The atmospheric metallicity of a giant planet describes its atmospheric inventory of elements heavier than hydrogen and helium. It must specify a convention: an elemental abundance ratio relative to hydrogen, an enhancement relative to a reference composition, or a total heavy-element mass fraction are related but different quantities. A quoted enrichment also needs to identify its reference, such as the host star.
Atmospheric retrieval estimates molecular abundances, which must then be connected to elemental inventories through a model of chemical equilibrium or disequilibrium chemistry in an exoplanet atmosphere. Atmospheric condensate rainout can hide elements in particles below the observed levels. Thus an atmospheric enrichment need not directly equal the heavy-element fraction of the whole exoplanet interior.
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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