Nitrogen as a Tracer of Giant Planet Formation. I. A Universal Deep Adiabatic Profile and Semianalytical Predictions of Disequilibrium Ammonia Abundances in Warm Exoplanetary Atmospheres

Abstract

A major motivation of spectroscopic observations of giant exoplanets is to unveil planet formation processes from atmospheric compositions. Several recent studies suggested that atmospheric nitrogen, like carbon and oxygen, can provide important constraints on planetary formation environments. Since nitrogen chemistry can be far from thermochemical equilibrium in warm atmospheres, we extensively investigate under what conditions, and with what assumptions, the observable NH3 abundances can diagnose an atmosphere’s bulk nitrogen abundance. In the first paper of this series, we investigate atmospheric T–P profiles across equilibrium temperature, surface gravity, intrinsic temperature, atmospheric metallicity, and C/O ratio using a 1D radiative–convective equilibrium model. Models with the same intrinsic temperature and surface gravity coincide with a shared “universal” adiabat in the deep atmosphere, across a wide equilibrium temperature range (250–1200 K), which is not seen in hotter or cooler models. We explain this behavior in terms of the classic “radiative zero solution” and then establish a semianalytical T–P profile of the deep atmospheres of warm exoplanets. This profile is then used to predict vertically quenched NH3 abundances. At solar metallicity, our results show that the quenched NH3 abundance only coincides with the bulk nitrogen abundance (within 10%) at low intrinsic temperature, corresponding to a planet with a sub-Jupiter mass (≲1 M J) and old age (≳1 Gyr). If a planet has a high-metallicity (≳10× solar) atmosphere, the quenched NH3 abundance significantly underestimates the bulk nitrogen abundance at almost all planetary masses and ages. We suggest modeling and observational strategies to improve the assessment of bulk nitrogen from NH3.

Photochemically produced SO2 in the atmosphere of WASP-39b

“…We further perform photochemical calculations using VULCAN with a grid of temperature profiles across planetary equilibrium temperatures 600-2,000 K, adopted from the 1D radiative-convective equilibrium models applied in ref. 39, in which an internal temperature of 100 K with perfect heat redistribution and gravity g = 1,000 cm s −2 are assumed. Apart from the thermal profiles, we keep the rest of the planetary parameters the same as the WASP-39b model in this work, including stellar UV irradiation.…”

“…b, The average VMR between 10 and 0.01 mbar as a function of planetary equilibrium temperature with temperature profiles adopted from ref. 39 (see text for the setup). The dotted grey line marks approximately the required SO 2 concentration to be detectable with WASP-39b parameters.…”

Volatile-to-sulfur Ratios Can Recover a Gas Giant’s Accretion History

“…Other axes beyond C/O may therefore be necessary if we hope to determine how and where a given planet may have formed. Although numerous groups have explored the dependence of atmospheric nitrogen abundance (as parameterized by N/O) on a planet's formation and accretion (Turrini et al 2021;Schneider & Bitsch 2021b;Ohno & Fortney 2023, measuring a planet's N abundance is only feasible at temperatures cooler than that of most hot Jupiters (since at T > 1000 K most N is tied up in N 2 ).…”

“…In this work we have not explored planetary nitrogen abundance, but it may also be an important axis. For example, the nitrogen-to-oxygen ratio in gas and solids should vary by several orders of magnitude from 1 to 100 au (Ohno & Fortney 2023). And if planet formation is dominated by planetesimal accretion, then the sulfur-to-nitrogen ratio may be a useful probe of whether a gas giant accreted most of its mass via solids or gas (Turrini et al 2021).…”

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

“…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.…”