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

“5 billion years ago) haze was likely biologically mediated via CH4 produced from methanogenesis, one of the earliest metabolisms (Woese and Fox, 1977; Ueno et al., 2006).”

“Since the full chemical scheme that leads to aerosol formation is not well understood despite both laboratory and theoretical studies ( e.g. , Hallquist et al. , 2009; Hicks et al. , 2015), it is assumed that C 4 H 2 and C 5 H 4 condense directly to haze particles (called HCAER and HCAER2 in…”

“The Line-by-Line Absorption Coefficients (LBLABC) code, a companion to SMART, creates line-by-line absorption files for input gas mixing ratios and temperature-pressure profiles using HITRAN 2012 line lists (Rothman et al. , 2013).”

“Spherical particles use a full Mie phase function, while fractal particles employ a Henyey-Greenstein phase function (Henyey and Greenstein, 1941).”

“Thus, S-MIF is generally regarded as robust evidence for an anoxic Archean atmosphere.”

“The version we use here is based directly on that used by Kopparapu et al. (2013).”

“In addition, several abiotic processes including serpentinization (the hydration of ultramafic rocks, mainly olivine and pyroxenes) can form methane (Kelley et al. , 2005; Etiope and Sherwood Lollar, 2013; Guzma´n-Marmolejo et al. , 2013), but the biotic flux of methane to the Archean atmosphere was…”

“Following the mechanism proposed for the formation of Titan’s hydrocarbon haze (Allen et al. , 1980; Yung et al. , 1984), every model of hydrocarbon haze formation in early Earth’s atmosphere—including ours—has assumed that aerosol formation will occur through the formation of acetylene (C 2 H 2 )…”

“Unlike the hazes that may exist around exoplanets with thick hydrogen-dominated atmospheres (Sing et al., 2011; Knutson et al., 2014; Kreidberg et al., 2014), the Archean (3.”

“After the GOE, all the sulfur in the atmosphere would have been oxidized into a single exit channel, eliminating any fractionation created in the atmosphere (Pavlov and Kasting, 2002).”

“There is evidence that Archean stromatolitic communities lived in inter- and supratidal zones (Allwood et al., 2006; Noffke and Awramik, 2013) experiencing frequent, sometimes extended, exposure to the surface environment, and it has been suggested that microbial mats existed on land as early as 2.”

“A d -2- stream method is used for radiative transfer (Toon et al. , 1989).”

“In cases where an icy surface is used, we use an albedo from the USGS Digital Spectral Library (Clark et al. , 2007).”

“…been assumed that Proterozoic Earth’s surface received less UV than the Archean due to the rise of oxygen (O 2 ) and ozone ( e.g. , Rugheimer et al. , 2015), but a recent study of chromium isotopes suggests that the Proterozoic O 2 mixing ratio was, at most, 0.1% PAL (Planavsky et al. , 2014).”

“Refractive indices, shown in the bottom panels, are presented from information in Khare et al. (1984a). 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.”

“…) and toluene (C 6 H 5 CH 3 ), 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. , 2007; Lo´pez-Puertas et al. , 2013).”

“…processes including serpentinization (the hydration of ultramafic rocks, mainly olivine and pyroxenes) can form methane (Kelley et al. , 2005; Etiope and Sherwood Lollar, 2013; Guzma´n-Marmolejo et al. , 2013), but the biotic flux of methane to the Archean atmosphere was likely much higher…”

“…thick hydrogen-dominated atmospheres (Sing et al. , 2011; Knutson et al. , 2014; Kreidberg et al. , 2014), the Archean (3.8–2.5 billion years ago) haze was likely biologically mediated via CH 4 produced from methanogenesis, one of the earliest metabolisms (Woese and Fox, 1977; Ueno et al. , 2006).”

“This humidity parameterization is further described by Pavlov et al. (2000).”

“…(Kopparapu et al. , 2013), the climate of early Mars (Ramirez et al. , 2013), the atmospheres of Earth-like planets around various stellar types (Segura et al. , 2003, 2005, 2010; Rugheimer et al. , 2013), clouds in exoplanet THE PALE ORANGE DOT atmospheres (Kitzmann et al. , 2010, 2011a), and…”

“While the precise mechanism (or mechanisms) that produces S-MIF is unknown, photolysis of sulfur gases in an anoxic atmosphere is the only known mechanism that produces large-magnitude D 33 S and D 36 S seen in the rock record (Farquhar et al. , 2001, 2007).”

