IDENTIFYING PLANETARY BIOSIGNATURE IMPOSTORS: SPECTRAL FEATURES OF CO AND O₄ RESULTING FROM ABIOTIC O₂/O₃ PRODUCTION

O₂ and O₃ have been long considered the most robust individual biosignature gases in a planetary atmosphere, yet multiple mechanisms that may produce them in the absence of life have been described. However, these abiotic planetary mechanisms modify the environment in potentially identifiable ways. Here we briefly discuss two of the most detectable spectral discriminants for abiotic O₂/O₃: CO and O₄. We produce the first explicit self-consistent simulations of these spectral discriminants as they may be seen by James Webb Space Telescope (JWST). If JWST-NIRISS and/or NIRSpec observe CO (2.35, 4.6 microm) in conjunction with CO₂ (1.6, 2.0, 4.3 microm) in the transmission spectrum of a terrestrial planet it could indicate robust CO₂ photolysis and suggest that a future detection of O₂ or O₃ might not be biogenic. Strong O₄ bands seen in transmission at 1.06 and 1.27 microm could be diagnostic of a post-runaway O₂-dominated atmosphere from massive H-escape. We find that for these false positive scenarios, CO at 2.35 microm, CO₂ at 2.0 and 4.3 microm, and O₄ at 1.27 microm are all stronger features in transmission than O₂/O₃ and could be detected with S/Ns ≳ 3 for an Earth-size planet orbiting a nearby 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 microm) by a next generation direct-imaging telescope such as LUVOIR/HDST or HabEx and would indicate an oxygen atmosphere too massive to be biologically produced.