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Extending the Astrobiological ‘Red Edge’

A useful exercise for learning how to look for life elsewhere is to try to find it right here on Earth. Thus Carl Sagan’s observations of our planet via data taken during the 1993 flyby of the Galileo spacecraft, which was doing a gravity assist maneuver enroute to Jupiter. Sagan and team found pigments on the Earth’s surface with a sharply defined edge in the red part of the spectrum. What he was looking at was the reflection of light off vegetation. The ‘red edge’ has become well known in astrobiology circles and is considered a potential biosignature.

On Earth, vegetation is the most abundant reflecting surface indicating life (vegetation covers about 60% of present-day Earth’s land surface). The increase in reflectance shows up at about 700 nm, varying in strength depending upon the species of plant. But as Jack O’Malley-James and Lisa Kaltenegger (Cornell University/Carl Sagan Institute) point out, photosynthetic structures containing chlorophyll are found not just in vegetation but also in lichens, corals, algae and cyanobacteria.

This is helpful, because for anyone looking at the early Earth, the vegetation red edge would have been apparent only after the advent of land plants, while if we can detect a similar feature in other forms of photosynthetic life (call this a photosynthetic red edge, or PRE), we can extend the ability to detect such life back as far as 2 billion years and more (in the case of cyanobacteria). According to the scientists, lichen probably emerged at about the same time as algae, 1 billion years ago, while corals and modern vegetation begin to appear no earlier than 725 million years back.

This offers a much wider ‘window’ in which to observe red edge features on other worlds. The authors’ new paper in Astrophysical Journal Letters looks at what could produce a PRE spectrum aside from land vegetation and asks whether features like these would be detectable. Says Kaltenegger:

“If an alien had used color to observe if our Earth had life, that alien would see very different colors throughout our planet’s history – going back billions of years – when different life forms dominated Earth’s surface. Astronomers had concentrated only on vegetation before, but with a better color palette, researchers can now look beyond a half-billion years and up to 2.5 billion years back on Earth’s history to match like periods on exoplanets.”

Image: To understand where exoplanets are in their own evolution, astronomers can use Earth’s biological milestones as a Rosetta stone. Credit: Wendy Kenigsberg/Cornell Brand Communications.

The paper models how a planet’s spectrum would change depending upon the dominant organism on the surface. O’Malley-James refers to the authors’ use of the early Earth in this analysis as a kind of Rosetta Stone, one that extends back halfway as far as the Earth itself. Examining the spectra of Earth-like planets modeled with four different organisms — cyanobacteria, algae, and lichen, as well as deciduous vegetation (lichen, for example, would have cast a sage to mint green color, a distinctive red-edge signature of photosynthesis), the authors show that the addition of an atmosphere and clouds to the model can mask individual features but still produce enough data to reach a broader conclusion: From the paper:

…for similar surface coverage the PRE signal of other organisms that could be dominant on the surface of an exoplanet can be similar in strength to the signal produced by modern vegetation for Earth in our models, which is approximated using deciduous tree reflectance producing an estimated reflectance increase of ∼4% (Table 1), falling within the lower end of the range of values (1%–10%) given for Earth’s VRE,,, Figure 1 shows that individually the different organisms can be distinguished with high spectral resolution. However, once we add a present-day Earth atmosphere as well as clouds to the model…, the individually distinguishing slope of the reflectivity of the organisms is no longer apparent. Thus a red edge detection, while not being specific to any one form of photosynthetic organism, can indicate a wider range of organisms than only vegetation.

Image: This is Figure 1 from the paper. Caption: Examples of red edge features—the increase in reflectance caused by chlorophyll, highlighted in the shaded region—exhibited by (A) corals, (B) deciduous vegetation (trees; representative of the present-day red edge feature in Earth’s spectrum), (C) the photosynthetic sea slug, Elysia viridis, (D) lichen (Acarospora sp.), (E) algae (Rhodosorus marinus), (F) cyanobacteria (Chroococcidiopsis sp.). Credit: Jack O’Malley-James/Lisa Kaltenegger.

The red edge would be a difficult biosignature detection but not beyond the reach of high-precision instruments as we move to the next generation of observatories. It also provides another tool for biodetection that in combination with atmosphere analysis offers a multi-pronged approach to our remote probing for life, lessening the potential ambiguity of the results.

