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

Abstract

Hydrogen sulfide (H _2 S) is the fifth most abundant molecule observed in the coma of comet 67P/Churyumov–Gerasimenko (67P). Prior to its incorporation into cometary materials, H _2 S likely underwent ultraviolet (UV) radiation exposure, which is thought to initiate a complex sulfur chemistry. We present an investigation into the UV photochemistry of H _2 S ices using infrared, Raman, and mass spectrometry techniques. Our study reveals the production of complex sulfur allotropes ranging from S _2 to S _6 , alongside polysulfanes (H _2 S _n , n = 2–3). Temperature-programmed desorption measurements postirradiation of H _2 S exhibit two peaks for S _2 molecules: a broad peak between 80 and 140 K and a distinct peak at ∼245 K. Notably, larger allotropes S _3 –S _5 exclusively display the 245 K peak. Furthermore, ROSINA measurements of the S _2 /H _2 S ratio during dust impact events and previously reported S _2 /H _2 S ratios in the undisturbed coma are compared to our laboratory-determined S _2 /H _2 S values. This analysis identifies two distinct sources of S _2 : a volatile S _2 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 S _2 to the initial H _2 S for both the volatile component and the refractory component at 245 K with both measurements conducted at an irradiation incident fluence of 2.25 × 10 ^17 photons cm ^−2 . These laboratory-derived S _2 /H _2 S ratios exhibit concordance with ROSINA measurements. When extrapolated to incident fluences anticipated in molecular clouds, this photoprocessing mechanism offers a plausible explanation for the measured S _2 /H _2 S ratio in comet 67P.

Keywords

• Comets
• Laboratory astrophysics
• Astrochemistry
• Small Solar System bodies