The Puzzling Chemical Composition of Gj 436b's Atmosphere: Influence of Tidal Heating on the Chemistry

The dissipation of the tidal energy deposited on eccentric planets may induce a heating of the planet that affects its atmospheric thermal structure. Here we study the influence of tidal heating on the atmospheric composition of the eccentric ( e = 0.16) "hot Neptune" GJ 436b, for which inconclusive chemical abundances are retrieved from multiwavelength photometric observations carried out during primary transit and secondary eclipse. We build up a one-dimensional model of GJ 436b's atmosphere in the vertical direction and compute the pressure–temperature and molecular abundances profiles for various plausible internal temperatures of the planet (up to 560 K) and metallicities (from solar to 100 times solar), using a radiative-convective model and a chemical model which includes thermochemical kinetics, vertical mixing, and photochemistry. We find that the CO/CH 4 abundance ratio increases with metallicity and tidal heating, and ranges from 1/20 to 1000 within the ranges of metallicity and internal temperature explored. Water vapor locks most of the oxygen and reaches a very high abundance, whatever the metallicity and internal temperature of the planet. The CO 2 /H 2 O abundance ratio increases dramatically with metallicity, and takes values between 10 −5 –10 −4 with solar elemental abundances and ∼0.1 for a metallicity 100 times solar. None of the atmospheric models based on solid physical and chemical grounds provide a fully satisfactory agreement with available observational data, although the comparison of calculated spectra and observations seems to point to models with a high metallicity and efficient tidal heating, in which high CO/CH 4 abundance ratios and warm temperatures in the dayside atmosphere are favored.