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Cited by 333Space Science and Extraterrestrial LifeAstro and Planetary SciencePlanetary Science and Exploration

The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth

Open access
Arney, Giada, Domagal-Goldman, Shawn D., Meadows, Victoria S., Wolf, Eric T., Schwieterman, Edward, Charnay, Benjamin, Claire, Mark, Hébrard, Eric +1 more·2016·Astrobiology
DOI 10.1089/ast.2015.1422PMC PMC5148108
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Abstract

Recognizing whether a planet can support life is a primary goal of future exoplanet spectral characterization missions, but past research on habitability assessment has largely ignored the vastly different conditions that have existed in our planet's long habitable history. This study presents simulations of a habitable yet dramatically different phase of Earth's history, when the atmosphere contained a Titan-like, organic-rich haze. Prior work has claimed a haze-rich Archean Earth (3.8–2.5 billion years ago) would be frozen due to the haze's cooling effects. However, no previous studies have self-consistently taken into account climate, photochemistry, and fractal hazes. Here, we demonstrate using coupled climate-photochemical-microphysical simulations that hazes can cool the planet's surface by about 20 K, but habitable conditions with liquid surface water could be maintained with a relatively thick haze layer (τ ∼ 5 at 200 nm) even with the fainter young Sun. We find that optically thicker hazes are self-limiting due to their self-shielding properties, preventing catastrophic cooling of the planet. Hazes may even enhance planetary habitability through UV shielding, reducing surface UV flux by about 97% compared to a haze-free planet and potentially allowing survival of land-based organisms 2.7–2.6 billion years ago. The broad UV absorption signature produced by this haze may be visible across interstellar distances, allowing characterization of similar hazy exoplanets. The haze in Archean Earth's atmosphere was strongly dependent on biologically produced methane, and we propose that hydrocarbon haze may be a novel type of spectral biosignature on planets with substantial levels of CO 2 . Hazy Archean Earth is the most alien world for which we have geochemical constraints on environmental conditions, providing a useful analogue for similar habitable, anoxic exoplanets. Key Words: Haze—Archean Earth—Exoplanets—Spectra—Biosignatures—Planetary habitability. Astrobiology 16, 873–899.

AI snapshot

Written from the paper’s full text.

TL;DR

Coupled climate-photochemical-microphysical simulations of hazy Archean Earth show that organic haze can cool the surface by about 20 K but still permit habitable temperatures with liquid surface water, because thick hazes self-limit via UV self-shielding. The haze also strongly shields surface UV and produces distinctive spectral features, suggesting hydrocarbon haze may be a biologically mediated spectral biosignature on anoxic, CO₂-rich exoplanets.

Key findings

  • Hazy Archean climates were cold but likely habitable: for Cases A–D, stabilized surface temperatures with haze were 263, 274, 262, and 251 K, and with ice-albedo feedback 257, 271, 257, and 241 K, all above the adopted 248 K habitability threshold; haze cooling reached up to ∼20 K, with maximum haze optical thickness leveling off at τ ∼ 5 at 200 nm.
  • Haze is self-limiting: optically thicker hazes shield CH₄ from photolysis, shutting down haze formation and preventing catastrophic cooling; the CH₄/CO₂ ratios needed to initiate haze formation were 0.18, 0.15, 0.19, and 0.28 for Cases A–D.
  • UV shielding was substantial: for Case B with CH₄/CO₂ = 0.21, surface UVC flux was about 0.03 W/m² at SZA = 60° and 0.22 W/m² at SZA = 0°, compared with 0.93 and 2.62 W/m² for the haze-free case, a reduction of about 97%; this may have allowed survival of land-based organisms 2.7–2.6 billion years ago.
  • Hazy Archean Earth would be a “pale orange dot,” with a broad UV absorption signature, a 6 μm transit-transmission haze feature, and a weaker 3 μm feature; the CH₄ flux needed to initiate haze was about 1 × 10¹¹ to 3 × 10¹¹ molecules/cm²/s, far above estimated maximum abiotic serpentinization fluxes of 6.8 × 10⁸ and 1.3 × 10⁹ molecules/cm²/s for 1 and 5 Earth-mass rocky planets.

Methods

  • Used the coupled 1-D photochemical-climate model Atmos and the 1-D radiative transfer model SMART; the photochemical model has 200 plane-parallel layers from the surface to 100 km, 76 chemical species, and fractal hydrocarbon haze microphysics, while the climate model uses correlated-k absorption and a δ-2-stream radiative transfer scheme, iterating between models to convergence.
  • Simulated four Archean cases spanning pCO₂ = 0.0036–0.018 bar and surface pressure = 0.5–1 bar, with solar constant at 2.7 Ga = 0.81 S/S₀, surface O₂ = 1.0 × 10⁻⁸, and varied CH₄/CO₂; about 60 coupled Atmos runs were required.
  • Generated synthetic reflectance, thermal emission, and transit transmission spectra with SMART, including binned haze particle modes and, for Case B, tests with water clouds.

Limitations

  • Stated in paper: the climate model could only be run to about 80 km while the photochemical model extends to 100 km, with isoprofiles above the climate grid; sensitivity tests showed <5% difference in largest haze particles and <0.5 K surface temperature effect. Older CH₄ coefficients may overestimate surface temperature by ≲5 K, while the model underpredicts Archean temperature by about 2–5 K compared with 3-D models, so these may cancel.
  • Stated in paper: the 1-D model lacks atmospheric circulation, precipitation, cloud formation, cloud scattering and absorption, and long-wave cloud radiative forcing; comparison with GCMs suggests the presented temperatures may be underestimates by about 3–5 K, and ice-albedo feedback, haze deposition, haze circulation, and cloud feedbacks are left to future GCM studies.
  • Stated in paper and evident from text: haze formation chemistry is not well understood, and the model assumes C₄H₂ and C₅H₄ condense directly to haze particles, possibly overestimating aerosol formation; only one Archean haze refractive index measurement exists, at 532 nm, and the study uses Titan-like Khare optical constants; NH₃ was not included in the photochemical scheme; nominal haze spectra exclude water clouds.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1089/ast.2015.1422

Cites

138 of 138 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 147 cited sources checked · 0 supported · 0 flagged

  • Findingown data

    Hazes can cool Archean Earth's surface by about 20 K.

