Cassini/VIMS observation of an Io post-eclipse brightening event

During the Cassini–Jupiter flyby, VIMS observed Io at different phase angles, both in full sunlight and in eclipse. By using the sunlight measurements, we were able to produce phase curves in the visual through all the near infrared wavelengths covered by the VIMS instrument (0.85–5.1 μm). The phase angle spanned from ∼2° to ∼120°. The measurements, done just after Io emerged from Jupiterʹs shadow, show an increase of about 15% in Ioʹs reflectance with respect to what would be predicted by the phase curve. This behavior is observed at wavelengths >1.2 μm. Moreover, just after emergence from eclipse an increase of about 25% is observed in the depth of SO2 frost bands at 4.07 and 4.35 μm. At 0.879<λ<1.04 μm the brightening is 10–24%. Below λ=0.879 μm the brightening, if present, should be less than the precision of our measurements (∼5%). Apparently, these observations are not explained neither by a diverse spatial distribution of SO2 on the Ioʹ surface nor by atmospheric SO2 condensation on the surface during the eclipse.