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Interpretations of exoplanetary transmission spectra have been undermined byapparent obscuration due to clouds/hazes. Debate rages on whether weak H$_2$Ofeatures seen in exoplanet spectra are due to clouds or inherently depletedoxygen. Assertions of solar H$_2$O abundances have relied on making a priorimodel assumptions, e.g. chemical/radiative equilibrium. In this work, weattempt to address this problem with a new retrieval paradigm for transmissionspectra. We introduce POSEIDON, a two-dimensional atmospheric retrievalalgorithm including generalised inhomogeneous clouds. We demonstrate that thisprescription allows one to break vital degeneracies between clouds andprominent molecular abundances. We apply POSEIDON to the best transmissionspectrum presently available, for the hot Jupiter HD 209458b, uncovering newinsights into its atmosphere at the day-night terminator. We extensivelyexplore the parameter space with an unprecedented 10$^8$ models, spanning thecontinuum from fully cloudy to cloud-free atmospheres, in a fully Bayesianretrieval framework. We report the first detection of nitrogen chemistry(NH$_3$ and/or HCN) in an exoplanet atmosphere at 3.7-7.7$\sigma$ confidence,non-uniform cloud coverage at 4.5-5.4$\sigma$, high-altitude hazes at$>$3$\sigma$, and sub-solar H$_2$O at $\gtrsim$3-5$\sigma$, depending on theassumed cloud distribution. We detect NH$_3$ at 3.3$\sigma$ and 4.9$\sigma$ forfully cloudy and cloud-free scenarios, respectively. For the model with thehighest Bayesian evidence, we constrain H$_2$O at 5-15 ppm (0.01-0.03$\times$solar) and NH$_3$ at 0.01-2.7 ppm, strongly suggesting disequilibrium chemistryand cautioning against equilibrium assumptions. Our results herald new promisefor retrieving cloudy atmospheres using high-precision HST and JWST spectra.

HD 209458b in New Light: Evidence of Nitrogen Chemistry, Patchy Clouds and Sub-Solar Water