Abiotic O₂ Levels on Planets around F, G, K, and M Stars: Effects of Lightning-produced Catalysts in Eliminating Oxygen False Positives

Abstract

Over the last few years, a number of authors have suggested that, under certain circumstances, molecular oxygen (O2) or ozone (O3) generated by abiotic processes may accumulate to detectable concentrations in a habitable terrestrial planet’s atmosphere, producing so-called “false positives” for life. But the models have occasionally disagreed with each other, with some predicting false positives, and some not, for the same apparent set of circumstances. We show here that photochemical false positives derive either from inconsistencies in the treatment of atmospheric and global redox balance or from the treatment (or lack thereof) of lightning. For habitable terrestrial planets with even trace amounts of atmospheric N2, NO produced by lightning catalyzes the recombination of CO and O derived from CO2 photolysis and should be sufficient to eliminate all reported false positives. Molecular oxygen thus remains a useful biosignature gas for Earth-like extrasolar planets, provided that the planet resides within the conventional liquid water habitable zone and has not experienced distinctly non-Earth-like, irrecoverable water loss.

O2- and CO-rich Atmospheres for Potentially Habitable Environments on TRAPPIST-1 Planets

“…The model has also been used to determine the photochemical oxygen buildup in abiotic atmospheres (Hu et al 2012;James & Hu 2018). For the latter purpose, the model has been compared with other photochemistry model in detail and we have found good agreement (Gao et al 2015;Harman et al 2018). The photochemistry model solves the one-dimensional chemical-transport equation for 111 O, H, C, N, and S species including sulfur and sulfuric acid aerosols.…”

“…To simulate the effect of lightning, we adopt a terrestrial production rate of NO from lightning, 6´10 8 cm -2 s -1 (Schumann & Huntrieser 2007), which is already greater than the majority of the parameter space explored previously for N2-CO2-H2O atmospheres (Wong et al 2017;Harman et al 2018). In addition, it is found that in an atmosphere more O2-rich than Earth's atmosphere, the NO production rate from lightning can be higher, up to ~10 9 cm -2 s -1 (Harman et al 2018). We also consider this higher production rate in a sensitivity study.…”

“…This would make the model results sensitive to the choice of the deposition velocity of O3. Some of the previous studies assume O3 to be the short-lived species in photochemical equilibrium (Harman et al 2015;Harman et al 2018), thus removing the need to specify a deposition velocity. On Earth, the deposition velocity of O3 is very different between the land and the ocean.…”

“…The onset of the O2-CO runaway is not sensitive to the lightning rate or the deposition velocity of O3, and is mildly sensitive to the spectral shape. NO in an N2-CO2-O2 atmosphere (Wong et al 2017;Harman et al 2018;Rimmer & Helling 2016). Reactive nitrogen species facilitate cycles of OH and HO2 in Earth's stratosphere and have been suggested to reduce the abundance of accumulated O2 in CO2-rich planetary atmospheres by many orders of magnitude (Harman et al 2018).…”

“…NO in an N2-CO2-O2 atmosphere (Wong et al 2017;Harman et al 2018;Rimmer & Helling 2016). Reactive nitrogen species facilitate cycles of OH and HO2 in Earth's stratosphere and have been suggested to reduce the abundance of accumulated O2 in CO2-rich planetary atmospheres by many orders of magnitude (Harman et al 2018). A main catalytic cycle enabled by reactive nitrogen species is NO + HO2 ® NO2 + OH, NO2 + hv ® NO + O, which is followed by CO + OH ® CO2 + H. Our model indicates that the effect of lightning, however, is limited to within a factor of ~2.…”