Cosmic Ray Irradiation of Interstellar Ices on Sulfur-Rich Grains: A Possible Source of Sulfur-Bearing Molecules.

The major reservoir of sulfur in dense interstellar clouds is still largely unknown, although a growing body of evidence suggests that it may exist in a refractory form (i.e., as minerals or allotropes of the element). Therefore, it is possible that the irradiation of sulfur-free interstellar ices on top of sulfur-rich refractory grain components by cosmic rays or stellar winds may result in the formation of simple inorganic sulfur molecules that could be readily detected by ground- or space-borne telescopes. In this study, we have irradiated neat ices of O2, CO, CO2, and H2O on top of layers of allotropic sulfur at 20 K using 1 MeV He+ ions as a mimic of space radiation. Experiments with CO2 and H2O ices were also repeated at 70 K to provide data obtained under conditions more relevant to icy bodies in the outer solar system for comparative purposes. We have found qualitative mid-infrared spectroscopic evidence for the synthesis of SO2, CS2, OCS, and H2SO4 hydrates, but not H2S, in our experiments and have quantified the efficiency of their formation by calculating the G-value (i.e., the number of molecules formed per 100 eV of energy deposited) for each ice-refractory system. Overall, SO2 and CS2 are the most commonly observed products in our experiments, although the highest G-value was that for H2SO4 hydrates formed as a result of the irradiation of H2O ice on top of sulfur at 70 K. An important outcome of our study is that our experimental results are consistent with recent observational surveys that suggest SO2 formation in interstellar ices proceeds primarily via an "energetic" route involving radiolytic processes, while OCS forms as a result of "nonenergetic" processes such as atom or radical addition reactions.