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INTRODUCTION Our view of planetary ring particles and the characteristics of their thermal emission has undergone a major paradigm shift since the arrival of Cassini at Saturn. Our understanding of the microstructure and microphysics of the rings has evolved from rings randomly filled with individual particles to Saturn's A and B rings containing particles that tend to clump into transient structures of characteristic sizes and orientations. The dynamics and evolution of rings strongly depend on the outcome of interparticle collisions and on the self-gravity of the rings. Energy loss, mass transfer, and sticking probability for relevant impact velocities will favor either aggregation or disruption and erosion of particles, modifying the size distribution and velocity dispersion, and thus the dynamics and structure of the rings. The thermal response of a ring is determined by absorbed and emitted radiation or conducted heat within the particles. The radiation source functions depend upon the ring structure. Energy sources include direct, reflected and scattered solar light, mutual heating by neighboring ring particles, and thermal and visible radiation from Saturn. Because of mutual shading and heating between particles, the thermal emission is determined not only by the physical properties of the ring particles, but also by the structural and dynamical properties of the ring disk itself. Friction in mutual dissipative collisions between particles, due to their irregular surfaces, transforms orbital kinetic energy into spin. The particle surface temperature and its thermal emission are expected to vary on the surface along the rotation axis and azimuthally. Ring particles, as they collide into one another, are tumbling around the ring mid-plane with a vertical excursion governed by the local ring dynamics. The thermal history of a particle along its orbit is then an indicator of vertical dynamics. The particle is conditioned by the time it spends in sunlight and in the planetary shadow. At the exit of the shadow, its ability to warm up is a function of the thermal inertia. Any difference in the heating curves between the lit and unlit sides should reveal the time each particle spends on each side. |
