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19 April 2024 |
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Thermochemistry and Photochemistry in Cooler Hydrogen Dominated Extrasolar Planets: The Case of GJ436b | | Michael R Line
; Gautam Vasisht
; Pin Chen
; D. Angerhausen
; Yuk. L. Yung
; | | Date: |
16 Apr 2011 | | Abstract: | We introduce a new 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 ab initio 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 {it Spitzer} measurements with retrieval have shown that methane
is driven strongly out of equilibrium and is deeply depleted on the dayside of
GJ 436b, whereas quenched carbon monoxide is abundant. This is surprising
because GJ 436b 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 GJ 436, 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
0.01-1 bars (which attack CH$_4$), but we cannot as yet identify an efficient
means to produce these hot atoms. | | Source: | arXiv, 1104.3183 | | Services: | Forum | Review | PDF | Favorites | |
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