  At the solar equinox in August 2009, the Composite Infrared Spectrometer
(CIRS) onboard Cassini showed the lowest Saturn's ring temperatures ever
observed. Detailed radiative transfer models show that the observed equinox
temperatures of Saturn's A ring are much higher than model predictions as long
as only the flux from Saturn is taken into account. This indicates that the A
ring was not completely cooled down at the equinox. We develop a simple
seasonal model for ring temperatures and first assume that the internal density
and the thermal inertia of a ring particle are uniform with depth. The particle
size is estimated to be 1-2 m. The seasonal thermal inertia is found to be
30-50 Jm$^{-2}$K$^{-1}$s$^{-1/2}$ in the middle A ring whereas it is $\sim$ 10
Jm$^{-2}$K$^{-1}$s$^{-1/2}$ or as low as the diurnal thermal inertia in the
inner and outermost regions of the A ring. An additional internal structure
model, in which a particle has a high density core surrounded by a fluffy
regolith mantle, shows that the core radius relative to the particle radius is
about 0.9 for the middle A ring and is much less for the inner and outer
regions of the A ring. This means that the radial variation of the internal
density of ring particles exists across the A ring. Some mechanisms may be
confining dense particles in the middle A ring against viscous diffusion.
Alternatively, the (middle) A ring might have recently formed ($<$ 10$^{8}$ yr)
by destruction of an icy satellite, so that dense particles have not yet
diffused over the A ring and regolith mantles of particles have not grown
thick. Our model results also indicate that the composition of the core is
predominantly water ice, not rock.
