We present a secondary eclipse observation for the hot Jupiter HD 189733b across the wavelength range 290–570 nm made using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope .
We measure geometric albedos of A _ { g } = 0.40 \pm 0.12 across 290–450 nm and A _ { g } < 0.12 across 450–570 nm 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 \sim 450 nm .
Our best-fit albedo values imply that HD 189733b would appear a deep blue color at visible wavelengths .