Disequilibrium carbon, oxygen, and nitrogen chemistry in the atmospheres of HD189733b and HD209458b

Julianne I. Moses, C. Visscher, J. J. Fortney, A. P. Showman, N. K. Lewis, C. A. Griffith, S. J. Klippenstein, M. Shabram, A. J. Friedson, M. S. Marley, R. S. Freedman

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293 Scopus citations

Abstract

We have developed a one-dimensional photochemical and thermochemical kinetics and diffusion model to study the effects of disequilibrium chemistry on the atmospheric composition of "hot-Jupiter" exoplanets. Here we investigate the coupled chemistry of neutral carbon, hydrogen, oxygen, and nitrogen species on HD189733b and HD209458b 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 the cooler HD189733b; however, the warmer stratospheric temperatures on HD209458b 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 an mbar due to transport-induced quenching, but CH4 and NH 3 are photochemically removed at higher altitudes. Disequilibrium chemistry also enhances atomic species, unsaturated hydrocarbons (particularly C2H2), some nitriles (particularly HCN), and radicals like OH, CH3, and NH2. In contrast, CO, H2O, N 2, and CO2 more closely follow their equilibrium profiles, except at pressures ≲1μbar, where CO, H2O, and N2 are photochemically destroyed and CO2 is produced before its eventual high-altitude destruction. The enhanced abundances of CH4, NH3, and HCN are expected to affect the spectral signatures and thermal profiles of HD189733b and other relatively cool, transiting exoplanets. We examine the sensitivity of our results to the assumed temperature structure and eddy diffusion coefficients and discuss further observational consequences of these models.

Original languageEnglish (US)
Article number15
JournalAstrophysical Journal
Volume737
Issue number1
DOIs
StatePublished - Aug 10 2011

Keywords

  • planetary systems
  • planets and satellites: atmospheres
  • planets and satellites: composition
  • planets and satellites: individual (HD 189733b, HD 209458b)
  • stars: individual (HD 189733, HD 209458)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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