  The thermal inertia values of Saturn's main rings (the A, B, and C rings and
the Cassini division) are derived by applying our thermal model to azimuthally
scanned spectra taken by the Cassini Composite Infrared Spectrometer (CIRS).
Model fits show the thermal inertia of ring particles to be 16, 13, 20, and 11
Jm$^{-2}$K$^{-1}$s$^{-1/2}$ for the A, B, and C rings, and the Cassini
division, respectively. However, there are systematic deviations between
modeled and observed temperatures in Saturn's shadow depending on solar phase
angle, and these deviations indicate that the apparent thermal inertia
increases with solar phase angle. This dependence is likely to be explained if
large slowly spinning particles have lower thermal inertia values than those
for small fast spinning particles because the thermal emission of slow rotators
is relatively stronger than that of fast rotators at low phase and vise versa.
Additional parameter fits, which assume that slow and fast rotators have
different thermal inertia values, show the derived thermal inertia values of
slow (fast) rotators to be 8 (77), 8 (27), 9 (34), 5 (55)
Jm$^{-2}$K$^{-1}$s$^{-1/2}$ for the A, B, and C rings, and the Cassini
division, respectively. The values for fast rotators are still much smaller
than those for solid ice with no porosity. Thus, fast rotators are likely to
have surface regolith layers, but these may not be as fluffy as those for slow
rotators, probably because the capability of holding regolith particles is
limited for fast rotators due to the strong centrifugal force on surfaces of
fast rotators. Other additional parameter fits, in which radii of fast rotators
are varied, indicate that particles less than $\sim$ 1 cm should not occupy
more than a half of the cross section for the A, B, and C rings.
