Indications for very high metallicity and absence of methane in the eccentric exo-Saturn WASP-117b

We investigate the atmospheric composition of the long-period (Porb = 10 days) eccentric exo-Saturn WASP-117b. WASP-117b could be similar in atmospheric temperature and chemistry to WASP-107b. In mass and radius, WASP-117b is similar to WASP-39b, which allows a comparative study of these planets. Methods. We analyzed a near-infrared transmission spectrum of WASP-117b taken with the Hubble Space Telescope (HST) WFC3 G141, which was reduced with two independent pipelines. High-resolution measurements were taken with VLT/ESPRESSO in the optical. Results. We report the robust (3-) detection of a water spectral feature. In a 1D atmosphere model with isothermal temperature, uniform cloud deck, and equilibrium chemistry, the Bayesian evidence of a retrieval analysis of the transmission spectrum indicates a preference for a high atmospheric metallicity [Fe=H] = 2:58+0:26 -0:37 and clear skies. The data are also consistent with a lower metallicity composition [Fe=H] > 1:75 and a cloud deck between 10-2:2 and 10-5:1 bar, but with weaker Bayesian preference. We retrieve a low CH4 abundance of >10-4 volume fraction within 1 and >2 10-1 volume fraction within 3. We cannot constrain the equilibrium temperature between theoretically imposed limits of 700 and 1000 K. Further observations are needed to confirm quenching of CH4 with Kzz 108 cm2 s-1.We report indications of Na and K in the VLT/ESPRESSO high-resolution spectrum with substantial Bayesian evidence in combination with HST data.