The paper is O’Malley-James & Kaltenegger, “Expanding the Timeline for Earth’s Photosynthetic Red Edge Biosignature,” Astrophysical Journal Letters Vol. 879, No. 2 (10 July 2019). Abstract.

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{ 15 comments… add one }
  • Gary Wilson July 12, 2019, 14:39

    A very interesting idea. How many planetary spectra do we have to date from rocky planets in the habitable zone? And how often do spectra include the 700-800 nm range? I see you suggest it could happen as we move to the next generation of observatories Paul but have any such spectra been collected yet?

    • Paul Gilster July 12, 2019, 14:58

      Here’s a good piece by Sara Seager that goes into spectroscopy and life detection on rocky worlds:

      https://www.pnas.org/content/111/35/12634

      Right now transmission spectroscopy is limited to gas giants close to their stars, though as we’ve just seen, smaller planets of Neptune scale are starting to yield data. But it will take a new generation of instruments to do this kind of work for small rocky worlds in the habitable zone. We’re also moving toward imaging smaller worlds. From Seager:

      “To be confident of finding a large enough pool of exoplanets to search for biosignature gases, we require the ability to directly image exoplanets orbiting 1,000 or more of the nearest Sun-like stars. The concept is that only with a large pool of Earth-like planets may we gain a probabilistic confidence of the existence of biosignature gases by mitigating the inevitability of false positives. Surveying a large number of stars will require a next-generation space telescope beyond JWST (an optical-wavelength telescope with a large diameter likely exceeding 10 m) (12). Studies are ongoing within the astronomy community to outline the mission concept and technology investment required.”

  • Michael July 12, 2019, 17:29

    There are a wide range of wavelengths that organisms use for energy and protection, there are less in longer wavelengths which I find strange as the sun was a lot dimmer when life first started out.

    https://www.geomar.de/en/research/fb2/fb2-bi/infrastructure/hplc-analyses/

  • Gary Wilson July 12, 2019, 18:07

    Thanks so much for that Paul. It will be very interesting to read. I have a lot of questions about the technique.

  • Robin Datta July 12, 2019, 19:11

    If the constraints of physics and chemistry force alien biology and alien photosynthesis into a mold approximating life on earth, then we would be less likely to miss them.
    The Full Palette of Photosynthesis

  • Alex Tolley July 13, 2019, 0:21

    …photosynthetic structures containing chlorophyll are found not just in vegetation but also in lichens, corals, algae and cyanobacteria.

    It isn’t entirely clear that lichen, corals have symbiotic algae, not that these organisms have chloroplasts in their cells. When the algae die in lichen, the lichen typically turns white. Similarly, with coral reefs, if the algae die or are expelled, the coral turn white (bleach). The sea slug is different in that it eats algae and then uses the algal genes to manufacture chlorophyll so that it can fix carbon directly.

    While I buy that cyanobacteria and algae will be widespread and dense enough to generate the red edge signal, I doubt lichens and corals will be sufficient to do so. As for sea slugs, any signal they produce would be insignificant compared to the algal source of the chlorophyll.

    For oceanic life, water is going to rapidly absorb the light at this “red edge” diminishing the signal. This is why corals, fish and invertebrates with red coloration look dark at even modest depths unless a light is shone on them to reveal their colors.

    In sum, what this suggests to me is that this extension of the red edge spectral signature to periods before plants fully invade the land is going to be a difficult task. dense algal blooms near the surface and around coastlines I can imagine will be detectable by this method, but even then, it will be a faint signal compared to terrestrial vegetation. If the chlorophylls on Earth prove to be unique in their light absorption mechanism, then this red edge may not even exist on many other exoplanets where carbon fixation has taken a different biochemical route.

  • djlactin July 13, 2019, 4:40

    The first photosynthesizers were purple (purple sulfur bacteria) and the predated cyanobacteria by over half a billion years (https://en.wikipedia.org/wiki/Evolution_of_photosynthesis, https://en.wikipedia.org/wiki/Purple_bacteria), so they would not show a ‘red edge’. (They are also aquatic, so the problem is deeper yet.) Also, any pigment (by definition) absorbs a portion of incident light, and a huge number of them exist, so I am not sure that we can assume that alien photosynthesizers will use chlorophyll and be green.

