Deriving mixing ratios of heavier neutral species in Saturn's ionosphere from light ion measurements and helium chemistry

Helium ions, He⁺, react only slowly with molecular hydrogen. A consequence of this is that He⁺ ions produced by, for example, photoionization of He in H₂-dominated ionospheres, such as those of Jupiter and Saturn, can have principal loss mechanisms other than through reactions with molecular hydrogen even if the other reactants prevail in rather small volume mixing ratios. The Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini mission operated in open-source ion mode during a few of the passages through Saturn’s upper atmosphere throughout the proximal orbits in 2017. Due to the high spacecraft velocity, exceeding 30 km/s, the retrieval of ion number densities was limited to light ion species with masses (for singly charged species) of < 8 Da. Direct measurements of mixing ratios of neutral species heavier than helium (such as H₂O, CH₄, NH₃, N₂,CO₂ and CO) in Saturn's equatorial ionosphere are sparse and their retrieval was in part complicated by adsorption effects.

We seek to make an independent estimate of the mixing ratios of volatiles other than H₂ and He by making use of a simple model focusing on the production and loss balance of helium ions. We first consider two models to estimate the local production rate of He⁺ from the measured density profiles of He and H₂ and show that these give estimates in reasonable agreement with each other. Then we show that the calculated concentration of He⁺ exceeds the observed values by up to two orders of magnitude if we only account for the loss of He⁺ ions through reactions with molecular hydrogen. We take this as a strong indicator that the principal loss mechanism of He⁺ in Saturn’s ionosphere is through reactions with other species than H₂, whose overall mixing ratio is denoted f_X.  Based on the assumption of photochemical equilibrium at altitudes below 2500 km, we can then proceed by estimating f_X to closest approach for Cassini's proximal orbits 288 and 292. Our derived mixing ratios for the inbound part of orbits 288 and 292 are in reasonable agreement with the direct measurements from INMS around closest approach and subceed them at higher altitudes. Comparisons with results from other studies potentially suggest an increased water influx around equatorial latitudes.