The Detectability of CH$_4$/CO$_2$/CO and N$_2$O Biosignatures through Reflection Spectroscopy of Terrestrial Exoplanets

TL;DR

This study assesses the detectability of key biosignature gases (CH$_4$, CO$_2$, CO, N$_2$O) in the reflected spectra of Earth-like exoplanets from different geological eras, informing future observational strategies.

Contribution

It provides the first detailed simulation-based analysis of detecting these biosignatures in Archean and Proterozoic Earth analogs across a broad wavelength range.

Findings

CH$_4$ is easily detectable in both Archean and Proterozoic spectra.

CO detection is challenging at low VMR in Archean atmospheres.

N$_2$O is detectable at higher VMR and longer wavelengths in Proterozoic atmospheres.

Abstract

The chemical makeup of Earth's atmosphere during the Archean (4 Ga-2.5 Ga) and Proterozoic eon (2.5 Ga-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 like 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 $\mbox{ExoReL$^\Re$}$ 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 $\mu$m. We show that it is challenging to detect CO in an Archean atmosphere for volume mixing ratio (VMR) $\leq$ 10%, but CH$_4$ is readily detectable for both the full wavelength span and truncated ranges cut at 1.7$\mu$m and 1.6$\mu$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 $\geq 1.4\mu$m, but undetectable for VMR $\leq 10^{-4}$ . 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.