“Additionally, although we anticipate planetary diversity in the exoplanet population, existing spectral studies are largely focused on the observables of modern-day Earth ( e.g. , Sagan et al. , 1993; Woolf et al. , 2002; Robinson et al. , 2011, 2014a).”

“Our solar spectrum was modeled by Chance and Kurucz (2010) and was scaled by the solar evolution model (Claire et al. , 2012) mentioned previously.”

“Additional information about fractal particles and their geometry can be found in the works of, for example, Ko¨ylu¨ et al. (1995) and Brasil et al. (1999).”

“Negative values of d (13)Corg below -37& are typically interpreted as evidence for methanogenesis (biological methane production) followed by subsequent incorporation into sediments by methanotrophy (methane consumption), which imparts a strongly negative d(13)Corg because organisms preferentially uptake the (12)C (Ur…”

“It has been suggested that the Rayleigh scattering slope could be used to constrain atmospheric pressure on exoplanets (Benneke and Seager, 2012), but this would not be possible on planets with hydrocarbon hazes due to these strong short-wavelength absorption effects.”

“…(Kopparapu et al. , 2013), the climate of early Mars (Ramirez et al. , 2013), the atmospheres of Earth-like planets around various stellar types (Segura et al. , 2003, 2005, 2010; Rugheimer et al. , 2013), clouds in exoplanet THE PALE ORANGE DOT atmospheres (Kitzmann et al. , 2010, 2011a), and…”

“Unlike the hazes that may exist around exoplanets with thick hydrogen-dominated atmospheres (Sing et al. , 2011; Knutson et al. , 2014; Kreidberg et al. , 2014), the Archean (3.8–2.5 billion years ago) haze was likely biologically mediated via CH 4 produced from methanogenesis, one of the earliest…”

“Following Sagan and Chyba (1997), Wolf and Toon calculated a NH 3 atmospheric lifetime of 7 · 10 7 years for a solar incident flux at a 45 (cid:2) angle assuming s * 11 at 200 nm.”

“Our climate model was originally developed by Kasting and Ackerman (1986).”

“The solar zenith angles (SZAs) used in the climate and photo-chemical 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 (cid:2) in the photochemical model and SZA = 60 (cid:2) in the climate model.”

“…also has a long heritage of being used to study photochemistry in nonhazy atmospheres ( e.g. , Kasting and Donahue, 1980; Pavlov and Kasting, 2002; Ono et al. , 2003; Segura et al. , 2003, 2005, 2007, 2010; Zahnle et al. , 2006; Grenfell et al. , 2007; Catling et al. , 2010; Domagal-Goldman et…”

“…al. (1984a) to allow us to draw comparisons with previous works involving our suite of models (Pavlov et al. , 2001b; Domagal-Goldman et al. , 2008; Haqq-Misra et al. , 2008; Zerkle et al. , 2012; Kurzweil et al. , 2013; Claire et al. , 2014), as well as the Wolf and Toon (2010) study, which all…”

“D line-by-line fully multiple scattering radiative transfer model (Meadows and Crisp, 1996; Crisp, 1997).”

“Unlike the hazes that may exist around exoplanets with thick hydrogen-dominated atmospheres (Sing et al., 2011; Knutson et al., 2014; Kreidberg et al., 2014), the Archean (3.”

“Additional information about fractal particles and their geometry can be found in the works of, for example, Ko¨ylu¨ et al. (1995) and Brasil et al. (1999).”

“…Kasting and Donahue, 1980; Pavlov and Kasting, 2002; Ono et al. , 2003; Segura et al. , 2003, 2005, 2007, 2010; Zahnle et al. , 2006; Grenfell et al. , 2007; Catling et al. , 2010; Domagal-Goldman et al. , 2011, 2014; Rugheimer et al. , 2013, 2015; Harman et al. , 2015; Schwieterman et al. , 2016).”

“…rocks, mainly olivine and pyroxenes) can form methane (Kelley et al. , 2005; Etiope and Sherwood Lollar, 2013; Guzma´n-Marmolejo et al. , 2013), but the biotic flux of methane to the Archean atmosphere was likely much higher than the abiotic flux (Kharecha et al. , 2005), as it is on Earth today.”

“While the precise mechanism (or mechanisms) that produces S-MIF is unknown, photolysis of sulfur gases in an anoxic atmosphere is the only known mechanism that produces large-magnitude D 33 S and D 36 S seen in the rock record (Farquhar et al. , 2001, 2007).”

