Reflection and Shadowing from Saturn’s Rings: Influence on Photochemisty and Heating

Cassini orbited Saturn for over thirteen years, nearly a half Saturn year. This corresponded to a seasonal configuration where the sub-solar point was at ~24°S at the time of Saturn Orbit Insertion (July 1, 2004) and ~27°N at the time of Cassini’s Grand Finale. During this period, the ring shadow moved southward from covering a substantial area of the northern hemisphere to covering a large swath of territory south of the equator as solstice approached. At equinox, the rings project a small sliver of shadow at low latitudes. At its maximum extent, the ring shadow can reach as far as 48°N/S at the meridian (~58°N/S at the terminator).

Both ultraviolet and visible sunlight penetrating into any particular latitude will vary greatly depending on both Saturn’s season, the optical thickness of each ring system and its reflective properties. In essence, the rings act like both semi-transparent Venetian blinds over the atmosphere of Saturn. At the same time, the illuminated side of the rings reflect ultraviolet and visible solar photons onto the fully illuminated hemisphere of the planet. This acts to enhance both photochemistry and heating and potentially enhancing seasonal effects. Lastly, the rings, having a temperature themselves, provides a source of thermal photons impinging onto the atmosphere and a possible source of heating.

The projection of the oscillating ring shadow onto the planet has been derived as a function of season. In addition, detailed calculations of geometric parameters important for light scattering from the rings onto an oblate planet have been worked for a fine grid of latitudes and longitudes. We will focus on how these geometric parameters can be used for both photochemical (UV) and thermal balance (visible and infrared) radiative transfer calculations as a function of season.

The impact of these augmentations on production and loss rates of hydrocarbons (e.g. acetylene, ethane, and propane), ammonia, phosphine, and hazes will be examined and attempt to explain several Saturn observations conducted by Cassini. Comparison with Jupiter, where seasonal effects are known to be insignificant, will be made.

Acknowledgements: The research described in this paper was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Government sponsorship is acknowledged.