THERMOCHEMICAL AND PHOTOCHEMICAL KINETICS IN COOLER HYDROGEN-DOMINATED EXTRASOLAR PLANETS: A METHANE-POOR GJ436b?
Abstract
We introduce a thermochemical kinetics and photochemical model. We use high-temperature bidirectional reaction rates for important H, C, O, and N reactions (most importantly for CH{sub 4} to CO interconversion), allowing us to attain thermochemical equilibrium, deep in an atmosphere, purely kinetically. This allows the chemical modeling of an entire atmosphere, from deep-atmosphere thermochemical equilibrium to the photochemically dominated regime. We use our model to explore the atmospheric chemistry of cooler (T{sub eff} < 10{sup 3} K) extrasolar giant planets. In particular, we choose to model the nearby hot-Neptune GJ436b, the only planet in this temperature regime for which spectroscopic measurements and estimates of chemical abundances now exist. Recent Spitzer measurements with retrieval have shown that methane is driven strongly out of equilibrium and is deeply depleted on the day side of GJ436b, whereas quenched carbon monoxide is abundant. This is surprising because GJ436b is cooler than many of the heavily irradiated hot Jovians and thermally favorable for CH{sub 4}, and thus requires an efficient mechanism for destroying it. We include realistic estimates of ultraviolet flux from the parent dM star GJ436, to bound the direct photolysis and photosensitized depletion of CH{sub 4}. While our models indicate fairly rich disequilibriummore »
- Authors:
-
- California Institute of Technology, Pasadena, CA 91106 (United States)
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States)
- Hamburger Sternwarte, Universitaet Hamburg, 21029 Hamburg (Germany)
- Publication Date:
- OSTI Identifier:
- 21582961
- Resource Type:
- Journal Article
- Journal Name:
- Astrophysical Journal
- Additional Journal Information:
- Journal Volume: 738; Journal Issue: 1; Other Information: DOI: 10.1088/0004-637X/738/1/32; Journal ID: ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
- Subject:
- 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ATMOSPHERIC CHEMISTRY; CARBON MONOXIDE; HYDROGEN; METHANE; PHOTOCHEMISTRY; PLANETS; REACTION KINETICS; SIMULATION; ALKANES; CARBON COMPOUNDS; CARBON OXIDES; CHALCOGENIDES; CHEMISTRY; ELEMENTS; HYDROCARBONS; KINETICS; NONMETALS; ORGANIC COMPOUNDS; OXIDES; OXYGEN COMPOUNDS
Citation Formats
Line, Michael R, Yung, Yuk L, Vasisht, Gautam, Chen, Pin, and Angerhausen, D. THERMOCHEMICAL AND PHOTOCHEMICAL KINETICS IN COOLER HYDROGEN-DOMINATED EXTRASOLAR PLANETS: A METHANE-POOR GJ436b?. United States: N. p., 2011.
Web. doi:10.1088/0004-637X/738/1/32.
Line, Michael R, Yung, Yuk L, Vasisht, Gautam, Chen, Pin, & Angerhausen, D. THERMOCHEMICAL AND PHOTOCHEMICAL KINETICS IN COOLER HYDROGEN-DOMINATED EXTRASOLAR PLANETS: A METHANE-POOR GJ436b?. United States. https://doi.org/10.1088/0004-637X/738/1/32
Line, Michael R, Yung, Yuk L, Vasisht, Gautam, Chen, Pin, and Angerhausen, D. 2011.
"THERMOCHEMICAL AND PHOTOCHEMICAL KINETICS IN COOLER HYDROGEN-DOMINATED EXTRASOLAR PLANETS: A METHANE-POOR GJ436b?". United States. https://doi.org/10.1088/0004-637X/738/1/32.
@article{osti_21582961,
title = {THERMOCHEMICAL AND PHOTOCHEMICAL KINETICS IN COOLER HYDROGEN-DOMINATED EXTRASOLAR PLANETS: A METHANE-POOR GJ436b?},
author = {Line, Michael R and Yung, Yuk L and Vasisht, Gautam and Chen, Pin and Angerhausen, D},
abstractNote = {We introduce a thermochemical kinetics and photochemical model. We use high-temperature bidirectional reaction rates for important H, C, O, and N reactions (most importantly for CH{sub 4} to CO interconversion), allowing us to attain thermochemical equilibrium, deep in an atmosphere, purely kinetically. This allows the chemical modeling of an entire atmosphere, from deep-atmosphere thermochemical equilibrium to the photochemically dominated regime. We use our model to explore the atmospheric chemistry of cooler (T{sub eff} < 10{sup 3} K) extrasolar giant planets. In particular, we choose to model the nearby hot-Neptune GJ436b, the only planet in this temperature regime for which spectroscopic measurements and estimates of chemical abundances now exist. Recent Spitzer measurements with retrieval have shown that methane is driven strongly out of equilibrium and is deeply depleted on the day side of GJ436b, whereas quenched carbon monoxide is abundant. This is surprising because GJ436b is cooler than many of the heavily irradiated hot Jovians and thermally favorable for CH{sub 4}, and thus requires an efficient mechanism for destroying it. We include realistic estimates of ultraviolet flux from the parent dM star GJ436, to bound the direct photolysis and photosensitized depletion of CH{sub 4}. While our models indicate fairly rich disequilibrium conditions are likely in cooler exoplanets over a range of planetary metallicities, we are unable to generate the conditions for substantial CH{sub 4} destruction. One possibility is an anomalous source of abundant H atoms between 0.01 and 1 bars (which attack CH{sub 4}), but we cannot as yet identify an efficient means to produce these hot atoms.},
doi = {10.1088/0004-637X/738/1/32},
url = {https://www.osti.gov/biblio/21582961},
journal = {Astrophysical Journal},
issn = {0004-637X},
number = 1,
volume = 738,
place = {United States},
year = {Thu Sep 01 00:00:00 EDT 2011},
month = {Thu Sep 01 00:00:00 EDT 2011}
}