Cassini’s Imaging Science Subsystem (ISS) and its Visual and Infrared Mapping Spectrometer (VIMS) both revealed that Saturn’s north polar region underwent a significant evolution in color and cloud structure between 2012 and 2017. At visual wavelengths the region between the polar hexagon and the eye changed from a dark blue/green to a moderately brighter gold color, with even more dramatic (up to 10×) reflectivity changes at near-IR wavelengths. Using combined spectra from 350 nm to 5.12 microns, and constraints from both reflected sunlight and thermal emission, we found that all the observed changes can be reproduced by a four-layer aerosol model consisting of a chromophore-bearing stratospheric haze near 50 mbar, a deeper haze of putative diphosphine particles typically near 300 mbar, an ammonia cloud layer between 0.4 and 1.3 bar, and a deeper cloud, possibly composed of a mix of NH4SH and water ice particles, in the 3 to 5 bar region. We found that between 2013 and 2016, the effective pressures of most layers changed very little, with the main exception being the NH3 ice layer, which decreased from about 1 bar to 0.4 bar near the edge of the eye, but returned to 1 bar inside the eye. Inside the hexagon there were big increases in optical depth, by factors of 4-10 for the putative diphosphine layer. At the beginning of this period the eye was very dark compared to surrounding latitudes and remained very dark, with generally very low optical depths compared to the region outside the eye. The color change from blue/green to orange inside most of the hexagon region can be explained almost entirely by changes in the stratospheric haze layer, which increased its particle size, imaginary index peak, and optical depth by factors of 2, 3, and 4 respectively. A plausible mechanism for these temporal increases is the action of photochemistry as the north polar region became increasingly exposed to solar UV radiation.
This research was supported by NASA CDAP Grant 80NSSC18K0966.