Research ArticlePLANETARY SCIENCE

Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life

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Science Advances  24 Jan 2018:
Vol. 4, no. 1, eaao5747
DOI: 10.1126/sciadv.aao5747
  • Fig. 1 Schematic of methodology for calculating atmosphere-ocean disequilibrium.

    We quantify the disequilibrium of the atmosphere-ocean system by calculating the difference in Gibbs energy between the initial and final states. The species in this particular example show the important reactions to produce equilibrium for the Phanerozoic atmosphere-ocean system, namely, the reaction of N2, O2, and liquid water to form nitric acid, and methane oxidation to CO2 and H2O. Red species denote gases that change when reacted to equilibrium, whereas green species are created by equilibration. Details of aqueous carbonate system speciation are not shown.

  • Fig. 2 The evolution of Earth’s atmosphere-ocean disequilibrium through time, as measured by available Gibbs free energy.

    The blue shaded regions show the evolution of Earth’s atmosphere-ocean disequilibrium. The wide ranges in the Archean and Proterozoic span our minimum and maximum disequilibrium scenarios. The large ranges are attributable to uncertainties in the atmospheric composition in each eon, mainly uncertain Pch4 in the Archean and uncertain Po2 in the Proterozoic. The two shadings for the Proterozoic represent different assumptions about atmospheric oxygen levels that represent divergent views in the current literature. Darker blue denotes Po2 > 2% PAL (present atmospheric level), whereas lighter blue denotes Po2 < 2% PAL. We calculate a secular increase in Earth’s atmosphere-ocean disequilibrium over Earth history, correlated with the history of atmospheric oxygen. The black dashed line shows the upper bound of the Earth’s atmosphere-only disequilibrium through time. We also include the modern (photochemically produced) disequilibria of Mars (red dashed) and Titan (blue dashed) for comparison (25). The abiotically produced disequilibria of all the other solar system planets are ≪1 J/mol (25).

  • Fig. 3 Atmosphere-ocean disequilibrium in the Proterozoic (maximum disequilibrium scenario).

    Blue bars denote assumed initial abundances from the literature, and green bars denote equilibrium abundances calculated using Gibbs free energy minimization. Subplots separate (A) atmospheric species and (B) ocean species. The most important contribution to Proterozoic disequilibrium is the coexistence of atmospheric oxygen, nitrogen, and liquid water. These three species are lessened in abundance by reaction to equilibrium to form aqueous H+ and Embedded Image. Changes in carbonate speciation caused by the decrease in ocean pH also contribute to the overall Gibbs energy change.

  • Fig. 4 Atmosphere-ocean disequilibrium in the Archean (maximum disequilibrium scenario).

    Blue bars denote assumed initial abundances from the literature, and green bars denote equilibrium abundances calculated using Gibbs free energy minimization. Subplots separate (A) atmospheric species and (B) ocean species. The most important contribution to Archean disequilibrium is the coexistence of atmospheric CH4, N2, CO2, and liquid water. These four species are lessened in abundance by reaction to equilibrium to form aqueous Embedded Image and Embedded Image. Oxidation of CO and H2 also contributes to the overall Gibbs energy change.

  • Fig. 5 Sensitivity of Archean disequilibrium to bicarbonate molality and ammonium molality in ocean, quantities that are probably impossible to directly observe for exoplanets.

    Colors shows fraction of methane depleted in equilibrium, as determined by semianalytic calculations. In this case, Pco2 = 0.49 bar, Pn2 = 0.5 bar, and Pch4 = 0.01 bar. As can be seen, most parts of parameter space have high CH4 depletion, that is, CH4 is in disequilibrium. Thus, unless both bicarbonate and ammonium molalities are extremely large, detectable quantities of methane are out of equilibrium with an N2-CO2 atmosphere and ocean. The white dashed line box denotes the plausible Archean range.

  • Fig. 6 Probability distribution for maximum abiotic methane production from serpentinization on Earth-like planets.

    This distribution was generated by sampling generous ranges for crustal production rates, FeO wt %, maximum fractional conversion of FeO to H2, and maximum fractional conversion of H2 to CH4, and then calculating the resultant methane flux 1 million times (see the main text). The modern biological flux (58) and plausible biological Archean flux (59) far exceed the maximum possible abiotic flux. These results support the hypothesis that the co-detection of abundant CH4 and CO2 on a habitable exoplanet is a plausible biosignature.

