Implications of the Cassini Grand Finale INMS measurements for atmospheric chemistry on Saturn (and vice versa)

In situ measurements from the Ion and Neutral Mass Spectrometer (INMS) instrument during the Grand Finale stage of the Cassini mission have revealed surprisingly large amounts of ring material flowing into Saturn’s equatorial upper atmosphere. Our photochemical models indicate that the composition of Saturn’s ionosphere and neutral stratosphere would be significantly affected by this incoming material if a large fraction of the ring debris were entering the atmosphere as vapor, or if solid ring particles were vaporized during entry. However, the observational consequences predicted by these models are not evident in our surveys of Cassini infrared and ultraviolet remote-sensing data from the last years of the Cassini mission. This lack of evidence for ring-derived molecules in the lower thermosphere and stratosphere suggests that either the inflow inferred by INMS derives from a recent dynamical event in the ring system that occurred a few months to a few years before the 2017 measurements, or that the incoming material derives predominantly from small dust particles (less than 100 nm radius) that are entering the atmosphere slowly enough to avoid ablating. If a recent but continuing ring event is the culprit, the abundance of HC₃N and HCN could build up to become observable within the next few years, and the CO₂ abundance could be enhanced over what was seen during the Cassini mission. Future infrared observations or upper limits for these molecules in Saturn’s stratosphere could shed light on the origin, timing, magnitude, and nature of Saturn’s ring-atmosphere interaction. Regarding this point, we note that JWST observations from Saturn in 2022 did not see any evidence for the expected species (Fletcher et al. 2023), further reinforcing the dust hypothesis or the very transient nature of the event.

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