ABIOTIC OXYGEN-DOMINATED ATMOSPHERES ON TERRESTRIAL HABITABLE ZONE PLANETS

“Extensive atmospheric O2 buildup due to H2O photolysis followed by H escape may also occur on planets that enter a runaway greenhouse state (Ingersoll 1969; Kasting 1988; Leconte et al. 2013).”

“Our present work could also strengthen the results of Wordsworth and Pierrehumbert (2014) and Tian et al. (2014), which indicate that oxygen is not a reliable biosignature; in fact, planets with such elevated quantities of O 2 may be uninhabitable.”

“…from robust photolysis of CO2 or a history of extreme hydrogen escape and O2 buildup (e.g., DomagalGoldman and Meadows, 2010; Hu et al., 2012; DomagalGoldman et al., 2014; Tian et al., 2014; Wordsworth and Pierrehumbert, 2014; Gao et al., 2015; Harman et al., 2015; Luger and Barnes, 2015).”

“A low inventory of non-condensing gases such as N2 in a planet’s atmosphere could prevent cold trapping of H2O in the upper atmosphere, increasing the likelihood of abiotic O2-rich worlds (Wordsworth & Pierrehumbert, 2014).”

“Because the water loss is a strong function of the planet’s atmospheric composition, this potential abiotic O2 generation mechanism can affect planets orbiting any type of star (Wordsworth and Pierrehumbert, 2014).”

“The atmospheres of the terrestrial exoplanets we study around cool stars will undoubtedly be even more diverse than those in the solar system (Meadows & Seager 2010; Wordsworth & Pierrehumbert 2014).”

“Hypothetical planets that lack noncondensable N2 (if that is cosmochemically plausible) and build up O2 from water vapor destruction and loss (Wordsworth and Pierrehumbert, 2014) would lack the N4 dimer signatures and may have strong O4.”

“This processes can even lead to the build up of abiotic O2-dominated atmospheres (Wordsworth and Pierrehumbert 2014).”

“…that the variation in the noncondensable gas fraction in the atmosphere (specifically the N2 amount) can affect the OLR and consequently the habitability limits through moist and runaway greenhouse (Nakajima et al. 1992; Pierrehumbert 2010; Kopparapu et al. 2014; Wordsworth & Pierrehumbert 2014).”

“Conversely, for Earth’s current surface temperature, a reduction in our current N2 partial pressure by a factor of*10 would be enough to move a planet into the moist upper atmosphere regime, where it would be susceptible to water loss and abiotic O2 buildup (Wordsworth and Pierrehumbert, 2014).”

“…subject of O 2 false positives has been considered with growing interest (Schindler and Kasting, 2000; Selsis et al. , 2002; Le´ger et al. , 2011; Hu et al. , 2012; Domagal-Goldman et al. , 2014; Tian et al. , 2014; Wordsworth and Pierrehumbert, 2014; Harman et al. , 2015; Luger and Barnes, 2015).”

“At this point, the planet would have been safe from further H2O loss if atmospheric water vapor were cold trapped in the troposphere by a sufficient inventory of noncondensable gases, such as N2 (Wordsworth and Pierrehumbert, 2014).”

“This keeps us out of the parameter space in which the background pressure is low and water loss is rapid (Wordsworth & Pierrehumbert 2014).”

“Wordsworth & Pierrehumbert (2014) showed that water-rich planets with low inventories of non-condensible gases like N2 may also abiotically oxygenate.”

“Wordsworth and Pierrehumbert (2014) investigated oxygen and ozone formation in CO2-rich low-pressure atmospheres.”

“…some habitable zone planets by photolysis and H evaporation (e.g. Bolmont et al. 2017), perhaps leading to abiotic oxygen-dominated atmospheres (Wordsworth & Pierrehumbert 2014; Luger & Barnes 2015), while in other planets the evaporation might prevent the atmosphere of an out-gassing planet…”

“Several recent studies extend this type of modeling to atmospheric loss on habitable zone exoplanets with H2O-rich atmospheres (Wordsworth et al. 2013; Ramirez & Kaltenegger 2014; Wordsworth & Pierrehumbert 2014; Luger & Barnes 2015; Tian & Ida 2015).”

