Observing spatial and temporal variations in the atmospheric chemistry of rocky exoplanets: Prospects for mid-infrared spectroscopy

Context. Future telescopes such as the Large Interferometer For Exoplanets (LIFE) will enable the unprecedented characterisation of the atmospheres of nearby rocky exoplanets, probing mid-infrared signatures of key molecules (e.g. CO2, H2O, O3, and CH4). Whilst 4D spatial and temporal variations of Earth as an exoplanet are below spectroscopic detection limits, such variability is strongly planet-specific. Aims. We investigated LIFE’s ability to detect 4D spatial and temporal variability in the atmospheres of tidally locked exoplanets. Methods. We created daily synthetic LIFE observations of Proxima Centauri b in a 1:1 and an eccentric 3:2 spin-orbit resonance (SOR), using LIFESIM on spectra from daily 3D climate-chemistry model (CCM) outputs of an aquaplanet with Earth-like composition. The spectra assume an inclination of 70◦. Results. Hemispheric distributions of temperature, clouds, and chemical species determine spectral signatures and variability with orbital phase angle. Such variability dictates the extent to which parameters (e.g. radius, temperature, or chemical abundances) can be reliably inferred from snapshot spectra at arbitrary viewing geometries. In the 1:1 SOR, the MIR spectra vary significantly with viewing geometry and indirectly probe atmospheric circulation. Nightside temperature inversions generate O3, CO2, and H2O emission features, though these lie below LIFE’s detection threshold; instead, O3 features disappear at certain phase angles. In contrast, the 3:2 SOR yields a more homogeneous atmosphere with weaker phase variability but enhanced bolometric flux due to eccentric heating. Phase-resolved LIFE observations confidently distinguish between the SORs and capture seasonal O3 variability for golden targets such as Proxima Centauri b. In the case of abiotic O2 and O3 build-up, the O3 variability presents a potential false positive scenario. Conclusions. Hence, LIFE can disentangle different spin-orbit states and resolve 4D atmospheric variability, enabling the daily characterisation of the 4D physical and chemical state of nearby terrestrial worlds. Importantly, this characterisation requires phase-resolved rather than snapshot spectra.