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).”

“…al. 2018; Fleming et al. 2020); the lighter hydrogen is lost to space, while the heavier oxygen remains behind, either reacting with the surface or accumulating in the atmosphere and creating a biosignature false positive (see, e.g., Wordsworth & Pierrehumbert 2013, 2014; Wordsworth et al. 2018).”

“g studies suggests that ozone can be formed from abiotically produced oxygen (e.g., Selsis et al. 2002, Segura et al. 2007, Domagal-Goldman and Meadows 2010; DomagalGoldman et al. 2014). In addition, Wordsworth and Pierrehumbert (2014) propose a (non-chemical) abiotic oxygen formation process based on hydrogen escape…”

“While several authors [7,11,42] have argued that the presence of atmospheric oxygen in various forms may not by itself be a robust biosignature, this question remains an important topic of debate.”

“…during the early phase of their evolution through interaction with the most abundant molecular species, but its long-term effect on the habitability of planets is complex and remains an open question (Wordsworth & Pierrehumbert 2014; Luger & Barnes 2015; Bolmont et al. 2016; Owen & Mohanty 2016).”

“Moreover, the background nitrogen pressure is not only important for maintaining habitability on a planet (Goldblatt et al. 2009; Wordsworth and Pierrehumbert 2014), but atmospheric nitrogen pressure may reflect its speciation in the mantle and can directly influence the amount of oxygen released…”

“…atmospheres themselves are of potential interest for very water-rich planets, but it is unclear if such atmospheres can exist except as transients because the absence of cold-trapping generally leads to hydrogen escape and buildup of a noncondensible O2 atmosphere (Wordsworth & Pierrehumbert 2014).”

“This view, however, has been challenged by the argument that abiotic water photolysis on a habitable-zone planet can also produce an oxygendominated atmosphere and thus may result in a false positive for life (Wordsworth and Pierrehumbert, 2014).”

“On Venus, it has been proposed that oxygen liberated by H2O photolysis and water loss (described in Section 3) could have been absorbed by the mantle, increasing its oxygen fugacity and releasing N2 to the atmosphere (Wordsworth and Pierrehumbert 2014).”

“But confirming their biogenic origin will require ruling out their possible production via abiotic processes such as geology or atmospheric chemistry (Domagal-Goldman et al., 2014; Tian et al., 2014; Wordsworth and Pierrehumbert, 2014; Harman et al., 2015).”

“…biosignatures, although recently that idea has been called into question; it has become more apparent that oxygen present in a planetary atmosphere may not necessarily indicate any sort of biogenic process (Domagal-Goldman et al. 2014; Wordsworth & Pierrehumbert 2014; Luger & Barnes 2015).”

“…ozone in a terrestrial planet’s atmosphere through photolysis of H2O or CO2, for planets e.g. with low-H2/high-CO2 atmospheres (Domagal-Goldman et al. 2014), with lowpressure atmospheres (Wordsworth & Pierrehumbert 2014) or CO2-rich atmospheres orbiting M dwarfs (Hu et al. 2012; Tian et al. 2014).”

“Most known abiotic production mechanisms produce a very high atmospheric O/H ratio, which leads to a lack of water vapor (Gao et al. (2015)) and clouds (Wordsworth & Pierrehumbert (2014)), and so can be ruled out if atmospheric water content is accurately measured.”

“For the gentler water loss mechanism outlined by Wordsworth and Pierrehumbert (2014), measuring the amount of N 2 directly is very difficult, but much like O 2 there exists a spectrally active dimer (N 2 -”

“Such a process also leads to build up of abiotic oxygen, which has been argued to suggest uninhabitable conditions [39].”

“ting H atoms can escape to space. The extent to which this occurs depends on the strength of the tropopause cold trap, which depends on the partial pressure of non-condensing gases in the atmosphere (Wordsworth and Pierrehumbert, 2014; Kleinböhl et al., 2018). Thus diffusion-limited escape of H can be tied to troposph…”

“For this study, we considered only low noncondensable oxygen false positives (Wordsworth and Pierrehumbert, 2014), waterworld oxygen false positives (Krissansen-Totton et al., 2021), photochemical oxygen false positives (Gao et al., 2015), methane degassing false positives (Wogan et al., 2020), and…”

“…limits on the oxygen abundance would imply, suggesting that there are unknown mechanisms for enriching the ratio of O 3 : O 2 in planetary atmospheres ( 11 , 12 ); (iii) oxygenated atmospheres may be produced on planets as a result of passing through the runaway greenhouse transition ( 13 , 14 ).”

“A number of models suggest that high XUV fluxes play a vital role in the buildup of massive O2 atmospheres by way of desiccation and photolysis (Kasting 1988; Wordsworth & Pierrehumbert 2014; Ramirez & Kaltenegger 2014; Luger & Barnes 2015; Bolmont et al. 2017; Palubski et al. 2020).”

“However, due to the limitations of atmospheric characterization for such distant systems, any detected biosignatures could also result from abiotic processes [1] and Fig.”

