Observability of molecular species in a nitrogen dominated atmosphere for 55 Cancri e.

One of the key goals of exoplanet science is the atmospheric characterisation of super-Earths. Atmospheric abundances provide insight on the formation and evolution of those planets and help to put our own rocky planets in context. Observations on 55 Cancri e point towards a N-dominated atmosphere. In this paper we explore this possibility, showing which will be the most abundant gases and observable species in emission and transmission spectroscopy of such an atmosphere. We use analytical arguments and observed parameters to estimate the possible thermal profile of the atmosphere and test three different extreme possibilities. The chemistry is calculated using equilibrium calculations and adopting Titan's elemental abundances as a potential N-dominated atmospheric composition. We also test the effect of different N/O ratios in the atmosphere. Emission and transmission spectra are computed and showed with a resolution relevant to future missions suitable to observe super-Earths (e.g. JWST, ARIEL). We find that even though N$_2$ is the most abundant molecule in the atmosphere followed by H$_2$ and CO, the transmission spectra shows strong features of NH3 and HCN, and CO and HCN dominate emission spectra. We also show that a decrease in the N/O ratio leads to stronger H2O, CO and CO2 and weaker NH3 and HCN features. A larger N/O is also more consistent with observations. Our exploration of a N-atmosphere for 55 Cancri e serve as a guide to understand such atmospheres and provide a reference for future observations.