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Technosignatures: Looking to Planetary Atmospheres

While we often think about so-called Dysonian SETI, which looks for signatures of technology in our astronomical data, as a search for Dyson spheres, the parameter space it defines is getting to be quite wide. A technosignature has to be both observable as well as unique, to distinguish it from natural phenomena. Scientists working this aspect of SETI have considered not just waste heat (a number of searches for distinctive infrared signatures of Dyson spheres have been run), but also artificial illumination, technological features on planetary surfaces, artifacts not associated with a planet, stellar pollution and megastructures.

Thus the classic Dyson sphere, a star enclosed by a swarm or even shell of technologies to take maximum advantage of its output, is only one option for SETI research. As Ravi Kopparapu (NASA GSFC) and colleagues point out in an upcoming paper, we can also cross interestingly from biosignature searches to technosignatures by looking at planetary atmospheres.

Biosignature science is the more developed of the two fields, though we’re seeing a lot of activity in technosignature work, the robust nature of which can be seen in the extensive references the Kopparapu team identifies. As applied to atmospheres, a search for technosignatures can involve looking for various forms of pollution that flag industrial activity.

To my knowledge, most work on atmospheric pollution has targeted chlorofluorocarbons (CFCs), a useful choice because there is no biological source here, although our own use of CFCs occurred in a fairly brief window and for a specific purpose (refrigeration). The NASA work targets the much more ubiquitous nitrogen dioxide (NO2), which can be a by-product of an industrial process and in general is produced by any form of combustion.

As Kopparapu notes:

“In the lower atmosphere (about 10 to 15 kilometers or around 6.2 to 9.3 miles), NO2 from human activities dominate compared to non-human sources. Therefore, observing NO2 on a habitable planet could potentially indicate the presence of an industrialized civilization.”

Adds Giada Arney, a co-author on the paper and a colleague of Kopparapu at GSFC:

“On Earth, about 76 percent of NO2 emissions are due to industrial activity. If we observe NO2 on another planet, we will have to run models to estimate the maximum possible NO2 emissions one could have just from non-industrial sources. If we observe more NO2 than our models suggest is plausible from non-industrial sources, then the rest of the NO2 might be attributed to industrial activity. Yet there is always a possibility of a false positive in the search for life beyond Earth, and future work will be needed to ensure confidence in distinguishing true positives from false positives.”

Image: Artist’s illustration of a technologically advanced exoplanet. The colors are exaggerated to show the industrial pollution, which otherwise is not visible. Credit: NASA/Jay Freidlander.

This is evidently the first time NO2 has been examined in technosignature terms. The scientists deploy a cloud-free 1-dimensional photochemical model that uses the atmospheric temperature profile of today’s Earth to examine possible mixing ratio profiles of nitrogen oxide compounds on a planet orbiting several stellar types, one of them being a G-class star like the Sun, the others being a K6V and two M-dwarfs, one of these being Proxima Centauri. The authors then calculate the observability of these NO2 features, considering observing platforms like the James Webb Space Telescope and the projected Large UV/Optical/IR Surveyor (LUVOIR) instrument.

Usefully, atmospheric NO2 strongly absorbs some wavelengths of visible light, and the authors’ calculations show that an Earth-like planet orbiting a star like the Sun could be studied from as far as 30 light years away and an NO2 signature detected even with a civilization producing the pollutant at roughly the same levels we do today. This would involve observing at visible wavelengths over the course of at least 400 hours, which parallels what the Hubble instrument needed to produce its well-known Deep Field observations.

But adding yet more interest to K-class stars, whose fortunes as future targets for bio- and technosignature observations seem to be rising, is the fact that stars cooler than the Sun should generate a stronger NO2 signal. These stars produce less ultraviolet light that can break down NO2. As to M-dwarfs, we have this:

Further work is needed to explore the detectability of NO2 on Earth-like planets around M-dwarfs in direct imaging observations in the near-IR with ground-based 30 m class telescopes. NO2 concentrations increase on planets around cooler stars due to reduced availability of short-wavelength photons that can photolyze NO2 . Non-detectability at longer observation times could place upper limits on the amount [of] NO2 present on M-dwarf HZ planets like Prox Cen b.

Where work will proceed is in the model used to make these calculations, which will need to be more complex, as the paper acknowledges:

…when we prescribe water-ice and liquid water clouds, there is a moderate decrease in the SNR of the geometric albedo spectrum from LUVOIR-15 m, with present Earth-level NO2 concentration on an Earth-like planet around a Sun-like star at 10 pc. Clouds and aerosols can reduce the detectability and could mimic the NO2 feature, posing a challenge to the unique identification of this signature. This highlights the need for performing these calculations with a 3-D climate model which can simulate variability of the cloud cover and atmospheric dynamics self-consistently.

