Photo-oxidation of SO2 and of SO2 trapped in amorphous water ice studied by IR spectroscopy.: Implications for Jupiter's satellite Europa

Fourier Transform Infrared Spectroscopy (FTIR), is used to characterize thin films of sulfur dioxide trapped in excess of amorphous water ice and to study the photo-oxidation of pure solid SO2 or of SO2 diluted in water ice. These laboratory experiments were undertaken in an attempt to find information on the formation of hydrated sulfuric acid detected on Europa's surface. Vibrational spectra at 30 K of mixed films containing SO2 with an excess of water depend on the deposition conditions. A metastable (SO2)(m)(H2O)(n) phase and a solid solution of SO2 in water ice have been evidenced. Irradiation at 266 nm of mixed films SO2/O-3/H2O produces sulfur trioxide and hydrogen peroxide without traces of sulfuric acid. Irradiation of films of pure SO2, under 200 nm, leads to sulfur trioxide and to ozone. When SO2 is diluted in amorphous water ice, new weak bands that appear after irradiation suggest the formation of sulfuric acid.