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

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 + , H 2 + , H 3 + , 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 H 3 + densities requires that they chemically react with one or more heavy neutral molecular species that have mixing ratios of about 100 ppm.