Prospects for detecting oxygen, water, and chlorophyll on an exo-Earth

The goal of finding and characterizing nearby Earth-like planets is driving many NASA high-contrast flagship mission concepts, the latest of which is known as the Advanced Technology Large-Aperture Space Telescope (ATLAST). In this article, we calculate the optimal spectral resolution $R=\lambda/\delta\lambda$ and minimum signal-to-noise ratio per spectral bin (SNR), two central design requirements for a high-contrast space mission, in order to detect signatures of water, oxygen, and chlorophyll on an Earth twin. We first develop a minimally parametric model and demonstrate its ability to fit synthetic and observed Earth spectra; this allows us to measure the statistical evidence for each component's presence. We find that water is the easiest to detect, requiring a resolution $R \gtrsim 20$, while the optimal resolution for oxygen is likely to be closer to $R = 150$, somewhat higher than the canonical value in the literature. At these resolutions, detecting oxygen will require $\sim$2 times the SNR as water. Chlorophyll requires $\sim$6 times the SNR as oxygen for an Earth twin, only falling to oxygen-like levels of detectability for a low cloud cover and/or a large vegetation covering fraction. This suggests designing a mission for sensitivity to oxygen and adopting a multi-tiered observing strategy, first targeting water, then oxygen on the more favorable planets, and finally chlorophyll on only the most promising worlds.