Is ozone a reliable proxy for molecular oxygen?: III. the impact of CH4on the O2O3relationship for Earth-like atmospheres

Thea Kozakis; João M. Mendonça; Lars A. Buchhave; Luisa M. Lara

doi:10.1051/0004-6361/202556015

Is ozone a reliable proxy for molecular oxygen? III. the impact of CH₄on the O₂O₃relationship for Earth-like atmospheres

Thea Kozakis^*
, João M. Mendonça
, Lars A. Buchhave
, Luisa M. Lara

^*Corresponding author for this work

CSIC - Institute of Astrophysics of Andalusia

Research output: Contribution to journal › Journal article › Research › peer-review

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Abstract

In the search for life in the Universe, molecular oxygen (O₂) combined with a reducing species, such as methane (CH₄), is considered a promising disequilibrium biosignature. In cases where it would be difficult or impossible to detect O₂ (such as in the mid-IR or low O₂ levels), it has been suggested that ozone (O₃), the photochemical product of O₂, could be used as a proxy for determining the abundance of O₂. As the O₂–O₃ relationship is known to be nonlinear, the goal of this series of papers is to explore how it would change for different host stars and atmospheric compositions and learning how to use O₃ to infer O₂. We used photochemistry and climate modeling to further explore the O₂–O₃ relationship by modeling Earth-like planets with the present atmospheric level (PAL) of O₂ between 0.01% and 150%, along with high and low CH₄ abundances of 1000% and 10% PAL, respectively. Methane is of interest not only because it is a biosignature, but it is also the source of hydrogen atoms for hydrogen oxide (HO_x), which destroys O₃ through catalytic cycles, and acts as a catalyst for the smog mechanism of O₃ formation in the lower atmosphere. We find that varying CH₄ causes changes to the O₂–O₃ relationship in ways that are highly dependent on both the host star and O₂ abundance. A striking result for high CH₄ models in high O₂ atmospheres around hotter hosts is that enough CH₄ is efficiently converted into H₂O to significantly impact stratospheric temperatures, and therefore the formation and destruction rates of O₃. Changes in HO_x have also been shown to influence both the HO_x catalytic cycle and production of smog O₃, causing variations in harmful UV reaching the surface, as well as changes in the 9.6 μm O₃ feature in emission spectra. This study further demonstrates the need to explore the O₂–O₃ relationship in different atmospheric compositions in order to use O₃ as a reliable proxy for O₂ in future observations.

Original language	English
Article number	A254
Journal	Astronomy and Astrophysics
Volume	701
Number of pages	16
ISSN	0004-6361
DOIs	https://doi.org/10.1051/0004-6361/202556015
Publication status	Published - 2025

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SDG 13 Climate Action

Keywords

Astrobiology
Planets and satellites: atmospheres
Planets and satellites: terrestrial planets

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@article{4c070b4127c345948d9ca1546d2ad104,

title = "Is ozone a reliable proxy for molecular oxygen?: III. the impact of CH4on the O2O3relationship for Earth-like atmospheres",

abstract = "In the search for life in the Universe, molecular oxygen (O2) combined with a reducing species, such as methane (CH4), is considered a promising disequilibrium biosignature. In cases where it would be difficult or impossible to detect O2 (such as in the mid-IR or low O2 levels), it has been suggested that ozone (O3), the photochemical product of O2, could be used as a proxy for determining the abundance of O2. As the O2–O3 relationship is known to be nonlinear, the goal of this series of papers is to explore how it would change for different host stars and atmospheric compositions and learning how to use O3 to infer O2. We used photochemistry and climate modeling to further explore the O2–O3 relationship by modeling Earth-like planets with the present atmospheric level (PAL) of O2 between 0.01\% and 150\%, along with high and low CH4 abundances of 1000\% and 10\% PAL, respectively. Methane is of interest not only because it is a biosignature, but it is also the source of hydrogen atoms for hydrogen oxide (HOx), which destroys O3 through catalytic cycles, and acts as a catalyst for the smog mechanism of O3 formation in the lower atmosphere. We find that varying CH4 causes changes to the O2–O3 relationship in ways that are highly dependent on both the host star and O2 abundance. A striking result for high CH4 models in high O2 atmospheres around hotter hosts is that enough CH4 is efficiently converted into H2O to significantly impact stratospheric temperatures, and therefore the formation and destruction rates of O3. Changes in HOx have also been shown to influence both the HOx catalytic cycle and production of smog O3, causing variations in harmful UV reaching the surface, as well as changes in the 9.6 μm O3 feature in emission spectra. This study further demonstrates the need to explore the O2–O3 relationship in different atmospheric compositions in order to use O3 as a reliable proxy for O2 in future observations.",

keywords = "Astrobiology, Planets and satellites: atmospheres, Planets and satellites: terrestrial planets",

author = "Thea Kozakis and Mendon{\c c}a, \{Jo{\~a}o M.\} and Buchhave, \{Lars A.\} and Lara, \{Luisa M.\}",

note = "Publisher Copyright: {\textcopyright} The Authors 2025.",

year = "2025",

doi = "10.1051/0004-6361/202556015",

language = "English",

volume = "701",

journal = "Astronomy and Astrophysics",

issn = "0004-6361",

publisher = "EDP Sciences",

}

TY - JOUR

T1 - Is ozone a reliable proxy for molecular oxygen?

