Presentation Information
[U17-P04]Coupled photochemistry-ecosystem modeling of reducing Earth-like planets: Implications for methane-based biosignatures
*Tomoya Kimura1, Yoshiaki Endo1, Yasuto Watanabe2, Kazumi Ozaki1 (1.Institute of SCIENCE TOKYO, 2.National Institute for Environmental Studies)
More than 6000 exoplanets have been discovered to date, including dozens located within the habitable zone (HZ). Proposed missions such as NASA's Habitable Worlds Observatory (HWO) identify the search for biosignatures through atmospheric spectroscopy of rocky planets orbiting Sun-like stars as a primary scientific objective. However, planetary atmospheric compositions are also influenced by abiotic processes, including atmospheric photochemistry, volcanic outgassing, and water-rock reactions. Therefore, distinguishing true biosignatures from false positives requires a systematic and quantitative understanding of the interactions between planetary environments and biological activity.
Molecular oxygen (O2) is often considered as one of the promising exoplanet biosignatures. However, Earth's present-day oxidizing atmosphere emerged as a consequence of oxygenic photosynthesis, a highly complex metabolic innovation that may not be universal among habitable planets. Geological evidence further indicates that establishment of modern atmospheric oxygen levels on Earth required more than 2 billion years after the emergence of oxygenic photosynthesis. From the perspective of life’s origin and early evolution, reducing atmospheric conditions may represent a more natural initial state. Therefore, planets with reducing atmospheres analogous to early Earth provide an important framework for understanding the general concept of habitable worlds.
In this study, we developed a theoretical model that integrates volcanic outgassing, surface carbon cycling, climate, atmospheric photochemistry, and anaerobic microbial ecosystems. Using this integrated framework, we quantitatively evaluated the impact of biological activity on atmospheric composition under reducing conditions. Specifically, we conducted a suite of numerical experiments for hypothetical Earth-like planets modeled within the HZ, varying the semi-major axis, outgassing flux, stellar spectral type (F, G, and K-type stars), and ecosystem structure. Special attention was given to methane (CH4), a proposed biosignature gas in reducing atmospheres, to determine the conditions under which it accumulates to high concentrations and to assess strategies for excluding false positives.
Our simulations show that biological activity increases atmospheric CH4 concentrations across the entire HZ. Even when primary productivity is extremely low (~1% of current marine biosphere), nonlinear atmospheric-ecosystem feedbacks allow CH4 to accumulate to high levels (>1000 ppmv). This CH4 amplification is particularly pronounced around K-type stars, where lower ultraviolet radiation extends the photochemical lifetime of CH4, creating favorable observational conditions. In contrast, under abiotic conditions, carbon monoxide (CO) tends to accumulate near the outer edge of the HZ, potentially leading to a so-called CO runaway state. In the presence of anaerobic microorganisms, however, CO is efficiently consumed and converted to CH4, suppressing CO runaway and resulting in a CO/CH4 ratio below 1.
Based on these results, we argue that the coexistence of elevated CH4 concentrations and a low CO/CH4 ratio (< 1) strengthens the CH4-based biosignatures by helping exclude false positives in reducing atmospheres. This diagnostic is particularly pronounced on planets located near the outer edge of the HZ, where methane is shielded by the ultraviolet-blocking effect of high CO2 concentrations. Our findings provide a theoretical framework for interpreting atmospheric observations by future missions such as HWO, especially when combined with orbital constraints and environmental context (e.g., the presence of H2O and CO2).
Molecular oxygen (O2) is often considered as one of the promising exoplanet biosignatures. However, Earth's present-day oxidizing atmosphere emerged as a consequence of oxygenic photosynthesis, a highly complex metabolic innovation that may not be universal among habitable planets. Geological evidence further indicates that establishment of modern atmospheric oxygen levels on Earth required more than 2 billion years after the emergence of oxygenic photosynthesis. From the perspective of life’s origin and early evolution, reducing atmospheric conditions may represent a more natural initial state. Therefore, planets with reducing atmospheres analogous to early Earth provide an important framework for understanding the general concept of habitable worlds.
In this study, we developed a theoretical model that integrates volcanic outgassing, surface carbon cycling, climate, atmospheric photochemistry, and anaerobic microbial ecosystems. Using this integrated framework, we quantitatively evaluated the impact of biological activity on atmospheric composition under reducing conditions. Specifically, we conducted a suite of numerical experiments for hypothetical Earth-like planets modeled within the HZ, varying the semi-major axis, outgassing flux, stellar spectral type (F, G, and K-type stars), and ecosystem structure. Special attention was given to methane (CH4), a proposed biosignature gas in reducing atmospheres, to determine the conditions under which it accumulates to high concentrations and to assess strategies for excluding false positives.
Our simulations show that biological activity increases atmospheric CH4 concentrations across the entire HZ. Even when primary productivity is extremely low (~1% of current marine biosphere), nonlinear atmospheric-ecosystem feedbacks allow CH4 to accumulate to high levels (>1000 ppmv). This CH4 amplification is particularly pronounced around K-type stars, where lower ultraviolet radiation extends the photochemical lifetime of CH4, creating favorable observational conditions. In contrast, under abiotic conditions, carbon monoxide (CO) tends to accumulate near the outer edge of the HZ, potentially leading to a so-called CO runaway state. In the presence of anaerobic microorganisms, however, CO is efficiently consumed and converted to CH4, suppressing CO runaway and resulting in a CO/CH4 ratio below 1.
Based on these results, we argue that the coexistence of elevated CH4 concentrations and a low CO/CH4 ratio (< 1) strengthens the CH4-based biosignatures by helping exclude false positives in reducing atmospheres. This diagnostic is particularly pronounced on planets located near the outer edge of the HZ, where methane is shielded by the ultraviolet-blocking effect of high CO2 concentrations. Our findings provide a theoretical framework for interpreting atmospheric observations by future missions such as HWO, especially when combined with orbital constraints and environmental context (e.g., the presence of H2O and CO2).