“Note that the version of the model used here can simulate atmospheres ranging from extremely anoxic (pO 2 = 10 - 14 ) to modern-day O 2 levels (Zahnle et al. , 2006).”

“The version we use here is based directly on that used by Kopparapu et al. (2013).”

“The maximum radius of a spherical haze particle ( i.e. , a haze ‘‘monomer’’) is set to 0.05 l m, the same nominal value used by Wolf and Toon (2010) and similar to the size of the monomers of Titan’s fractal haze aggregates (Rannou et al. , 1997; Tomasko et al. , 2008).”

“While the climatic effects of this haze have been studied ( e.g. , Pavlov et al. , 2001b), impacts of haze on Archean Earth’s habitability have not been previously investigated using tightly coupled climate-photochemical models.”

“…particle microphysics) is run first to generate an initial atmospheric state based on user-specified boundary conditions [gas mixing ratios or fluxes, the solar constant at 2.7 Ga (Claire et al. , 2012), the stellar spectral type, total atmospheric pressure, the initial temperature-pressure profile].”

“Those spectral studies that consider Archean Earth and anoxic planets have not examined hazes (Meadows, 2006; Kaltenegger et al. , 2007; Domagal-Goldman et al. , 2011).”

“Negative values of d (13)Corg below -37& are typically interpreted as evidence for methanogenesis (biological methane production) followed by subsequent incorporation into sediments by methanotrophy (methane consumption), which imparts a strongly negative d(13)Corg because organisms preferentially uptake the (12)C (Ur…”

“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.”

“…communities lived in inter-and su-pratidal zones (Allwood et al. , 2006; Noffke and Awramik, 2013) experiencing frequent, sometimes extended, exposure to the surface environment, and it has been suggested that microbial mats existed on land as early as 2.7–2.6 Ga (Watanabe et al. , 2000).”

“Additionally, although we anticipate planetary diversity in the exoplanet population, existing spectral studies are largely focused on the observables of modern-day Earth ( e.g. , Sagan et al. , 1993; Woolf et al. , 2002; Robinson et al. , 2011, 2014a).”

“Additionally, stratospheric and mesospheric circulation patterns on Earth presently impact high-altitude aerosol distributions by transporting particles preferentially to the poles (Bardeen et al. , 2008).”

“…pressure (pCO 2 ) in the Archean at 2.7 Ga to be be-tween 0.0036 and 0.018 bar [10–50 · the present atmospheric level (PAL)] (Driese et al. , 2011), while recent estimates of Archean surface pressure ( P surf ) are consistent with values as low as 0.5 bar (Som et al. , 2012; Marty et al. , 2013).”

“…Kasting and Donahue, 1980; Pavlov and Kasting, 2002; Ono et al. , 2003; Segura et al. , 2003, 2005, 2007, 2010; Zahnle et al. , 2006; Grenfell et al. , 2007; Catling et al. , 2010; Domagal-Goldman et al. , 2011, 2014; Rugheimer et al. , 2013, 2015; Harman et al. , 2015; Schwieterman et al. , 2016).”

“…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; Ho¨rst et al. , 2012; Trainer, 2013)—see also our discussion of haze formation pathways in Section 4.4.”

“…pressure (pCO 2 ) in the Archean at 2.7 Ga to be be-tween 0.0036 and 0.018 bar [10–50 · the present atmospheric level (PAL)] (Driese et al. , 2011), while recent estimates of Archean surface pressure ( P surf ) are consistent with values as low as 0.5 bar (Som et al. , 2012; Marty et al. , 2013).”

“Negative values of d (13)Corg below -37& are typically interpreted as evidence for methanogenesis (biological methane production) followed by subsequent incorporation into sediments by methanotrophy (methane consumption), which imparts a strongly negative d(13)Corg because organisms preferentially uptake the (12)C (Ur…”

“Additionally, although we anticipate planetary diversity in the exoplanet population, existing spectral studies are largely focused on the observables of modern-day Earth ( e.g. , Sagan et al. , 1993; Woolf et al. , 2002; Robinson et al. , 2011, 2014a).”

