Models of Saturn's Equatorial Ionosphere Based on In Situ Data From Cassini's Grand Finale

Abstract

AbstractWe present new models of Saturn's equatorial ionosphere based on the first in situ measurements of its upper atmosphere. The neutral spectrum measured by Cassini's Ion and Neutral Mass Spectrometer, which includes substantial methane, ammonia, and organics in addition to the anticipated molecular hydrogen, helium, and water, serves as input for unexpectedly complex ionospheric chemistry. Heavy molecular ions are found to dominate Saturn's equatorial low‐altitude ionosphere, with a mean ion mass of 11 Da. Key molecular ions include H3O+ and HCO+; other abundant heavy ions depend upon the makeup of the mass 28 neutral species, which cannot be uniquely determined. Ion and Neutral Mass Spectrometer neutral species lead to generally good agreement between modeled and observed plasma densities, though poor reproduction of measured H+ and H3+ variability and an overabundance of modeled H3+ potentially hint at missing physical processes in the model, including a loss process that affects H3+ but not H+.