Is Ozone a Reliable Proxy for Molecular Oxygen? II. The impact of N${}_2$O on the O${}_2$-O${}_3$ relationship for Earth-like atmospheres

Abstract

Molecular oxygen (O2) will be an important molecule in the search for biosignatures in terrestrial planetary atmospheres in the coming decades. In particular, O2 combined with a reducing gas is thought to be strong evidence for disequilibrium caused by surface life. However, there are circumstances where it would be very difficult or impossible to detect O2, in which cases it has been suggested that ozone (O3), the photochemical product of O2, could be used instead. Unfortunately, the O2-O3 relationship is highly nonlinear and dependent on the host star, as shown in detail in the first paper in this series. We explore the O2-O3 relationship around G0V-M5V host stars, using climate/photochemistry modeling to simulate atmospheres while varying abundances of O2 and nitrous oxide (N2O). N2O is of particular importance to the O2-O3 relationship not just because it is produced biologically, but because it is the primary source of nitrogen oxides (NOx), which fuel the NOx catalytic cycle which destroys O3, and the smog mechanism that produces O3. We vary the O2 mixing ratio from 0.01-150% present atmospheric level (PAL), and N2O abundances of 10% and 1000% PAL. We find that varying N2O impacts the O2-O3 relationship differently depending strongly on both the host star and the amount of atmospheric O2. Planets orbiting hotter hosts with strong UV fluxes efficiently convert N2O into NOx, often depleting a significant amount of O3 via faster NOx catalytic cycles. However, for cooler hosts and low O2 levels we find that increasing N2O can lead to an increase of overall O3 due to the smog mechanism producing O3 in the lower atmosphere. Variations in O3 result in significant changes in the amount of harmful UV reaching the surfaces of the model planets as well as the strength of the 9.6 $\mu$m O3 emission spectral feature, demonstrating potential impacts on habitability and future observations.

Final version as published in A & A

References in corpus (13)

• The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth
• Exoplanet Reflected Light Spectroscopy with PICASO
• Sensitivity of Biosignatures on Earth-like Planets orbiting in the Habitable Zone of Cool M-Dwarf Stars to varying Stellar UV Radiation and Surface Biomass Emissions
• Large Interferometer For Exoplanets (LIFE): V. Diagnostic potential of a mid-infrared space-interferometer for studying Earth analogs
• Large Interferometer For Exoplanets (LIFE): III. Spectral resolution, wavelength range and sensitivity requirements based on atmospheric retrieval analyses of an exo-Earth
• Sensitive Probing of Exoplanetary Oxygen via Mid Infrared Collisional Absorption
• Effects of UV stellar spectral uncertainty on the chemistry of terrestrial atmospheres
• Is ozone a reliable proxy for molecular oxygen? I. The O2-O3 relationship for Earth-like atmospheres
• Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach
• Lightning-induced chemistry on tidally-locked Earth-like exoplanets
• Large Interferometer For Exoplanets (LIFE): IX. Assessing the Impact of Clouds on Atmospheric Retrievals at Mid-Infrared Wavelengths with a Venus-Twin Exoplanet
• Large Interferometer For Exoplanets (LIFE): XIII. The Value of Combining Thermal Emission and Reflected Light for the Characterization of Earth Twins
• Potential of Ozone Formation by the Smog Mechanism to shield the surface of the Early Earth from UV radiation?

Cited by in corpus (1)

• Is ozone a reliable proxy for molecular oxygen? III. The impact of CH${}_4$ on the O${}_2$-O${}_3$ relationship for Earth-like atmospheres