Detecting and Characterizing Water Vapor in the Atmospheres of Earth Analogs through Observation of the 0.94 μm Feature in Reflected Light

The characterization of rocky, Earth-like planets is an important goal for future large ground- and space-based telescopes. In support of developing an efficient observational strategy, we have applied Bayesian statistical inference to interpret the albedo spectrum of cloudy true-Earth analogs that include a diverse spread in their atmospheric water vapor mixing ratios. We focus on detecting water-bearing worlds by characterizing their atmospheric water vapor content via the strong 0.94 μm H₂O absorption feature, with several observational configurations. Water vapor is an essential signpost when assessing planetary habitability, and determining its presence is important in vetting whether planets are suitable for hosting life. We find that R = 140 spectroscopy of the absorption feature combined with a same-phase green-optical photometric point at 0.525-0.575 μm is capable of distinguishing worlds with less than 0.1× Earth-like water vapor levels from worlds with 1× Earth-like levels or greater at a signal-to-noise ratio of 5 or better with 2σ confidence. This configuration can differentiate between 0.01× and 0.1× Earth-like levels when the signal-to-noise ratio is 10 or better at the same confidence. However, strong constraints on the water vapor mixing ratio remained elusive with this configuration even at a signal-to-noise of 15. We find that adding the same-phase optical photometric point does not significantly help characterize the H₂O mixing ratio, but does enable an upper limit on atmospheric ozone levels. Finally, we find that a 0.94 μm photometric point, instead of spectroscopy, combined with the green-optical point, fails to produce meaningful information about atmospheric water content.