Thermodynamic state of SO2 on Io's surface

Abstract

It has been suggested that surface conditions on Io might be conducive to significant SO2 adsorption on sulphur or alkali sulphides. A number of spectroscopic arguments for and against the high abundance of a SO2 adsorbate (versus frost) on Io have been made. We have measured SO2 adsorption isotherms on particulate sulphur and approached the question of SO2 adsorbate/ice from a thermodynamic perspective. Because of formidable experimental difficulties, data were not obtained at the very low temperatures and PSO2 characteristic of Io. However, data were obtained over a wide range of pressures and temperatures somewhat higher that Io's and extrapolated to Io surface conditions. Errors in estimating adsorptive capacity accrue through extrapolation of the adsorption isotherms; however, more significant is the addition of the solid SO2 equilibrium vapor pressure curve to the phase diagram. The formation of ice places a strict upper limit on adsorptive coverage at any temperature. We calculate that limit for Io based upon our data and show that no more than 1.4 × 10−2 monolayers of SO2 can adsorb on sulphur at Io temperatures. Given the assumption that sulphur forms the primary adsorbent on Io's surface, or the assumption that the adsorptive capacity of the other adsorbents is not substantially greater than that of sulphur, this explains the fact that the ν1 + ν3 band center position is in better agreement with that of frost than adsorbate.