Spatial and seasonal variations in Saturn's haze and vertical phosphine distribution at 3 microns from 2005 to 2010

We acquired spectra of Saturn from two complementary instruments: the SpeX spectrograph at NASA's Infrared Telescope Facility and the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft. We obtained these spectra between 2005 and 2010, when Saturn's southern hemisphere transitioned from summer to fall, at two distinct latitudes: 15° S (equatorial zone) and 30°S (south equatorial belt). We extracted these spectra in the wavelength range 2.4-3.2 mum, where tropospheric haze and phosphine gas absorb reflected sunlight. We examined the spatial and seasonal variations in Saturn's haze and vertical phosphine distribution. No previous work had modeled variations in both atmospheric components together. We accomplished this by producing synthetic spectra using the adding-doubling method to characterize radiative transfer through atmospheric layers and the correlated k distribution method to model molecular absorption. Studying Saturn's haze helps us understand chemical and thermodynamic processes and Saturn's energy balance. As a disequilibrium species, the presence of phosphine indicates continual replenishment via upwelling from below Saturn's cloud decks. Therefore, studying phosphine is important as a tracer of vertical dynamics. Compared to the tropospheric haze at 30°S, we found that the haze at 15°S was denser, optically thicker, located higher in Saturn's atmosphere, and composed of aerosols with a larger effective radius but a smaller imaginary refractive index. We determined, at both latitudes from 2005 to 2010, that the haze became less dense and descended deeper into Saturn's atmosphere and the radius of the haze aerosols decreased. By comparing our modeled imaginary refractive indices with lab results, we excluded pure ammonia ice as the composition of the aerosols. We found that Saturn's vertical phosphine distribution followed a power law in the troposphere such that the mixing ratio decreased with height. We also determined that the phosphine abundance was greater at 15° S than at 30°S during all years and varied slightly at both latitudes from year to year but not seasonally. As Saturn progresses from its recent equinox to the northern hemisphere's summer solstice, we plan to expand both our spatial and seasonal coverage. We will continue to capitalize on Cassini's unprecedented view of Saturn.