Research Article

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

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Science  14 Jun 2019:
Vol. 364, Issue 6445, eaau1017
DOI: 10.1126/science.aau1017

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Cassini's last look at Saturn's rings

During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin.

Science, this issue p. 1046, p. eaat2965, p. eaat2349, p. eaau1017; see also p. 1028

Structured Abstract

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.

False-color images of Saturn’s rings.

(Top) A mosaic showing Daphnis in the Keeler gap on the lit side of the rings, with three wave crests of the structure raised by Daphnis in the gap’s outer edge. (Middle and bottom) Visual and Infrared Mapping Spectrometer radial scans across the lit side of the main rings, displayed as false-color images. Reddish colors signify a higher fraction of components other than water ice. The boxed region in the middle panel indicates the location of the bottom panel.

Abstract

Saturn’s rings are an accessible exemplar of an astrophysical disk, tracing the Saturn system’s dynamical processes and history. We present close-range remote-sensing observations of the main rings from the Cassini spacecraft. We find detailed sculpting of the rings by embedded masses, and banded texture belts throughout the rings. Saturn-orbiting streams of material impact the F ring. There are fine-scaled correlations among optical depth, spectral properties, and temperature in the B ring, but anticorrelations within strong density waves in the A ring. There is no spectral distinction between plateaux and the rest of the C ring, whereas the region outward of the Keeler gap is spectrally distinct from nearby regions. These results likely indicate that radial stratification of particle physical properties, rather than compositional differences, is responsible for producing these ring structures.

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