    “Here, we demonstrate using coupled climate-photochemical-microphysical simulations that hazes can cool the planet's surface by about 20 K”
    Quote in the paperSubstantial claim
  • Findingown data

    Optically thicker hazes are self-limiting due to their self-shielding properties, preventing catastrophic cooling of the planet.

    “We find that optically thicker hazes are self-limiting due to their self-shielding properties, preventing catastrophic cooling of the planet.”
    Quote in the paperSubstantial claim
  • Findingown data

    Habitable conditions with liquid surface water could be maintained with a relatively thick haze layer (τ ∼ 5 at 200 nm) even with the fainter young Sun.

    “but habitable conditions with liquid surface water could be maintained with a relatively thick haze layer (τ ∼ 5 at 200 nm) even with the fainter young Sun”
    Quote in the paperSubstantial claim
  • Findingown data

    Hazes reduce surface UV flux by about 97% compared to a haze-free planet.

    “reducing surface UV flux by about 97% compared to a haze-free planet”
    Quote in the paperSubstantial claim
  • Findingown data

    The haze can produce a stronger UV shield compared to the low O2 atmosphere (0.1% PAL) proposed recently by Planavsky et al. (2014) for our atmospheric assumptions.

    “The haze can produce a stronger UV shield compared to the low O2 atmosphere (0.1% PAL) proposed recently by Planavsky et al. (2014) for our atmospheric assumptions.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Planavsky et al., 2014

      Not checked in the free run.

  • Classified as probably not a substantial claim. 40
  • May combine two claims. 58

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingnone given

    Larger haze particles than 2 μm do not exist in the simulated atmospheres due to rainout.

    “The particle mode sizes we use span from 0.001 to 2 μm; larger particles do not exist in our atmospheres due to rainout.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingreasoning

    The largest particles produced by the 250 K atmosphere have a geometric radius of 0.8 μm compared to 0.65 μm radius particles for the 200 K atmosphere.

    “The largest particles produced by the 250 K atmosphere have a geometric radius of 0.8 μm compared to 0.65 μm radius particles for the 200 K atmosphere.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    The surface UVC flux under a haze for Case B (CH4/CO2 = 0.21) would have been about 0.03 W/m2 for SZA = 60° and 0.22 W/m2 for SZA = 0°.

    “In contrast, the surface UVC flux under a haze for Case B (CH4/CO2 = 0.21) would have been about 0.03 W/m2 for SZA = 60° and 0.22 W/m2 for SZA = 0°.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    A modest amount of O2 in the Proterozoic (1% PAL) produces a stronger UV shield than the Archean haze, but the haze cuts out more UVA and UVB radiation than ozone in all situations.

    “A modest amount of O2 in the Proterozoic (1% PAL) produces a stronger UV shield than the Archean haze, but the haze shown here cuts out more UVA (320–400 nm) and UVB (280–320 nm) radiation than ozone in all situations.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    At short wavelengths in direct imaging, haze absorption decreases the planet's brightness; scattering brightens the planet at longer wavelengths.

    “At short wavelengths in direct imaging, haze absorption decreases the planet's brightness; scattering brightens the planet at longer wavelengths.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingreasoning

    There is a maximum haze optical thickness and a minimum temperature from haze-induced cooling for each atmosphere.

    “Thus, we find there is a maximum haze optical thickness—and a minimum temperature from haze-induced cooling—for each atmosphere.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingreasoning

    The other cases produce similar results for UV shielding because they have similar optical thicknesses.

    “This table only includes Case B, but the other cases produce similar results for UV shielding because they have similar optical thicknesses.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    According to the study's assumptions, Proterozoic Earth with 0.1% PAL O2 would have received 0.57 W/m2 of UVC at the surface, so in this case, the Archean hazy UV shield was stronger.

    “According to these assumptions, Proterozoic Earth with 0.1% PAL O2 would have received 0.57 W/m2 of UVC at the surface, so in this case, the Archean hazy UV shield was stronger.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    Even using the conventional habitability threshold of 273 K, Cases A-C have a hazy solution space where Tsurf > 273 K, and Case B stabilizes at Tsurf = 274 K with its thickest haze.

    “Note that even using the conventional habitability threshold of 273 K, Cases A–C have a hazy solution space where Tsurf > 273 K, and Case B stabilizes at Tsurf = 274 K with its thickest haze.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingreasoning

    Hazy Archean climates were cold but most likely habitable.

    “We find that hazy Archean climates were cold but most likely habitable (Fig. 4).”
    Quote in the paperSubstantial claim
    • Rests on an assumption: A planet with any nonzero fraction of open ocean is habitable.

  • May combine two claims.

    Findingown data

    For the haze-free Case B atmosphere (CH4/CO2 = 0.1), the models calculate the flux of UVC at the surface as about 0.93 W/m2 for SZA = 60° and 2.62 W/m2 for SZA = 0°.

    “For the haze-free Case B atmosphere (CH4/CO2 = 0.1), our models calculate the flux of UVC at the surface as about 0.93 W/m2 for a SZA of 60° and 2.62 W/m2 for SZA = 0°.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The temperatures of planets A, B, C, and D with ice-albedo feedbacks are 257, 271, 257, and 241 K, a decrease of 3-10 K compared with simulations with the nominal albedo.

    “The temperatures of planets A, B, C, and D with ice-albedo feedbacks are 257, 271, 257, and 241 K, a decrease of 3–10 K compared with simulations with the nominal albedo.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingown data

    These spectral features can also be seen in the peaks of the haze imaginary refractive index.

    “These features can also be seen in the peaks of the haze imaginary refractive index (Fig. 2).”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    For the CAM nominal Archean atmosphere, the CAM produces a global average surface temperature of 287.9 K, while the authors' model produces 285.3 K, a difference of 2.6 K.

    “For this world, the CAM produces a global average surface temperature of 287.9 K. Our model produces 285.3 K for this configuration, a difference of 2.6 K.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingcitation

    The buildup of C2H6 in the authors' spectra is not inconsistent with the results of Domagal-Goldman et al. (2011).

    “its buildup in our spectra is not inconsistent with the results of Domagal-Goldman et al. (2011)”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Domagal-Goldman et al. (2011)

      Not checked in the free run.

