2019AJ....158...27L

The James Webb Space Telescope (JWST) will offer the first opportunity to characterize terrestrial exoplanets with sufficient precision to identify high mean molecular weight atmospheres, and TRAPPIST-1's seven known transiting Earth-sized planets are particularly favorable targets. To assist community preparations for JWST observations, we use simulations of plausible post-ocean-loss and habitable environments for the TRAPPIST-1 exoplanets, and test simulations of all bright object time-series spectroscopy modes and all Mid-Infrared Instrument photometry filters to determine optimal observing strategies for atmospheric detection and characterization using both transmission and emission observations. We find that transmission spectroscopy with the Near-Infrared Spectrograph Prism is optimal for detecting terrestrial, CO₂-containing atmospheres, potentially in fewer than 10 transits for all seven TRAPPIST-1 planets, if they lack high-altitude aerosols. If the TRAPPIST-1 planets possess Venus-like H₂SO₄ aerosols, up to 12 times more transits may be required to detect an atmosphere. We present optimal instruments and observing modes for the detection of individual molecular species in a given terrestrial atmosphere and an observational strategy for discriminating between evolutionary states. We find that water may be prohibitively difficult to detect in both Venus-like and habitable atmospheres, due to its presence lower in the atmosphere where transmission spectra are less sensitive. Although the presence of biogenic O₂ and O₃ will be extremely challenging to detect, abiotically produced oxygen from past ocean loss may be detectable for all seven TRAPPIST-1 planets via O₂-O₂ collisionally induced absorption at 1.06 and 1.27 µm, or via NIR O₃ features for the outer three planets. Our results constitute a suite of hypotheses on the nature and detectability of highly evolved terrestrial exoplanet atmospheres that may be tested with JWST.