  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.
