2022ApJ...930...62G

The search for signs of life on other worlds has largely focused on terrestrial planets. Recent work, however, argues that life could exist in the atmospheres of temperate sub-Neptunes. Here we evaluate the usefulness of carbon dioxide isotopologues as evidence of aerial life. Carbon isotopes are of particular interest, as metabolic processes preferentially use the lighter ¹²C over ¹³C. In principle, the upcoming James Webb Space Telescope (JWST) will be able to spectrally resolve the ¹²C and ¹³C isotopologues of CO₂, but not CO and CH₄. We simulated observations of CO₂ isotopologues in the H₂-dominated atmospheres of our nearest (<40 pc), temperate (equilibrium temperature of 250-350 K) sub-Neptunes with M-dwarf host stars. We find ¹³CO₂ and ¹²CO₂ distinguishable if the atmosphere is H₂ dominated with a few percentage points of CO₂ for the most idealized target with an Earth-like composition of the two most abundant isotopologues, ¹²CO₂ and ¹³CO₂. With a Neptune-like metallicity of 100x solar and a C/O of 0.55, we are unable to distinguish between ¹³CO₂ and ¹²CO₂ in the atmospheres of temperate sub-Neptunes. If atmospheric composition largely follows metallicity scaling, the concentration of CO₂ in a H₂-dominated atmosphere will be too low to distinguish CO₂ isotopologues with JWST. In contrast, at higher metallicities, there will be more CO₂, but the smaller atmospheric scale height makes the measurement impossible. Carbon dioxide isotopologues are unlikely to be useful biosignature gases for the JWST era. Instead, isotopologue measurements should be used to evaluate formation mechanisms of planets and exosystems.