“…— most prevalent among them O 2 — have come under more scrutiny due to the risks for false positives ( S. D. Domagal-Goldman et al. 2014; R. Wordsworth & R. Pierrehumbert 2014; C. E. Harman & S. Domagal-Goldman 2018; V. S. Meadows et al. 2018 ) and false negatives ( C. T. Reinhard et al.…”

“…chemistry (Herbort et al., 2020; Schaefer et al., 2012; Yu et al., 2021); atmospheric escape (Dong et al., 2018; Kislyakova et al., 2015; Kite & Barnett, 2020; Tian, 2015; Wordsworth & Pierrehumbert, 2014); or volcanism (Gaillard & Scaillet, 2014; Kadoya & Tajika, 2015; Liggins et al., 2020;…”

“The nitrogen content in the atmosphere plays a significant role in the rate of water–loss from a planet (e.g. Wordsworth & Pierrehumbert 2013, 2014).”

“In the case of an N2-dominated atmosphere, it has been shown that if the partial pressure of N2 is too small, water vapor cannot be trapped in the troposphere and the upper part of the atmosphere would be wet (Wordsworth & Pierrehumbert 2014).”

“…greenhouse effect after planet formation due to the superluminous pre-main sequence phase of M-dwarfs (Luger & Barnes 2015), or in atmospheres with low amounts of noncondensing gases without a cold trap (Wordsworth & Pierrehumbert 2014; see also Schwieterman et al. 2018 for a recent review).”

“Some calculations with climate and photochemistry models suggest abiotic sources for O2 and O3 (Segura et al., 2007; Domagal-Goldman et al., 2014; Wordsworth and Pierrehumbert, 2014), although such a ‘false positive’ spectral signature might be discernable from inhabited planets by the detection of species such as CO…”

“The possibility of this configuration is strengthened by the possibility of abiotic oxygen-dominated atmospheres on terrestrial HZ planets (Wordsworth & Pierrehumbert 2014).”

“Our study suggests that the pN2 of extrasolar rocky planets in habitable zones may provide useful constraints not only on present climate (Vladilo et al., 2013; Wordsworth & Pierrehumbert, 2014) but also on the biogeochemical processes that have acted on such planets.”

“Atmospheric pressure is included in the decision tree to rule out a low pressure atmosphere false positive for O 2 that can leave interpretation of O 2 in absence of a detection of CH 4 ambiguous for modern Earth (Wordsworth & Pierrehumbert 2014).”

“Additionally, planets with global surface oceans may accumulate abiotic O 2 with no geologic sinks, resulting in a false positive biosigna-ture (Wordsworth and Pierrehumbert 2014; Kleinböhl et al. 2018; Krissansen-Totton et al. 2021b).”

“But these signatures are ambiguous – does an O 2 detection mean life, or just photodissociation of water [Wordsworth and Pierrehumbert, 2014]?”

“…interact with oxygen at all but that either affect the climate (and therefore the abundance of oxygen-containing condensable molecules, like water, in the atmosphere) (Wordsworth and Pierrehumbert, 2014) or that affect the transport of ultraviolet light through the atmosphere (Rimmer et al., 2019).”

“Higher concentrations of ozone due to either biological or abiotic (Wordsworth & Pierrehumbert 2014; Luger & Barnes 2015) oxygen production can also enhance the shortwave absorption from Sun-like stars.”

“The former is not a good choice because abiotic oxygen might arise as result of photodissociation of water molecules (Wordsworth and Pierrehumbert 2014); the latter is not a good choice because the controversial discovery of methane on Mars is still so difficult to understand that a new paper on…”

“Ocean salinity can also be important in modulating the climate of rocky planets (Olson et al., 2022).”

“We can also consider other gas disequilibria as an indicator for life: CO 2 and CH 4 may be an alternative pair of biosignature disequilibrium gases that were first proposed for Earth’s O 2 -poor ancient Archean atmosphere (Krissansen-Totton et al. 2018; Meadows et al. 2023).”

“Measuring atmospheric biosignatures of terrestrial exoplanets and interpreting atmospheric features in terms of such chemical disequilib-ria, however, are non-trivial tasks 50,55–57 .”

“…for longer continuous integration times, and tidally-locked planets at moderate insolations typically have sea ice fractions above 0.5 due to the cold night-side, with open ocean only found near the center of the day-side (Pierrehumbert, 2011; Heng and Vogt, 2011; Yang et al., 2013, 2014).”

“52–55 In this study, we focus on a different regime where these Proterozoic Earth-like atmospheres are weakly oxygenated and fundamentally assume that O 2 is biologically sourced at the 0.1% and 1% PAL, but the emphasis on evaluating species detections based on the atmospheric and environmental…”

“As O 3 abundance is more pronounced on our TRAPPIST-1e scenario than on Earth (see Fig.”

“The super-luminous pre-main sequence of M-dwarfs may drive extreme rates of H 2 O photolysis and hydrogen escape, causing O 2 buildup that could later mimic an in-habited planet with oxygen production via photosynthesis (R. Luger & R. Barnes 2015; R. Wordsworth & R. Pierrehumbert 2014).”

“This is consistent with previous work published by other authors discussing habitable zone placement (e.g., Kasting, 1995; Luger & Barnes, 2015; Meadows, 2017; Wordsworth & Pierrehumbert, 2014).”

“If O2 is abiogenically produced by the high-energy irradiation of surface oceans, there could be a mass-independent effect distinct from the effect of oxygenic photosynthesis (Wordsworth & Pierrehumbert 2014).”

“…phase water loss around M-stars [Luger and Barnes, 2015] and slow hydrogen escape from thin atmospheres lacking noncondensible gases [Wordsworth and Pierrehumbert, 2014] (previous scenarios invoking CO2 photolysis [Tian et al., 2014; Harman et al., 2015] have recently been shown to result from a…”