We present Hubble Space Telescope optical and near-IR transmission spectra of the transiting hot-Jupiter WASP-31b .
The spectrum covers 0.3–1.7 \mu m at a resolution R \sim 70 , which we combine with Spitzer photometry to cover the full-optical to IR .
The spectrum is dominated by a cloud-deck with a flat transmission spectrum which is apparent at wavelengths > 0.52 \mu m. The cloud deck is present at high altitudes and low pressures , as it covers the majority of the expected optical Na line and near-IR H _ { 2 } O features .
While Na I absorption is not clearly identified , the resulting spectrum does show a very strong potassium feature detected at the 4.2- \sigma confidence level .
Broadened alkali wings are not detected , indicating pressures below \sim 10 mbar .
The lack of Na and strong K is the first indication of a sub-solar Na/K abundance ratio in a planetary atmosphere ( ln [ Na/K ] = -3.3 \pm 2.8 ) , which could potentially be explained by Na condensation on the planet ’ s night side , or primordial abundance variations .
A strong Rayleigh scattering signature is detected at short wavelengths , with a 4- \sigma significant slope .
Two distinct aerosol size populations can explain the spectra , with a smaller sub-micron size grain population reaching high altitudes producing a blue Rayleigh scattering signature on top of a larger , lower-lying population responsible for the flat cloud deck at longer wavelengths .
We estimate that the atmospheric circulation is sufficiently strong to mix micron size particles upward to the required 1–10 mbar pressures , necessary to explain the cloud deck .
These results further confirm the importance of clouds in hot-Jupiters , which can potentially dominate the overall spectra and may alter the abundances of key gaseous species .