tokens: O 2 and O 3 are also central to the design of instruments for other future ground- and space-based telescopes (Domagal-Goldman et al. 2014 ; Harman et al. 2015 ; Meadows 2017 ; Meadows et al. 2017 ). Here, we argue that O 2 and O 3 are reliable bioindicators under certain conditions. We outline what those conditions are, and discuss the secondary measurements and constraints necessary to demonstrate the presence of such conditions on another world. 2. False Positives in the Literature For the purposes of this paper, a “false positive” for life is defined as any abiotically derived O 2 concentration that exceeds that estimated to follow the GOE (Harman et al. 2015 ). If the low estimates by Planavsky et al. ( 2014 ) are correct, then that level is at or below 0.1% PAL. Possible mechanisms by which that abiotic O 2 level might be achieved have been recently reviewed by Meadows ( 2017 ) and Meadows et al. ( 2017 ). Broadly speaking, these scenarios can be divided into two categories: (1) false positives driven by changes in the redox budget of a planet’s combined atmosphere/ocean system (as defined in Catling Kasting 2017 , Ch. 8), and (2) false positives driven by photochemistry within the atmosphere. Examples of the redox-driven false positives include planets that experience substantial water loss because they are located inside the inner edge of the habitable zone (Kasting 1988 ; Kasting et al. 1993a ), or their host (M) stars are relatively bright during their pre-main sequence lifetimes (Ramirez Kaltenegger 2014 ; Luger Barnes 2015 ; Tian 2015 ), or they lack non-condensable gases like N 2 that help keep water vapor contained in the lower atmosphere (Wordsworth Pierrehumbert 2014 ). In each of these cases, hydrogen is irreversibly lost to space, oxidizing the atmosphere and oceans and, ultimately, the planet itself. The ultimate fate of this oxygen is uncertain but it could potentially be drawn down to undetectable concentrations by absorption into the mantle (Wordsworth et al. 2018 ). Other types of redox-driven false positives essentially correspond to hypothetical planetary scenarios that lack a volcanic source of reduced gases (e.g., Hu et al. 2012 ; Domagal-Goldman et al. 2014 ; Gao et al. 2015 ). If volcanic outgassing is assumed to be nearly zero, or the recombination of CO and O 2 is inefficient, or there are no surface sinks for O 2 , then O 2 can build up in the atmosphere. If reduced volcanic gas emissions are entirely neglected, O 2 can and must accumulate as H 2 O is photolyzed and the resulting hydrogen escapes to space. But we argue that a false positive driven by such low outgassing rates is unlikely if the planet is habitable, so that liquid water can facilitate reactions of O 2 with its crust, and if the planet remains volcanically active for long time periods, as the Earth has done. (We return to this last point in Section 6 .) The false positive scenario described by Hu et al. ( 2012 ) is more complicated in that it has
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