  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 H$_2$S ice samples and in H$_2$O:H$_2$S 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 H$_2$S$_x$ species is
efficient, not only in pure H$_2$S ice samples, but also in water-rich ice
samples. Large sulfur chains are formed more efficiently at low temperatures
($\approx$10 K), while high temperatures ($\approx$50 K) favour the formation
of short sulfur chains. Mass spectra of H$_2$S$_x$, x~=~2-6, species are
presented for the first time. Their analysis suggests that H$_2$S$_x$ species
are favoured in comparison with S$_x$ chains. Nevertheless, the detection of
several S$_x^+$ fragments at high temperatures in H$_2$S:H$_2$O ice mixtures
suggests the presence of S$_8$ 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) H$_2$S$_4$ and H$_2$S$_5$ to be
responsible of those S$_3^+$ and S$_4^+$ cations, respectively, or 2) S$_8$
species, sublimating and being fragmented in the mass spectrometer. If S$_8$ is
the parent molecule, then S$_5^+$ and S$_6^+$ cations could be also detected in
future missions by broadening the mass spectrometer range.
