The Detectability of CH4/CO2/CO and N2O Biosignatures Through Reflection Spectroscopy of Terrestrial Exoplanets

The chemical makeup of Earth’s atmosphere during the Archean (4–2.5 Ga) and Proterozoic eon (2.5–0.5 Ga) contrast considerably with the present-day: the Archean was rich in carbon dioxide and methane, and the Proterozoic had potentially higher amounts of nitrous oxide. CO2 and CH4 in an Archean Eart...

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Bibliographic Details
Published in:The Astronomical journal 2024-12, Vol.168 (6), p.292
Main Authors: Tokadjian, Armen, Hu, Renyu, Damiano, Mario
Format: Article
Language:English
Subjects:
Atmosphere
Bayesian statistics
Biosignatures
Carbon dioxide
Earth
Earth analogs
Exoplanet atmospheres
Extrasolar planets
Extrasolar rocky planets
Extraterrestrial life
Geological processes
Methane
Mixing ratio
Nitrous oxide
Posterior distribution
Spectroscopy
Terrestrial planets
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Summary:The chemical makeup of Earth’s atmosphere during the Archean (4–2.5 Ga) and Proterozoic eon (2.5–0.5 Ga) contrast considerably with the present-day: the Archean was rich in carbon dioxide and methane, and the Proterozoic had potentially higher amounts of nitrous oxide. CO2 and CH4 in an Archean Earth analog may be a compelling biosignature because their coexistence implies methane replenishment at rates unlikely to be abiotic. However, CH4 can also be produced through geological processes, and setting constraints on volcanic molecules such as CO may help address this ambiguity. N2O in a Proterozoic Earth analog may be evidence of life because N2O production on Earth is mostly biological. Motivated by these ideas, we use the code EXORELR to generate forward models and simulate spectral retrievals of an Archean and Proterozoic Earth-like planet to determine the detectability of CH4, CO2, CO, and N2O in their reflected light spectrum for wavelength range 0.25–1.8 μm. We show that it is challenging to detect CO in an Archean atmosphere for volume mixing ratio (VMR) ≤ 10%, but CH4 is readily detectable for both the full wavelength span and truncated ranges cut at 1.7 μm and 1.6 μm, although for the latter two cases the dominant gas of the atmosphere is misidentified. Meanwhile, N2O in a Proterozoic atmosphere is detectable for VMR = 10−3 and long wavelength cutoff ≥1.4 μm, but undetectable for VMR ≤ 10−4 . The results presented here will be useful for the strategic design of the future Habitable Worlds Observatory and the components needed to potentially distinguish between inhabited and lifeless planets.
ISSN:0004-6256
1538-3881
DOI:10.3847/1538-3881/ad88eb