DISEQUILIBRIUM CARBON, OXYGEN, AND NITROGEN CHEMISTRY IN THE ATMOSPHERES OF HD 189733b AND HD 209458b

By J. Moses, C. Visscher, Jon Fortney, A. Showman, N. Lewis, C. Griffith, S. Klippenstein, M. Shabram, A. Friedson, M. Marley and 1 more. 2011

318 citations recorded by the source corpus.

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 HD 189733b and HD 209458b 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 HD 189733b; however, the warmer stratospheric temperatures on HD 209458b 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 NH3 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, N2, and CO2 more closely follow their equilibrium profiles, except at pressures ≲1 μbar, 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 HD 189733b 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/3e52388e71c075fe5af739572468dd41db159758

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. Jon Fortney
3. A. Showman
4. M. Line
5. D. Sing
6. D. Deming
7. J. D'esert
8. H. Knutson

Citations in the corpus

• C/O RATIO AS A DIMENSION FOR CHARACTERIZING EXOPLANETARY ATMOSPHERES (2012, background): “While Pq and Tq can be computed for planet-specific models based on non-equilibrium chemistry (e.g. Madhusudhan & Seager 2011; Moses et al. 2011), we adopt generic values for our current purpose.”
• A SYSTEMATIC RETRIEVAL ANALYSIS OF SECONDARY ECLIPSE SPECTRA. II. A UNIFORM ANALYSIS OF NINE PLANETS AND THEIR C TO O RATIOS (2013, methodology): “We also show a temperature profile from Moses et al. (2011), which is generally cooler than our derived spread.”
• THE IMPRINT OF EXOPLANET FORMATION HISTORY ON OBSERVABLE PRESENT-DAY SPECTRA OF HOT JUPITERS (2016, background): “1) of particles of a size of up to 0.1-1 µm in H–He-dominated, Jupiter-like planets with vertical eddy diffusion values typically found in GCM simulations (see, e.g. Moses et al. 2011; Agúndez et al. 2014).”
• 3.6 AND 4.5 μm PHASE CURVES AND EVIDENCE FOR NON-EQUILIBRIUM CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HD 189733b (2012, background): “Moses et al. (2011) and Visscher & Moses (2011) find that their non-equilibrium models for this planet’s day side have more methane and less CO than the equilibrium predictions, as expected in the limit of strong vertical mixing (Kzz = 10 11).”
• HST hot-Jupiter transmission spectral survey: evidence for aerosols and lack of TiO in the atmosphere of WASP-12b (2013, background): “Hydrocarbons in hot Jupiters models have been explored in several studies including Liang et al. (2004), Zahnle et al. (2009) and Moses et al. (2011).”
• Equilibrium chemistry down to 100 K (2017, background): “However, recent kinetic chemical models (Visscher et al. 2006, 2010; Zahnle et al. 2009; Line et al. 2010; Moses et al. 2011; Kopparapu et al. 2012; Venot et al. 2012) have shown that in hot exoplanet atmospheres (T >∼ 1200 K), the chemical timescales are in fact quite short, and hence…”
• VULCAN: An Open-source, Validated Chemical Kinetics Python Code for Exoplanetary Atmospheres (2016, methodology): “For our current C/H/O network, we include 29 molecules and radicals with up to two carbon atoms: H, H2, O, OH, H2O, CH, C, 3CH2 (triplet), CH3, CH4, C2, C2H2, C2H3, C2H, C2H4, C2H5, C2H6, CO, CO2, CH2OH, H2CO, HCO, CH3O, CH3OH, CH3CO, O2, H2CCO, HCCO and He.”
• H2O abundances and cloud properties in ten hot giant exoplanets (2018, background): “While the lower level in the atmosphere of 100 bar is well below the observable photosphere, the upper level of 10−6 bar marks the region where molecules are photodissociated (Moses et al. 2011, 2013; Moses 2014) and hence no longer contribute opacity.”
• PHOTOCHEMISTRY IN TERRESTRIAL EXOPLANET ATMOSPHERES. I. PHOTOCHEMISTRY MODEL AND BENCHMARK CASES (2012, background): “These reactions may be important for modeling hot Jupiters (e.g., Moses et al. 2011), but they should not be included in the computation of low-temperature cases, e.g., for Earth-like planets.”
• The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations (2012, background): “Equilibrium-chemistry models of hot Jupiters suggest that the opacity at 4.5 µm should be higher than at 3.6 µm (the non-equilibrium models of Moses et al. 2011, find a similar opacity in the two bands, but significantly underpredict the flux at longer wavelengths measured with Spitzer for HD…”
• Re-analysis of ammonia spectra: Updating the HITRAN 14NH3 database (2013, background): “Its presence has also been predicted in exoplanets [12,13], but is yet to be detected.”
• A SYSTEMATIC RETRIEVAL ANALYSIS OF SECONDARY ECLIPSE SPECTRA. I. A COMPARISON OF ATMOSPHERIC RETRIEVAL TECHNIQUES (2013, background): “First, vertical mixing will smooth out the mixing ratio profiles over the thermal infrared photosphere (Line et al. 2010, Moses et al. 2011, Line et al. 2011), and secondly current observations simply do not have the information content to warrant the retrieval of vertical mixing ratio information…”