Empirical Photochemical Modeling of Saturn’s Ionization Balance Including Grain Charging

Abstract

We present a semianalytical photochemical model of Saturn’s near-equatorial ionosphere and adapt it to two regions (∼2200 and ∼1700 km above the 1 bar level) probed during the inbound portion of Cassini’s orbit 292 (2017 September 9). The model uses as input the measured concentrations of molecular hydrogen, hydrogen ion species, and free electrons, as well as the measured electron temperature. The output includes upper limits, or constraints, on the mixing ratios of two families of molecules, on ion concentrations, and on the attachment rates of electrons and ions onto dust grains. The model suggests mixing ratios of the two molecular families that, particularly near ∼1700 km, differ notably from what independent measurements by the Ion Neutral Mass Spectrometer suggest. Possibly connected to this, the model suggests an electron-depleted plasma with a level of electron depletion of around 50%. This is in qualitative agreement with interpretations of Radio Plasma Wave Science/Langmuir Probe measurements, but an additional conundrum arises in the fact that a coherent photochemical equilibrium scenario then relies on a dust component with typical grain radii smaller than 3 Å.

Saturn’s atmospheric response to the large influx of ring material inferred from Cassini INMS measurements

“…This modeldata mismatch suggests that the ring vapor could not have not penetrated very far into the stratosphere by the end of the Cassini mission, or CIRS would have seen evidence for the oxygen-bearing species and HCN and HC 3 N, if not additional nitrogen species and hydrocarbon perturbations. On the other hand, the INMS measurements produce clear signals of heavy molecules in Saturn's thermosphere that do not appear to be an instrumental artifact (Waite et al, 2018;Miller et al, 2020;Serigano et al, 2022), and the INMS-inferred ring vapor influx rates are consistent to first order with ion and electron densities inferred for the ionosphere (e.g., see the models above and section 5.1, as well as Moore et al, 2018;Cravens et al, 2019;Persoon et al, 2019;Hadid et al, 2019;Morooka et al, 2019;Dreyer et al, 2021;Vigren et al, 2022). One possibility that could potentially reconcile the stratospheric results with the thermospheric results is if the inferred ring vapor during the Grand Finale were the result of a recent disruptive event in the rings that generated a recent, transient inflow of dust and vapor.…”

“…The dominance of HCNH + at the main peak in our Case A model is consistent with the combined INMS and RPWS data analysis that points to a heavy ion being the dominant species at the main peak (Cravens et al, 2019); it also satisfies the combined RPWS/LP and INMS analysis that suggests that the main-peak ion should have a recombination rate coefficient ≲3 × 10 −7 cm 3 s −1 if the ion densities inferred by Morooka et al (2019) are correct (Dreyer et al, 2021) (but see Johansson et al 2022 for an alternate interpretation). However, the H + /H 3 + ratio predicted by the model at 3 × 10 −7 mbar, the deepest point in the trajectory of orbit 292, is much smaller than is observed (Waite et al, 2018;Moore et al, 2018), and the observed ion and electron densities at this pressure level are inconsistent with the INMS-inferred abundance of neutral species that are able to react with H + and maintain photochemical equilibrium (Vigren et al, 2022). These issues are not limited to Case A and are discussed more completely in section 5.1.…”

“…We assume the electron and neutral temperatures are equal in our models (for a justification of this assumption, see Moore et al, 2008). The influence of grain charging and greater electron temperatures is considered in the models of Vigren et al (2022); briefly, some electron depletion and grain charging is predicted if electron temperatures are as high as indicated by Morooka et al (2019) but these effects become much less significant if electron and ion temperatures are assumed to be equal.…”

“…Our diurnally varying model results for the ionosphere at a local time near noon are presented in Figs. 16 and 17 and compared with number densities of various ions measured by INMS (Waite et al, 2018;Moore et al, 2018;Vigren et al, 2022) and number densities of free electrons determined from RPWS/LP observations (Wahlund et al, 2018;Morooka et al, 2019;Persoon et al, 2019;Hadid et al, 2019) at two different atmospheric pressures. Because of the high encounter velocity of the spacecraft with respect to Saturn's atmosphere, only the densities of the lightest ions H + , H 2 + , H 3 + , and He + were recorded by INMS (Waite et al, 2018).…”

“…Some of the consequences of this infalling ring-derived vapor and dust on Saturn's ionosphere have been discussed previously. Briefly, the inflow of neutral molecules from the rings causes heavy molecular ions with relatively short chemical lifetimes to supplant H + and H 3 + as the dominant ions near Saturn's main ionospheric peak at pressures ∼10 −6 -10 −7 mbar (Moore et al, 2018;Waite et al, 2018;Cravens et al, 2019;Dreyer et al, 2021;Chadney et al, 2022;Vigren et al, 2022), and incoming ring dust may influence ionospheric structure and charge balance (Mitchell et al, 2018;Hadid et al, 2019;Persoon et al, 2019;Morooka et al, 2019;Vigren et al, 2022;Johansson et al, 2022). From model-data comparisons and photochemical-equilibrium arguments, Dreyer et al (2021) find that the dominant heavy ion at low latitudes near 1700 km altitude should have an effective electron-recombination rate coefficient of ≲ 3 × 10 −7 at an electron temperature of 300 K, which makes HCO + a prime candidate for the dominant ion at the main peak (see also Moore et al, 2018).…”