Chemical variation with altitude and longitude on exo-Neptunes: Predictions for Ariel phase-curve observations

Using two-dimensional (2D) thermal structure models and pseudo-2D chemical kinetics models, we explore how atmospheric temperatures and composition change as a function of altitude and longitude within the equatorial regions of close-in transiting Neptune-class exoplanets at different distances from their host stars. Our models predict that the day-night stratospheric temperature contrasts increase with increasing planetary effective temperatures T_eff and that the atmospheric composition changes significantly with T_eff. We find that horizontal transport-induced quenching is very effective in our simulated exo-Neptune atmospheres, acting to homogenize the vertical profiles of species abundances with longitude at stratospheric pressures where infrared observations are sensitive. Our models have important implications for planetary emission observations as a function of orbital phase with the Ariel mission. Cooler solar-composition exo-Neptunes with T_eff = 500-700 K are strongly affected by photochemistry and other disequilibrium chemical processes, but their predicted variations in infrared emission spectra with orbital phase are relatively small, making them less robust phase-curve targets for Ariel observations. Hot solar-composition exo-Neptunes with T_eff ≥ 1300 K exhibit strong variations in infrared emission with orbital phase, making them great targets for constraining global temperatures, energy-balance details, atmospheric dynamics, and the presence of certain high-temperature atmospheric constituents. However, such high-temperature exo-Neptunes are arguably less interesting from an atmospheric chemistry standpoint, with spectral signatures being dominated by a small number of species whose abundances are expected to be constant with longitude and consistent with thermochemical equilibrium. Solar-composition exo-Neptunes with T_eff = 900-1100 K reside in an interesting intermediate regime, with infrared phase curve variations being affected by both temperature and composition variations, albeit at smaller predicted phase-curve amplitudes than for the hotter planets. This interesting intermediate regime shifts to smaller temperatures as atmospheric metallicity is increased, making cool higher-metallicity Neptune-class planets appropriate targets for Ariel phase-curve observations.