  Sulfur is an abundant element in the cosmos and it is thus an important
contributor to astrochemistry in the interstellar medium and in the Solar
System. Astronomical observations of the gas and of the solid phases in the
dense interstellar/circumstellar regions have evidenced that sulfur is
underabundant. The hypothesis to explain such a circumstance is that it is
incorporated in some species in the solid phase (i.e. as frozen gases and/or
refractory solids) and/or in the gas phase, which for different reasons have
not been observed so far. Here we wish to give a contribution to the field by
studying the chemistry induced by thermal and energetic processing of frozen
mixtures of sulfur dioxide (one of the most abundant sulfur-bearing molecules
observed so far) and water. We present the results of a series of laboratory
experiments concerning thermal processing of different H2O:SO2 mixtures and ion
bombardment 30 keV He$^+$ of the same mixtures. We used in situ FTIR
spectroscopy to investigate the induced effects. The results indicate that
ionic species such as HSO$_{3}^{-}$, HSO$_{4}^{-}$, and S$_2$O$_{5}^{2-}$ are
easily produced. Energetic processing also produces SO$_3$ polymers and a
sulfurous refractory residue. The produced ionic species exhibit spectral
features in a region that, in astronomical spectra of dense molecular clouds,
is dominated by strong silicate absorption. However, such a dominant feature is
associated with some spectral features, some of which have not yet been
identified. We suggest adding the sulfur-bearing ionic species to the list of
candidates to help explain some of those features.
