Is ozone a reliable proxy for molecular oxygen? II. The impact of N2O on the O2-O3 relationship for Earth-like atmospheres
Is ozone a reliable proxy for molecular oxygen? II. The impact of N2O on the O2-O3 relationship for Earth-like atmospheres
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 µm O3 emission spectral feature, demonstrating potential impacts on habitability and future observations.
astro-ph.EP, planets and satellites: terrestrial planets, planets and satellites: atmospheres, astrobiology, planetary systems
Kozakis, Thea
8c823f29-3f3a-4d8a-ba87-ceda4be9e6f5
Mendonça, João M.
cb29fe08-eb94-4fad-8eba-eac1c5de491b
Buchhave, Lars A.
09bc47d1-865f-4f71-b25a-51ad6371e3f8
Lara, Luisa M.
79c67bdd-d21c-42e9-8c0d-9899552560e1
Kozakis, Thea
8c823f29-3f3a-4d8a-ba87-ceda4be9e6f5
Mendonça, João M.
cb29fe08-eb94-4fad-8eba-eac1c5de491b
Buchhave, Lars A.
09bc47d1-865f-4f71-b25a-51ad6371e3f8
Lara, Luisa M.
79c67bdd-d21c-42e9-8c0d-9899552560e1
Kozakis, Thea, Mendonça, João M., Buchhave, Lars A. and Lara, Luisa M.
(2025)
Is ozone a reliable proxy for molecular oxygen? II. The impact of N2O on the O2-O3 relationship for Earth-like atmospheres.
A&A, 699, [A247].
(doi:10.1051/0004-6361/202555289).
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 µm O3 emission spectral feature, demonstrating potential impacts on habitability and future observations.
Text
2505.23279v2
- Author's Original
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aa55289-25
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Accepted/In Press date: 27 May 2025
e-pub ahead of print date: 18 July 2025
Keywords:
astro-ph.EP, planets and satellites: terrestrial planets, planets and satellites: atmospheres, astrobiology, planetary systems
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Local EPrints ID: 505071
URI: http://eprints.soton.ac.uk/id/eprint/505071
ISSN: 0004-6361
PURE UUID: 3c351e08-1656-4942-8eea-db12c94297c0
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Date deposited: 25 Sep 2025 17:03
Last modified: 26 Sep 2025 02:18
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Author:
Thea Kozakis
Author:
João M. Mendonça
Author:
Lars A. Buchhave
Author:
Luisa M. Lara
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