Formation and desorption of sulfur chains (H₂Sₓ and Sₓ) in cometary ice: effects of ice composition and temperature

The reservoir of sulfur accounting for sulfur depletion in the gas of dense clouds and circumstellar regions is still unclear. One possibility is the formation of sulfur chains, which would be difficult to detect by spectroscopic techniques. This work explores the formation of sulfur chains experimentally, both in pure H₂S ice samples and in H₂O: H₂S ice mixtures. An ultra-high vacuum chamber, ISAC, eqquipped with FTIR and QMS, was used for the experiments. Our results show that the formation of H₂Sₓ species is efficient, not only in pure H₂S ice samples, but also in water-rich ice samples. Large sulfur chains are formed more efficiently at low temperatures (10 K), while high temperatures (50 K) favour the formation of short sulfur chains. Mass spectra of H₂Sₓ, x~=~2-6, species are presented for the first time. Their analysis suggests that H₂Sₓ species are favoured in comparison with Sₓ chains. Nevertheless, the detection of several Sₓ^+ fragments at high temperatures in H₂S: H₂O ice mixtures suggests the presence of S₈ in the irradiated ice samples, which could sublimate from 260~K. ROSINA instrument data from the cometary Rosetta mission detected mass-to-charge ratios 96 and 128. Comparing these detections with our experiments, we propose two alternatives: 1) H₂S₄ and H₂S₅ to be responsible of those S₃^+ and S₄^+ cations, respectively, or 2) S₈ species, sublimating and being fragmented in the mass spectrometer. If S₈ is the parent molecule, then S₅^+ and S₆^+ cations could be also detected in future missions by broadening the mass spectrometer range.