  • Geoffrey Hillend July 14, 2019, 17:40

    Chlorophyll is extremely practical since it uses Sunlight, water and Co2 and makes oxygen. These are very abundant with Earthlike environmental conditions and we can assume that nature works through necessary everywhere. I’ll admit I like to stick to general principles with the idea that the environmental restrictions for life apply everywhere, so that we can assume that life have a very convergent evolution everywhere not only in the galaxy, but also in the entire universe. Consequently, the DNA should be the same on all planets. I like this idea since we could have many Earth twins in different stages of evolution which are the same as Earth has experienced in it’s history. We could have a dinosaur planet which would be experiencing it’s Cretaceous period with ferns and dinosaurs and it would have a strong red edge spectral signal. This planet might be hard to find though without our limited technology of today and the near future.

    • Alexander Tolley July 14, 2019, 23:14

      I like this idea since we could have many Earth twins in different stages of evolution which are the same as Earth has experienced in it’s history.

      I don’t agree. Since you then mention dinosaurs, you should be aware that the chordates were a rather small lineage that may not have survived. Without this lineage, almost the whole spectrum of animal life from fish onwards would have been absent. Earth would have been populated by the various invertebrate lineages only, resulting in a very different evolutionary history. Even allowing for the effects of convergent evolution, the resulting lifeforms would be very different even if they had superficial similarities to familiar animals.

      Regarding chlorophyll. It is quite a complex molecule and we just don’t know if it has optimized for certain wavelengths of latched onto them through chance. We do know that plants have a number of accessory pigments, a good example is red algae. These pigments will change the reflectance spectra of plants. Can we be sure that terrestrial chlorophyll types are universal, especially under different star types, or how common accessory pigments may be?

  • Geoffrey Hillend July 15, 2019, 17:03

    It is true that if the chordates did not survive, then we would have only invertebrates, but the only way that would happen if there was something to stop them like a catastrophic environmental change which is unlikely to happen on every Earth twin. Since nature works through mutation to adapt to new environments going from the sea onto land, or fish and amphibians to reptiles, birds, dinosaurs, mammals etc Sea creatures which have bones or are vertebrate will mutate to move onto land should be convergent everywhere in the universe. Mammals survived even through there was a giant meteor impact so I don’t see how vertebrates would not survive assuming nature is goal oriented and mutation is not totally random but purposeful with the potential to adapt to all physical environments especially new ones where there is no competition like from sea to land. If the chordates did not survive, then fish and amphibians or new sea life mutations would occur to be more adaptable and come onto the land at a latter and safer time. There were the snowball Earth periods and times with less oxygen before the Cambrian explosion that were less adaptable.

    There are more stars that are moving off the main sequence so we are the new kids on the block so there might not be a lot of Earth twins in with parallel an earlier period in history on our Earth, but the potential for a small number of them to be in our galaxy is high. New stars and solar systems are being born regularly so we can have systems of many different ages.

    I will agree with the idea that chlorophyll is light dependent since G class stars like our Sun radiate mostly in the visible spectrum and I have an a priori, intuitive expectation to limit the search for life to our exact physical parameters: the planet must have a G class star, and Earth sized body with a Moon and magnetic field be in the life belt etc. I don’t think we will find any life on planets around M dwarfs that receive high amounts of x rays, ultra violet. I use those parameters to rule out any false positives or gases like Co2, O2, etc which are not the result of life but only planetary chemistry.

  • Alex Tolley July 16, 2019, 1:20

    …assuming nature is goal oriented and mutation is not totally random but purposeful…

    This is not how evolution works. Life does not perceive a peak in the fitness landscape and try to scale it. Mutations are random, and individuals with beneficial mutations differentially survive, influencing the the replication of that change in t4eh population.

    For example, birds weren’t reptiles that saw an advantage to flying. Adaptations that led to wings had other [reproductive] advantages. Eventually, wings allowed flight which offered different advantages. But as we know, some birds returned to flightlessness as other mutations led to different advantages.

