  WASP-39b is a hot Saturn-mass exoplanet with a predicted clear atmosphere
based on observations in the optical and infrared. Here we complete the
transmission spectrum of the atmosphere with observations in the near-infrared
(NIR) over three water absorption features with the Hubble Space Telescope
(HST) Wide Field Camera 3 (WFC3) G102 (0.8-1.1 microns) and G141 (1.1-1.7
microns) spectroscopic grisms. We measure the predicted high amplitude H2O
feature centered at 1.4 microns, and the smaller amplitude features at 0.95 and
1.2 microns, with a maximum water absorption amplitude of 2.4 planetary scale
heights. We incorporate these new NIR measurements into previously published
observational measurements to complete the transmission spectrum from 0.3-5
microns. From these observed water features, combined with features in the
optical and IR, we retrieve a well constrained temperature Teq = 1030(+30,-20)
K, and atmospheric metallicity 151 (+48,-46)x solar which is relatively high
with respect to the currently established mass-metallicity trends. This new
measurement in the Saturn-mass range hints at further diversity in the planet
formation process relative to our solar system giants.
