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.