Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life

Krissansen‐Totton, Joshua; Olson, Stephanie L.; Catling, David C.

doi:10.1126/sciadv.aao5747

Cited by 153 publications

(178 citation statements)

References 101 publications

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“…Biological fixation tends to decrease pN 2 , suggesting that pN 2 was not lower in the prebiotic era than in the Archaean (Johnson & Goldblatt, 2017). Krissansen-Totton, Olson, and Catling (2018) calculate that weathering restricted pCO 2 ≤ 1 bar at 4 Ga. We consequently retain 6.5 × 10 8 cm −2 s −1 of Wong et al (2017) as the upper bound on NO − X but caution that if pCO 2 were higher or pN 2 lower than what we consider, NO − X could have been up to an order of magnitude higher. For more details, see supporting information Text S4.…”

Section: Kinetic Steady Statementioning

confidence: 96%

Nitrogen Oxide Concentrations in Natural Waters on Early Earth

Ranjan

Todd

Rimmer

et al. 2019

Geochem Geophys Geosyst

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A key challenge in origins‐of‐life studies is estimating the abundances of species relevant to the chemical pathways proposed to have contributed to the emergence of life on early Earth. Dissolved nitrogen oxide anions ( NOX−), in particular nitrate ( NO3−) and nitrite ( NO2−), have been invoked in diverse origins‐of‐life chemistry, from the oligomerization of RNA to the emergence of protometabolism. Recent work has calculated the supply of NOX− from the prebiotic atmosphere to the ocean and reported steady state [ NOX−] to be high across all plausible parameter space. These findings rest on the assumption that NOX− is stable in natural waters unless processed at a hydrothermal vent. Here, we show that NOX− is unstable in the reducing environment of early Earth. Sinks due to ultraviolet photolysis and reactions with reduced iron (Fe2+) suppress [ NOX−] by several orders of magnitude relative to past predictions. For pH = 6.5–8 and T = 0–50 °C, we find that it is most probable that [ NOX−] <1μM in the prebiotic ocean. On the other hand, prebiotic ponds with favorable drainage characteristics may have sustained [ NOX−] ≥1μM. As on modern Earth, most NOX− on prebiotic Earth should have been present as NO3−, due to its much greater stability. These findings inform the kind of prebiotic chemistries that would have been possible on early Earth. We discuss the implications for proposed prebiotic chemistries and highlight the need for further studies of NOX− kinetics to reduce the considerable uncertainties in predicting [ NOX−] on early Earth.

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Section: Kinetic Steady Statementioning

confidence: 96%

Nitrogen Oxide Concentrations in Natural Waters on Early Earth

Ranjan

Todd

Rimmer

et al. 2019

Geochem Geophys Geosyst

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“…For each prebiotic and biotic atmosphere, we calculate the atmosphere-ocean chemical disequilibrium with Gibbs energy minimization, using code described previously (Krissansen-Totton, et al 2018c). Given the chemical composition of an atmosphere-ocean system, the code reacts all molecules and atoms to thermodynamic equilibrium.…”

Section: Quantification Of Chemical Disequilibriummentioning

confidence: 99%

“…Additionally, we assumed atmospheric temperature and pressure to be 25°C and 1 bar respectively. Chemical disequilibrium is fairly insensitive to ocean composition, atmospheric pressure and temperature Krissansen-Totton, et al 2018c); consequently, order of magnitude errors in these assumptions will result in a fairly small error (well within a factor of ~2) in the available Gibbs energy.…”

Section: Quantification Of Chemical Disequilibriummentioning

confidence: 99%

“…The CH2O produced could have been processed by heterotrophs and methanogens yielding CH4, which would have added to the Archean CO2-N2-CH4-H2O disequilibrium without the need for additional volcanic outgassing (Krissansen-Totton, et al 2018c). Figure 3 does not explicitly capture this effect because the evolutionary history of anoxygenic photosynthesis is uncertain, but Archean disequilibrium estimates allow for such photosynthesis (Krissansen-Totton, et al 2018c).…”

