2020AJ....160..288F

The atmospheric pressure-temperature profiles for transiting giant planets cross a range of chemical transitions. Here we show that the particular shapes of these irradiated profiles for warm giant planets below ∼1300 K lead to striking differences in the behavior of nonequilibrium chemistry compared to brown dwarfs of similar temperatures. Our particular focus is H₂O, CO, CH₄, CO₂, and NH₃ in Jupiter- and Neptune-class planets. We show that the cooling history of a planet, which depends most significantly on planetary mass and age, can have a dominant effect on abundances in the visible atmosphere, often swamping trends one might expect based on T_eq alone. The onset of detectable CH₄ in spectra can be delayed to lower T_eq for some planets compared to equilibrium, or pushed to higher T_eq. The detectability of NH₃ is typically enhanced compared to equilibrium expectations, which is opposite to the brown dwarf case. We find that both CH₄ and NH₃ can become detectable at around the same T_eq (at T_eq values that vary with mass and metallicity), whereas these "onset" temperatures are widely spaced for brown dwarfs. We suggest observational strategies to search for atmospheric trends and stress that nonequilibrium chemistry and clouds can serve as probes of atmospheric physics. As examples of atmospheric complexity, we assess three Neptune-class planets, GJ 436b, GJ 3470b, and WASP-107, all around T_eq = 700 K. Tidal heating due to eccentricity damping in all three planets heats the deep atmosphere by thousands of degrees and may explain the absence of CH₄ in these cool atmospheres. Atmospheric abundances must be interpreted in the context of physical characteristics of the planet.