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

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. The retrieval of number densities of volatiles like H₂O, CH₄, NH₃, N₂ and CO were 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. We proceed with a brief survey of chemical reaction databases highlighting that it seems reasonable to consider an effective rate constant of k₁ ≈ (1.0 ± 0.5)*10^-9 cm³ s^-1 for reactions involving the neutralization of He⁺ in reactions with H₂O, CH₄, NH₃, N₂ and CO. This allows us to estimate the mixing ratio of these molecules across an altitude profile. Our results are compatible with the average values reported by Miller et al. (2020) and show indications of enhanced mixing ratios towards lower altitudes and/or near equatorial latitudes.