Disequilibrium Carbon, Oxygen, and Nitrogen Chemistry in the Atmospheres of HD 189733b and HD 209458b (1102.0063v2)

Abstract: We have developed 1-D photochemical and thermochemical kinetics and diffusion models for the transiting exoplanets HD 189733b and HD 209458b to study the effects of disequilibrium chemistry on the atmospheric composition of "hot Jupiters." Here we investigate the coupled chemistry of neutral carbon, hydrogen, oxygen, and nitrogen species, and we compare the model results with existing transit and eclipse observations. We find that the vertical profiles of molecular constituents are significantly affected by transport-induced quenching and photochemistry, particularly on cooler HD 189733b; however, the warmer stratospheric temperatures on HD 209458b can help maintain thermochemical equilibrium and reduce the effects of disequilibrium chemistry. For both planets, the methane and ammonia mole fractions are found to be enhanced over their equilibrium values at pressures of a few bar to less than a mbar due to transport-induced quenching, but CH4 and NH3 are photochemically removed at higher altitudes. Atomic species, unsaturated hydrocarbons (particularly C2H2), some nitriles (particularly HCN), and radicals like OH, CH3, and NH2 are enhanced overequilibrium predictions because of quenching and photochemistry. In contrast, CO, H2O, N2, and CO2 more closely follow their equilibrium profiles, except at pressures < 1 microbar, where CO, H2O, and N2 are photochemically destroyed and CO2 is produced before its eventual high-altitude destruction. The enhanced abundances of HCN, CH4, and NH3 in particular are expected to affect the spectral signatures and thermal profiles HD 189733b and other, relatively cool, close-in transiting exoplanets. We examine the sensitivity of our results to the assumed temperature structure and eddy diffusion coefficientss and discuss further observational consequences of these models.

• The paper demonstrates that transport-induced quenching and photochemistry significantly alter molecular abundances in hot Jupiter atmospheres.
• The authors employ a one-dimensional kinetics and diffusion model to quantify deviations from equilibrium chemistry for HD 189733b and HD 209458b.
• The findings underscore the need for refined high-temperature kinetics data and improved laboratory measurements to enhance exoplanet atmospheric interpretations.

Analyzing Disequilibrium Chemistry on HD 189733b and HD 209458b

The paper by Moses et al. presents a detailed examination of disequilibrium chemistry in the atmospheres of two well-known hot Jupiter exoplanets, HD 189733b and HD 209458b. Utilizing a one-dimensional photochemical and thermochemical kinetics and diffusion model, the authors investigate the complex interplay of carbon, hydrogen, oxygen, and nitrogen chemistry under nonequilibrium conditions. This research has implications for both theoretical understanding and observational interpretation of exoplanetary atmospheres, particularly in assessing chemical composition from transit and eclipse observations.

The research indicates that disequilibrium processes, such as transport-induced quenching and photochemistry, significantly influence atmospheric composition. Transport-induced quenching, which occurs when vertical transport timescales are shorter than chemical conversion timescales, freezes the molecular abundances at values different from their equilibrium predictions. This process is more pronounced on cooler exoplanets like HD 189733b compared to warmer ones like HD 209458b, where stratospheric thermal inversions help maintain equilibrium at high altitudes.

Key findings of the paper include the enhanced abundance of transport-quenched species such as methane (CH₄) and ammonia (NH₃) over their equilibrium values in certain atmospheric layers, although these species are efficiently removed by photochemical processes at higher altitudes. The research also highlights the significant role of photochemistry in driving the abundance of atomic species like H, C, and O, as well as radicals and compounds such as CH₃, OH, C₂H₂, and HCN, thereby altering atmospheric spectra.

The presence of disequilibrium products has observational consequences, as these species can alter spectral signatures, influencing both transit and secondary eclipse observations. Notably, while photochemistry impacts HD 189733b more significantly due to its cooler temperatures, its effects are relatively muted for the hotter HD 209458b.

From a broader perspective, this work emphasizes the importance of integrating disequilibrium chemistry into models for a more accurate interpretation of exoplanetary atmospheres. The research encourages the inclusion of high-temperature kinetics data, as well as enhanced laboratory measurements of molecular line parameters, to refine the understanding and prediction of atmospheric compositions. The paper anticipates the potential of future telescopic missions such as JWST to provide richer datasets for testing and refining such complex atmospheric models.

In conclusion, the paper by Moses et al. underscores the complexity of atmospheric chemistry on hot Jupiters and the necessity of considering disequilibrium processes to interpret observations accurately. The enhanced understanding of these processes sets the stage for more detailed future studies and improved observational strategies.