The Deep Blue Color of HD189733b: Albedo Measurements with Hubble Space Telescope/Space Telescope Imaging Spectrograph at Visible Wavelengths

We present a secondary eclipse observation for the hot Jupiter HD189733b across the wavelength range 290-570nm made using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We measure geometric albedos of Ag = 0.40 \pm 0.12 across 290-450nm and Ag < 0.12 across 450-570nm at 1-sigma confidence. The albedo decrease toward longer wavelengths is also apparent when using six wavelength bins over the same wavelength range. This can be interpreted as evidence for optically thick reflective clouds on the dayside hemisphere with sodium absorption suppressing the scattered light signal beyond ~450nm. Our best-fit albedo values imply that HD189733b would appear a deep blue color at visible wavelengths.