  • May combine two claims.

    Findingown data

    The authors achieve the closest match to the Charnay et al. results for a modern continental land mass arrangement, with GCM average albedo 0.34 and temperature 283.7 K, close to their result of 281.1 K.

    “We achieve the closest match to the Charnay et al. results for a modern continental land mass arrangement: in the GCM, this yields an average albedo of 0.34 and a temperature of 283.7 K, close to our result of 281.1 K for this configuration.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Charnay et al.

      Not checked in the free run.

  • May combine two claims.

    Findingown data

    The authors' global average 1-D result for that case is 282 K, which is 5 K cooler than the LMD result.

    “This is comparable to, but 5 K warmer than, our global average 1-D result of 282 K.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingown data

    There is a very weak haze feature near 3 μm in transit transmission.

    “There is also a very weak haze feature near 3 μm in transit transmission that can be most easily seen in Fig. 10.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Findingnone given

    The other cases produce similar spectra to Case B.

    “the other cases produce similar spectra as discussed below.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingown data

    Of all the optical constants plotted in Fig. 14, the Khare indices produce the closest match to the Hasenkopf Archean real refractive index (n) near 532 nm and a reasonable match to the Hasenkopf Archean imaginary refractive index (k), agreeing to within approximately 40% near 532 nm.

    “of all the optical constants plotted in Fig. 14, the Khare indices actually produce the closest (although still too low) match to the Hasenkopf Archean real refractive index (n) near 532 nm and produce a reasonable match to the Hasenkopf Archean imaginary refractive index (k), agreeing to within approximately 40% near 532 nm.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingcitation

    The authors find their maximum haze thickness levels off at τ ∼ 5 at 200 nm, which results in a significantly shorter NH3 lifetime of 1 × 104 years.

    “Following Wolf and Toon, we find our maximum haze thickness levels off at τ ∼ 5 at 200 nm, which results in a significantly shorter NH3 lifetime of 1 × 104 years, although we did not include NH3 in our photochemical scheme.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Findingcitation

    The temperature difference between Khare and Khare-Hasenkopf is smaller than the difference from using spherical versus fractal particles, which can result in temperature differences >10 K.

    “This difference in temperature is smaller than the difference of using spherical versus fractal particles: our comparison to the Haqq-Misra et al. (2008) study in Section 4.2 shows that particle shape can result in temperature differences >10 K.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Haqq-Misra et al. (2008)

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingnone given

    The 1% CH4 Mahjoub imaginary refractive index agrees to within 5% of the Hasenkopf Archean measurement near 532 nm.

    “Note the 1% CH4 Mahjoub imaginary refractive index agrees to within 5% of the Hasenkopf Archean measurement near 532 nm.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationnone given

    Hydrocarbon haze has profound spectral impacts for both reflected light and transit transmission spectra.

    “As we will show, hydrocarbon haze has profound spectral impacts for both reflected light and transit transmission spectra.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationown data

    Hazes may enhance planetary habitability through UV shielding, potentially allowing survival of land-based organisms 2.7-2.6 billion years ago.

    “Hazes may even enhance planetary habitability through UV shielding, reducing surface UV flux by about 97% compared to a haze-free planet and potentially allowing survival of land-based organisms 2.7–2.6 billion years ago.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Modeling work suggests a longer Mesoarchean to Neoarchean hazy period between 3.2 and 2.7 Ga.

    “the modeling work of Domagal-Goldman et al. (2008) suggests a longer Mesoarchean to Neoarchean hazy period between 3.2 and 2.7 Ga.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Domagal-Goldman et al., 2008

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    S8 is the most important species to rain out S-MIF from the atmosphere, and S-MIF is more easily preserved under reducing conditions.

    “S8 is the most important species to rain out S-MIF from the atmosphere; because a more reducing atmosphere enhances the ability of S8 to polymerize, S-MIF is more easily preserved under reducing conditions (Zahnle et al., 2006).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Zahnle et al., 2006

      Not checked in the free run.

  • May combine two claims.

    Premisereasoning

    The S-MIF signal is variable throughout the Archean and vanishes completely once O2 builds up to non-negligible levels after the GOE at 2.5 Ga.

    “The S-MIF signal is variable throughout the Archean, and it vanishes completely once O2 builds up to non-negligible levels in the atmosphere after the GOE at 2.5 Ga.”
    Quote in the paperSubstantial claim

  • Classified as probably not a substantial claim.

    Premisecitation

    Several studies have proposed links between haze and the mass-independent sulfur isotope fractionation signal preserved before the Great Oxygenation Event at about 2.5 Ga.

    “Several studies have proposed links between haze and the mass-independent sulfur isotope fractionation signal (S-MIF) (Farquhar et al., 2000) preserved in the geological record before the Great Oxygenation Event (GOE) at about 2.5 Ga”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Farquhar et al., 2000

      Not checked in the free run.

    • cites Domagal-Goldman et al., 2008

      Not checked in the free run.

    • cites Zerkle et al., 2012

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    The broad UV absorption signature produced by this haze may be visible across interstellar distances, allowing characterization of similar hazy exoplanets.

    “The broad UV absorption signature produced by this haze may be visible across interstellar distances, allowing characterization of similar hazy exoplanets.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    An organic haze could have formed in Earth's early anoxic atmosphere and strongly interacted with visible and UV radiation, cooling the climate.

    “Early in Earth's history, an anoxic atmosphere could have supported the formation of an organic haze (Pavlov et al., 2001a; Trainer et al., 2004, 2006; DeWitt et al., 2009; Hasenkopf et al., 2010; Zerkle et al., 2012; Kurzweil et al., 2013; Claire et al., 2014; Izon et al., 2015) that strongly interacted with visible and UV radiation, cooling the planet's climate (Pavlov et al., 2001b; Domagal-Gol”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Pavlov et al., 2001a

      Not checked in the free run.

    • cites Trainer et al., 2004, 2006

      Not checked in the free run.

    • cites DeWitt et al., 2009

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Study design and conductnone given

    The climate model is considered converged when the change in temperature between time steps and change in flux out the top of the atmosphere are sufficiently small (typically on the order of 1 × 10−5).

