Table 1.

Summary of Known Abiotic O₂ Generation Mechanisms

Mechanism	Action	Targets affected	Potential O2 produced	Potential O3 produced	Spectral discriminant	References
O2 runaway from a superluminous pre-main sequence star	Massive H2O evaporation and photolysis during the host star's superluminous pre-main sequence phase	HZ planets orbiting late-type M dwarfs	100s of bar, depending on initial water inventory	Possible, after complete loss of H2O	O4 in transmission (NIR) and direct imaging (visible + NIR)	Luger and Barnes, 2015; Tian, 2015
Lack of noncondensable gases	Lack of cold trap allows water to enter stratosphere and be photolyzed	HZ planets orbiting any stellar type	15% O2	Not calculated	Quantification of O2, and N2 abundance via the N2-N2 collisional pair at 4.1 μm	Wordsworth and Pierrehumbert, 2014
Desiccated planets	Lack of water inhibits catalytic recombination of CO2	HZ planets orbiting late-type M dwarfs, also volatile-poor planets	15% O2	0.2 times Earth's for M dwarfs	Absence of H2O absorption in direct imaging. O3 looks similar to Earth's.	Gao et al.,2015
Photochemical production from CO2 photolysis	High stellar FUV/NUV, reduction of O2 sinks	HZ planets orbiting K and M dwarfs	<0.02% for F and G star planets	0.15–0.01 times Earth's for M dwarf planets depending on sinks	Presence of CO, CO2, M dwarf spectral host	Harman et al.,2015
			<6% for M dwarf planets with O2-saturated oceans
Photochemical production on CO2-rich, H-poor planets	High stellar FUV/MUV photolysis CO2 and produces O3	HZ planets orbiting F dwarfs and some M dwarfs	40 ppm	0.1 times Earth's for M dwarfs	Presence of CO, absence of H-bearing gases such as CH4	Domagal-Goldman et al.,2014
Photochemical production from CO2 photolysis and stellar inhibition of recombination	High stellar FUV/NUV destroys HOx species and inhibits CO2 recombination	HZ planets orbiting M dwarfs, CO2-rich (<10%) atmospheres	0.2% for M dwarfs with high FUV/NUV ratios	0.06 times Earth's for M dwarfs	Presence of CO, CO2, high FUV/NUV ratio for the parent star with low absolute NUV	Tian et al.,2014
Photochemical production from CO2 photolysis	CO2 photolysis and no CO2 or CH4 surface flux	1 bar CO2-rich (90%) atmospheres orbiting a G2V	0.1% O2	0.3 times Earth's for G dwarfs	Presence of CO, CO2	Hu et al.,2012

HZ = habitable zone.