“…2015), supporting theoretical studies on the causes and consequences of photochemical haze formation in the atmosphere (Pavlov et al. , 2001a, 2001b; Domagal-Goldman et al. , 2008; Haqq-Misra et al. , 2008; Kurzweil et al. , 2013; Claire et al. , 2014) as well as experimental data (Trainer et al.…”

“…Kasting and Donahue, 1980; Pavlov and Kasting, 2002; Ono et al. , 2003; Segura et al. , 2003, 2005, 2007, 2010; Zahnle et al. , 2006; Grenfell et al. , 2007; Catling et al. , 2010; Domagal-Goldman et al. , 2011, 2014; Rugheimer et al. , 2013, 2015; Harman et al. , 2015; Schwieterman et al. , 2016).”

“Because of this, planets orbiting stars emitting a high proportion of radiation at NIR wavelengths are harder to freeze (Shields et al. , 2013).”

“Those spectral studies that consider Archean Earth and anoxic planets have not examined hazes (Meadows, 2006; Kaltenegger et al. , 2007; Domagal-Goldman et al. , 2011).”

“…of 1 and 5 Earth masses, respectively (Guzma´n-Marmolejo et al. , 2013), although there has been earlier speculation of higher abiotic production rates (Kasting, 2005; Shaw, 2008), especially if ancient seafloor spreading rates were faster or the amount of iron-rich ancient seafloor rock was…”

“Note that photo-synthetic organisms would not likely have been photon limited by the lower light levels under the haze: the lower light limit for red algae is 6 · 10 15 photons/m 2 /s (Littler et al. , 1986).”

“By using the updated habitability threshold of T surf > 248 K, all our cases even with thick hazes are habitable. be consistent with a reported glaciation event at 2.9 Ga (Young et al. , 1998) as a previous study has suggested (Domagal-Goldman et al. , 2008).”

“Furthermore, Abbot et al. (2011) argued that ocean open belts can remain climatologically stable, even if the ice latitude is reduced to 5–15 (cid:2) .”

“…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; Ho¨rst et al. , 2012; Trainer, 2013)—see also our discussion of haze formation pathways in Section 4.4.”

“On the other hand, if organisms remained protected by some other UV shield such as minerals, layers of overlying microbial mat, or water (Cockell, 1998), changes in UV radiation levels should not affect them as strongly, so the colder conditions created by the haze might have been the larger source…”

“…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-Goldman et al. , 2008; Haqq-Misra et al. , 2008; Wolf and Toon, 2010; Ha-senkopf et al. , 2011).”

“…) and toluene (C 6 H 5 CH 3 ), 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. , 2007; Lo´pez-Puertas et al. , 2013).”

“This model also has a long heritage of being used to study photochemistry in nonhazy atmospheres ( e.g. , Kasting and Donahue, 1980; Pavlov and Kasting, 2002; Ono et al. , 2003; Segura et al. , 2003, 2005, 2007, 2010; Zahnle et al. , 2006; Grenfell et al. , 2007; Catling et al. , 2010;…”

“SMART has been validated against observations of multiple solar system planets (Robinson et al. , 2011; Arney et al. , 2014).”

“Aerosol formation follows the method used in Kasting et al. (1989) and described and updated in Pavlov et al. (2001b).”

“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)—see also our discussion of ha…”

“…of being used to study photochemistry in nonhazy atmospheres ( e.g. , Kasting and Donahue, 1980; Pavlov and Kasting, 2002; Ono et al. , 2003; Segura et al. , 2003, 2005, 2007, 2010; Zahnle et al. , 2006; Grenfell et al. , 2007; Catling et al. , 2010; Domagal-Goldman et al. , 2011, 2014;…”

“Recent paleosol measurements have constrained the CO 2 partial pressure (pCO 2 ) in the Archean at 2.7 Ga to be be-tween 0.0036 and 0.018 bar [10–50 · the present atmospheric level (PAL)] (Driese et al. , 2011), while recent estimates of Archean surface pressure ( P surf ) are consistent with values as low as 0.5 bar…”

“There is evidence that Archean stromatolitic communities lived in inter-and su-pratidal zones (Allwood et al. , 2006; Noffke and Awramik, 2013) experiencing frequent, sometimes extended, exposure to the surface environment, and it has been suggested that microbial mats existed on land as early as…”

“D photochemical code developed originally by Kasting et al. (1979), but the version we use here was significantly modernized and updated by Zahnle et al. (2006) and uses the haze formation scheme described by Pavlov et al. (2001b).”

