Thermal Properties Of Saturns Rings

The temperature of any given ring particle is the result of a balance between energy absorbed by the particle and energy re-radiated by the particle. The energy absorbed by a ring particle is dependent on several factors: the amount of sunlight that is incident on the ring particle (either direct, reflected, or scattered), the amount of thermal radiation received from other ring particles or from Saturn itself, the total reflectivity (bolometric Bond albedo, integrated over all directions and over all wavelengths) of the ring particle, the spin rate of the ring particle, and the size of the particle.

The amount of sunlight that reaches a particle is dependent on the tilt angle of the rings (from 0° to 26.73°) and on the optical thickness of the ring region. As one might suspect, the greater the tilt angle, the more sunlight reaches any given ring particle, at least for optically thick rings like the A and B rings. The absolute temperature of the A and B rings is nearly twice as high (90 K to 100 K) when the ring tilt angle is at its highest compared with that (^50 K) when the rings are edge-on to the Sun. Contrast that with the temperatures of the C ring and Cassini Division, where measured temperatures are between 80 K and 90 K and almost no temperature variation with ring tilt angle is observed [48]. The probable cause of the uniform temperatures with tilt angle for the C ring and Cassini Division is the low optical thickness of these rings, which probably means that interparticle effects are small.

The C ring and Cassini Division particles also have lower reflectivity at visible wavelengths than do the A- and B-ring particles and therefore absorb more of the radiation incident on them. This alone, however, is insufficient to explain their higher measured temperatures. In addition, the temperatures must be higher (as observed from Earth [49]) due to the presence of particles that are large enough and slowly rotating enough that little heat is conducted from their illuminated sides to their unilluminated sides. That conclusion seems to be borne out by Cassini observations; moreover, Cassini data may be starting to show us variations in the spin rate of particles from place to place (Chapter 10).

The C ring and Cassini Division have essentially the same measured temperatures whether observed from the illuminated or unilluminated face of the rings. The dark faces of the A and B rings are considerably lower (about 56 K [50]) than their illuminated faces, except at very low tilt angles. Particles on the unilluminated face generally transition to the illuminated face at least once each orbit around the planet. The lower temperatures imply that the ring particles can cool quickly and heat quickly in response to changing energy input (i.e., they are composed of material with low thermal inertia) and that they also have highly insulating surfaces that do not allow heat to be conducted clear around or through the interior of the particles. This is also consistent with the observation of substantially lower temperatures in the A, B, and C rings after passage through Saturn's shadow than before shadow entry [51].

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