    The chordate phylum is just one of many. There is no particular reason to believe that it would be a priori a successful lineage compared to other phyla. It could easily have become extinct during its early period. There is no reason to believe that the chordate body plan would have evolved again after extinction. More likely, any niche advantage it had would have been occupied by species of other phyla.

    In the distant future, we may find body plans that offer very different advantages. Your logic would then be forced to answer why did that body plan not evolve on Earth too as convergent evolution would suggest. Evolution shows how contingent life is, based on random mutations and natural selection. Our own evolution at the cellular level was contingent on the capture of prokaryotes that became our mitochondria. Why didn’t the host cell just appropriate the genes for oxidative metabolism instead, just as some invertebrates seemed to have done if they have multiple life stages? There is no logic to life, it is purely phenomenological. IMO, the only explanatory feature that has stood the test of time is that natural selection as an algorithm that drives evolution.

  • Geoffrey Hillend July 16, 2019, 17:11

    I agree with natural selection and the randomness of the DNA. What I meant by goal oriented is the mutations make life flexible and adaptable. Nature works through necessity, so it has some limitations and contains the potential to adapt to all environments within certain limitations. The environments control the DNA in natural selection. Consequently, all the environmental changes that have occurred on our Earth will be the same on all Earth twins with a Moon, therefore, the DNA will be forced to produce the same forms since a twin Earth will have the same exact geology, the plate tectonics, continental drift and the same supercontinents which have come together and broken apart several times in the past 4.5 billion years which makes the environment predictable. There is a limited possibility of chemistry, weather and environmental conditions which makes the life forms predictable since the environment is predictable. Feathers are light which is influenced by gravity and weight. They could also be used as display but in order exploit a new environment, the air and flight, feathers had to be light.

    As a result, I don’t think I will see any spectral signature for chlorophyll on an Earth sized exoplanet in the life belt around an M dwarf star due their history of intense solar radiation and proximity to the star. This is just a prediction based these above contingencies etc. for life and I still think we should look at those stars anyway to rule them out which is what scientists always do anyway.

    Consequently, I do think there is a dinosaur planet in our galaxy. We might need a larger, more powerful space telescope to find it.

  • Harry R Ray July 17, 2019, 10:07

    Imaging the “red edge” of an Earth-sized exoplanet is far off in the future, but imaging an Earth-sized planet around a nearby star now appears to be entirely possible RIGHT NOW(and potentially may even have been done ALREADY)! Unbeknownst to me, amid all of the hoopla regarding NEAR last summer, ANOTHER instrument was installed at the same time on a DIFFERENT telescope. Go to http://www.portaltotheuniverse.org and click on “The First Photograph Of Another Earth”. and scroll down to the following quote: “Now the first image of such a world might be within reach. Last summer, a team of astronomers from the University of California, Santa Barbara installed a powerful new kind of camera on the eight meter Subaru telescope…” and then scroll back to read the entire article. This new “MKID” technology is only able to observe very faint light sources, so it does NOT compete with NEAR as far as Alpha Centauri A and B are concerned because both stars are far too bright. Howevfer Proxima Centauri seams to be IDEAI for this new instrument. Paul Gilster: even though the instrument Has been installed for over a year now, I have absolutely no idea whether it is FULLY OPERATIONAL yet. Please inquire as to the status of this instrument and post a reply to this comment ASAP! Thanks.

    • Harry R Ray July 18, 2019, 9:47

      MKID stands for Microwave Kinetic Inductance Detectors. For details, google Subaru telescope/MKID and click on the item at the TOP of the list page: SCExAO: MKID camera-Subaru telescope.

  • Mike Serfas July 18, 2019, 18:06

    The Arxiv link seems more useful: https://arxiv.org/ftp/arxiv/papers/1907/1907.05245.pdf The Seager, 2005 reference is worth following up. A point worth noting is that the “red edge” comes about because autotrophs evolve some mechanism to exploit light to drive a chemical reaction, and that reaction takes a threshold amount of energy. Any wavelength longer than this cannot be used, so trees reflect away the redder 40% of the Sun’s light rather than waste more of the energy in the rest. I would think this level would be set according to the star – after all, chlorophyll f actually exists on Earth. It may be interesting to consider how solar panel manufacturers deal with band gap considerations to try to guess whether life on other worlds might deal with the problem more effectively than on ours.

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