Nitrogen as a Tracer of Giant Planet Formation. II. Comprehensive Study of Nitrogen Photochemistry and Implications for Observing NH₃ 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 NH3 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 NH3 abundance coincides with the bulk nitrogen abundance only at sub-Jupiter-mass (≲1M 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 NH3 abundance computed by VULCAN and thus helps to infer the bulk nitrogen abundance from a retrieved NH3 abundance. By computing transmission and emission spectra, we predict that the equilibrium temperature range of 400–1000 K is optimal for detecting NH3 because NH3 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, NH3 leaves observable signatures of ∼50 ppm at 1.5, 2.1, and 11 μm in transmission spectra and >300–100 ppm at 6 and 11 μm in emission spectra. The photodissociation of NH3 leads HCN to replace NH3 at low pressures. However, the low HCN column densities lead to much weaker absorption features than for NH3. The NH3 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.

Using a Quench Level Approximation to Estimate the Effect of Metallicity on the Abundances of N-bearing Species in H2-dominated Atmospheres

“…For a higher value of K zz , the sweet spot expands toward a higher value of T equi (Figure B4 in Appendix B). Our results are similar to those in Ohno & Fortney (2023b). For the higher T equi , the photochemistry efficiently depletes the NH 3 ; however, the effect of this is beyond the scope of this study, and we refer the reader to Ohno & Fortney (2023b).…”

“…The transit signature of NH 3 /HCN is around 50/100-200/300 ppm for the mixing ratio of ≈10 −6 for a solar elemental composition Ohno & Fortney 2023b). Despite the low abundance of N-bearing species, recent work shows the potential capability of JWST in observing N-bearing species Ohno & Fortney 2023b). It is found that the HCN signature becomes negligible for HCN/H 2 O < 10 −2 .…”

“…The mixing ratio of HCN remains small (≈10 −8 -10 −9 ) in thermochemical equilibrium. The transit signature of NH 3 /HCN is around 50/100-200/300 ppm for the mixing ratio of ≈10 −6 for a solar elemental composition Ohno & Fortney 2023b). Despite the low abundance of N-bearing species, recent work shows the potential capability of JWST in observing N-bearing species Ohno & Fortney 2023b).…”

“…As a result, there is a sweet spot for photochemically produced HCN between 800 and 1400 K (Baeyens et al 2022). In an atmosphere with low temperature and high vertical mixing, photochemically produced HCN can diffuse into the higher pressure region (P > 10 −4 bar) and can imprint its signature on the transmission spectra (Moses et al 2011;Madhusudhan et al 2016;Ohno & Fortney 2023b). Some studies incorporate the zonal wind (mixing along the latitude) and meridional wind (mixing along the longitude) and found that NH 3 and HCN can be greatly affected due to horizontal mixing.…”

“…NH 3 is highly photoactive, and the long timescale of the chemical conversion of NH 3 N 2 makes its abundance prone to change due to photochemistry and atmospheric mixing. It has been shown that disequilibrium processes can increase the abundance of NH 3 and HCN at the photospheric pressure by several orders of magnitude in the infrared photosphere (Zahnle et al 2009;Line et al 2011;Moses et al 2011;Madhusudhan 2012;Moses et al 2013a;Heng & Lyons 2016;Tsai et al 2018;Ohno & Fortney 2023a, 2023b. Moses et al (2011) studied the nitrogen chemistry of two exoplanets, HD 189733 b and HD 209458 b, and compared the results of their model with transit and eclipse observations.…”