“Early Earth’s atmosphere would have contained negligible O 2 but significant amounts of CO 2 ( e.g. , Kasting, 1993; Driese et al. , 2011), whereas Titan’s atmosphere is extremely reducing (de Kok et al. , 2007).”

“However, as we have argued, a mean surface temperature of the freezing point of water is not a useful threshold for global habitability (Charnay et al. , 2013; Shields et al. , 2013; Wolf and Toon, 2013; Kunze et al. , 2014), so some of the Haqq-Misra et al. spherical haze results may actually be…”

“…climate of early Mars (Ramirez et al. , 2013), the atmospheres of Earth-like planets around various stellar types (Segura et al. , 2003, 2005, 2010; Rugheimer et al. , 2013), clouds in exoplanet THE PALE ORANGE DOT atmospheres (Kitzmann et al. , 2010, 2011a), and the climate of early Earth…”

“D studies have shown that Archean Earth can maintain an open ocean fraction of > 50% for T GAT ‡ 260 K and an equatorial open ocean belt for T GAT ‡ 248 K (Wolf and Toon, 2013; Charnay et al. , 2013).”

“The planet colors were calculated using the ‘‘Spectral Color Spreadsheet’’ from brucelindbloom.com with the same method used by Charnay et al. (2015) for GJ 1214b.”

“Refractive indices, shown in the bottom panels, are presented from information in Khare et al. (1984a). 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.”

“28 lm) wavelength range (Pierson et al., 1992; Dillon and Castenholz, 1999), but in the modern atmosphere, UVC is fully blocked by O2 and ozone.”

“…Kasting and Donahue, 1980; Pavlov and Kasting, 2002; Ono et al. , 2003; Segura et al. , 2003, 2005, 2007, 2010; Zahnle et al. , 2006; Grenfell et al. , 2007; Catling et al. , 2010; Domagal-Goldman et al. , 2011, 2014; Rugheimer et al. , 2013, 2015; Harman et al. , 2015; Schwieterman et al. , 2016).”

“This model also has a long heritage of being used to study photochemistry in nonhazy atmospheres (e.g., Kasting and Donahue, 1980; Pavlov and Kasting, 2002; Ono et al., 2003; Segura et al., 2003, 2005, 2007, 2010; Zahnle et al., 2006; Grenfell et al., 2007; Catling et al., 2010; Domagal-Goldman et al., 2011, 2014; Rug…”

“In Earth-like clear-sky atmospheres, the minimum atmospheric altitude transit observations are able to probe will typically be limited by refraction (Garcı´a Mun˜oz et al. , 2012; Be´tre´mieux and Kaltenegger, 2014; Misra et al. , 2014b), but in hazy atmospheres, haze controls the minimum effective…”

“…the minimum atmospheric altitude transit observations are able to probe will typically be limited by refraction (Garcı´a Mun˜oz et al. , 2012; Be´tre´mieux and Kaltenegger, 2014; Misra et al. , 2014b), but in hazy atmospheres, haze controls the minimum effective tangent height, especially…”

“Additionally, although we anticipate planetary diversity in the exoplanet population, existing spectral studies are largely focused on the observables of modern-day Earth ( e.g. , Sagan et al. , 1993; Woolf et al. , 2002; Robinson et al. , 2011, 2014a).”

“…of 1 and 5 Earth masses, respectively (Guzma´n-Marmolejo et al. , 2013), although there has been earlier speculation of higher abiotic production rates (Kasting, 2005; Shaw, 2008), especially if ancient seafloor spreading rates were faster or the amount of iron-rich ancient seafloor rock was greater.”

“…ranges indicate disagreement with the Khare measurements (Ramirez et al. , 2002; Tran et al. , 2003; Vuitton et al. , 2009; Hasenkopf et al. , 2010; Imanaka et al. , 2012; Mahjoub et al. , 2012; Sciamma-O’Brien et al. , 2012), although these measurements themselves show considerable variation…”

“D line-by-line fully multiple scattering radiative transfer model (Meadows and Crisp, 1996; Crisp, 1997).”

“…Kasting and Donahue, 1980; Pavlov and Kasting, 2002; Ono et al. , 2003; Segura et al. , 2003, 2005, 2007, 2010; Zahnle et al. , 2006; Grenfell et al. , 2007; Catling et al. , 2010; Domagal-Goldman et al. , 2011, 2014; Rugheimer et al. , 2013, 2015; Harman et al. , 2015; Schwieterman et al. , 2016).”

