Photochemistry in Saturn's Ring Shadowed Atmosphere: Production Rates of Key Atmospheric Molecules and Preliminary Analysis of Observations

Cassini has been orbiting Saturn for over eight years. During this epoch, the ring shadow has moved from shading a large portion of the northern hemisphere (the ring plane was inclined by ~24 degrees relative to the Sun-Saturn vector) to shading mid-latitudes south of the equator and continues southward. At its maximum extent, the projection of the ring plane shadow onto Saturn can reach as far as 48N (~58N at the terminator). The net result, is that the intensity of both ultraviolet and visible sunlight penetrating onto any particular northern/southern latitude will vary depending on Saturn's tilt relative to the Sun and the optical thickness of each ring system. Our previous work has examined the variation of the solar flux as a function of solar inclination, i.e. season on Saturn. Here we report on the impact of the oscillating ring shadow on the photolysis and production rates of key hydrocarbons in Saturn's stratosphere and upper troposphere, including ethane, acetylene, propane, benzene. We investigate the impact on production and loss rates of the long-lived, photochemical hydrocarbons leading to haze formation at several latitudes over one Saturn year. Similarly, we assess the impact on the abundance of phosphine, a disequilibrium species whose presence in the upper troposphere is a tracer of convection processes in the deep atmosphere. Along with the above, we present preliminary analysis of Cassini's UVIS and VIMS datasets that provide an estimate of the evolving haze content of the northern hemisphere. We will also compare our model results to abundances determined from previously released CIRS observations. The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.