We have measured the dayside spectrum of HD 189733b between 1.5 and 2.5 µm using the NICMOS instrument on the Hubble Space Telescope. The emergent spectrum contains significant modulation, which we attribute to the presence of molecular bands seen in absorption. We find that water (H 2 O), carbon monoxide (CO), and carbon dioxide (CO 2 ) are needed to explain the observations, and we are able to estimate the mixing ratios for these molecules. We also find temperature decreases with altitude in the ∼ 0.01 < P < ∼ 1 bar region of the dayside near-infrared photosphere and set an upper limit to the dayside abundance of methane (CH 4 ) at these pressures.

Using the NICMOS instrument on the Hubble Space Telescope, we have measured the dayside spectrum of HD 209458b between 1.5-2.5 µm. The emergent spectrum is dominated by features due to the presence of methane (CH 4 ) and water vapor (H 2 O), with smaller contributions from carbon dioxide (CO 2 ). Combining this near-infrared spectrum with existing mid-infrared measurements shows the existence of a temperature inversion and confirms the interpretation of previous photometry measurements. We find a family of plausible solutions for the molecular abundance and detailed temperature profile. Observationally resolving the ambiguity between abundance and temperature requires either (1) improved wavelength coverage or spectral resolution of the dayside emission spectrum, or (2) a transmission spectrum where abundance determinations are less sensitive to the temperature structure.

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.

and is difficult to explain with models that assume local thermodynamic equilibrium

It has been shown that spectroscopy of transiting extrasolar planets can potentially provide a wealth of information about their atmospheres. Herein, we set up the inverse problem in spectroscopic retrieval. We use non-linear optimal estimation to retrieve the atmospheric state (pioneered for Earth sounding by Rodgers 1976Rodgers , 2000. The formulation quantifies the the degrees-of-freedom and information content of the spectrum with respect to geophysical parameters; herein, we focus specifically on temperature and composition. First, we apply the technique to synthetic near infrared spectra, and explore the influence of spectral signal-to-noise ratio and resolution (the two important parameters when designing a future instrument) on the information content of the data. As expected, we find that the number of retrievable parameters increase with increasing signal-to-noise and resolution, although the gains quickly level off for large values. Second, we apply the methods to the previously studied atmosphere of HD 189733b, and compare the results of our retrieval with those obtained by others. Subject headings: planetary systems -planets and satellites: atmospheres -radiative transfermethods: data analysis-planets and satellites: individual (HD189733b)