WUPPE observed Io once on the ASTRO-1 mission and thrice on the ASTRO-2 mission.
The ASTRO-1 observation and the first one on ASTRO-2 do not have
polarimetry, but the spectra are usable.
These observations were all made as offsets during HUT observations
of the emission from the Io torus (the belt of gas associated with Io).
The albedo (reflectance) measurements are consistent with 25% of the surface being covered by sulfur dioxide frost.
The two observations with polarization measurements differ in Io orbital phase by 20°. The difference in polarization between the two may be due to spots on the surface near known volcanoes being different in polarizing properties than the average of the whole surface. With only two observations there is ambiguity in which locations on the surface are causing the variation.
These results have appeared in the Astronomical Journal.