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

TL;DR

This study evaluates the Large Interferometer For Exoplanets (LIFE) capability to detect and interpret 4D atmospheric variability on nearby rocky exoplanets, focusing on Proxima Centauri b in 1:1 and 3:2 spin-orbit resonances. By combining a 4D climate-chemistry model (UM-UKCA) with mid-infrared spectral synthesis (PSG GlobES) and LIFE signal simulations (LIFEsim), the authors show that phase-resolved MIR spectroscopy can distinguish spin-orbit states and reveal daily 4D atmospheric states, including temperature, clouds, and O3 distributions. They demonstrate significant phase-dependent variability in O3, CO2, and H2O features, with strong phase contrasts in the synchronous case and more homogeneous emission in the eccentric case, while also highlighting potential abiotic O2/O3 false positives. The work emphasizes the necessity of phase-resolved observations, robust 4D modelling, and carefully planned observing strategies to exploit LIFE's capability for characterising nearby terrestrial worlds and assessing biosignatures.

Abstract

Future telescopes such as the Large Interferometer For Exoplanets (LIFE) will enable mid-infrared characterisation of the atmospheres of nearby rocky exoplanets. Whilst 4D spatial and temporal variations of Earth as an exoplanet are below spectroscopic detection limits, such variability is planet-specific. We investigate LIFE's ability to detect 4D variability in the atmospheres of tidally locked exoplanets. We create 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 output of an aquaplanet with Earth-like composition. 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 can be reliably inferred from snapshot spectra at arbitrary viewing geometries. In the 1:1 SOR, 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, and 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 like Proxima Centauri b. In case of abiotic O2/O3 build-up, the O3 variability presents a potential false positive scenario. Hence, LIFE can disentangle different spin-orbit states and resolve 4D atmospheric variability, enabling daily characterisation of the 4D physical and chemical state of nearby terrestrial worlds. Importantly, this characterisation requires phase-resolved rather than snapshot spectra.

Figures (9)

• Figure 1: Observed hemispheric distributions as a function of $\theta(t)$ for the 1:1 SOR (top) and the 3:2 SOR with an eccentricity of 0.3 (bottom): surface temperature (blue-red), vertically integrated water vapour or H$_2$O (g) column density (white-blue), vertically integrated total cloud path (white-grey), and vertically integrated O$_3$ column density (viridis). The longitude of perihelion of the 3:2 SOR is at 102.94$^\circ$. For both SORs, we use four extreme cases of $\theta(t)$ in Table \ref{['tab:phase_angles_pcb']}. The distributions vary spatially and temporally, showing the orbital evolution of the climate and chemistry and effects of viewing geometry.
• Figure 2: Phase angle evolution of the hemispheric means across the observed hemisphere of Proxima Centauri b in 1:1 SOR (navy) and 3:2 SOR (orange), for the quantities shown in Figure \ref{['fig:pcb_3dchem_distrib_phase']} and $\overline{\chi_{O3, Strat}}$, the mean volume mixing ratio of O$_3$ in the stratosphere. The hemispheric means are given in Tables \ref{['tab:mean_3d_diagnostics_11']} and \ref{['tab:mean_3d_diagnostics_32']}.
• Figure 3: Simulated spectral radiance at the top of the atmosphere for Proxima Centauri b in (a) 1:1 SOR and (b) 3:2 SOR, for $\theta(t)$ as shown in Table \ref{['tab:phase_angles_pcb']}. Spectra were created using PSG and the GlobES tool (see Section \ref{['sec:PSG_description']} for details). We also include blackbody curves at different temperatures for comparison and grey rectangles for important molecular and collision-induced absorption (CIA) features. For panel (b), the solid and dashed lines represent the first and second orbits around the host star, respectively, and together correspond to the length of a full day for the 3:2 SOR.
• Figure 4: Example simulated LIFE observation for the 1:1 resonance case (left) and 3:2 resonance case (right) assuming an integration time of 24 hours. The grey area represents the 1-$\sigma$ sensitivity; the dark grey error bars show an individual simulated observation. Lower panel: Statistical significance of the detected differences between different phases.
• Figure 5: Pairs of simulated LIFE spectra for the 1:1 SOR at similar distances from phase angles of 90 and 270$^{\circ}$, illustrating the effects of the atmospheric circulation. Panel a shows spectra for phase angles 98 and 265$^{\circ}$, panel b for 130 and 233$^{\circ}$, and panel c for 66 and 297$^{\circ}$.