The three parameter groups are local temperature, local pressure, and elemental composition. The elemental composition includes both atmospheric metallicity of a giant planet and atmospheric carbon-to-oxygen ratio; these are independent compositional controls, so a specification by and alone is incomplete. Given these inputs, thermochemical equilibrium minimizes Gibbs free energy with element conservation.
- The carbon monoxide-to-methane partition. For , cooler gas and, at fixed mole fractions, larger pressure favour the side with fewer molecules, whereas hotter gas generally favours CO. Thus relatively hot photospheres commonly show CO near and , while cooler conditions favour methane bands near . The exact boundary depends on composition and pressure; carbon monoxide–methane quenching can invalidate an equilibrium diagnosis.
- Water absorption, for example around and , traces the oxygen left after CO forms. In oxygen-rich gas with , water is often prominent; carbon-rich gas can have much less water and relatively more methane or hydrogen cyanide. Condensation and high-temperature dissociation are exceptions to a simple elemental bookkeeping argument.
- Carbon dioxide, with a strong band near , is sensitive to enrichment and oxidation chemistry. In a hydrogen-dominated trace-species regime, gives at fixed , so CO2 can grow roughly quadratically with heavy-element enrichment before the trace approximation fails.
CO/CH4, H2O, and CO2 bands provide complementary temperature and elemental-abundance diagnostics, but their observed strengths also depend on the atmospheric pressure-temperature profile, exoplanet clouds and observing geometry. A spectral feature is not by itself a direct equilibrium abundance.
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