The Jovian plasma ion heating of any atmospheric gas over Io is shown to be important for determining the temperature structure and the location of the exobase and for limiting diffusive separation in Io's atmosphere. This is used, along with the plasma supply rate, to estimate an average column of gas over the trailing hemisphere of ~3¿1016 SO2/cm2 with exobase at ~1.4RIo. ¿ American Geophysical Union 1989