Evolution of Saturn's north polar color and cloud structure between 2012 and 2017 inferred from Cassini VIMS and ISS observations

Abstract: Cassini/ISS imagery and Cassini/VIMS spectral imaging observations from 0.35 to 5.12 microns show that between 2012 and 2017 the region poleward of the Saturn's northern hexagon changed from dark blue/green to a moderately brighter gold color, except for the inner eye region (88.2 deg - 90 deg N), which remained relatively unchanged. These and even more dramatic near-IR changes can be reproduced by an aerosol model of four compact layers consisting of a stratospheric haze at an effective pressure near 50 mbar, a deeper haze of putative diphosphine particles typically near 300 mbar, an ammonia cloud layer with a base pressure between 0.4 bar and 1.3 bar, and a deeper cloud of a possible mix of NH4SH and water ice particles within the 2.7 to 4.5 bar region. Our analysis of the background clouds between the discrete features shows that between 2013 and 2016 the effective pressures of most layers changed very little, except for the ammonia ice layer, which decreased from about 1 bar to 0.4 bar near the edge of the eye, but increased to 1 bar inside the eye. Inside the hexagon there were large increases in optical depth, by up to a factor of 10 near the eye for the putative diphosphine layer and by a factor of four over most of the hexagon interior. Inside the eye, aerosol optical depths were very low, suggesting downwelling motions. The high contrast between eye and surroundings in 2016 was due to substantial increases in optical depths outside the eye. The color change from blue/green to gold inside most of the hexagon region can be explained in our model almost entirely by changes in the stratospheric haze, which increased between 2013 and 2016 by a factor of four in optical depth and by almost a factor of three in the short-wavelength peak imaginary index.