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 ${{\rm\small{EXOREL}}}^{{\mathfrak{R}}}$ 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.

Keywords

• Exoplanet atmospheres
• Bayesian statistics
• Spectroscopy
• Biosignatures
• Posterior distribution
• Extrasolar rocky planets