  • Table 1 Assumed initial atmosphere-ocean composition for Archean and Proterozoic.
    Archean rangeProterozoic range
    Atmospheric
    species
    Mixing ratioReference/explanationMixing ratioReference/explanation
    Minimum
    disequilibrium
    Maximum
    disequilibrium
    Minimum
    disequilibrium
    Maximum
    disequilibrium
    N2(g)0.980.5Mixing ratios sum to 10.990.86Mixing ratios sum to 1
    O2(g)1 × 10−102 × 10−7(39)0.00010.03(14, 76)
    CH4(g)0.00010.01(77)3 × 10−61 × 10−4(78, 79)
    CO2(g)0.0010.5(80, 81)0.00010.1(80, 81)
    H2(g)00.0001(59)02 × 10−6(82)
    N2O(g)00No denitrification so negligible production01 × 10−6(79, 83)
    NH3(g)01 × 10−9(84)00Negligible in bulk atmosphere
    O3(g)00Negligible in bulk atmosphere00Negligible in bulk atmosphere (79)
    CO(g)00.001(59)02 × 10−7(82)
    Ocean speciesMolality (mmol/kg)Reference/explanationMolality (mmol/kg)Reference/explanation
    Minimum
    disequilibrium
    Maximum
    disequilibrium
    Minimum
    disequilibrium
    Maximum
    disequilibrium
    Na+550586Charge balance547549Charge balance
    Cl546546Modern value546546Modern value
    Embedded Image00.2(85, 86)0.255(87, 88)
    H2S00.004In euxinic oceans using a Black Sea analog*00.004In euxinic oceans using
    a Black Sea analog*
    Embedded Image00.050Set by phosphorus assuming Redfield ratios and
    the presence of N fixation, given that N fixation
    evolved early (89)
    00.050Set by phosphorus
    assuming Redfield ratios
    Embedded Image00Anoxic bulk ocean00Anoxic bulk ocean
    Alkalinity440(29); Krissansen-Totton et al., in preparation1.03.0(29); Krissansen-Totton
    et al., in preparation
    pH8.06.3Carbon chemistry equilibrium8.46.0Carbon chemistry
    equilibrium

    *The concentration of H2S in the Black Sea is around 400 μmol/kg (90). However, <1 to 10% of the Precambrian seafloor was euxinic (91), which implies <1% euxinia by volume because euxinic continental slopes are much shallower than the deep ocean. Thus, we assumed 4 μmol/kg as an upper limit for bulk ocean aqueous H2S.

    †The maximum subsurface concentration of Embedded Image is ultimately controlled by flux of phosphate from continental weathering. Assuming a modern dissolved phosphate abundance of ~2 μmol/kg, this implies an ammonium abundance of 32 μmol/kg assuming a 1:16 Redfield ratio. Results are largely insensitive to initial abundances of Embedded Image (Fig. 5).

    ‡pH is calculated from alkalinity and Pco2 assuming chemical equilibrium.

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/1/eaao5747/DC1

      Supplementary Text

      section S1. The omission of solids, redox-sensitive nonvolatile aqueous species, and ocean heterogeneity

      section S2. Full results and Aspen Plus validation

      section S3. Reactions associated with Precambrian disequilibria and their Gibbs energy contributions

      section S4. Sensitivity of available Gibbs energy to difficult-to-observe variables

      section S5. Abiotic CH4 formation from high-temperature processes

      section S6. Atmospheric kinetics of methane destruction

      fig. S1. Atmosphere-ocean disequilibrium in the Proterozoic (minimum disequilibrium scenario).

      fig. S2. Atmosphere-ocean disequilibrium in the Archean (minimum disequilibrium scenario).

      fig. S3. Relationship between methane fluxes and atmospheric abundances.

      table S1. Proterozoic maximum disequilibrium.

      table S2. Proterozoic minimum disequilibrium.

      table S3. Proterozoic disequilibrium with 2% PAL of O2.

      table S4. Archean maximum disequilibrium.

      table S5. Archean minimum disequilibrium.

      table S6. Reactions contributing to Proterozoic disequilibrium.

      table S7. Reactions contributing to Archean disequilibrium.

      table S8. Sensitivity of Archean disequilibrium to difficult-to-observe variables.

      References (92105)

    • Supplementary Materials

      This PDF file includes:

      • Supplementary Text
      • section S1. The omission of solids, redox-sensitive nonvolatile aqueous species, and ocean heterogeneity
      • section S2. Full results and Aspen Plus validation
      • section S3. Reactions associated with Precambrian disequilibria and their Gibbs energy contributions
      • section S4. Sensitivity of available Gibbs energy to difficult-to-observe variables
      • section S5. Abiotic CH4 formation from high-temperature processes
      • section S6. Atmospheric kinetics of methane destruction
      • fig. S1. Atmosphere-ocean disequilibrium in the Proterozoic (minimum disequilibrium scenario).
      • fig. S2. Atmosphere-ocean disequilibrium in the Archean (minimum disequilibrium scenario).
      • fig. S3. Relationship between methane fluxes and atmospheric abundances.
      • table S1. Proterozoic maximum disequilibrium.
      • table S2. Proterozoic minimum disequilibrium.
      • table S3. Proterozoic disequilibrium with 2% PAL of O2.
      • table S4. Archean maximum disequilibrium.
      • table S5. Archean minimum disequilibrium.
      • table S6. Reactions contributing to Proterozoic disequilibrium.
      • table S7. Reactions contributing to Archean disequilibrium.
      • table S8. Sensitivity of Archean disequilibrium to difficult-to-observe variables.
      • References (92–105)

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