Voyager search for Posteclipse brightening on Io

Observations of three eclipse reappearances of Io were made during the two Voyager encounters. No posteclipse brightening of the type reported by some Earth-based observers—a brightening by some 10% just after eclipse which gradually disappears on a time scale of 10 to 15 min—was detected. Our negative result has a number of implications. First, it suggests that large areas of the surface of Io might not consist predominantly of yellow elemental sulfur (S ₈), because the reflectance of this material changes measurably over the temperature range experienced by the surface of Io following an eclipse. Other allotropes or sulfur compounds are probably responsible for the yellow color of some areas of Io. Second, at most locations the amount of SO ₂ vapor above the surface at local noon must be considerably less than the 0.2 cm-atm measured by IRIS near the erupting volcano Loki, on March 5, 1979. We note that our data suggest a slight posteclipse brightening of the south polar region of Io, but with an amplitude (∼3%) and a time scale (about 3 min) quite distinct from those reported for the classical whole-disk phenomenon. This small possible effect needs to be confirmed by detailed picture differencing, but if real, it appears to involve both the brightening and darkening of small localized areas.