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S. F. Bass (Oxford), J. I. Moses (LPI)
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 including standard hydrogen- and hydrocarbon-ion chemistry, we investigate the effects of a meteoritic influx of oxygen (e.g., H2O, CO, CO2, and/or O) and metals (e.g., Mg). The Infrared Space Observatory (ISO) observations of H2O and CO2 in Saturn's stratosphere are used to constrain the influx of meteoritic material, results from a recent stellar occultation are used to constrain the background atmospheric structure, and ISO observations of stratospheric hydrocarbons are used to constrain the neutral composition and eddy diffusion coefficient profile. As expected, the topside ionosphere of Saturn is dominated by H+, with H3+ becoming important just below the electron-density peak. We find that metal ions, represented here by Mg+, may dominate over hydrocarbon ions in Saturn's lower ionosphere. The introduction of sinusoidal winds in the model, designed to simulate the effects of gravity waves, leads to sharp peaks in the electron-density profiles in the lower ionosphere. The location and magnitude of the sharp lower ionospheric layers observed with the Voyager radio occultation experiments may help constrain wind parameters and physical properties such as size, volatility, density, and composition of interplanetary debris in the outer solar system. Preliminary modeling indicates that micrometeoroids near 10 AU may be smaller, fluffier, and more volatile than particles near 1 AU.