    “The climate model is considered converged when the change in temperature between time steps and change in flux out the top of the atmosphere are sufficiently small (typically on the order of 1 × 10−5).”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Study design and conductown data

    The climate model could not be run to convergence using the same top-of-atmosphere pressure used for the photochemical model; it ran up to about 80 km for 1 bar atmospheres.

    “We have been unable to run the climate model to convergence using the same top-of-atmosphere pressure used for the photochemical model: the photochemical model extends to 100 km, but we have only been able to successfully run the climate model up to about 80 km for our 1 bar atmospheres.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Δ36S/Δ33S in Archean sedimentary rocks is generally around −1, with stratigraphic variations interpreted as evidence of changes to the S-MIF production mechanism from atmospheric composition changes, suggesting haze influence.

    “Δ36S/Δ33S in Archean sedimentary rocks is generally around −1, but stratigraphic variations in this slope have been observed in the geological record and interpreted as evidence of changes to the S-MIF production mechanism resulting from changes in atmospheric composition (Zerkle et al., 2012; Kurzweil et al., 2013; Izon et al., 2015), suggesting the influence of haze.”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Zerkle et al., 2012

      Not checked in the free run.

    • cites Kurzweil et al., 2013

      Not checked in the free run.

    • cites Izon et al., 2015

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Fractal particles produce more extinction in the ultraviolet but less in the visible and near infrared compared to spherical particles.

    “In general, compared to spherical particles, fractal particles produce more extinction in the ultraviolet (UV) but less in the visible and near infrared (NIR).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Wolf and Toon, 2010

      Not checked in the free run.

  • Classified as probably not a substantial claim. May combine two claims.

    Premisecitation

    A change in the slope of Δ36S/Δ33S from −1.5 to −0.9 at 2.71 Ga is interpreted to be caused by a decrease in the CH4/CO2 ratio at 2.71 Ga, possibly indicating an organic haze was present for some period of time prior to 2.71 Ga and cleared afterward.

    “they discuss a change in the slope of Δ36S/Δ33S from −1.5 to −0.9 at 2.71 Ga and interpret this to be caused by a decrease in the CH4/CO2 ratio at 2.71 Ga, possibly indicating an organic haze was present for some period of time prior to 2.71 Ga and cleared afterward.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Kurzweil et al., 2013

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Study design and conductnone given

    The photochemical model is considered converged when redox is conserved and a re-run of the model using last run's output as initial conditions occurs quickly (i.e., <50 time steps).

    “The photochemical model is considered converged when redox is conserved and a re-run of the model using last run's output as initial conditions occurs quickly (i.e., <50 time steps).”
    Quote in the paperNot a substantial claim

  • Classified as probably not a substantial claim.

    Interpretationcitation

    An upper atmosphere haze that decreased tropospheric SO2 photolysis by blocking UV photons and cooled the planet could explain both the glaciation and the decrease in S-MIF.

    “An upper atmosphere haze that decreased tropospheric SO2 photolysis by blocking UV photons and cooled the planet could explain both the glaciation and the decrease in S-MIF.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Domagal-Goldman et al., 2008

      Not checked in the free run.

    • cites Haqq-Misra et al., 2008

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Haze-free and reducing atmospheric conditions dominated after 2.71 Ga, with haze reappearing in brief intervals of time.

    “In this interpretation, haze-free and reducing atmospheric conditions dominated after 2.71 Ga, with haze reappearing in brief intervals of time as suggested by the Zerkle et al. (2012) and Izon et al. (2015) measurements.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Zerkle et al., 2012

      Not checked in the free run.

    • cites Izon et al., 2015

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductreasoning

    The model may overestimate the rate of aerosol formation.

    “In a real atmosphere, the molecules would be larger before aerosols condense, and back-reactions should occur, so this model may overestimate the rate of aerosol formation.”
    Quote in the paperNot a substantial claim
    • The authors state this as an assumption: C4H2 and C5H4 condense directly to haze particles.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    A mode standard deviation of 1.5 is realistic for an aerosol distribution.

    “Spherical particles in SMART are binned according to log-normal size distributions using the radii mentioned previously and a mode standard deviation of 1.5, which is realistic for an aerosol distribution (Tolfo, 1977).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Tolfo, 1977

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductcitation

    The version of the photochemical model used here can simulate atmospheres ranging from extremely anoxic (pO2 = 10−14) to modern-day O2 levels.

    “Note that the version of the model used here can simulate atmospheres ranging from extremely anoxic (pO2 = 10−14) to modern-day O2 levels (Zahnle et al., 2006).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Zahnle et al., 2006

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Fractals tend to be more forward scattering in the visible and NIR and more isotropically scattering in the UV compared to equal-mass spherical particles.

    “In addition, fractals tend to be more forward scattering in the visible and NIR and more isotropically scattering in the UV compared to equal-mass spherical particles.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Wolf and Toon, 2010

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Δ33S typically correlates with enrichments in δ34S and with depletions in δ36S.

    “Δ33S typically correlates with enrichments in δ34S and with depletions in δ36S”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Claire et al., 2014

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Interpretationreasoning

    The end of the cold period typified by low Δ33S may be due to a decrease in the atmospheric CH4/CO2 ratio, which would have cleared any haze present in the atmosphere.

    “In this conceptual model, the end of the cold period typified by low Δ33S may be due to a decrease in the atmospheric CH4/CO2 ratio, which would have cleared any haze present in the atmosphere.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Claire et al., 2014

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Fractal particles have different spectral properties and climatic effects than spherical particles.

    “Unlike previous studies of the Archean climate under a haze, we use realistic fractal (rather than spherical) particles, which have different spectral properties and climatic effects.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Wolf and Toon, 2010

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    An increase in magnitude of S-MIF signals after 2.73 Ga occurred during a prolonged negative shift in δ13Corg, suggesting enhanced biological methane activity at this time.

    “Kurzweil et al. (2013) noted that an increase in magnitude of S-MIF signals after 2.73 Ga (Thomazo et al., 2009) occurred during a prolonged negative shift in δ13Corg, suggesting enhanced biological methane activity at this time.”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Kurzweil et al., 2013

      Not checked in the free run.

    • cites Thomazo et al., 2009

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    More recent 3-D studies have shown that Archean Earth can maintain an open ocean fraction of >50% for TGAT ≥ 260 K and an equatorial open ocean belt for TGAT ≥ 248 K.

