Close-range remote sensing of Saturn’s rings during Cassini’s ring-grazing orbits and Grand Finale

INTRODUCTION Saturn’s rings are an accessible exemplar of astrophysical disk processes and a delicate tracer of the Saturn system’s dynamical processes and history. RATIONALE During its ring grazing orbits and Grand Finale, the Cassini spacecraft passed very close to Saturn’s main rings and obtained very high–spatial-resolution images, spectral scans, and temperature scans. RESULTS We find structures related to the detailed sculpting of rings by embedded masses, including structures near the moon Daphnis that have apparently experienced markedly different perturbations compared to the surrounding ring material, and complex structure elements within the largest propeller-shaped disturbances. Interpreting certain such elements in terms of the Hill radius yields diameters of 1.0 to 1.6 km for the largest propeller-causing moons. Several classes of subkilometer structure in the ring, which we call textures, are found in well-defined radial bands, which in many cases are difficult to correlate with other ring properties. The plateaux in the C ring exhibit a characteristic streaky texture. We hypothesize that these textures indicate variation in properties that affect the results of particle-particle collisions. Medium-strength density waves neither alter the spectral characteristics of the region surrounding them nor exclude swarms of propellers from their vicinity, as the largest-density waves are known to do. We also confirm that even the strongest bending waves (such as Mimas 5:3) do not exhibit any signs of spectral halos. However, medium-strength density waves do exhibit clumpy texture in their troughs, and they also alter the propeller size distribution. “Mini-jets” in the F ring are found in clusters, whose members evolve in lockstep with each other. This provides the strongest evidence yet that impacts onto the rings are commonly due to (Saturn-orbiting) streams of material, rather than lone impacting objects. The distinct light-scattering characteristics of the narrow region outward of the Keeler gap—weaker water ice absorption bands, higher reflectivity, grayish rather than reddish color—transition abruptly from the rest of the A ring, although different degrees of abruptness are seen in the visible and the near-infrared. The combination of weaker water-ice band depths with higher reflectivity is difficult to understand. Water-ice band depth and general color slope are closely correlated with optical depth, and temperature is anticorrelated (that is, denser regions are colder), even in sharply banded regions, down to the spatial resolution limit of ~3 km px −1 . However, the narrow bright bands in the C ring, called plateaux, have similar color slopes and water-ice band depths to those of the surrounding C ring, despite their marked difference in brightness. Furthermore, denser regions are warmer in some fine-scaled structures, including C-ring plateaux and structure in the B ring, both on the lit side only, and strong waves in the A ring on both the lit and unlit sides. CONCLUSION The rings are sculpted by embedded masses, producing structure visible down to our resolution limit. Correlations of spectral properties and temperature with optical depth are tight at many locations, although exceptions are found that deepen puzzles in certain regions. Many of these results are likely related to radial stratification in particle properties, rather than in chemical composition or surface mass density.