The Evolution of Activity and Chemical Composition in Rosetta’s Comet Targets across Multiple Apparitions: Complications for CS₂ as the CS Parent in Comet Nuclei

Jupiter-family comets are ephemeral small bodies injected into the inner solar system from the Kuiper Belt, doomed to either sublimate all their volatiles and become inert or violently shatter from the activity. We investigate two target candidates of the ESA Rosetta mission, comets 46P/Wirtanen and 67P/Churyumov-Gerasimenko, which had favorable apparitions for Earth-based observations in 2018-2019 and 2021, respectively. Using the Hubble Space Telescope STIS and COS instruments, we observed OH and CS emissions to characterize production rates of H₂O and CS, established Af ρ values, and placed upper limits on the production rate of C₂ and its parent. We find CS/H₂O relative abundances that are significantly (5σ-7σ) larger than previous remote near-UV (NUV) measurements of 46P and 67P at similar heliocentric distances and CS/H₂O values larger than those obtained via contemporaneous submillimeter observations for the same apparitions. We also find that for 67P the remote derivations of CS₂/H₂O ratios are substantially (∼50×) higher than the values measured by the ROSINA mass spectrometer on board the Rosetta spacecraft for all NUV-derived CS₂ production rates. The discrepancy points toward an unidentified CS parent or parents with contributing factors from uncertainties with the fluorescence efficiencies of the CS (0,0) band of the A ¹Π-X ¹Σ⁺ system around 2580 Å. Given the significance of understanding the chemistry and dissociation physics of sulfur-bearing molecules in comets for tracing planetesimal formation environments, as well as the limited studies in this area, we propose several hypotheses to explain this discrepancy and outline future studies to address these issues.