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

Abstract 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. CO 2 and CH 4 in an Archean Earth analog may be a compelling biosignature because their coexistence implies methane replenishment at rates unlikely to be abiotic. However, CH 4 can also be produced through geological processes, and setting constraints on volcanic molecules such as CO may help address this ambiguity. N 2 O in a Proterozoic Earth analog may be evidence of life because N 2 O production on Earth is mostly biological. Motivated by these ideas, we use the code <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathsize="small" mathvariant="normal">EXOREL</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="fraktur">R</mml:mi> </mml:mrow> </mml:msup> </mml:math> to generate forward models and simulate spectral retrievals of an Archean and Proterozoic Earth-like planet to determine the detectability of CH 4 , CO 2 , CO, and N 2 O in their reflected light spectrum for wavelength range 0.25–1.8 μ m. We show that it is challenging to detect CO in an Archean atmosphere for volume mixing ratio (VMR) ≤ 10%, but CH 4 is readily detectable for both the full wavelength span and truncated ranges cut at 1.7 μ m and 1.6 μ m, although for the latter two cases the dominant gas of the atmosphere is misidentified. Meanwhile, N 2 O in a Proterozoic atmosphere is detectable for VMR = 10 −3 and long wavelength cutoff ≥1.4 μ m, 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.