= Reference-pressure degeneracy in transmission spectroscopy
The <reference-pressure degeneracy in transmission spectroscopy> is an ambiguity between the <pressure> assigned to a reference planetary <radius> and an inferred absorbing abundance. In a thin isothermal <ideal gas>, the <slant optical depth of an isothermal atmosphere> is approximately proportional to $X P_0 e^{-z/H}$, where $X$ is an absorber's number fraction, $P_0$ the <pressure> at the reference <radius>, and $H$ the <atmospheric scale height>. Increasing $X$ while decreasing $P_0$ by the same factor can therefore preserve a simple <exoplanet transmission spectrum>.
A measured molecular band need not determine an absolute mixing ratio when the reference <pressure> is assumed rather than constrained. Additional spectral information with a different <pressure> dependence can help, but the degeneracy is model- and data-dependent.<The theory of transmission spectra revisited by Kevin Heng and Daniel Kitzmann>[Abstract: reference radius, reference pressure, and water-abundance degeneracy in WFC3 spectra]{ref}
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