Chemical kinetics on extrasolar planets

Abstract

Chemical kinetics plays an important role in controlling the atmospheric composition of all planetary atmospheres, including those of extrasolar planets. For the hottest exoplanets, the composition can closely follow thermochemical-equilibrium predictions, at least in the visible and infrared photosphere at dayside (eclipse) conditions. However, for atmospheric temperatures approximately <2000K, and in the uppermost atmosphere at any temperature, chemical kinetics matters. The two key mechanisms by which kinetic processes drive an exoplanet atmosphere out of equilibrium are photochemistry and transport-induced quenching. I review these disequilibrium processes in detail, discuss observational consequences and examine some of the current evidence for kinetic processes on extrasolar planets.

Keywords: atmospheric chemistry; chemical kinetics; exoplanets; extrasolar planets; photochemistry; planetary atmospheres.

Figure 1.

Mole fractions (volume mixing ratios) of (a) CH₄, (b) NH₃, (c) C₂H₂, and (d) HCN from chemical models of HD 189733b, assuming thermochemical equilibrium (green dashed curves), or including photochemical kinetics and transport with the Moses et al. [62] reaction mechanism (red solid curves) or with the Venot et al. [66] reaction mechanism (blue solid curves). The dayside thermal structure and nominal eddy diffusion coefficient profile from [62] were used throughout the modeling [see also 66,90]. Note that the kinetics models begin to diverge from the equilibrium profiles at different depths due to differences in the adopted reaction mechanism. The resulting “quenched” abundances therefore differ between models, despite the same assumptions concerning atmospheric transport. See Venot et al. [66] for similar figures. Online version is in color.

Figure 2.

Thermal profiles for the hypothetical “hot”, “warm”, and “cool” exoplanets (as labeled) used in the chemical models shown in Fig. 3. The gray dashed lines represent the equal-abundance curves for CH₄-CO and NH₃-N₂. Profiles to the right of these curves are within the N₂ and/or CO stability fields. The dot-dashed lines show the condensation curves for MgSiO₃, Mg₂SiO₄, and Fe (solid, liquid) [112]. Online version is in color.

Figure 3.

Mole fraction (volume mixing ratio) profiles for our generic cool (top), warm (middle), and hot (bottom) exoplanets, assuming thermochemical equilibrium (left) or thermochemical and photochemical kinetics and transport (right). All models assume a solar elemental composition, and the transport models assume a uniform eddy diffusion coefficient of 10⁹ cm² s⁻¹. Note the decrease in the the importance of CH₄ and NH₃ going from the “cool” to the “hot” exoplanet. Online version is in color.

Figure 4.

(Left) Synthetic transit spectra for solar-composition HD 189733b models assuming thermochemical equilibrium (blue) or considering thermochemical/photochemical kinetics and transport (red). Absorption depth is plotted as the square of the apparent planet-to-star radius ratio (figure is modified from [62], with spectral calculations from J. J. Fortney). (Right) Synthetic eclipse emission spectra for solar-composition HD 189733b models assuming thermochemical equilibrium (blue) or considering thermochemical/photochemical kinetics and transport (red), plotted in terms of the flux of the planet divided by the flux of the star (figure is modified from [62], with spectral calculations from C. A. Griffith). Disequilibrium molecules responsible for absorption bands in the emission spectrum are labeled specifically. Online version is in color.

Figure 5.

Synthetic eclipse spectra for HD 189733b from a Moses et al. [63] disequilibrium chemistry model that assumes a C/O ratio of 0.7 and a metallicity of 4× solar (solid black line), compared with Spitzer broadband photometric points at 3.6 and 4.5 μm [126] and 5.6, 8, 16, and 24 μm [135, 125, 136] (large red circles with error bars), with Spitzer IRS spectra [29] (medium green circles with error bars), and with HST/NICMOS spectra [33] (small blue circles with error bars). The gray circles without error bars represent the model results convolved over the Spitzer broadband channels. The insert in the upper left shows the thermal profile adopted in the modeling (figure is modified from [63], with spectral calculations from N. Madhusudhan). Online version is in color.