Nitrogen as a Tracer of Giant Planet Formation. II. Comprehensive Study of Nitrogen Photochemistry and Implications for Observing NH 3 and HCN in Transmission and Emission Spectra

Abstract

Atmospheric nitrogen may provide important constraints on giant planet formation. Following our semianalytical work, we further pursue the relation between observable NH 3 and an atmosphere’s bulk nitrogen abundance by applying the photochemical kinetics model VULCAN across planetary equilibrium temperature, mass, age, eddy diffusion coefficient, atmospheric composition, and stellar spectral type. We confirm that the quenched NH 3 abundance coincides with the bulk nitrogen abundance only at sub-Jupiter-mass (≲1 M J ) planets and old ages (≳1 Gyr) for solar composition atmospheres, highlighting important caveats for inferring atmospheric nitrogen abundances. Our semianalytical model reproduces the quenched NH 3 abundance computed by VULCAN and thus helps to infer the bulk nitrogen abundance from a retrieved NH 3 abundance. By computing transmission and emission spectra, we predict that the equilibrium temperature range of 400–1000 K is optimal for detecting NH 3 because NH 3 depletion by thermochemistry and photochemistry is significant at hotter planets whereas entire spectral features become weak at colder planets. For Jupiter-mass planets around Sun-like stars in this temperature range, NH 3 leaves observable signatures of ∼50 ppm at 1.5, 2.1, and 11 μ m in transmission spectra and >300–100 ppm at 6 and 11 μ m in emission spectra. The photodissociation of NH 3 leads HCN to replace NH 3 at low pressures. However, the low HCN column densities lead to much weaker absorption features than for NH 3 . The NH 3 features are readily accessible to JWST observations to constrain atmospheric nitrogen abundances, which may open a new avenue to understanding the formation processes of giant exoplanets.