The effects of external material on the chemistry and structure of Saturn's ionosphere

Abstract

We have developed a one‐dimensional coupled ion‐neutral photochemical model for Saturn's upper atmosphere to better understand the structure and chemistry of Saturn's ionosphere. In addition to modeling the chemistry of hydrogen and hydrocarbon ions, we investigate the effects of an oxygen and metal influx from ring or meteoric sources. The Infrared Space Observatory observations of H2O and CO2 in Saturn's stratosphere are used to constrain the influx of extraplanetary material. As expected, the topside ionosphere of Saturn is dominated by H+, with H3+ prevailing just below the electron density peak. When micrometeoroid ablation is considered, we find that metal ions, represented here by Mg+, can take the place of hydrocarbon ions as the major ionic species in the lower ionosphere. The models then exhibit a characteristic double peak, with H+ creating the high‐altitude peak and Mg+ the low‐altitude peak. A pronounced gap forms between the two peaks, especially at night, when H3+ ions rapidly recombine. Neutral winds and electric fields in the presence of magnetic fields can cause vertical plasma motion that can shift the location of both electron density peaks. In addition, multiple sharp layers in the electron density profile can form in the lower ionosphere when oscillatory vertical drifts are introduced into the model to simulate the effects of atmospheric gravity waves. The location and magnitude of the “main peak” as well as the sharper lower‐ionospheric layers observed with the Voyager and Pioneer radio occultation experiments (and eventually with similar Cassini observations) can help constrain the atmospheric structure, wind profiles, or electric field properties in Saturn's upper atmosphere.