Dual Sources of S2 Observed in Comet 67P: Insights from Comparing ROSINA Measurements and Laboratory Simulations

Hydrogen sulfide (H2S) is the fifth most abundant molecule found in the coma of comet 67P/Churyumov-Gerasimenko (67P). Prior to its incorporation into cometary materials, H2S likely underwent ultraviolet (UV) radiation exposure, which is thought to initiate a complex sulfur chemistry. In this study, we present a comprehensive investigation into the UV photochemistry of H2S ice films using IR, Raman, and mass spectrometry techniques. Our study reveals the production of complex sulfur allotropes ranging from S2 to S6, alongside polysulfanes (H2Sn, n=2-3). Temperature Programmed Desorption (TPD) measurements post-irradiation exhibit two peaks for S2 molecules: a broad peak between 80 K and 140 K and a distinct peak at ~ 245 K. Notably, larger allotropes S3-S5 exclusively display the 245 K peak. Furthermore, ROSINA measurements of the S2/H2S ratio during dust impact events and previously reported S2/H2S ratios in the undisturbed coma are compared to our laboratory determined S2/H2S values. This analysis identifies two distinct sources of S2: a volatile S2 potentially sublimated directly from the comet's surface, and a secondary source likely resulting from fragmentation of larger sulfur chains during dust impacts. We determined the ratio of produced S2 to the initial H2S for both the volatile component, denoted as (S2/H2S)volatile, and the refractory component at 245 K, labeled as (S2/H2S)refractory, with both measurements conducted at an irradiation fluence of 2.25 1017 photons.cm-2. Remarkably, these laboratory-derived S2/H2S ratios exhibit concordance with ROSINA measurements. When extrapolated to irradiation fluences anticipated in molecular clouds, this photo-processing mechanism offers a plausible explanation for the measured S2/H2S ratio in comet 67P.