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Observations suggest an abundance of water and paucity of methane in themajority of observed exoplanetary atmospheres. We isolate the effect ofatmospheric processes to investigate possible causes. Previously, we studiedthe effect of effective temperature, surface gravity, metallicity,carbon-to-oxygen ratio, and stellar type assuming cloud-free thermochemicalequilibrium and disequilibrium chemistry. However, under these assumptions,methane remains a persisting spectral feature in the transmission spectra ofexoplanets over a certain parameter space, the Methane Valley. In this work weinvestigate the role of clouds on this domain and we find that clouds changethe spectral appearance of methane in two direct ways: 1) by heating-up thephotosphere of colder planets, and 2) by obscuring molecular features. Thepresence of clouds also affects methane features indirectly: 1) cloud heatingresults in more evaporation of condensates and hence releases additionaloxygen, causing water dominated spectra of colder carbon-poor exoplanets, and2) HCN/CO production results in a suppression of depleted methane features bythese molecules. The presence of HCN/CO and a lack of methane could be anindication of cloud formation on hot exoplanets. Cloud heating can also depleteammonia. Therefore, a simultaneous depletion of methane and ammonia is notunique to photochemical processes. We propose that the best targets for methanedetection are likely to be massive but smaller planets with a temperaturearound 1450 K orbiting colder stars. We also construct Spitzer syntheticcolor-maps and find that clouds can explain some of the high contrastobservations by IRAC's channel 1 and 2.

The role of clouds on the depletion of methane and water dominance in the transmission spectra of irradiated exoplanets