“…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 (Domagal-Goldman et al. , 2008; Zerkle et al. , 2012; Kurzweil et al. , 2013; Claire et al. , 2014; Izon et al. , 2015).”

“Additionally, although we anticipate planetary diversity in the exoplanet population, existing spectral studies are largely focused on the observables of modern-day Earth ( e.g. , Sagan et al. , 1993; Woolf et al. , 2002; Robinson et al. , 2011, 2014a).”

“…more recent measurements of hydrocarbon refractive indices over more restricted wavelength ranges indicate disagreement with the Khare measurements (Ramirez et al. , 2002; Tran et al. , 2003; Vuitton et al. , 2009; Hasenkopf et al. , 2010; Imanaka et al. , 2012; Mahjoub et al. , 2012;…”

“Kurzweil et al. (2013) noted that an increase in magnitude of S-MIF signals after 2.73 Ga (Thomazo et al. , 2009) oc-curred during a prolonged negative shift in d 13 C org , suggesting enhanced biological methane activity at this time.”

“The mean field approximation we use for fractal scattering has been validated against scattering by silica fractal aggregates (Botet et al. , 1997) and Titan’s hazes (Rannou et al. , 1997; Larson et al. , 2015).”

“Following the mechanism proposed for the formation of Titan’s hydrocarbon haze (Allen et al. , 1980; Yung et al. , 1984), every model of hydrocarbon haze formation in early Earth’s atmosphere—including ours—has assumed that aerosol formation will occur through the formation of acetylene (C 2 H 2 )…”

“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.”

“…primary nonbiological source, has been estimated as 6.8 · 10 8 and 1.3 · 10 9 molecules/cm 2 /s for rocky planets of 1 and 5 Earth masses, respectively (Guzma´n-Marmolejo et al. , 2013), although there has been earlier speculation of higher abiotic production rates (Kasting, 2005; Shaw, 2008),…”

“The mean field approximation we use for fractal scattering has been validated against scattering by silica fractal aggregates (Botet et al. , 1997) and Titan’s hazes (Rannou et al. , 1997; Larson et al. , 2015).”

“…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…”

“We compare these values to the tolerances of Chloroflexus aurantiacus (Pierson et al. , 1992), a deep-branching, mat-forming anoxygenic phototroph with UV resistance that has been studied as an analogue for Archean phototrophs.”

“…(Ramirez et al. , 2013), the atmospheres of Earth-like planets around various stellar types (Segura et al. , 2003, 2005, 2010; Rugheimer et al. , 2013), clouds in exoplanet THE PALE ORANGE DOT atmospheres (Kitzmann et al. , 2010, 2011a), and the climate of early Earth (Haqq-Misra et al. , 2008).”

“…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…”

“To generate transit transmission spectra, we use the SMART-T model (Misra et al. , 2014a, 2014b).”

“…transit observations are able to probe will typically be limited by refraction (Garcı´a Mun˜oz et al. , 2012; Be´tre´mieux and Kaltenegger, 2014; Misra et al. , 2014b), but in hazy atmospheres, haze controls the minimum effective tangent height, especially at shorter wavelengths where it…”

“Absorption from the haze itself can be seen as the ‘‘bump’’ in the ‘‘thick’’ haze (CH 4 /CO 2 = 0.21) transit transmission spectrum at 6 l m, a wavelength region accessible with the James Webb Space Telescope (Wright et al. , 2004).”

“…, 2001a, 2001b; Domagal-Goldman et al. , 2008; Haqq-Misra et al. , 2008; Kurzweil et al. , 2013; Claire et al. , 2014) as well as experimental data (Trainer et al. , 2004, 2006; DeWitt et al. , 2009; Hasenkopf et al. , 2010, 2011) and theory on their potential radiative effects (Wolf and Toon,…”

“…with the Khare measurements (Ramirez et al. , 2002; Tran et al. , 2003; Vuitton et al. , 2009; Hasenkopf et al. , 2010; Imanaka et al. , 2012; Mahjoub et al. , 2012; Sciamma-O’Brien et al. , 2012), although these measurements themselves show considerable variation among each other (Fig.”

