Since the Saturn orbit insertion of the Cassini spacecraft in mid-2004, the Cassini composite infrared spectrometer (CIRS) measured temperatures of Saturn's main rings at various observational geometries.
In the present study, we apply our new thermal model . Icarus 201, 634-654) for fitting to the early phase Cassini data (Spilker, L.J., and 11 colleagues [2006]. Planet. Space Sci. 54, 1167Sci. 54, -1176)). Our model is based on classical radiative transfer and takes into account the heat transport due to particle motion in the azimuthal and vertical directions. The model assumes a bimodal size distribution consisting of small fast rotators and large slow rotators. We estimated the bolometric Bond albedo, A V , the fraction of fast rotators in cross section, f fast , and the thermal inertia, C, by the data fitting at every radius from the inner C ring to the outer A ring. The albedo A V is 0.1-0.4, 0.5-0.7, 0.4, 0.5 for the C ring, the B ring, the Cassini division, and the A ring, respectively. The fraction f fast depends on the ratio of scale height of fast rotators to that of slow rotators, h r . When h r = 1, f fast is roughly half for the entire rings, except for the A ring, where f fast increases from 0.5 to 0.9 with increasing saturnocentric radius. When h r increases from 1 to 3, f fast decreases by 0.2-0.4 for the B and A rings while no change in f fast is seen for the optically thin C ring and Cassini division. The large f fast seen in the outer A ring probably indicates that a large number of small particles detach from large particles in high velocity collisions due to satellite perturbations or self-gravity wakes. The thermal inertia, C, is constrained from the efficiency of the vertical heat transport due to particle motion between the lit and unlit faces, and is coupled with the type of vertical motion. We found that in most regions, except for the mid B ring, sinusoidal vertical motion without bouncing is more reasonable than cycloidal motion assuming bouncing at the midplane, because the latter motion gives too large C as compared with previous estimations. For the mid B ring, where the optical depth is highest in Saturn's rings, cycloidal vertical motion is more reasonable than sinusoidal vertical motion which gives too small C.