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Biosignatures: The Case for Nitrous Oxide

Are we overlooking a potential biosignature? A new study makes the case that nitrous oxide could be a valuable indicator of life on other worlds, and one that can be detected with current and future instrumentation. In today’s essay, Don Wilkins takes a close look at the paper. A retired aerospace engineer with thirty-five years experience in designing, developing, testing, manufacturing and deploying avionics, Don tells me he has been an avid supporter of space flight and exploration all the way back to the days of Project Mercury. Based in St. Louis, where he is an adjunct instructor of electronics at Washington University, Don holds twelve patents and is involved with the university’s efforts at increasing participation in science, technology, engineering, and math.

by Don Wilkins

Biosignatures, specific signals produced by life, are the focus of intense study within the astronomical community. Gases such as nitrogen (N2), oxygen and methane are sought in planetary atmospheres as their large scale production is linked to life, although abiotic sources contribute to false positives. The James Webb Space Telescope (JWST) provides a new, powerful tool to explore the atmospheres of temperate terrestrial exoplanets in the approximately known 5,000 exoplanetary systems.

A paper by a UC Riverside, University of Maryland and NASA team led by Dr. Edward Schwieterman proposes the biosignature list expand to include nitrous oxygen (N2O), the so-called laughing gas once employed by anesthesiologists. Nitrous oxide is excluded from the current list of biosignatures. The gas, even though produced in quantity by biological processes, would be difficult to detect across interstellar ranges at current N2O concentrations in Earth’s atmosphere.

Dr. Schwieterman and his team believe the exclusion of nitrous oxide is not merited and propose several conditions where N2O could accumulate in quantities and be detected by current sensors. The paper notes that large quantities of atmospheric N2O can accumulate if:

1. Ocean conditions allow significant biological release of N2O.

2. The target exoplanet orbits a late K-dwarf or inactive early M dwarf, comparably low level UV emitters. The reduction rate of N2O into N2 and nitrous oxides is proportional to its exposure to UV.

3. Metal enzymatic catalysts, such as copper, are not abundant enough to partially terminate the reduction process. During the Proterozoic (approximately 2500 to 540 million years ago), high concentrations of atmospheric N2O produced by biological processes may have been present as copper catalysts were not readily available to reduce the gas.

4. The nitrous oxide reductase enzyme, the last step of the denitrification metabolism that yields fixed N2 from N2O, never evolves.

5. Phosphorous in a form suitable for biological activities is limited, although this shortage would adversely affect the possibility of life.

Dr. Schwieterman and his team examined contributors to false positives. A small amount of nitrous oxide and nitrogen dioxide (NO2) is created by lightning. The presence of NO2 is a strong indicator the N2O is produced by an abiotic source.

Another source of false positives is chemodenitrification, N2O production through abiotic reduction of nitric oxide (NOx) by ferrous iron.

Stellar activity can produce abiotic N2O production. This false positive is associated with young and magnetically active stars.

Image: This is Figure 13 from the paper. Caption: A concept image illustrating the interpretability of N2O as a biosignature in the context of the planetary environment. The left panel illustrates a scenario like the modern Earth, with a high-oxygen atmosphere and N2O generated overwhelmingly via partial biological denitrification. In this case, the simultaneous presence of O2, O3 , N2O, and CH4 indicates a strong chemical disequilibrium. The middle panel illustrates a weakly oxygenated planetary environment like the Proterozoic Earth, with N2O generated both by partial biological denitrification and by chemodenitrification of nitrogenous intermediates (likely substantially biogenic) in a redox stratified ocean. In this case, molecular oxygen (O2 ) and methane may have concentrations that are too low to detect directly, but detectable N2O and O3 would be a strong biosignature. A false positive is unlikely, because an abiotic O2 atmosphere would be unstable in combination with a reducing ocean. The right panel illustrates the most likely false-positive scenario, where an active star splits N2 via SPEs [solar proton events], resulting in photochemically produced N2O. This scenario would predict additional photochemical products, such as HCN, that would be indicative of abiotic origins. Stellar characterization would confirm the magnitude of the stellar activity. Vigorous atmospheric production of NOx species could be inferred from spectrally active NO2. Credit: Schwieterman et al.

Several candidates for a N2O search tightly orbit TRAPPIST-1 (the name is derived from Transiting Planets and Planetesimals Small Telescope, a Belgian robotic telescope at the European Southern Observatory’s installation at La Silla, Chile). An ultracool red dwarf older than Earth’s Sun, TRAPPIST-1 is a type M star, 40 light years distant from Earth. Seven rocky exoplanets are found here, likely made of materials similar to Earth – iron, oxygen, magnesium, and silicon – but these worlds are 8% less dense than Earth. This implies that the ratio between the constituent materials on the TRAPPIST-1 worlds differs from the ratio on Earth.