“…of hydrocarbon refractive indices over more restricted wavelength ranges indicate disagreement with the Khare measurements (Ramirez et al. , 2002; Tran et al. , 2003; Vuitton et al. , 2009; Hasenkopf et al. , 2010; Imanaka et al. , 2012; Mahjoub et al. , 2012; Sciamma-O’Brien et al. , 2012),…”

“Studies have shown the formation of HCN, CH 3 CN, and other nitrile gas species are formed using the same type of UV source in a CH 4 /N 2 gas mixture, thus corroborating the indirect nitrogen photochemistry (Trainer et al. , 2012; Yoon et al. , 2014).”

“…have argued, a mean surface temperature of the freezing point of water is not a useful threshold for global habitability (Charnay et al. , 2013; Shields et al. , 2013; Wolf and Toon, 2013; Kunze et al. , 2014), so some of the Haqq-Misra et al. spherical haze results may actually be ‘‘habitable.’’”

“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).”

“8 billion years of Earth’s formation, a common mechanism for S-MIF production was already established in the atmosphere (Thomassot et al., 2015).”

“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)—see also our discussion of ha…”

“Ho¨rst and Tolbert (2013) measured a similar effective particle density, 0.65 g/ cm 3 , for a 0.1% CH 4 haze experiment using a UV lamp.”

“…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; Ho¨rst et al. , 2012; Trainer, 2013)—see also our discussion of haze formation pathways in Section 4.4.”

“Studies have shown the formation of HCN, CH3CN, and other nitrile gas species are formed using the same type of UV source in a CH4/N2 gas mixture, thus corroborating the indirect nitrogen photochemistry (Trainer et al., 2012; Yoon et al., 2014).”

“…(Ramirez et al. , 2013), the atmospheres of Earth-like planets around various stellar types (Segura et al. , 2003, 2005, 2010; Rugheimer et al. , 2013), clouds in exoplanet THE PALE ORANGE DOT atmospheres (Kitzmann et al. , 2010, 2011a), and the climate of early Earth (Haqq-Misra et al. , 2008).”

“Our SZA = 60 (cid:2) flux, 0.03 W/m 2 , is low enough to allow growth of Chloroflexus aurantiacus over the length of a day in the late Archean [about 18–19 h for a day-night cycle; Denis et al. (2002)].”

“Subsequent studies using this model or other versions of it to study fractal haze formation include those of Harman et al. (2013), Kurzweil et al. (2013), and Claire et al. (2014), with the latter two of these studies also derived from the same Zahnle et al. (2006) model branch used here.”

“, 2015) that strongly interacted with visible and UV radiation, cooling the planet’s climate (Pavlov et al., 2001b; Domagal-Goldman et al., 2008; Haqq-Misra et al., 2008; Wolf and Toon, 2010; Hasenkopf et al., 2011).”

“The mean field approximation we use for fractal scattering has been validated against scattering by silica fractal aggregates (Botet et al. , 1997) and Titan’s hazes (Rannou et al. , 1997; Larson et al. , 2015).”

“Colors and photometric bands have been considered as indicators of Earth-like worlds (Traub, 2003; Crow et al. , 2011; Krissansen-Totton et al. , 2016), but hazy Archean Earth suggests that not all Earth-like planets will be pale blue dots.”

“Those spectral studies that consider Archean Earth and anoxic planets have not examined hazes (Meadows, 2006; Kaltenegger et al. , 2007; Domagal-Goldman et al. , 2011).”

“…indices over more restricted wavelength ranges indicate disagreement with the Khare measurements (Ramirez et al. , 2002; Tran et al. , 2003; Vuitton et al. , 2009; Hasenkopf et al. , 2010; Imanaka et al. , 2012; Mahjoub et al. , 2012; Sciamma-O’Brien et al. , 2012), although these…”

“Our older CH 4 coefficients may overestimate the surface temperature by ( 5 K at the CH 4 mixing ratios used here (Byrne and Goldblatt, 2015).”

“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).”

“Since the full chemical scheme that leads to aerosol formation is not well understood despite both laboratory and theoretical studies (e.g., Hallquist et al., 2009; Hicks et al., 2015), it is assumed that C4H2 and C5H4 condense directly to haze particles (called HCAER and HCAER2 in Supplementary Table S1).”

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

“…al., 2013), a purple Archean ocean due to photosynthetic purple bacteria (Sanromá et al., 2014); an orange Archean Earth due to an organic-rich atmospheric haze (Arney et al., 2016); and trajectories of O2 fluxes over geologic time given estimated atmospheric concentrations (Gebauer et al., 2017).”

“Such hazes may have been present during early periods of Earth’s history when the atmosphere was more reducing (Arney et al., 2016).”