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

Abstract

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.

Parent Volatile Outgassing Associations in Cometary Nuclei: Synthesizing Rosetta Measurements and Ground-based Observations

“…Atomic sulfur measurements are common for comets in the UV, although their interpretation is not straightforward due to the required radiative transfer modeling. The in situ measurements by the ROSINA instrument showed that 67P's measurements of atomic sulfur relative to water could not be easily traced to any sulfur-bearing species, therefore prompting more questions about the sources of elemental sulfur in comets (Calmonte et al 2016;Biver et al 2022;Noonan et al 2023). Due to their strong transitions, multiple sulfur-bearing molecules are commonly studied in comets from remote observations.…”

“…Due to their strong transitions, multiple sulfur-bearing molecules are commonly studied in comets from remote observations. In the UV, S, S 2 , and CS have been detected in multiple comets so far (e.g., Meier & A'Hearn 1997;Reylé & Boice 2003;Noonan et al 2023); in the near-IR, OCS has been detected in multiple comets with transitions at ∼4.8 μm (Saki et al 2020); and at radio wavelengths, species such as H 2 S, SO, SO 2 , H 2 CS, OCS, and CS have strong transitions (Boissier et al 2007; see Table 3 in Biver et al 2022). These molecules have been measured in dozens of comets (e.g., see Boissier et al 2007, Table 3 in Biver et al 2022, Table 5 in Le Roy et al 2015, Table 1 in Bockelée-Morvan et al 2004, and Table A2 in Calmonte et al 2016 for a list of detected sulfur-bearing species and their range of abundances in comets).…”

“…Ward et al (2012) showed that thermal oxygen atoms are quite efficient at converting CS 2 ice to OCS at ISM conditions, in fact, so much so that it is difficult to maintain any CS 2 reservoir. However, this may not be the case at protoplanetary disk densities, where the lack of sulfur-bearing molecule detections leaves chemical models poorly constrained (Semenov et al 2018;Noonan et al 2023).…”

“…The Space Telescope Imaging Spectrograph on the Hubble Space Telescope measured the sulfur-bearing species CS in comet 67P during the 2021 apparition (Noonan et al 2023) and discovered significant variations from both Rosetta measurements (Calmonte et al 2016) and radio observations (Coulson et al 2020;Biver et al 2021). Higher levels of CS/H 2 O (derived from CS and OH) were seen compared to in situ measurements, challenging the previous assumption that CS is a direct product of CS 2 dissociation, as it was also shown in radio millimeter and submillimeter observations of comets.…”

The Return of the Rosetta Target: Keck Near-infrared Observations of Comet 67P/Churyumov–Gerasimenko in 2021

“…On 2021 December 16 Q(H 2 O) ≈ 1.5 × 10 28 s −1 based the Keck measurements. Five days earlier, observations of OH with the Hubble Space Telescope resulted in significantly lower production: Q(H 2 O) ≈ 0.3 × 10 28 s −1 (Noonan et al 2023).…”