Photochemistry in Terrestrial Exoplanet Atmospheres. III. Photochemistry and Thermochemistry in Thick Atmospheres on Super Earths and Mini Neptunes

Some super Earths and mini Neptunes will likely have thick atmospheres that are not H_2-dominated. We have developed a photochemistry-thermochemistry kinetic-transport model for exploring the compositions of thick atmospheres on super Earths and mini Neptunes, applicable for both H_2-dominated atmospheres and non-H_2-dominated atmospheres. Using this model to study thick atmospheres for wide ranges of temperatures and elemental abundances, we classify them into hydrogen-rich atmospheres, water-rich atmospheres, oxygen-rich atmospheres, and hydrocarbon-rich atmospheres. We find that carbon has to be in the form of CO_2 rather than CH_4 or CO in a H_2-depleted water-dominated thick atmosphere and that the preferred loss of light elements from an oxygen-poor carbon-rich atmosphere leads to the formation of unsaturated hydrocarbons (C_2H_2 and C_2H_4). We apply our self-consistent atmosphere models to compute spectra and diagnostic features for known transiting low-mass exoplanets GJ 1214 b, HD 97658 b, and 55 Cnc e. For GJ 1214 b, we find that (1) C_2H_2 features at 1.0 and 1.5 μm in transmission and C_2H_2 and C_2H_4 features at 9-14 μm in thermal emission are diagnostic for hydrocarbon-rich atmospheres; (2) a detection of water-vapor features and a confirmation of the nonexistence of methane features would provide sufficient evidence for a water-dominated atmosphere. In general, our simulations show that chemical stability has to be taken into account when interpreting the spectrum of a super Earth/mini Neptune. Water-dominated atmospheres only exist for carbon to oxygen ratios much lower than the solar ratio, suggesting that this kind of atmospheres could be rare