It is possible each of the four outer, cooler planets incorporates a large core, a mantle and a planet girding ocean. Finding N2O in the atmosphere of these worlds would be a significant clue pointing to the existence of life.

Image: A planet’s density is determined by its composition as well as its size: Gravity compresses the material a planet is made of, increasing the planet’s density. Uncompressed density adjusts for the effect of gravity and can reveal how the composition of various planets compare. Credit: NASA/JPL-Caltech.

The conditions around the four outer planets of TRAPPIST-1 appear ideal for a search for biological nitrous oxide. The authors use TRAPPIST 1e as a test case. From the paper:

We used the biogeochemical model cGENIE to inform the maximum plausible N2O fluxes for an Earthlike biosphere, which could be 1–2 orders of magnitude larger than those on present-day Earth, assuming nutrient-rich oceans and evolutionary or environmental conditions that limit the last step in the denitrification process. Even for maximal biospheric N2O fluxes of 100 Tmol yr−1, an Earthlike atmosphere will never enter an N2O runaway, but would attain much larger concentrations than those found on Earth today. We find that late K-dwarf and inactive early M-dwarf stars can maintain the highest N2O levels at any given surface flux, potentially exceeding 1000 ppm. We show that for N2O fluxes of 10–100 Tmol yr−1, JWST could detect N2O at 2.9 μm on TRAPPIST-1e within its mission lifetime.

As we look toward future instrumentation, K-dwarfs turn out to be particularly susceptible to this analysis:

Terrestrial planets orbiting K-dwarf stars are particularly appealing targets for N2O searches with future MIR [mid-IR] missions, due to favorable planet–star angular separations and because N2O fluxes of only 2 to 3 times those of Earth’s modern global average can produce N2O signatures comparable to those of O3.

However, additional research to strengthen the case for N2O as a biosignature is needed. Evaluation of the biological, geological and chemical combinations producing large quantities of N2O can determine suitable exoplanet candidates where a search for nitrous oxide will support findings of life.

The paper is Edward W. Schwieterman et al., “Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach,” The Astrophysical Journal 937:109 (22pp), 2022 October 1 (full text).

{ 8 comments… add one }
  • Alex Tolley October 25, 2022, 16:50

    However, additional research to strengthen the case for N2O as a biosignature is needed. Evaluation of the biological, geological and chemical combinations producing large quantities of N2O can determine suitable exoplanet candidates where a search for nitrous oxide will support findings of life.

    Unlike CH4:O2 disequilibrium, it isn’t clear that even natural, abiogenic N2O production has some natural ratio with other nitrogen gases – NH3, NO2, N2, that is tiny compared to instances of biogenic as well as biogenic excess production. This signature depends on denitrification enzymes failing to evolve, as well as conditions for abiogenic denitrification being absent, and abiogenic N2O production being largely absent too.

    At first blush, it seems that a lot of conditions need to be met to rule out false positives, and avoid false negatives. It could easily cause confusion if a CH4:O2 biosignature was found, but N2O was absent as it is largely the case on Earth. Conversely, a lack of CH4LO2, or pre-oxygenic photosynthesis atmosphere of mainly N2:CH4: CO2 (but no CO) but absent a N2O biosignature (because it was reduced to NH3) would imply no life despite there being a diverse bacterial life.

    The use of N2O as a potential biosignature is problematic, as this journal article suggests:
    Abiotic Nitrous Oxide Production From Sediments and Brine of Don Juan Pond, Wright Valley Antarctica, at Mars Analog Temperatures (−40°C)

    From the abstract:

    We conclude that abiotic N2O production is possible on Mars and on other extraterrestrial bodies and exoplanets. Thus, the presence of atmospheric N2O on these bodies should not be taken by itself as an indicator of microbial life.

    N2O might be a supporting biosignature, but it isn’t clear to me at this stage whether its use would enhance or complicate existing biosignatures as an imperfect proxy for terrestrial-like life. Finding clear disequilibria in gases as the late James Lovelock indicated with his Gaia hypothesis seems like the best bet when those disequilibria are marked.

