The evolution of infalling sulfur species in Titan’s atmosphere

Abstract

Aims. We studied the hypothesis that micrometeorites and Enceladus' plume activity could carry sulfur-bearing species into the upper atmosphere of Titan, in a manner similar to oxygen-bearing species. Methods. We have developed a detailed photochemical model of sulfur compounds in the atmosphere of Titan that couples hydrocarbon, nitrogen, oxygen, and sulfur chemistries. Results. Photochemical processes produce mainly CS and H 2 CS in the upper atmosphere of Titan and C 3 S, H 2 S and CH 3 SH in the lower atmosphere. Mole fractions of these compounds depend significantly on the source of sulfur species. Conclusions. A possible future detection of CS (or the determination of a low upper limit) could be used to distinguish the two scenarios for the origin of sulfur species, which then could help to differentiate the various scenarios for the origin of H 2 O, CO, and CO 2 in the stratosphere of Titan.

The Origin of Titan’s External Oxygen: Further Constraints from ALMA Upper Limits on CS and CH2NH

“…Based on the upper limits in Table 1, scenarios A1 and B are consistent with our observations, but scenario A2 with the higher H 2 O flux is not consistent and can be rejected. Therefore, if the model of Hickson et al (2014) is correct, this implies that the lower H 2 O flux inferred from Herschel/ PACS (Moreno et al 2012) is more consistent with our upper limits than the higher flux implied by Cottini et al (2012). Bauduin et al (2018) investigate whether the discrepancy between Herschel/PACS and Cassini/CIRS determinations could be due to different analysis techniques by re-analyzing both data sets using a common retrieval scheme.…”

“…We used ALMA observations of Titan at ∼340GHz to determine a new upper limit of 0.0074ppb for CS in the stratosphere assuming a uniform profile above a 100km altitude condensation level, or 0.0256ppb for a profile that is uniform above a 200km photochemical sink level. We also calculated upper limit scale factors for photochemical model profiles predicted by Hickson et al (2014). The upper limits allow us to constrain the origin of external oxygen into Titan's atmosphere based on the S/O ratio in the two main source candidates: Enceladus and micrometeorites.…”

“…The primary aim of this study is to provide constraints on Titan's external oxygen source using CS as a tracer, which we can now address using our upper limits. Hickson et al (2014) consider three external source scenarios: (1) A1, micrometeorite source with external fluxes of 5.2×10 5 cm −2 s −1 for H 2 O and 1.6×10 6 cm −2 s −1 for O, which has an H 2 O flux consistent with the Herschel/PACS water vapor measurements by Moreno et al (2012); (2) A2, micrometeorite source with external fluxes of 2.6× 10 6 cm −2 s −1 for H 2 O and zero for O, which has a ∼4 times greater H 2 O flux consistent with the Cassini/CIRS water vapor measurement by Cottini et al (2012); and (3) B, Enceladus source with external fluxes of 6.5×10 5 cm −2 s −1 for H 2 O, and 1.6×10 6 cm −2 s −1 for O. For the micrometeorite sources (A1, A2) the abundance of sulphur species relative to H 2 O was assumed to be 1.5×10 −2 for H 2 S, 4×10 −3 for OCS, and 2×10 −3 for CS.…”

“…Right panels in each column show the measured spectrum (gray) with a synthetic spectrum corresponding to the 3σ upper limit. The upper limits are summarized inTable 1.scaling photochemical model profiles fromLavvas et al (2008),Lellouch et al (2010),Hickson et al (2014),Loison et al (2015), andVuitton et al (2018)…”

Titan's atmosphere and climate

“…Upper limits on PH 3 and H 2 S were obtained from Cassini CIRS [ Nixon et al , ] and additional efforts using ALMA to search for sulfur‐bearing species are ongoing. Photochemical models indicate that better constraints on sulfur‐bearing species may provide insight regarding external sources of material to Titan's atmosphere such as Enceladus, micrometeorites, and comets [ Hickson et al , ]. Phosphorous and sulfur, in addition to CHON, are important from an astrobiological perspective and to improve our understanding of Titan's formation and evolution.…”