    “However, more recent 3-D studies have shown that Archean Earth can maintain an open ocean fraction of >50% for TGAT ≥ 260 K and an equatorial open ocean belt for TGAT ≥ 248 K (Wolf and Toon, 2013; Charnay et al., 2013).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Wolf and Toon, 2013

      Not checked in the free run.

    • cites Charnay et al., 2013

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    These ice-albedo temperatures may be underestimates because once haze forms, deposition of dark hydrocarbons onto ice-covered areas will lower the albedo of the ice, which may then melt the ice, reverting parts of the surface back to ocean water.

    “These ice-albedo temperatures may be underestimates because once haze forms, deposition of dark hydrocarbons onto ice-covered areas will lower the albedo of the ice. This decreased ice albedo may then melt the ice, reverting parts of the surface back to ocean water.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationcitation

    This might lead to cooling of the planet or even warming depending on cloud particle size and the altitude-and therefore temperature-of the cloud layer.

    “This might lead to cooling of the planet or even warming depending on cloud particle size and the altitude—and therefore temperature—of the cloud layer (Goldblatt and Zahnle, 2011).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Goldblatt and Zahnle, 2011

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation

    A recent study of chromium isotopes suggests that the Proterozoic O2 mixing ratio was, at most, 0.1% PAL.

    “but a recent study of chromium isotopes suggests that the Proterozoic O2 mixing ratio was, at most, 0.1% PAL (Planavsky et al., 2014).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Planavsky et al., 2014

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    Later purported hazy periods around 2.7 Ga and between 2.65 and 2.5 Ga are not associated with glaciations and may be consistent with the thinner-haze solution space of Cases A and C or even the thickest haze solution space of the warmer Case B.

    “Later purported hazy periods around 2.7 Ga (Kurzweil et al., 2013; Izon et al., 2015) and between 2.65 and 2.5 Ga (Zerkle et al., 2012) are not associated with glaciations and may be consistent with the thinner-haze solution space of Cases A and C or even the thickest haze solution space of the warmer Case B.”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Kurzweil et al., 2013

      Not checked in the free run.

    • cites Izon et al., 2015

      Not checked in the free run.

    • cites Zerkle et al., 2012

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    DNA damage is most acute in the UVC (λ < 0.28 μm) wavelength range, but in the modern atmosphere, UVC is fully blocked by O2 and ozone.

    “DNA damage is most acute in the UVC (λ < 0.28 μm) wavelength range (Pierson et al., 1992; Dillon and Castenholz, 1999), but in the modern atmosphere, UVC is fully blocked by O2 and ozone.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Pierson et al., 1992

      Not checked in the free run.

    • cites Dillon and Castenholz, 1999

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Study design and conductcitation

    The surface albedo 0.32 includes the effect of clouds and reproduces the average temperature of present-day Earth (288 K) with modern atmospheric conditions.

    “Unless otherwise specified, the surface albedo used by the climate model is 0.32. This includes the effect of clouds, which is standard in this 1-D treatment (Kopparapu et al., 2013) and is the albedo that reproduces the average temperature of present-day Earth (288 K) with modern atmospheric conditions.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Kopparapu et al., 2013

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation

    Surface O2 mixing ratio is set to 1.0 × 10−8, consistent with Zerkle et al. (2012).

    “We set the mixing ratio of O2 at the surface to 1.0 × 10−8, consistent with the Zerkle et al. (2012) study.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Zerkle et al., 2012

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation

    Abbot et al. (2011) argued that ocean open belts can remain climatologically stable, even if the ice latitude is reduced to 5-15°.

    “Furthermore, Abbot et al. (2011) argued that ocean open belts can remain climatologically stable, even if the ice latitude is reduced to 5–15°.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Abbot et al., 2011

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing). May combine two claims.

    Interpretationreasoning

    Because methane-producing metabolisms evolved early and Earth's atmosphere was anoxic for about a billion years after the origin of life, pale orange dots may proliferate in the Galaxy if other habitable worlds evolve on similar paths to Earth.

    “Because methane-producing metabolisms evolved early and Earth's atmosphere was anoxic for about a billion years after the origin of life, pale orange dots may proliferate in the Galaxy if other habitable worlds evolve on similar paths to Earth.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: Methane-producing metabolisms evolved early.
    • Rests on an assumption: Earth's atmosphere was anoxic for about a billion years after the origin of life.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Study design and conductcitation

    The true cloud distribution on Archean Earth is unknown, and clouds may have had important climatic effects on early Earth.

    “Of course, the true cloud distribution on Archean Earth is unknown, and clouds may have had important climatic effects on our early planet (Goldblatt and Zahnle, 2011).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Goldblatt and Zahnle, 2011

      Not checked in the free run.

  • May combine two claims.

    Study design and conductcitation

    The solar zenith angles used in the climate and photochemical models were chosen to best represent globally averaged behavior, with SZA=45° in photochemical and SZA=60° in climate, tuned to reproduce modern-day Earth's average chemical profiles and climate.

    “The solar zenith angles (SZAs) used in the climate and photochemical models were chosen to best represent globally averaged behavior of the physics in each specific model, which Segura et al. (2003) found as SZA = 45° in the photochemical model and SZA = 60° in the climate model. These zenith angles are both tuned to reproduce modern-day Earth's average chemical profiles and climate, respectively.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Segura et al., 2003

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    The fractal nature of the hazes makes them strong absorbers at short wavelengths and therefore a potential shield against damaging UV radiation for the anoxic Archean.

    “The impact of these hazes on the biosphere goes beyond temperature reduction: their fractal nature makes them strong absorbers at short wavelengths and therefore a potential shield against damaging UV radiation for the anoxic Archean (Wolf and Toon, 2010), which would have received significantly more UV at the surface than the planet today (Rugheimer et al., 2015).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Wolf and Toon, 2010

      Not checked in the free run.

    • cites Rugheimer et al., 2015

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S2 (checkable).

    Interpretationreasoning

    The occurrence of haze in the habitable zone may be a way to flag interesting planets for careful follow-up study.

    “so the occurrence of haze in the habitable zone may be a way to flag interesting planets for careful follow-up study that would search for other indicators of life and quantify the concentration of CH4 and other gases.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Domagal-Goldman et al. (2011) showed much greater C2H6 accumulation on planets orbiting low-mass stars compared to worlds orbiting the Sun.

