Presentation #404.02 in the session Catching Big Air: Giant Exoplanet Atmospheres.
Exoplanets that receive stellar irradiance of approximately Earth’s or less have been discovered and many are suitable for spectral characterization. The temperate planets in the mini-Neptune and larger size range provide new opportunities to study the condensation of water and the photochemical processes of major volatiles in a reducing environment. Using a photochemical model, I will show that the photodissociation of NH3 in presence of CH4 results in HCN as the main photochemical product. This mechanism does not require high-energy photons and indicates that HCN should be common in the atmospheres of temperate and gas-rich exoplanets. Also, the photodissociation of CH4 together with H2O leads to CO and CO2, and the ratio between CO2 and CO is controlled by the photodissociation of H2O. The synthesis of hydrocarbon is also suppressed by the elemental H radical produced from the photodissociation of H2O. Temperate planets with super-solar atmospheric metallicity and appreciable internal heat may have additional CO and CO2 from the interior and less NH3 and thus less HCN. JWST provides a wide wavelength coverage in the infrared and the sensitivity to detect the equilibrium gases H2O, CH4, and NH3 and the photochemical gases HCN and CO2 in many cases. Spectral characterization of temperate and gas-rich exoplanets in transit thus promises to expand the types of molecules detected and improve our understanding of the workings of atmospheric chemistry in exoplanetary atmospheres.