The authors consider biosignatures and technosignatures to be “two sides of the same coin,” a nod to the fact that we should be able to search for each at the same time with the next generation of observatories. Finding the common ground between biosignature research and SETI seems overdue, for a positive result for either would demonstrate life’s emergence elsewhere in the universe, and that remains question number one.

The paper is Kopparapu et al., “Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology,” accepted at the Astrophysical Journal. (Preprint).

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{ 9 comments… add one }
  • ljk February 18, 2021, 13:09

    How do we know what is alien life?

    https://mashable.com/article/how-to-prove-alien-life/

    • Alex Tolley February 18, 2021, 16:50

      Is this the first good science article I have ever been exposed to on Mashable?

      “Unless there’s a rabbit jumping in front of a rover — and we have to make sure it’s not a robot — it’s going to be difficult,” said Nathalie Cabrol,

      But what a technosignature!

      I have a old-ish book on Mars life- Malcolm Walter: “The Search for Life on Mars” (pub. 1999). He writes quite extensively on the ALH 84001 meteorite and the controversy over the biosignatures. The best part of this slim volume is the strategy for where to search for life on Mars based on the experience of scientists working on Earth searching for microbial paleolife in the geology, such as stromatolites. It is much more complex than it seems to non-specialists.

  • Alex Tolley February 18, 2021, 16:14

    Just as with CFCs, I see NO2 as having a narrow window as an indicator of technological civilization. The bulk of terrestrial NO2 is due to internal combustion engines, especially diesel engines. These appeared at the end of the 19th century and are already looking to be phased out in some classes of engines within the next 50 years as it is a noxious gas that has adverse health effects.

    In cosmic time, ETI will either be in the stone age or in a stable, advanced technology age. If civilization collapses, it may result in a return to a pre-industrial age. IMO, therefore the window to detect such gas is but a sliver in the lifetime of any civilization. Any NO2 detection will be from natural processes.

    It might be worth looking for such gases, although if it is for the purpose of looking for techno-signatures, it seems to be more like looking for your lost keys under a lampost – because that is where the light is, not where you dropped them.

    Whether ETI is biological, machine-based, or both, I think looking for structures, especially in space is the best approach. How you detect them will depend on the technology we have available. Whether that technology is remote detection with interstellar probes, we will have the capabilities to achieve those needed technologies within the next 1000 years, assuming our civilization retains the capabilities and desire to do so.

    • Andrei February 18, 2021, 19:39

      I fully agree with your thoughts here, many countries actively work to phase out diesel engines. And some have set up the ambitious goal by not allowing any but electrical cars to be sold within the coming decade. There’s countries that might reach that goal even without having to legislate on the matter, such as Norway as most sold cars are electric already. While that country have a huge untapped clean power potential, they build windmills instead. Such will not help in rogue nations that are so insane they use fossil fuel to produce electricity – but I digress. Any sensible civ would go down a similar path, meaning that NO2 or any pollutant is unlikely to be a technosignature for any biological civ. It’s harder to tell how a robotic/machine culture would act, if they consider their ancestors to be kept they might create a zoo, while they consider that horrible oxygen atmosphere to be a hindrance – as it make them rust and corrode. And then replace the atmosphere with more sensible gas, then we would not recognize that world as habitable.
      This would not mean that we would be unable to detect a civ with remote means, beside the more obvious one of mega projects in space, a planet with high power production / use would be in thermal imbalance. And with higher resolution studies, not necessarily sharp enough to see details – still might be able to spot if a night side is brighter than it’s supposed to be. But it’s definitely a harder task than looking for unusual gas in a planetary atmosphere.

  • Robin Datta February 18, 2021, 23:40

    In the latest episode of Event Horizon, in which John Michael Godier interviews Dr. Sara Seager
    Aliens are Hard to Find with Prof. Sara Seager
    the discussion includes includes planetary atmospheres and CFCs.

  • James Benford February 19, 2021, 16:40

    I continue to be skeptical that CFCs can be detected at interstellar distances when their < 1 ppb density here can barely be detected from LEO!

  • Geoffrey Hillend February 19, 2021, 17:23

    I thought we could detect carbon monoxide from car exhaust on an exoplanet, but after looking at the Aliens are Hard to Find with Prof. Sara Seager I realize it is only a trace gas and probably not detectable because there is not enough of it and the signal would be to small. CFC’s could be ambiguous.

    I was thinking about the false positive of oxygen and wondering what are the ideal conditions for an FP which is probably a lot of H2O and it’s photolysis? If there is no oxygen, then there won’t be any industrial NO2 or even any Nitrogen cycle without life or no biotic NO2.

  • ljk February 19, 2021, 18:32

    I wonder if the environmental havoc created by the humans inhabiting the existence of Silent Running could be detected at interstellar distances:

    https://www.bbc.com/culture/article/20210212-silent-running-the-sci-fi-that-predicted-modern-crises

  • ljk February 24, 2021, 17:42

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