37th DPS Meeting, 4-9 September 2005
Session 28 Extrasolar Planets
Oral, Tuesday, September 6, 2005, 4:20-6:00pm, Law LG19

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[28.03] Atmospheric Dynamics and Disequilibrium CO/CH4 Chemistry in the Atmosphere of the Transiting Planet HD 209458b

C.S. Cooper, A.P. Showman (University of Arizona, Department of Planetary Sciences / LPL)

Chemical equilibrium considerations predict that carbon will be present in the atmosphere of HD 209458b as either CO or CH4. Assuming a solar abundance distribution of elements, the concentrations of these species are controlled by temperature and pressure only. In cooler regions, however, vigorous mass transport can occur on time scales shorter than the time scale for interconversion between CO and CH4. This results in excess [CO]/[CH4] relative to the concentrations attained in chemical equilibrium.

In this work, we adapt the model of HD 209458b's meteorology used in Cooper & Showman (2005) to treat the disequilibrium chemistry of carbon. Cooper & Showman (2005) show that high-speed winds (~ 4 km s-1) and large temperature contrasts (~ 500 K) are plausible near the photosphere. We model CO and CH4 as passive tracers; i.e., they follow but do not disturb the fluid dynamics of the system. Our simulations will yield maps of the distribution of CO and CH4 to help guide efforts to detect the presence of carbon on this planet.

Though CO was not detected on HD 209458b (Deming et al. 2005) by transmission spectroscopy, its presence in the atmosphere cannot be ruled out. For example, CO could be present over much of the dayside hemisphere but depleted near the limbs. The distribution of CO in the atmosphere is diagnostic of temperature at the pressure levels probed by various CO absorption features (Bézard & Lellouch 2002). A positive CO detection would also rule out the hypothesis that the ratio C/O > 1 on this extrasolar planet (e.g., Fortney et al. 2005).

This research is supported by NSF grant AST-0307664 and NASA GSRP NGT5-50462.

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