Section: Life's Impact On Disequilibria Through Earth's Historymentioning

confidence: 99%

“…A full evaluation of disequilibrium biosignatures involves inferring the surface fluxes of disequilibrium gases (Krissansen-Totton, et al 2018c;Simoncini et al 2013) which is beyond the scope of this paper. Life is characterized by a surface source or sink of a gas that is unexplained by abiotic processes, although a single atmospheric abundance could be due to different sources and sinks.…”

Section: Atmosphere-ocean Near Equilibriummentioning

confidence: 99%

See 2 more Smart Citations

When is Chemical Disequilibrium in Earth-like Planetary Atmospheres a Biosignature versus an Anti-biosignature? Disequilibria from Dead to Living Worlds

Wogan

Catling

2020

ApJ

Self Cite

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2When is chemical disequilibrium in Earth-like planetary atmospheres a biosignature versus an anti-biosignature? Disequilibria from dead to living worlds. Abstract [249 words of a 250 limit]Chemical disequilibrium in exoplanetary atmospheres (detectable with remote spectroscopy) can indicate life. The modern Earth's atmosphere-ocean system has a much larger chemical disequilibrium than other solar system planets with atmospheres because of oxygenic photosynthesis. However, no analysis exists comparing disequilibrium on lifeless, prebiotic planets to disequilibrium on worlds with primitive chemotrophic biospheres that live off chemicals and not light. Here, we use a photochemical-microbial ecosystem model to calculate the atmosphere-ocean disequilibria of Earth with no life and with a chemotrophic biosphere. We show that the prebiotic Earth likely had a relatively large atmosphere-ocean disequilibrium due to the coexistence of water and volcanic H2, CO2, and CO. Subsequent chemotrophic life likely destroyed nearly all of the prebiotic disequilibrium through its metabolism, leaving a likely smaller disequilibrium between N2, CO2, CH4, and liquid water. So, disequilibrium fell with the rise of chemotrophic life then later rose with atmospheric oxygenation due to oxygenic photosynthesis. We conclude that big prebiotic disequilibrium between H2 and CO2 or CO and water is an anti-biosignature because these easily metabolized species can be eaten due to redox reactions with low activation energy barriers. However, large chemical disequilibrium can also be a biosignature when the disequilibrium arises from a chemical mixture with biologically insurmountable activation energy barriers, and clearly identifiable biogenic gases. The modern disequilibrium between O2, N2, and liquid water along with minor CH4 is such a case. Thus, the interpretation of disequilibrium requires context. With context, disequilibrium can be used to infer dead or living worlds.

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Life detection in a universe of false positives

Smith,

Mathis

2023

BioEssays

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Astrobiology aims to determine the distribution and diversity of life in the universe. But as the word “biosignature” suggests, what will be detected is not life itself, but an observation implicating living systems. Our limited access to other worlds suggests this observation is more likely to reflect out‐of‐equilibrium gasses than a writhing octopus. Yet, anything short of a writhing octopus will raise skepticism about what has been detected. Resolving that skepticism requires a theory to delineate processes due to life and those due to abiotic mechanisms. This poses an existential question for life detection: How do astrobiologists plan to detect life on exoplanets via features shared between non‐living and living systems? We argue that you cannot without an underlying theory of life. We illustrate this by analyzing the hypothetical detection of an “Earth 2.0” exoplanet. Without a theory of life, we argue the community should focus on identifying unambiguous features of life via four areas: examining life on Earth, building life in the lab, probing the solar system, and searching for technosignatures. Ultimately, we ask, what exactly do astrobiologists hope to learn by searching for life?

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Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life

Abstract: Coexisting methane and carbon dioxide in atmospheres of habitable planets represent a disequilibrium biosignature.

Cited by 153 publications

References 101 publications

Nitrogen Oxide Concentrations in Natural Waters on Early Earth

Nitrogen Oxide Concentrations in Natural Waters on Early Earth

When is Chemical Disequilibrium in Earth-like Planetary Atmospheres a Biosignature versus an Anti-biosignature? Disequilibria from Dead to Living Worlds

Life detection in a universe of false positives

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