Abstract
O(2) and O(3) 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 O(2)/O(3): CO and O(4). 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 μm) in conjunction with CO(2) (1.6, 2.0, 4.3 μm) in the transmission spectrum of a terrestrial planet it could indicate robust CO(2) photolysis and suggest that a future detection of O(2) or O(3) might not be biogenic. Strong O(4) bands seen in transmission at 1.06 and 1.27 μm could be diagnostic of a post-runaway O(2)-dominated atmosphere from massive H-escape. We find that for these false positive scenarios, CO at 2.35 μm, CO(2) at 2.0 and 4.3 μm, and O(4) at 1.27 μm are all stronger features in transmission than O(2)/O(3) 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. O(4) 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 μm) 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.