Impact of Surface Pressure on Nitrogen-Dominated USP Super-Earths Atmospheres

Abstract In this work, we compared the chemistry and emission spectra of nitrogen-dominated Ultra-Short-Period (USP) super-Earth atmospheres at various surface pressures (0.1 to 10 bar) in and out of chemical equilibrium. We used the VULCAN chemical kinetic code, which includes thermochemical kinetics, vertical transport, and photochemistry, linked with the petitRADTRANS radiative transfer model to predict emission spectra. Radiative-convective temperature-pressure profiles were computed with the HELIOS code. Using PandExo noise simulations, we explored the observability of disequilibrium effects with JWST. Our results showed that surface pressure significantly impacted temperature profiles, atmospheric abundances, and emission spectra. Disequilibrium signatures in cooler planets were detectable by targeting HCN, C 2 H 4 , and CO, while warmer planets showed CH 4 and HCN features using JWST’s NIRSpec and MIRI instruments. These species also provided insights into the presence of surfaces in nitrogen-dominated atmospheres, revealing details about atmospheric thickness.