Despite the recent detection of Io's SO2 atmosphere at millimetre
  wavelengths, several puzzling aspects remain. Modelling of these
  data suggest a hot, temporally stable, atmosphere with low fractional
  coverage consistent with upper limits set by UV observations.
  However, the model is difficult to reconcile with the Voyager 1 IRIS
  first detection of SO2 made in 1979 at 7.38 microns. IRIS provided
  only a point measurement and no disc-integrated measurement (which
  could be compared directly with mm and UV measurements) has yet been
  made. Millimetre observations suggest a small east-west (trailing-
  leading) asymmetry in the atmospheric pressure which can be tested by
  observations at several orbital phases. We propose to use SWS to
  observe the 7.38 micron line and measure the column abundance and
  temperature of the atmosphere at 6 orbital phases.

  Observations of high ionization states in the Io plasma torus
  provide strong constraints on the equilibrium of the system. The
  observed presence of OIII, for example, cannot be explained by a
  chemical equilibrium at the electron temperatures found well inside
  Io's orbit. Unlike the other major species in the torus, SIV, has
  no optical emissions. It is also controversial in that IUE
  observations of the 1406 A intercombination line gives a line
  intensity 4 times lower than predicted from Voyager 1 UV
  measurements. This line is relatively weak requiring many hours of
  observing time and smearing out longitudinal asymmetries (which
  may be quite strong) in SIV emission. The IR line of SIV at
  10.53 microns is 40 times stronger and easily detectable with ISO.
  We therefore propose to use SWS to observe the longitudinal
  variability of SIV, compare it to SIII emission (which can be
  obtained simultaneously), and investigate the chemical equilibrium
  of the torus.