Maintenance of atmospheric biosignatures across the inner habitable zone for earthlike planets

With ongoing missions such as the James Webb Space Telescope and planned initiatives such as the Large Interferometer For Exoplanets (LIFE), the detection and attribution of biosignatures in exoplanetary atmospheres increasingly becomes a point of focus. However, in order to assess how different biosignatures manifest themselves in the atmospheres of rocky exoplanets in contrast to our temperate Earth, improved insights into the maintenance of earthlike atmospheric biosignatures in different atmospheres are necessary. 

Here, we identify and investigate the main processes and possible couplings between atmospheric climate and photochemistry for earthlike planets across the inner habitable zone. We also study the detectability of the modelled spectral features with the LIFE simulator to assess how the atmospheres of planets with an earthlike biosphere may appear to future missions like the LIFE interferometer. 

We use the global-mean, stationary, coupled climate-chemistry column model, 1D-TERRA, to simulate the climate and chemistry of planetary atmospheres at different distances from the Sun, initially assuming Earth's planetary parameters and evolution. We run six scenarios: we assume rocky exoplanets with Earth’s biomass fluxes forming around the Sun with insolation from 100% to 150% in steps of 10%. From the resulting output of temperature and composition profiles, we calculate theoretical transmission and emission spectra using a radiative transfer model (GARLIC).

The models show moderate ocean evaporation as the planet moves closer to the Sun, which results in water-vapour-rich atmospheres with the partial pressures of steam ranging from about 0.01 bar (modern Earth insolation, S=1) up to 0.6 bar (S=1.5). In the latter model,  the global mean surface temperature increases to 365.4K. This is mainly due to the higher energy input and the enhanced greenhouse effect due to increased amounts of water vapour in the atmosphere. Regarding key atmospheric biosignatures, ozone, surprisingly, mostly survives in the middle atmosphere in all scenarios, mainly because hydrogen oxide abundances, a catalytic sink for ozone, are prevented from strongly increasing due to reactions with nitrogen oxides. Methane is strongly removed for insolations above 20% those of Earth, because rising water abundances strongly increase hydroxyl (OH) (via UV photolysis) the main sink for methane. Nitrous oxide (N2O) generally survives, mainly due to trade-off effects where enhanced photolytic loss on upper layers due to higher insolation is counterbalanced by stronger absorption of photons on the lower layers due to enhanced water from evaporation. Hydrogen escape rates are 0.690 Tg/yr for the highest insolation scenario. Abiotic oxygen production associated with atmospheric escape of atomic hydrogen as well as catalytic in-situ recycling of oxygen atoms present in HOx species, lead to an increase in the O2 vmr to 0.35 mol/mol on increasing solar insolation from S = 1.0-1.3. For all scenarios, the simulated transmission and emission spectra show clearly evident H2O and CH4 features in the near to mid IR, strong CO2 absorption around 15 microns, and O3 absorption at around 9.6 microns.