The Ion Composition of Saturn's Equatorial Ionosphere as Observed by Cassini

Abstract

The Cassini Orbiter made the first in situ measurements of the upper atmosphere and ionosphere of Saturn in 2017. The Ion and Neutral Mass Spectrometer (INMS) found molecular hydrogen and helium as well as minor species including water, methane, ammonia, and organics. INMS ion mode measurements of light ion species (H+, H2+, H3+, and He+) and Radio and Plasma Wave Science instrument measurements of electron densities are presented. A photochemical analysis of the INMS and Radio and Plasma Wave Science data indicates that the major ion species near the ionospheric peak must be heavy and molecular with a short chemical lifetime. A quantitative explanation of measured H+ and H3+ densities requires that they chemically react with one or more heavy neutral molecular species that have mixing ratios of about 100 ppm.

Saturn's Ionosphere: Electron Density Altitude Profiles and D‐Ring Interaction From The Cassini Grand Finale

“…Layer C , the region below 2,200 km, represents a chemical equilibrium region, dominated by chemical processes. Morooka et al () observe the dominance of negatively and positively charged heavy cluster ions in this altitude range, and Cravens et al () interpret it as a nondiffusive layer of heavy molecular ions. Moreover, recently the Ion‐Neutral Mass Spectrometer observations implied the presence of organic ions at those low altitudes (Waite et al, ), and the densities of positive dust grains measured by the Charge Energy Mass Spectrometer, designed to measure only the positive dust, are shown to be directly proportional to the heavy positive ion population (Mitchell et al, ) below 1,700 km.…”

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

“…No physics that could produce such layers is included in these preliminary 1-D model calculations. ing ratio for such a species is several times larger than the~10 À4 CH 4 mixing ratio and peaks near the equatorial plane (Cravens et al, 2018). None of the measured constituents is known to fit this requirement.…”

“…Figure 1 demonstrates that light ions are a minority in Saturn's low-altitude equatorial ionosphere. In order to explain the relatively minor contribution of H + and H 3 + ions in this regime, a high abundance (~10 À4 mixing ratio) of heavy molecular species must also be present (Cravens et al, 2018). INMS has measured a number of 10.1029/2018GL078162 abundant neutral species that are consistent with this requirement, but there is uncertainty in their identification, particularly for the 28-Da species.…”

Chemical interactions between Saturn’s atmosphere and its rings

“…The molecular volatiles (water, methane, ammonia, carbon dioxide) can easily convert the long-lived protons of the ionosphere into molecular ions with shorter lifetimes, decreasing the overall electron density (6), as described below. ions measured are the primary product of ionization in the Saturn ionosphere and are created by ionization of H 2 , the most abundant neutral, by solar extreme ultraviolet radiation (34). They are a good tracer of the ionization process.…”