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.^[ref]