 Disequilibrium chemistry in an exoplanet atmosphere

Disequilibrium chemistry in an exoplanet atmosphere occurs when transport or externally driven reactions prevent the composition from reaching local thermochemical equilibrium. Deep gas brought upward can retain its earlier composition if chemical reactions become too slow in cooler layers. At higher altitudes, atmospheric photochemistry can instead destroy or create species using stellar photons.

Models coupling chemical kinetics with vertical diffusion for HD 189733 b and HD 209458 b demonstrate that chemical quench levels and photochemical destruction can affect the same molecule at different heights.^[ref] An atmospheric retrieval that assumes equilibrium everywhere can consequently infer a different composition or atmospheric pressure-temperature profile from one that allows transport.