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O2 and O3 have been long considered the most robust individual biosignaturegases in a planetary atmosphere, yet multiple mechanisms that may produce themin the absence of life have been described. However, these abiotic planetarymechanisms modify the environment in potentially identifiable ways. Here webriefly discuss two of the most detectable spectral discriminants for abioticO2/O3: CO and O4. We produce the first explicit self-consistent simulations ofthese spectral discriminants as they may be seen by JWST. If JWST-NIRISS and/orNIRSpec observe CO (2.35, 4.6 um) in conjunction with CO2 (1.6, 2.0, 4.3 um) inthe transmission spectrum of a terrestrial planet it could indicate robust CO2photolysis and suggest that a future detection of O2 or O3 might not bebiogenic. Strong O4 bands seen in transmission at 1.06 and 1.27 um could bediagnostic of a post-runaway O2-dominated atmosphere from massive H-escape. Wefind that for these false positive scenarios, CO at 2.35 um, CO2 at 2.0 and 4.3um, and O4 at 1.27 um are all stronger features in transmission than O2/O3 andcould be detected with SNRs $\gtrsim$ 3 for an Earth-size planet orbiting anearby M dwarf star with as few as 10 transits, assuming photon-limited noise.O4 bands could also be sought in UV/VIS/NIR reflected light (at 0.345, 0.36,0.38, 0.445, 0.475, 0.53, 0.57, 0.63, 1.06, and 1.27 um) by a next generationdirect-imaging telescope such as LUVOIR/HDST or HabEx and would indicate anoxygen atmosphere too massive to be biologically produced.

Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting from Abiotic O2/O3 Production