“Recent research shows possible false positives may occur under certain geological settings (Domagal-Goldman et al. 2014; Tian et al. 2014; Wordsworth & Pierrehumbert 2014).”

“…H-escape and O2-build up on planets in the HZ during the pre-main sequence, especially for planets orbiting the latest type stars (Luger & Barnes 2015), and 3) abiotic O2 buildup due to H-escape from N2-poor atmospheres that lack tropospheric H2O cold traps (Wordsworth & Pierrehumbert 2014)1.”

“…can produce and destroy potential biosignatures (O2, O3, and CH4) and habitability indicators (H2O and CO2) in exoplanet atmospheres (Hu et al. 2012; Domagal-Goldman et al. 2014; Tian et al. 2014; Wordsworth & Pierrehumbert 2014; Gao et al. 2015; Harman et al. 2015; Luger & Barnes 2015).”

“A number of ‘pathological’ highO2 scenarios are now known, including those that involve: (1) hydrogen escape from atmospheres depleted in non-condensing gases (Wordsworth and Pierrehumbert, 2014), (2) CO2 photolysis in very dry atmospheres (Gao et al.”

“If the planet is able to retain its background atmosphere, the tropopause is expected to act as an efficient cold trap, preventing water from reaching the higher parts of the atmosphere (e.g., Wordsworth & Pierrehumbert 2014; Turbet et al. 2016).”

“010; Seager et al. 2012; Zahnle et al. 2013). Nonetheless, it has recently been shown that abiotic processes may lead to buildup of O 2 -dominated atmospheres on planets that lack life in some cases (Wordsworth and Pierrehumbert 2014; Luger and Barnes 2015; Schaefer et al. 2016). These cases constitute ‘false positive…”

“A planet with negligible N2 and Ar would enter the moist greenhouse limit even if it were in a Snowball state (Wordsworth and Pierrehumbert 2014).”

“One example would be the possible detection of O2, which on Earth has biological origins but on other worlds can be produced abiologically through the photodissociation of water vapour and escape of hydrogen (e.g. Wordsworth & Pierrehumbert 2014; Bolmont et al. 2017).”

“Furthermore, planets with very low atmospheric N2 will no longer cold trap H2O and hence will irreversibly oxidize via hydrogen loss, which can lead to the buildup of abiotic O2-dominated atmospheres in extreme cases (Wordsworth and Pierrehumbert, 2014).”

“…are a variety of ways to produce abiotic molecular oxygen from the photolysis of water and CO2 by strong UV flux (Gao et al. 2013; Tian et al. 2014; Wordsworth & Pierrehumbert 2014; Luger & Barnes 2015; Harman et al. 2015, 2018) on terrestrial exoplanets, especially on planets around M-dwarfs with…”

“…through photolysis in CO2-rich planets orbiting M dwarfs (Hu et al. 2012; Tian et al. 2014), around FGKM stars with low-H2/high-CO2 atmospheres (Domagal-Goldman et al. 2014), around pre-mainsequence M stars (Luger & Barnes 2015), and in low pressure atmospheres (Wordsworth & Pierrehumbert 2014).”

“ther way to potentially produce abiotic oxygen is via enhanced water vapor photolysis in the upper atmosphere (i.e., the cold trap), leading to the preferential escape of lighter H molecules to space [24]. This concept is essentially analogous to the “moist greenhouse” of Kasting [37] except that the moist greenhouse…”

“This reduces (but does not completely obviate; Wordsworth & Pierrehumbert 2014) the potential ambiguities regarding O2 as a biosignature for planets orbiting Sun-like stars.”

“It is telling that a range of falsepositive and false-negative scenarios for habitability and life have already been identified (e.g., Domagal-Goldman et al. 2014; Wordsworth & Pierrehumbert 2014; Reinhard et al. 2017).”

“On a completely abiotic planet orbiting in the habitable zone of a Sun-like star, O 2 can build up abiotically, but probably only under the condition that non-condensible gases (including N 2 ) are present in low amounts (Wordsworth and Pierrehumbert, 2014).”

