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Context. Sulfur is used as a tracer of the evolution from interstellar cloudsto stellar systems. However, most of the expected sulfur in molecular cloudsremains undetected. Sulfur disappears from the gas phase in two steps. Onefirst depletion occurs during the translucent phase, reducing the gas phasesulfur by 7-40 times, while the following freeze-out step occurs in molecularclouds, reducing it by another order of magnitude. This long-standing dilemmaawaits an explanation. Aims. The aim of this study is to understand under whichform the missing sulfur is hiding in molecular clouds. Depletion onto dustgrains is considered. Methods. Experimental simulations mimicking H2S iceUV-photoprocessing in molecular clouds were conducted. The ice was monitoredusing infrared spectroscopy and the desorbing molecules were measured byquadrupole mass spectrometry. Theoretical Monte Carlo simulations wereperformed for interpretation of the experimental results and extrapolation toastrophysical conditions. Results. H2S2 formation was observed duringirradiation at 8 K. Molecules H2Sx with x > 2 were also identified and found todesorb during warm-up, along with S2 to S4 species. Larger Sx molecules up toS8 are refractory at room temperature and remained on the substrate forming aresidue. Monte Carlo simulations were able to reproduce the molecules desorbingduring warming up, and found that residues are chains or sulfur consisting of6-7 atoms. Conclusions. We propose that S+ in translucent clouds contributesnotoriously to S depletion in denser regions by forming long S-chains on dustin few times 10^4 years. We suggest that the S2 to S4 molecules observed incomets are not produced by fragmentation of these large chains. Instead, theyprobably come either from UV-photoprocessing of H2S-bearing ice produced inmolecular clouds or from short S chains formed during the translucent cloudphase

Photoprocessing of H2S on dust grains: building S chains in translucent clouds and comets