    “which showed much greater C2H6 accumulation on planets orbiting low-mass stars compared to worlds orbiting the Sun.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Domagal-Goldman et al. (2011)

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation

    Guzmán-Marmolejo et al. (2013) suggested that an atmospheric CH4 concentration greater than 10 ppmv is suggestive of life.

    “Based on their calculations, Guzmán-Marmolejo et al. (2013) suggested that an atmospheric CH4 concentration greater than 10 ppmv is suggestive of life.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Guzmán-Marmolejo et al. (2013)

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    The Charnay et al. results for the same atmospheric properties but with an equatorial supercontinent result in the same overall planetary albedo (0.33) but a lower mean temperature of 285 K.

    “The Charnay et al. results for the same atmospheric properties but with an equatorial supercontinent result in the same overall planetary albedo (0.33) but a lower mean temperature of 285 K”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Charnay et al.

      Not checked in the free run.

  • May combine two claims.

    Interpretationown data

    An Archean haze would have profoundly impacted Earth's environment, habitability, and spectrum.

    “Our study shows how an Archean haze would have profoundly impacted our planet's environment, habitability, and spectrum.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Study design and conductreasoning

    The spectra in Section 3.3 do not include water clouds, making the albedos darker than in the Atmos parameterization.

    “To consider the spectral effect of haze without contamination from other atmospheric aerosols, the spectra in this section do not include water clouds, even though cloud albedo is implicit in the parameterization of the Atmos model's surface albedo. This makes the albedos of the planets whose spectra are shown in this section darker than those in the Atmos parameterization.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationnone given

    Abundant stratospheric water vapor would imply that the planet is in the midst of a moist or runaway greenhouse state and thus is not conventionally habitable.

    “Abundant stratospheric water vapor would imply that the planet is in the midst of a moist or runaway greenhouse state and thus is not conventionally habitable.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Laboratory experiments on organic haze formation have shown that haze-formation chemistry can involve the formation of important prebiotic molecules such as amino acids and nucleotide bases.

    “Laboratory experiments on organic haze formation have shown that haze-formation chemistry can involve the formation of important prebiotic molecules such as amino acids and nucleotide bases (Khare et al., 1986; McDonald et al., 1994; DeWitt et al., 2009; Hörst et al., 2012; Trainer, 2013)”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites Khare et al., 1986

      Not checked in the free run.

    • cites McDonald et al., 1994

      Not checked in the free run.

    • cites DeWitt et al., 2009

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Study design and conductown data

    The authors tested their model results against the CAM GCM nominal Archean atmosphere reported by Wolf and Toon (2013).

    “We also tested our model results against the Community Atmosphere Model (CAM) GCM nominal Archean atmosphere reported by Wolf and Toon (2013).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Wolf and Toon (2013)

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    C2H6 is a greenhouse gas and its ability to warm in a CH4- and haze-rich atmosphere has been discussed previously.

    “C2H6 is a greenhouse gas, and its ability to warm in a CH4- and haze-rich atmosphere has been discussed previously (Haqq-Misra et al., 2008).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Haqq-Misra et al., 2008

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    The comparison with 3-D models suggests the temperatures presented in this work are reasonable but may be underestimates by about 3-5 K.

    “Our comparison with these 3-D models suggests the temperatures we present in this work are reasonable but may be underestimates by about 3–5 K.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based). May combine two claims.

    Interpretationnone given

    Haze formation is likely considerably more complex and is still not well understood.

    “but haze formation is likely considerably more complex and is still not well understood.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    The Mahjoub study tested the impact of methane concentration in the gas phase on resultant hydrocarbon optical properties with gas mixtures containing 1%, 2%, 5%, and 10% CH4 in CH4-N2 mixtures.

    “The Mahjoub study tested the impact of methane concentration in the gas phase on the resultant hydrocarbon optical properties with gas mixtures containing 1%, 2%, 5%, and 10% CH4 in CH4-N2 mixtures.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Mahjoub et al.

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Premisecitation

    Following Sagan and Chyba (1997), Wolf and Toon calculated a NH3 atmospheric lifetime of 7 × 107 years for a solar incident flux at a 45° angle assuming τ ∼ 11 at 200 nm.

    “Following Sagan and Chyba (1997), Wolf and Toon calculated a NH3 atmospheric lifetime of 7 × 107 years for a solar incident flux at a 45° angle assuming τ ∼ 11 at 200 nm.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites Sagan and Chyba (1997)

      Not checked in the free run.

    • cites Wolf and Toon

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductcitation

    The authors examined the sensitivity of their nominal spectra to varied refractive indices measured by Mahjoub et al. (2012).

    “As an example and test of the impact different refractive indices have on our spectra, we examined the sensitivity of our nominal spectra to varied refractive indices measured by Mahjoub et al. (2012).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Mahjoub et al. (2012)

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Mass spectrometry of aerosol analogues formed with C/O = 0.1 indicates the formation of carbonyl and carboxyl groups rather than aromatic cycles and long-aliphatic chains, and suggests the formation of organic acids such as succinic acid (C4H6O4).

    “Mass spectrometry of aerosol analogues formed with C/O = 0.1 indicates the formation of carbonyl and carboxyl groups rather than aromatic cycles and long-aliphatic chains, and even suggests the formation of organic acids such as succinic acid (C4H6O4) (DeWitt et al., 2009).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites DeWitt et al., 2009

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    These results suggest that N2 chemical activation could be due to its reaction with the methylidyne (CH) radical formed from CH4 photolysis, to form two radical intermediates, diazomethyl HCNN and its isomer HNCN, which might then react to form HCN and other products.

    “These results suggest that N2 chemical activation could be due to its reaction with the methylidyne (CH) radical formed from CH4 photolysis, to form two radical intermediates, diazomethyl HCNN and its isomer HNCN, which might then react to form HCN and other products.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    The Khare refractive indices span an extremely wide wavelength range, from 0.02 to 920 μm, so only one set of optical constants is needed to cover all wavelengths relevant to photochemistry, climate, and spectra.