“ses, but collapse of other volatiles on a planet’s nightside, particularly H 2 O [e.g., Leconte et al. (2013); Menou (2013)], may be signicant and linked to the overall oxidation rate of the planet (Wordsworth and Pierrehumbert 2014). Future work needs to focus on the eciency of delivery/loss mechanisms for highly vol…”

“Wordsworth & Pierrehumbert (2013, 2014) argued that the degree to which a condensible gas (e.g., water) is transported from the surface is represented by = m p L m p c Tv v n n p where L, cp, and T are the specific latent heat of the condensing gas, the specific heat capacity at constant pressure…”

“shown that O2 may also build up abiotically in an exoplanet's atmosphere (Domagal-Goldman et al., 2014; Gao et al., 2015; Harman et al., 2015; Luger and Barnes, 2015; Tian et al., 2014; Wordsworth and Pierrehumbert, 2014).”

“Critically, all of them depend on eliminating the reducing gases that would otherwise consume oxygen (Hu et al., 2012; Tian et al., 2014; Wordsworth and Pierrehumbert, 2014; Gao et al., 2015; Harman et al., 2015; Luger and Barnes, 2015; Narita et al., 2015).”

“Complete water loss would take a few hundred million years (Watson et al. 1981), but may have been throttled by oxygen accumulation (Wordsworth & Pierrehumbert 2014).”

“However, these low N2 concentrations would likely result in abiotic accumulation of O2 from water loss (Wordsworth & Pierrehumbert 2014)—a redox-based false positive.”

“Wordsworth & Pierrehumbert (2014) point out that H2O-rich atmospheres without significant amounts of non-condensing gas, such as N2, are particularly vulnerable to this process, as the non-condensing gas creates a cold trap for H2 that aids retention.”

“Such thin, pure water-vapor atmospheres have been studied in connection with loss of water to space and formation of abiotic oxygen signatures (Wordsworth & Pierrehumbert 2014).”

“For instance, diatomic oxygen can, under some circumstances, also be produced abiotically (16).”

“Wordsworth and Pierrehumbert (2014) and Luger and Barnes (2015) outline scenarios whereby water loss on otherwise habitable planets could result in the accumulation of atmospheric oxygen.”

“…scenarios where nonliving processes might mimic life by producing oxygen (i.e., false positives; Domagal-Goldman et al. 2014; Tian et al. 2014; Wordsworth & Pierrehumbert 2014; Harman et al. 2015; Luger & Barnes 2015; Schwieterman et al. 2019), the circumstances are unusual and contextual…”

“Several of those mechanisms would be present on a young planet (e.g. Hu et al. 2012; Domagal-Goldman et al. 2014; Tian et al. 2014; Rameriz and Kaltenegger 2014; Wordsworth and Pierrehumbert 2014; Luger and Barns 2015; Harman et al. 2015; Gao et al. 2015; or see recent review by Meadows et al. 2018).”

“…research has identified several possible false positive mechanisms for abiotic production of oxygen on exoplanets, especially those orbiting M dwarf stars (Domagal-Goldman et al., 2014; Tian et al., 2014; Wordsworth & Pierrehumbert, 2014; Harman et al., 2015; Luger & Barnes, 2015; Hu et al., 2020).”

“…rapid water loss if atmospheric CO2 is sequestered in carbonates and the N2 background is eroded by nonthermal processes; the resulting H2O-dominated atmosphere will have a weak cold trap, potentially leading to high H escape rates (Wordsworth & Pierrehumbert 2014; Kleinböhl et al. 2018).”

“However, detection of these gases is not free of false-positive scenarios [12, 13, 14, 15].”

“This is similar to the mechanism that can lead to large amounts of oxygen in exoplanets without a cold trap (Wordsworth and Pierrehumbert, 2014).”

“(Planets with Sun-like host stars can also undergo this process, although in this case its effectiveness is likely more dependent on the composition of the planet’s atmosphere; Catling et al. 2001; Wordsworth & Pierrehumbert 2013, 2014.)”

“Detection of neither of these gases is, however, free of false-positive scenarios [26 – 29].”

