out that mathematics might look different to aliens, or that aliens might analyze the world through other lenses besides mathematics. He argues that humans evolved language to support our complex society and that languages on other planets would probably evolve for the same reason. He admits, however, that language could evolve for a reason incomprehensible to earthlings. In a later chapter, he even contradicts his main conceit that understanding evolution on Earth will allow us to understand other planets: What if we encountered a planet inhabited by designed artificial organisms, or robots, which could bypass natural selection?

    Kershenbaum recognizes that scientists are not the only group that have spent centuries speculating about alien life. He has a healthy respect for the work of science fiction writers, too, and his book is peppered with pop culture references ranging from “Guardians of the Galaxy” to “Arrival.” He charmingly refers to “Star Trek: Next Generation” as the “Shakespeare of science fiction.” His footnotes feature references to both the Bible and Richard Dawkins. The book also includes photographs and drawings to accentuate his points, of creatures ranging from man o’ war to ancient ammonites with delicate tendrils and shells. These features, paired with Kershenbaum’s friendly and undidactic tone, make his book readable and approachable.

    Ultimately, his goal is to encourage readers to ask the right questions about alien life, even if we can’t necessarily land on particular answers. Some of those questions include larger philosophical quandaries: Would aliens share the “human condition” with us, and what exactly is the human condition? What is an animal, what is an alien, what is personhood?

    These questions are important, Kershenbaum argues, because of humanity’s fraught history of grappling with those very issues regarding animals and other humans here at home. Perhaps, he says in his epilogue, while we wait to find aliens, we can ponder these big questions and apply the answers in new ways right here on Earth.

  • ljk May 6, 2021, 14:18

    May 6, 2021 at 11:31 AM by VICTOR TANGERMANN

    Scientists Claim to Spot Fungus Growing on Mars in NASA Rover Photos

    “Fungi thrive in radiation intense environments.”

    The hunt for life on Mars continues, with NASA’s latest rover Perseverance using its scientific instrumentation to scan the Jezero Crater, an area believed to be a dried up ancient lake, for any signs of ancient microbial life.

    But according to an international team of researchers, the space agencies other rovers may have already found signs of relatively advanced life — in the form of “fungus-like Martian specimens,” according to a new paper published in the journal Advances in Microbiology.

    The team, which includes researchers from the Harvard-Smithsonian Center for Astrophysics and George Mason University, believes they have found photographic evidence of a variety of fungus-like organisms, some resembling the shape of puffballs, a round cloud-like fungus found in abundance back here on Earth, on the Red Planet.

    Their evidence: images taken by NASA’s Opportunity and Curiosity rovers as well as the agency’s HiRISE high-resolution camera attached to the Mars Reconnaissance Orbiter.

    “Fungi thrive in radiation intense environments,” the team writes in its paper. “Sequential photos document that fungus-like Martian specimens emerge from the soil and increase in size, including those resembling puffballs.”

    “After obliteration of spherical specimens by the rover wheels, new sphericals — some with stalks — appeared atop the crests of old tracks,” the researchers write.

    The team went so far as to say that “black fungi-bacteria-like specimens also appeared atop the rovers.”

    They didn’t stop there: the team also examined photos taken by NASA’s HiRISE, and found evidence for “amorphous specimens within a crevice” that “changed shape and location then disappeared.”

    “It is well established that a variety of terrestrial organisms survive Mars-like conditions,” the team concludes. “Given the likelihood Earth has been seeding Mars with life and life has been repeatedly transferred between worlds, it would be surprising if there was no life on Mars.”

    The team argues that these Martian lifeforms “would have evolved on and already be adapted to the low temperatures, intermittent availability of water, low amounts of free oxygen, and high levels of radiation.”

    The researchers did caveat their findings, pointing out that “similarities in morphology are not proof of life,” and that “we cannot completely rule out minerals, weathering, and unknown geological forces that are unique to Mars and unknown and alien to Earth.”

    But it’s a wild conclusion nonetheless. The researchers’ peers will likely go over the paper with a fine-toothed comb, and likely shred the results — it’s not every day that researchers are willing to stick out their necks and claim to have found evidence of life on Mars.

    The rest of the article here:


    The link to the paper here:


    I recall the National Geographic Magazine article from 1955 on Mars where astronomers saw a “patch” of green the size of Texas and were certain that it was a giant field of plant life.

    We may know a lot more about the Red Planet in 2021 compared to 1955, but we also have a long way to go. Especially regarding any natives.

    Plus let us not forget this fellow…


  • ljk May 10, 2021, 11:44
  • Karl Kharas May 10, 2021, 20:55

    The text seems to assume that O2 is really easy to detect spectroscopically. But why? The puzzle I don’t understand here is how O2 can be detected spectroscopically. There’s no dipole in the visible or near infrared, or near UV. No dipole, so no dipole-allowed transitions. That’s why our atmosphere is essentially transparent in the visible and IR. What am I missing here?

  • Ronald June 23, 2021, 19:35

    But wouldn’t an (O2 rich) runaway greenhouse world and a desert world in the HZ be rather hot, as a telltale sign of abiotic condition? What I mean is the combined criteria of an O2 rich atmosphere ánd a moderate temperature. Ok, for waterworlds that could still be the case.