    “An additional advantage of the Khare refractive indices is that they span an extremely wide wavelength range, ranging from 0.02 to 920 μm, so only one set of optical constants is needed to cover all the wavelengths relevant to photochemistry, climate, and spectra.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Only one study, Hasenkopf et al. (2010), has measured an Archean Earth haze refractive index, and only at a single wavelength (532 nm).

    “Unfortunately, only one study, that of Hasenkopf et al. (2010), has measured an Archean Earth haze refractive index (as opposed to a titanian haze), and this was only done at a single wavelength (532 nm).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Hasenkopf et al. (2010)

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Early Earth's atmosphere would have contained negligible O2 but significant amounts of CO2, whereas Titan's atmosphere is extremely reducing.

    “Early Earth's atmosphere would have contained negligible O2 but significant amounts of CO2 (e.g., Kasting, 1993; Driese et al., 2011), whereas Titan's atmosphere is extremely reducing (de Kok et al., 2007).”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites Kasting, 1993

      Not checked in the free run.

    • cites Driese et al., 2011

      Not checked in the free run.

    • cites de Kok et al., 2007

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    In situ measurements by several instruments on board Cassini have found direct evidence for long hydrocarbons and nitriles chains, benzene (C6H6) and toluene (C6H5CH3), and indirect evidence for polycyclic aromatic hydrocarbons (PAHs) and nitrogen-containing polycyclic aromatic hydrocarbons (PANHs), indicating that these compounds might play a role in the formation of Titan's hazes.

    “In situ measurements by several instruments on board Cassini have found direct evidence for long hydrocarbons and nitriles chains, benzene (C6H6) and toluene (C6H5CH3), and indirect evidence for polycyclic aromatic hydrocarbons (PAHs) and nitrogen-containing polycyclic aromatic hydrocarbons (PANHs), indicating that these compounds might play a role in the formation of Titan's hazes (Waite et al.,”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites Waite et al., 2007

      Not checked in the free run.

    • cites López-Puertas et al., 2013

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Wolf and Toon (2010) commented on the possibility of a fractal hydrocarbon haze shielding ammonia (NH3) from photolysis, allowing this greenhouse gas to build up in the Archean atmosphere.

    “Wolf and Toon (2010) commented on the possibility of a fractal hydrocarbon haze shielding ammonia (NH3) from photolysis, allowing this greenhouse gas to build up in the Archean atmosphere.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Wolf and Toon (2010)

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation

    These results bring to light a significant but still unknown mechanism regarding the activation of nitrogen and its inclusion in oxygenated organics, providing a new and quantifiable source for these two elements into early Earth aerosols.

    “These results bring to light a significant but still unknown mechanism regarding the activation of nitrogen and its inclusion in oxygenated organics, thus providing a new and quantifiable source for these two elements into the early Earth aerosols.”
    Quote in the paperSubstantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    Discovering habitable exoplanets dissimilar to modern Earth will increase the diversity of known habitable environments.

    “Discovering habitable exoplanets dissimilar to modern Earth will increase the diversity of known habitable environments.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Study design and conductnone given

    The Khare-Hasenkopf spectrum does not account for differences expected in the spectral shape of Archean haze analogues across the UV-visible-IR relative to titanian haze analogues.

    “Of course this does not account for differences expected in the spectral shape of Archean haze analogues across the UV-visible-IR relative to the titanian haze analogues.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationreasoning

    The climatic effects of this haze could have been part of feedbacks between biological CH4 production, atmospheric chemistry, and surface UV radiation.

    “The climatic effects of this haze could have been part of feedbacks between biological CH4 production, atmospheric chemistry, and surface UV radiation.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Premisecitation

    Mahjoub et al. found that refractive indices have a strong dependency on CH4 concentration over 0.37-1 μm: the imaginary index of refraction (k) decreases with increasing CH4 concentration, and the real index of refraction (n) increases with CH4 for the compositions tested.

    “Mahjoub et al. found that refractive indices have a strong dependency on the CH4 concentration over 0.37–1 μm: results indicate that the imaginary index of refraction (k) decreases with increasing CH4 concentration, and the real index of refraction (n) increases with CH4 for the compositions tested.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Mahjoub et al.

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationreasoning

    Due to the properties of fractal hazes, these particles are relatively transparent at wavelengths longer than approximately the visible range.

    “Due to the properties of fractal hazes, these particles are relatively transparent at wavelengths longer than approximately the visible range”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Interpretationnone given

    The Mahjoub constants do not extend shortward of 0.37 μm, but variation at shorter wavelengths should be anticipated.

    “The Mahjoub constants do not extend shortward of 0.37 μm, but we should anticipate variation at these shorter wavelengths as well.”
    Quote in the paperNot a substantial claim
89 of 89

  • The authors state this as an assumption.

    Fixed isoprofiles are assumed for CO2 and N2 in the atmospheres considered here.

    “Fixed isoprofiles are assumed for CO2 and N2 in the atmospheres considered here.”

  • The authors state this as an assumption.

    C4H2 and C5H4 condense directly to haze particles.

    “it is assumed that C4H2 and C5H4 condense directly to haze particles (called HCAER and HCAER2 in Supplementary Table S1).”

  • The paper relies on this without support.

    A planet with any nonzero fraction of open ocean is habitable.

    “Since a planet with any nonzero fraction of open ocean is habitable, we regard these updated globally averaged temperatures—all of which are significantly below freezing—to be more realistic habitability thresholds than 273 K.”

  • The paper relies on this without support.

    Methane-producing metabolisms evolved early.

    “Because methane-producing metabolisms evolved early and Earth's atmosphere was anoxic for about a billion years after the origin of life, pale orange dots may proliferate in the Galaxy if other habitable worlds evolve on similar paths to Earth.”

  • The paper relies on this without support.

    Earth's atmosphere was anoxic for about a billion years after the origin of life.

    “Because methane-producing metabolisms evolved early and Earth's atmosphere was anoxic for about a billion years after the origin of life, pale orange dots may proliferate in the Galaxy if other habitable worlds evolve on similar paths to Earth.”

  • The paper relies on this without support.

    Water vapor clouds have an approximately gray opacity from the visible into the NIR.

    “water vapor clouds have an approximately gray opacity from the visible into the NIR.”

  • The paper relies on this without support.

    The tropopause on Earth is at roughly 10 km.

