Because SO2 gas in Io's atmosphere is transported poleward where it condenses, the absence of bright polar caps on Io is intriguing. In this paper, we show that the energetic plasma ions measured by the low energy charged particles (LECP) instrument on Voyager carry most of the plasma energy and they deposit sufficient energy in Io's surface to alter the molecular composition and hence the ''color'' of the surface. We use an extrapolation of the LECP energy flux measured by Voyager, together with the calculated surface condensation rate, to obtain the energy deposited per molecule condensed on the surface in the region above 50¿ latitude. Because vapor deposited SO2 is spectrally flat above ~0.4 μm, a small concentration of absorbers can efficiently darken the surface. Based on the laboratory data of Moore <1984>, a dose ~0.5--5 eV per molecule at 88 K produces significant color change in vapor-deposited SO2. At the poles the energy deposited is ~10 eV per molecule condensed and at 50¿ latitude it is ~1 eV per molecule condensed so that these regions will not appear as a bright frost. In contrast, the fractional deposition rate in this region of photodissociation products is small, and the sputter removal rate of SO2 is slower than the chemical alteration rate. ¿ American Geophysical Union 1996