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Thermochemical and Photochemical Kinetics in Cooler Hydrogen-dominated Extrasolar Planets: A Methane-poor GJ436b?

Line, Michael R. and Vasisht, Gautam and Chen, Pin and Angerhausen, D. and Yung, Yuk L. (2011) Thermochemical and Photochemical Kinetics in Cooler Hydrogen-dominated Extrasolar Planets: A Methane-poor GJ436b? Astrophysical Journal, 738 (1). Art. No. 32. ISSN 0004-637X.

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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_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_(eff) < 10^3 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_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_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_4 destruction. One possibility is an anomalous source of abundant H atoms between 0.01 and 1 bars (which attack CH_4), but we cannot as yet identify an efficient means to produce these hot atoms.

Item Type:Article
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URLURL TypeDescription DOIArticle Paper
Line, Michael R.0000-0002-2338-476X
Vasisht, Gautam0000-0002-1871-6264
Yung, Yuk L.0000-0002-4263-2562
Additional Information:© 2011 The American Astronomical Society. Received 2011 April 14; accepted 2011 June 3; published 2011 August 10 We thank Julie Moses, Channon Visscher, Karen Willacy, and M. C. Liang for useful chemistry discussions and tips. We also thank Xi Zhang, Heather Knutson, Mimi Gerstell, Mark Allen, the Yuk Yung Group, and the anonymous referee for reading the paper and providing valuable feedback. M. Line is supported by the JPL Graduate Fellowship funded by the JPL Research and Technology Development Program. P. Chen and G. Vasisht are supported by the JPL Research & Technology Development Program, and contributions herein were supported by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
Funding AgencyGrant Number
JPL Graduate FellowshipUNSPECIFIED
JPL Research and Technology Development ProgramUNSPECIFIED
Subject Keywords:astrochemistry; methods: numerical; planets and satellites: atmospheres; planets and satellites: composition; planets and satellites: individual (GJ436b)
Issue or Number:1
Record Number:CaltechAUTHORS:20140822-083020182
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Official Citation:Thermochemical and Photochemical Kinetics in Cooler Hydrogen-dominated Extrasolar Planets: A Methane-poor GJ436b? Michael R. Line et al. 2011 ApJ 738 32
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48790
Deposited By: Tony Diaz
Deposited On:22 Aug 2014 15:47
Last Modified:10 Oct 2019 22:52

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