“False-positive scenarios for Sunlike star planets with an Earth-like inventory of noncondensing gases have not yet been identified (Wordsworth & Pierrehumbert 2014; Harman et al. 2018; Meadows et al. 2018b); therefore, we expect that we will be able to interpret O2 and O3 detections confidently…”

“…as the abundances of key biosignatures (O2, O3, and CH4) and habitability indicators (CO2 and H2O) in exoplanet atmospheres around M dwarfs (Hu et al. 2012; Domagal-Goldman et al. 2014; Tian et al. 2013; Wordsworth & Pierrehumbert 2014; Gao et al. 2015; Harman et al. 2015; Luger & Barnes 2015).”

“We note here that there are possible biotic and abiotic sources for CH4, CO2, and even O2, which can be produced at low concentrations by photochemistry (see, e.g., Kasting & Catling 2003; Wordsworth & Pierrehumbert 2014).”

“Lifeless, terrestrial planets in the habitable zone of any type of star can develop oxygendominated atmospheres through photolysis of H2O (e.g., Wordsworth & Pierrehumbert 2014) or CO2 (e.g., Harman et al. 2015).”

“…can cause water vapor to be nondilute even at temperatures comparable to the present Earth; as suchatmospheres are prone to losing water to space and generating abiotic oxygen (Wordsworth & Pierrehumbert 2014), the dynamical features that could affect water loss are of considerable interest.”

“…<<1 % modern pO2 is difficult to reconcile with photochemical models that predict small fluctuations in O2 fluxes around this low baseline should result in either runaway oxygenation to ~10% modern or collapse to preGOE levels (Claire et al 2006; Goldblatt et al 2006; Zahnle et al 2006).”

“Accumulation of oxygen, however, may throttle hydrogen escape [Wordsworth and Pierrehumbert, 2014], and extensive hydration of the surface rocks is another potential sink for water [Matsui and Abe,”

“Wordsworth & Pierrehumbert (2014) showed that in a moist convective environment the temperature structure and hence the amount of water vapor that reaches the upper atmosphere depends largely on the amount of non-condensable gases in the atmosphere.”

“However, in theory, these oxidized minerals could form on lifeless terrestrial worlds whose near-surface environments have been oxidized via photochemical processes and atmospheric escape of hydrogen [35 – 37]; thus, their formation may not unambiguously reflect the presence of a biosphere.”

“Water vapor can be split either in the upper atmosphere due to UV-driven photolysis (e.g., Wordsworth & Pierrehumbert 2014; Luger & Barnes 2015; Meadows et al. 2018b; Wordsworth et al. 2018) 1 In some samples none of the planets have O2, so the p-value is undefined and we discard it.”

“α β β − + − I I ExP (1 10 ) /( ) (1 10 ) (8) i i i i i i NIR vis ( ) NIR”

“Discriminating false positives, where an abiotically driven signal could mimic a true biosignature, is especially crucial for interpreting chemical disequilibrium signatures considering false positives for O 2 been heavily studied [42, 43, 44, 45, 46].”

“…exoplanets is still beyond observational capabilities (Hedelt et al. 2013) and the interpretation of atmospheric features in terms of chemical disequilibrium driven by biological activity is uncertain (Segura et al. 2007; Wordsworth & Pierrehumbert 2014; Harman et al. 2015; Narita et al. 2015).”

“The loss of H atoms via hydrogen escape (explored in Paper III) will modify the redox chemistry of the atmosphere (e.g., Kasting, 1993; Wordsworth & Pierrehumbert, 2014), while drawdown processes which preferentially remove certain species (e.g., the carbonate-silicate cycle removing CO2) will…”

“This limits oxygen accumulation by providing a non-condensable background gas to throttle hydrogen escape at the cold trap (Kleinböhl et al., 2018; Wordsworth & Pierrehumbert, 2014).”

“Theabiotic O 2 -forming scenarios mentioned above all need either atmospheres initially enriched in water vapor to be photodissociated (Wordsworth and Pierrehumbert, 2014; Luger and Barnes, 2015) or sufficient CO 2 and small atmospheric hydrogen fractions to effectively maintain the…”

“…into a variety of scenarios, some with weak atmosphere or surface sinks for O2 such that any abiotic production can enable O2 to accumulate (Selsis et al. 2002; Léger et al. 2011; Hu et al. 2012; Domagal-Goldman et al. 2014; Wordsworth & Pierrehumbert 2014; Harman et al. 2015; Luger & Barnes 2015).”