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We present an analysis of seven primary transit observations of the hotNeptune GJ436b at 3.6, 4.5 and $8~\mu$m obtained with the Infrared Array Camera(IRAC) on the Spitzer Space Telescope. After correcting for systematic effects,we fitted the light curves using the Markov Chain Monte Carlo technique.Combining these new data with the EPOXI, HST and ground-based $V, I, H$ and$K_s$ published observations, the range $0.5-10~\mu$m can be covered. Due tothe low level of activity of GJ436, the effect of starspots on the combinationof transits at different epochs is negligible at the accuracy of the dataset.Representative climate models were calculated by using a three-dimensional,pseudo-spectral general circulation model with idealised thermal forcing.Simulated transit spectra of GJ436b were generated using line-by-line radiativetransfer models including the opacities of the molecular species expected to bepresent in such a planetary atmosphere. A new, ab-initio calculated, linelistfor hot ammonia has been used for the first time. The photometric data observedat multiple wavelengths can be interpreted with methane being the dominantabsorption after molecular hydrogen, possibly with minor contributions fromammonia, water and other molecules. No clear evidence of carbon monoxide anddioxide is found from transit photometry. We discuss this result in the lightof a recent paper where photochemical disequilibrium is hypothesised tointerpret secondary transit photometric data. We show that the emissionphotometric data are not incompatible with the presence of abundant methane,but further spectroscopic data are desirable to confirm this scenario.

Methane in the atmosphere of the transiting hot Neptune GJ436b?