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At the solar equinox in August 2009, the Composite Infrared Spectrometer(CIRS) onboard Cassini showed the lowest Saturn's ring temperatures everobserved. Detailed radiative transfer models show that the observed equinoxtemperatures of Saturn's A ring are much higher than model predictions as longas only the flux from Saturn is taken into account. This indicates that the Aring was not completely cooled down at the equinox. We develop a simpleseasonal model for ring temperatures and first assume that the internal densityand the thermal inertia of a ring particle are uniform with depth. The particlesize is estimated to be 1-2 m. The seasonal thermal inertia is found to be30-50 Jm$^{-2}$K$^{-1}$s$^{-1/2}$ in the middle A ring whereas it is $\sim$ 10Jm$^{-2}$K$^{-1}$s$^{-1/2}$ or as low as the diurnal thermal inertia in theinner and outermost regions of the A ring. An additional internal structuremodel, in which a particle has a high density core surrounded by a fluffyregolith mantle, shows that the core radius relative to the particle radius isabout 0.9 for the middle A ring and is much less for the inner and outerregions of the A ring. This means that the radial variation of the internaldensity of ring particles exists across the A ring. Some mechanisms may beconfining 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 yetdiffused over the A ring and regolith mantles of particles have not grownthick. Our model results also indicate that the composition of the core ispredominantly water ice, not rock.

Incomplete cooling down of Saturn's A ring at solar equinox: Implication for seasonal thermal inertia and internal structure of ring particles