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

Titan's atmospheric inventory of oxygen compounds (H₂O, CO₂, CO) are thought to result from photochemistry acting on externally supplied oxygen species (O⁺, OH, H₂O). These species potentially originate from two main sources: (1) cryogenic plumes from the active moon Enceladus and (2) micrometeoroid ablation. Enceladus is already suspected to be the major O⁺ source, which is required for CO creation. However, photochemical models also require H₂O and OH influx to reproduce observed quantities of CO₂ and H₂O. Here, we exploit sulphur as a tracer to investigate the oxygen source because it has very different relative abundances in micrometeorites (S/O ~ 10⁻²) and Enceladus' plumes (S/O ~ 10⁻⁵). Photochemical models predict most sulphur is converted to CS in the upper atmosphere, so we use Atacama Large Millimeter/submillimeter Array (ALMA) observations at ~340 GHz to search for CS emission. We determined stringent CS 3σ stratospheric upper limits of 0.0074 ppb (uniform above 100 km) and 0.0256 ppb (uniform above 200 km). These upper limits are not quite stringent enough to distinguish between Enceladus and micrometeorite sources at the 3σ level and a contribution from micrometeorites cannot be ruled out, especially if external flux is toward the lower end of current estimates. Only the high-flux micrometeorite source model of Hickson et al. can be rejected at 3σ. We determined a 3σ stratospheric upper limit for CH₂NH of 0.35 ppb, which suggests cosmic rays may have a smaller influence in the lower stratosphere than predicted by some photochemical models. Disk-averaged C₃H₄ and C₂H₅CN profiles were determined and are consistent with previous ALMA and Cassini/CIRS measurements.