ON THE VERTICAL THERMAL STRUCTURE OF IOS ATMOSPHERE

A radiative-thermal conduction model for the vertical thermal structur e of Io's atmosphere is developed with solar heating by SO2 absorption in UV and near-IR bands and non-LTE cooling by SO2 nu1, nu2, nu3, vib rational bands and rotational lines. The model predicts the existence of a mesopause in Io's atmosphere when the surface pressure exceeds ap proximately 10 nbar. The radiative time constant for establishing a me sosphere/mesopause on Io is only approximately 20 min, whereas the the rmospheric radiative time constant is about 1 hr. These time constants are significantly shorter than the diurnal time scale and competitive with dynamical time scales. In the thermosphere when solar UV heating dominates, the asymptotic thermospheric temperature is approximately 270 K, only 140 K greater than the surface temperature because at high altitudes non-LTE cooling by SO2 rotation lines exceeds cooling in th e nu2 vibrational band. Solar-heating-only models are incapable of gen erating warm enough atmospheres to satisfy the observational inference s from UV and especially millimeter-wave measurements. Joule heating d riven by the penetration of Jupiter's corotational electric field into Io's conducting ionosphere is demonstrated to be the dominant heating mechanism in the subnanobar regions of Io's atmosphere with temperatu res ranging from 150 to 1000 K as a function of decreasing pressure fr om 1 to 0.1 nbar. The asymptotic thermospheric temperature can attain a value as high as 1800 K. (C) 1994 Academic Press, Inc.