Prospects for Detecting Oxygen, Water, and Chlorophyll on an Exo-Earth

Abstract

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.

PNAS published, replaced with accepted version. Conclusions mostly unchanged, 5 page supporting information added

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