Atmospheric metallicity measures enrichment in elements heavier than hydrogen and helium relative to the host star. Solar-System giant planets suggest decreasing enrichment with increasing planetary mass, although clouds, condensation, and formation history complicate comparisons.
The atmospheric carbon-to-oxygen ratio compares the elemental abundances of carbon and oxygen in the atmosphere. It influences molecular composition under chemical equilibrium, but condensation, disequilibrium chemistry in an exoplanet atmosphere, and incomplete spectral coverage complicate its inference from molecular bands.
In hot hydrogen-rich thermochemical equilibrium, much carbon can be bound in carbon monoxide, consuming oxygen that is unavailable for water. At a C/O ratio near or above one, water can be strongly depleted. Cooler methane-dominated chemistry can release more oxygen for water at the same elemental inventory. Pressure, quenching, dissociation and atmospheric photochemistry can modify this temperature-dependent partition.
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