The effect of spectral resolution on biosignature detection via reflected light observations of the Earth through time

TL;DR

This study evaluates the spectral resolution needed for detecting biosignatures on Earth-like exoplanets, balancing detectability, exposure time, and noise, to inform the design of NASA's Habitable Worlds Observatory.

Contribution

It provides specific spectral resolution requirements across visible and near-IR wavelengths for biosignature detection, supporting current instrument baseline choices.

Findings

Visible resolution R=140 suffices for O2 detection in Phanerozoic atmospheres.

Higher resolutions may reduce exposure time for low-O2 atmospheres but require technological improvements.

Near-IR resolution R≥40 avoids CO2-CO degeneracy, with R=70 sufficient for all Earth-through-time cases.

Abstract

NASA's Habitable Worlds Observatory (HWO) will search for biosignatures on Earth-like exoplanets using reflected light spectroscopy. A critical instrument design parameter is resolving power, which must balance biosignature detectability against exposure time and detector noise constraints. We assess the resolving power needed to detect and characterize key biosignature gases and habitability indicators including O$_2$, O$_3$, H$_2$O, CH$_4$, CO$_2$ and CO across atmospheres representing the Archean, Proterozoic, and Phanerozoic Earth. We combine analytical detectability calculations spanning spectral resolutions ($\lambda/\Delta{\lambda}$) $R=20$-$5000$ with atmospheric retrievals using the rfast radiative transfer model and pyEDITH exposure time calculator for realistic wavelength-dependent noise modeling. In the visible ($0.4$-$1.0$ $\mu$m), the nominal resolution $R_{Vis}=140$ is sufficient for detecting O$_2$ in Phanerozoic-like atmospheres. Higher resolutions could theoretically reduce exposure times for low-O$_2$ Proterozoic atmospheres, but require $>10\times$ reductions in dark current and could increase H$_2$O detection exposure times by $\sim 2\times$, penalizing the foundational habitability constraint that anchors downstream biosignature searches. The most efficient path for low-O$_2$ atmospheres may instead be indirect inference via O$_3$, whose Hartley-Huggins bands are detectable at $R_{UV}\sim 7$. In the near-IR ($1.0$-$1.7$ $\mu$m), $R_{NIR}\geq40$ is necessary to avoid a degeneracy between CO$_2$ and CO that could produce false positive detections of abundant CO. The nominal $R_{NIR}=70$ is sufficient for characterizing all Earth-through-time cases. These results support HWO's current baseline resolution choices and provide actionable guidance for finalizing spectrometer requirements while maintaining technological feasibility for the search for life on exoplanets.