Retrieved Atmospheres and Inferred Surface Properties for Exoplanets Using Transmission and Reflected Light Spectroscopy

Future astrophysics missions will seek extraterrestrial life via transmission and direct imaging observations. To assess habitability and biosignatures, we need robust retrieval tools to analyze observed spectra, and infer surface and atmospheric properties with their uncertainties. We use a novel retrieval tool to assess accuracy in characterizing near-surface habitability and biosignatures via simulated transmission and direct imaging spectra, based on the Origins Space Telescope (Origins) and LUVOIR mission concepts. We assess our ability to discriminate between an Earth-like and a false-positive O_3 TRAPPIST-1 e with transmission spectroscopy. In reflected light, we assess the robustness of retrieval results to un-modeled cloud extinction. We find that assessing habitability using transmission spectra may be challenging due to relative insensitivity to surface temperature and near-surface H_2O abundances. Nonetheless, our order of magnitude H_2O constraints can discriminate extremely desiccated worlds. Direct imaging is insensitive to surface temperature and subject to the radius/albedo degeneracy, but this method proves highly sensitive to surface water abundance, achieving retrieval precision within 0.1 Origins-like transmission observations (t=40 hours) may detect the CO_2/CH_4 pair on M-dwarf planets and differentiate between biological and false positive O_3 using H_2O and abundant CO. In contrast, direct imaging observations with LUVOIR-A (t=10 hours) are better suited to constraining O_2 and O_3, and may be sensitive to wavelength-dependent water cloud features, but will struggle to detect modern Earth-like abundances of methane. For direct imaging, we weakly detect a stratospheric ozone bulge by fitting the near-UV wings of the Hartley band.