    “The tropopause on Earth is at roughly 10 km”

  • The paper relies on this without support.

    Geochemical evidence suggests hazy conditions were not present throughout the entire Archean.

    “geochemical evidence suggests hazy conditions were not present throughout the entire Archean”

  • The paper relies on this without support.

    The geochemical constraints on Archean CO2 concentration and geological constraints on surface pressure used in the study are valid.

    “consistent with geochemical constraints on CO2 concentration and geological constraints on surface pressure”

  • The paper relies on this without support.

    UVC radiation is harmful to unshielded life.

    “may have allowed survival of otherwise unshielded life at the surface of our Archean planet”
10 of 10

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • A cooling limit exists because as a haze becomes more UV absorbing, it shields CH4 molecules from the photolysis needed for further haze formation.

    “A cooling limit exists because as a haze becomes more UV absorbing, it shields CH4 molecules from the photolysis needed for further haze formation.”

    From The Archean atmosphere (2020) · cites this paper as [1]

  • Whether this shielding occurred depends on whether hazes existed and on the size and radiative properties of haze particles, which remain uncertain.

    “However, whether this shielding actually occurred depends on whether hazes actually existed and on the size and radiative properties of haze particles, which remain uncertain (1, 163).”

    From The Archean atmosphere (2020) · cites this paper as [1]

  • Too much CH4 relative to CO2 would create the high-altitude organic haze, which may cool Earth by up to ~20 K by absorbing incoming solar radiation and radiating energy back to space in a so-called anti-greenhouse effect.

    “However, too much CH4 relative to CO2 would create the high-altitude organic haze mentioned earlier, which may cool Earth by up to ~20 K (1, 215) by absorbing incoming solar radiation and radiating energy back to space in a so-called “anti-greenhouse effect” (216).”

    From The Archean atmosphere (2020) · cites this paper as [1]

  • Attenuation of sunlight saturates.

    “Thus, attenuation of sunlight saturates (1, 163).”

    From The Archean atmosphere (2020) · cites this paper as [1]

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Conflicting criteria for haze-particle growth

    Minor

    The methods say particles can grow when the coagulation lifetime is longer than the removal lifetime, but the results say growth occurs when the coagulation timescale is shorter than the removal timescale.

  • Sulfur isotope relationship uses the wrong isotope on its right-hand side

    Minor

    The text says δ36S is roughly twice δ34S, but the stated mass-dependent relationship makes δ36S proportional to itself instead.

  • Example does not give equal log-space bin distances

    Minor

    The stated binning rule uses distance in log space, but 0.33 μm is not halfway in log space between 0.3 and 0.4 μm. Those bins therefore would not each receive 50% of the mass under the stated rule.

3 of 3

Cites

Flagged first, then by citations

Research works printed but not linked: 5

  1. 10.1051/0004-6361/201014343 · DOI
  2. Schopf J.W., editor. (1983) Earth's Earliest Biosphere: Its Origin and Evolution, Princeton University Press, Princeton, NJ.
  3. Sciamma-O'Brien E., , Dahoo P.-R., , Hadamcik E., , Carrasco N., , Quirico E., , Szopa C., , and Cernogora G. (2012) Optical constants from 370nm to 900nm of Titan tholins produced in a low pressure RF plasma discharge. Icarus, 218:353–363.
  4. Traub W.A. (2003) Extrasolar planet characteristics in the visible wavelength range. In Proceedings of the Conference on Towards Other Earths: DARWIN/TPF and the Search for Extrasolar Terrestrial Planets, Heidelberg, Germany, pp 231–239.
  5. Workman J., editor. (2000) The Handbook of Organic Compounds, Three-Volume Set: NIR, IR, R, and UV-Vis Spectra Featuring Polymers and Surfactants, Academic Press, San Diego, CA.

Cited by

Sorted by citations

Authors
Arney, G., Domagal-Goldman, S. D., Meadows, V. S., Wolf, E. T., Schwieterman, E., Charnay, B., Claire, M., Hébrard, E., & Trainer, M. G.
Affiliations
  • Goddard Space Flight Center (US)
  • University of Washington (US)
  • University of Colorado Boulder (US)
  • University of California, Riverside (US)
  • University of St Andrews (GB)
  • NASA Goddard Space Flight Center
Corresponding author
Arney, Giada
ORCID
Charnay, Benjamin
Venue
Astrobiology
Publisher
SAGE Publications
Published
2016-11
Volume
16
Issue
11
Pages
873-899
ISSN
  • 1531-1074 (ISSN-L)
  • 1557-8070
DOI
10.1089/ast.2015.1422
PMC
PMC5148108
OpenAlex
W2534050968
Semantic Scholar
62da501fd3e70b940830188582e2e383e75193d3
Cited by
327
References
136
Citations by year
2026: 22 · 2025: 59 · 2024: 28 · 2023: 33 · 2022: 33 · 2021: 30 · 2020: 33 · 2019: 25 · 2018: 45 · 2017: 18 · 2016: 1
FWCI
14.17
Type
journal article
Language
en
Open access
Licence
Locations
  • Astrobiology (publishedVersion, cc-by-nc)
  • PubMed Central (publishedVersion, cc-by-nc)
  • PubMed (publishedVersion)
  • Open Research Exeter (University of Exeter) (acceptedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • HAL (submittedVersion)
  • eScholarship (California Digital Library) (submittedVersion, cc-by-nc)
  • St Andrews Research Repository (St Andrews Research Repository) (submittedVersion, cc-by-nc)
  • arXiv (Cornell University)
Funders
Natural Environment Research Council (NE/J022802/2)
Topics
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Space Science and Extraterrestrial Life
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astro and Planetary Science
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Planetary Science and Exploration
Keywords
Archean, atmospheric haze, habitability, faint young Sun, UV protection, biosignatures, methane, self-shielding, exoplanet atmospheres, surface cooling
MeSH terms
Atmosphere, Climate, Extraterrestrial Environment, Gases, Models, Theoretical, Particle Size, Spectrophotometry, Ultraviolet, Temperature, Ultraviolet Rays, Water, Earth, Planet, Exobiology

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-08.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-08.

Statistics and arithmetic are recomputed, not judged.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

What counts as a claim ›How we measure confidence ›How we check this ›

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