Large Interferometer For Exoplanets (LIFE). V. Diagnostic potential of a mid-infrared space interferometer for studying Earth analogs

Abstract

Context. An important future goal in exoplanetology is to detect and characterize potentially habitable planets. Concepts for future space missions have already been proposed: from a large UV-optical-infrared space mission for studies in reflected light, to the Large Interferometer for Exoplanets (LIFE) for analyzing the thermal portion of the planetary spectrum. Using nulling interferometry, LIFE will allow us to constrain the radius and effective temperature of (terrestrial) exoplanets, as well as provide unique information about their atmospheric structure and composition. Aims. We explore the potential of LIFE for characterizing emission spectra of Earth at various stages of its evolution. This allows us (1) to test the robustness of Bayesian atmospheric retrieval frameworks when branching out from a modern Earth scenario while still remaining in the realm of habitable (and inhabited) exoplanets, and (2) to refine the science requirements for LIFE for the detection and characterization of habitable, terrestrial exoplanets. Methods. We performed Bayesian retrievals on simulated spectra of eight different scenarios, which correspond to cloud-free and cloudy spectra of four different epochs of the evolution of the Earth. Assuming a distance of 10 pc and a Sun-like host star, we simulated observations obtained with LIFE using its simulator LIFEsim, considering all major astrophysical noise sources. Results. With the nominal spectral resolution (R = 50) and signal-to-noise ratio (assumed to be S/N = 10 at 11.2 μm), we can identify the main spectral features of all the analyzed scenarios (most notably CO2, H2O, O3, and CH4). This allows us to distinguish between inhabited and lifeless scenarios. Results suggest that O3 and CH4 in particular yield an improved abundance estimate by doubling the S/N from 10 to 20. Neglecting clouds in the retrieval still allows for a correct characterization of the atmospheric composition. However, correct cloud modeling is necessary to avoid biases in the retrieval of the correct thermal structure. Conclusions. From this analysis, we conclude that the baseline requirements for R and S/N are sufficient for LIFE to detect O3 and CH4 in the atmosphere of an Earth-like planet with an O2 abundance of around 2% in volume mixing ratio. Doubling the S/N would allow a clearer detection of these species at lower abundances. This information is relevant in terms of the LIFE mission planning. We also conclude that cloud-free retrievals of cloudy planets can be used to characterize the atmospheric composition of terrestrial habitable planets, but not the thermal structure of the atmosphere. From the inter-model comparison performed, we deduce that differences in the opacity tables (caused by, e.g., a different line wing treatment) may be an important source of systematic errors.