Sulfur-bearing molecules are highly reactive in the gas phase of the ISM .
However , the form in which most of the sulfur is locked onto interstellar dust grains is unknown .
By taking advantage of the short time-scales of shocks in young molecular outflows , one could track back the main form of sulfur in the ices .
In this paper , six transitions of H _ { 2 } S and its isotopologues in the L1157-B1 bowshock have been detected using data from the Herschel-CHESS survey and the IRAM-30m ASAI large program .
These detections are used to calculate the properties of H _ { 2 } S gas in L1157-B1 through use of a rotation diagram and to explore the possible carriers of sulfur on the grains .
The isotopologue detections allow the first calculation of the H _ { 2 } S deuteration fraction in an outflow from a low mass protostar .
The fractional abundance of H _ { 2 } S in the region is found to be 6.0 \times 10 ^ { -7 } and the deuteration fraction is 2 \times 10 ^ { -2 } .
In order to investigate the form of sulfur on the grains , a chemical model is run with four different networks , each with different branching ratios for the freeze out of sulfur bearing species into molecules such as OCS and H _ { 2 } S. It is found that the model best fits the data when at least half of each sulfur bearing species hydrogenates when freezing .
We therefore conclude that a significant fraction of sulfur in L1157-B1 is likely to be locked in H _ { 2 } S on the grains .