    • Jeff Wright October 27, 2022, 4:10

      In terms of abiogenesis-the bullet-like rhabdovirus makes me wonder if a powerful cosmic ray strike hitting goo left a template of sorts. I have wondered about vortices by smokers spinning things up-well I seem to have been vindicated in part-as per the phys.org story “simple machine may pave the way for more powerful cell phones and wi-fi’ -but it is the fluid handling of nanofibers that struck me. Tell me that channel can’t be inside some smoker’s concretions. The alembics of the deep-not a pond. Also-maybe we can get space elevator fibers now,

  • Afterthought October 26, 2022, 0:18

    Judging any of these methods requires an n of at least 1 besides earth.

  • Geoffrey Hillend October 26, 2022, 18:25

    Nitrous oxide is a biosignature, but without a lot of oxygen at the ground level which can only be made by life, there cannot be any NOx compounds. There are nitrifiers, the nitrosomona’s, and the de nitrifiers don’t always complete the oxidation process which ends up in molecular nitrogen. They stop halfway and make nitrous oxide. Reay 2015. Consequently, without life and oxygen there is no nitrous oxide which is a short lived gas due to photodissociation or photolysis of N2O.

    Nitrous oxide can be used as a biosignature gas only if oxygen is already found in the spectra. CH4 is another short lived gas, so I don’t think there will be any false positives. Without life there is no O2, CH4 and N2O.

  • Alex Tolley October 27, 2022, 9:52

    Without life there is no O2, CH4 and N2O.

    CH4 is produced abiotically, for example by serpentinization. You may recall the current question of whether the CH4 detected on Mars is of geologic or biotic origin.

    O2 is also produced abiotically primarily through photolysis of water by UV. Prior to oxygenic photosynthesis on Earth, this was the main source of O2 in the atmosphere. In extremist a planet could have an O2 atmosphere providing a false +ve.

  • don wilkins October 28, 2022, 10:12

    I agree, and believe the authors of the paper would agree, that nitrous oxide is not a good biomarker for terrestrial-type worlds. The reported suggestion relates to water worlds where “traditional” biomarkers might not be reliable. Underwater life might produce nitrous oxide which might be affected the star’s outputs producing a large signal possible detectable across interstellar space. Further study might find nitrous oxide to be an unreliable indicator which should lead to additional analysis of what would serve as a bioindicator for the worlds covered in water.

    • Alex Tolley October 28, 2022, 13:10

      I take your point that the authors focus on water worlds. However, the N2O signal still requires a number of conditions before it is useful as the authors discuss.

      If, a big if, we were to detect N2O on such a world, but with no other biomarker, we might hope this would end the speculation that water worlds do not have the conditions suitable for abiogenesis or even the geological carbon cycle to control the surface temperature. OTOH, could we be sure that N2O was a result of biological activity and not from some other geochemical process?

      Before oxygenic photosynthesis evolved, the CH4 emissions by terrestrial methanogens is/was far greater than geologic processes, so that it is thought that this could be a good biosignature candidate for life in the Archaean eon. The Martian CH4 is very low, so if it is biological, it must come from a small, isolated source, probably from below the surface.

      If oxygenic photosynthesis does not evolve, or the planet’s life evolution is still in the period before its evolution. or the equivalent of our Great Oxidation Event hasn’t yet occurred, then any O2 is from photolysis, and we are stuck with a possible CH4 and possible N2O biosignature, with most of the atmosphere a probable N2/CO2 mixture, and even possibly H2 if he planet is old enough and large enough.

      I am not certain, but algal chlorophyll might be enough to provide a spectral “red edge” signal or its equivalent.

      What I think we can be certain of is that there would be no use looking for a radio signal technosignature from a water world, nor any city lights or other signatures of a land-based civilization.

      But if we did find an N2O signature AND a technosignature, that might just indicate colonization by an ETI (floating cities?), and where the N2O might be a pollutant from combustion processes. Now that would be interesting!

      For an astrobiologist hoping to be the first to discover life outside our system, my sense is that a strong CH4:O2 signal is the one to detect as it has the least ambiguity even if it will have many false negatives. But you only need one positive detection to be first, the cataloging comes later.

  • Geoffrey Hillend October 28, 2022, 17:05

    Mars is not a good example because it took years to see the CH4 with a telescope. There is also nitrous oxide in the upper atmosphere of Mars from a recombination of N2 and O the photo dissociation of water and nitrogen. Assuming most Earth sized exoplanets should have water and an atmosphere due to their larger escape velocity and gravity, we can expect to find N2O and CH4 without life, but not oxygen. Without oxygen we will have to come to the conclusion that most likely, those trace gases are from abiotic sources which was my point. JWST should give us this information.

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