T2 - III. the impact of CH4on the O2O3relationship for Earth-like atmospheres

AU - Kozakis, Thea

AU - Mendonça, João M.

AU - Buchhave, Lars A.

AU - Lara, Luisa M.

PY - 2025

Y1 - 2025

N2 - In the search for life in the Universe, molecular oxygen (O2) combined with a reducing species, such as methane (CH4), is considered a promising disequilibrium biosignature. In cases where it would be difficult or impossible to detect O2 (such as in the mid-IR or low O2 levels), it has been suggested that ozone (O3), the photochemical product of O2, could be used as a proxy for determining the abundance of O2. As the O2–O3 relationship is known to be nonlinear, the goal of this series of papers is to explore how it would change for different host stars and atmospheric compositions and learning how to use O3 to infer O2. We used photochemistry and climate modeling to further explore the O2–O3 relationship by modeling Earth-like planets with the present atmospheric level (PAL) of O2 between 0.01% and 150%, along with high and low CH4 abundances of 1000% and 10% PAL, respectively. Methane is of interest not only because it is a biosignature, but it is also the source of hydrogen atoms for hydrogen oxide (HOx), which destroys O3 through catalytic cycles, and acts as a catalyst for the smog mechanism of O3 formation in the lower atmosphere. We find that varying CH4 causes changes to the O2–O3 relationship in ways that are highly dependent on both the host star and O2 abundance. A striking result for high CH4 models in high O2 atmospheres around hotter hosts is that enough CH4 is efficiently converted into H2O to significantly impact stratospheric temperatures, and therefore the formation and destruction rates of O3. Changes in HOx have also been shown to influence both the HOx catalytic cycle and production of smog O3, causing variations in harmful UV reaching the surface, as well as changes in the 9.6 μm O3 feature in emission spectra. This study further demonstrates the need to explore the O2–O3 relationship in different atmospheric compositions in order to use O3 as a reliable proxy for O2 in future observations.

AB - In the search for life in the Universe, molecular oxygen (O2) combined with a reducing species, such as methane (CH4), is considered a promising disequilibrium biosignature. In cases where it would be difficult or impossible to detect O2 (such as in the mid-IR or low O2 levels), it has been suggested that ozone (O3), the photochemical product of O2, could be used as a proxy for determining the abundance of O2. As the O2–O3 relationship is known to be nonlinear, the goal of this series of papers is to explore how it would change for different host stars and atmospheric compositions and learning how to use O3 to infer O2. We used photochemistry and climate modeling to further explore the O2–O3 relationship by modeling Earth-like planets with the present atmospheric level (PAL) of O2 between 0.01% and 150%, along with high and low CH4 abundances of 1000% and 10% PAL, respectively. Methane is of interest not only because it is a biosignature, but it is also the source of hydrogen atoms for hydrogen oxide (HOx), which destroys O3 through catalytic cycles, and acts as a catalyst for the smog mechanism of O3 formation in the lower atmosphere. We find that varying CH4 causes changes to the O2–O3 relationship in ways that are highly dependent on both the host star and O2 abundance. A striking result for high CH4 models in high O2 atmospheres around hotter hosts is that enough CH4 is efficiently converted into H2O to significantly impact stratospheric temperatures, and therefore the formation and destruction rates of O3. Changes in HOx have also been shown to influence both the HOx catalytic cycle and production of smog O3, causing variations in harmful UV reaching the surface, as well as changes in the 9.6 μm O3 feature in emission spectra. This study further demonstrates the need to explore the O2–O3 relationship in different atmospheric compositions in order to use O3 as a reliable proxy for O2 in future observations.

KW - Astrobiology

KW - Planets and satellites: atmospheres

KW - Planets and satellites: terrestrial planets

U2 - 10.1051/0004-6361/202556015

DO - 10.1051/0004-6361/202556015

M3 - Journal article

AN - SCOPUS:105017004889

SN - 0004-6361

VL - 701

JO - Astronomy and Astrophysics

JF - Astronomy and Astrophysics

M1 - A254

ER -

Is ozone a reliable proxy for molecular oxygen? III. the impact of CH4on the O2O3relationship for Earth-like atmospheres