The calculations of thermal structure for an SO2 atmosphere of Io lead to exospheric temperatures in 800--1200K range. The Pioneer 10 electron density profiles can be fit with an SO2 surface density of 1.2¿1011 cm-3 at 5:30 pm local time and exosphere temperature of 1030 ¿K. Low energy electrons provide the major ionization source but the solar UV absorption dominates the heating of the atmosphere due to the long wavelength absorption threshold of SO2 and large absorption cross-section.