Atmospheric chemistry on Uranus and Neptune

Abstract

Comparatively little is known about atmospheric chemistry on Uranus and Neptune, because remote spectral observations of these cold, distant 'Ice Giants' are challenging, and each planet has only been visited by a single spacecraft during brief flybys in the 1980s. Thermochemical equilibrium is expected to control the composition in the deeper, hotter regions of the atmosphere on both planets, but disequilibrium chemical processes such as transport-induced quenching and photochemistry alter the composition in the upper atmospheric regions that can be probed remotely. Surprising disparities in the abundance of disequilibrium chemical products between the two planets point to significant differences in atmospheric transport. The atmospheric composition of Uranus and Neptune can provide critical clues for unravelling details of planet formation and evolution, but only if it is fully understood how and why atmospheric constituents vary in a three-dimensional sense and how material coming in from outside the planet affects observed abundances. Future mission planning should take into account the key outstanding questions that remain unanswered about atmospheric chemistry on Uranus and Neptune, particularly those questions that pertain to planet formation and evolution, and those that address the complex, coupled atmospheric processes that operate on Ice Giants within our solar system and beyond. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.

Keywords: Neptune; Uranus; atmospheric chemistry; photochemistry; planetary atmospheres.

Figure 1.

(a) Global-average temperature–pressure profile of the atmospheres of Uranus and Neptune, with major regions of the atmosphere labelled (modified from Moses et al. [21]). (b) Thermochemical equilibrium prediction of the upper-tropospheric cloud structure on Uranus (modified from Hueso & Sánchez-Lavega [22]). The predicted mass mixing ratios of condensible gases are shown as coloured solid lines, and the maximum cloud density as solid black lines with colour-shaded regions. (Online version in colour.)

Figure 2.

Mole fraction profiles of some key species in the deep troposphere of Uranus (a) and Neptune (b), as predicted from a thermochemical kinetics and diffusion model [95,96]. The deep carbon and oxygen abundances are varied until the model reproduces the observed upper tropospheric CH₄ mixing ratio at the equator [44,45] and the observed upper-tropospheric CO mixing ratio (Neptune) or CO upper limit (Uranus) [53,78]. Figure modified from Venot et al. [95]. (Online version in colour.)

Figure 3.

Mixing ratios of H₂S (a) and CH₄ (b) as a function of latitude on Neptune from models that provide the best fits to the ALMA data of Tollefson et al. [28]. Figure from [28]. (Online version in colour.)

Figure 4.

Vertical mixing-ratio profiles for several stratospheric constituents on Uranus (a) and Neptune (b) as predicted from one-dimensional global-average photochemical models (coloured lines), compared to various observations (data points with associated error bars). The models for both planets include an external source of oxygen from the ablation of interplanetary dust [148], while the Neptune model includes a source of CO from a large cometary impact 200 years ago. The sharp drops in species mixing ratios in the lower stratosphere are caused by condensation. Figure modified from Moses & Poppe [148]. (Online version in colour.)

Figure 5.

Photochemical model predictions [21] for the column abundance of C₂H₆ (a,c) and C₂H₂ (b,d) above 0.1 mbar (a,b) and 1 mbar (c,d) on Neptune as a function of planetocentric latitude and season, where the season is represented by solar longitude L_s (L_s = 0° is northern vernal equinox, L_s = 90° is northern summer solstice, etc.). The white dashed line shows the season at the time of Voyager 2 encounter, and the white solid line shows the season at the current expected launch date (30 March 2021) for the James Webb Space Telescope (JWST). Figure modified from Moses et al. [21]. (Online version in colour.)

Figure 6.

(a) Photochemical model predictions for the time evolution of CO introduced from a large cometary impact on Neptune, along with a smaller steady source from the ablation of icy interplanetary dust grains (Figure modified from Moses & Poppe [148]). (b) Mixing ratio profiles derived for CS, HCN and CO from analyses of the millimetre and sub-millimetre observations of Moreno et al. [42] and Luszcz-Cook & de Pater [53]. Note that although the column abundance of CS is well constrained from the observations, the mixing-ratio profile is not, so multiple models are shown that all provide a good fit to the data [42] (figure modified from Moreno et al. [42]). (Online version in colour.)

Figure 7.

Ion chemistry on Uranus and Neptune. (a) Photochemical model for Neptune’s ionosphere, in which Mg⁺ ions form sharp layers in the lower ionosphere as a result of sinusoidal winds from a hypothetical atmospheric wave; figure modified from [191]. (c) Global photochemical model results for Uranus, illustrating the H₃⁺ column density as a function of latitude and solar local time (SLT); figure from [194]. (b) Photochemical model results for Neptune, illustrating the vertical profiles of major hydrogen and oxygen ions; figure from [150]. (d) Photochemical model results for Neptune, showing the dominant hydrocarbon ions in the lower ionosphere; figure from [150]. (Online version in colour.)