CIRS High-Resolution Thermal Scans and the Structure of Saturn's B Ring

Cassini's 126th flyby of Titan on 29 November 2016, sent the spacecraft on a trajectory that would take it within 10,000 kilometers of Saturn's F ring before a subsequent encounter on 22 April 2017 would send it on ballistic trajectory carrying it between Saturn's cloud tops and the planet's D ring. This geometry has proven very beneficial for high-resolution studies of the rings, not just because of Cassini's proximity to the rings, but also because of the spacecraft's high elevation angle above the rings, which reduces the foreshortening that tends to degrade resolution in the ring plane.We will report on several observations made by Cassini's Composite Infrared Spectrometer of Saturn's main rings at the high spatial resolutions enabled by the end-of-mission geometry, particulary the B ring, during the F-ring and proximal orbits. CIRS' three infrared detectors cover a combined spectral range of 10 to 1400 cm ^{-1} (1 mm down to 7 μm). We focus on data from Focal Plane 1, which covers the 10 to 600 cm ^{-1} range (1 mm to 16 μm). The apodized spectral resolution of the instrument can be varied from 15 to 0.5 cm ^{-1} (Flasar et al. 2004). FP1's wavelength range makes it well-suited to sensing thermal emission from objects at temperatures typical of Saturn's rings.Correlating temperatures retrieved from scans of that face of the rings exposed to direct solar illumination (the lit face) and the opposite (unlit) face with ring optical depth suggests differences in ring structure or particle transport between the lit and unlit rings in different regions of the B ring. We find that the temperature differential between the lit and unlit faces of the rings varies from 2-3 K in the most optically thin sections of the B ring's B1 region (ring radii of 92,000-99,000 km) up to 20 K in the optically thick portions of the B2 region of the B ring (ring radii of 99,000-104,500 km). Moreover, temperatures on the unlit side of the B ring's B3 region vary (ring radii of 104,500-110,000 km) by 5-6 K and are correlated with slight optical depth variations. There are no such correlated temperature variations on the lit side of the B3 region for reasons that are not yet clear. Ferrari et al. (2013) and Pilorz et al. (2015) published thorough analyses of the thermal throughput across this optically thick ring. We will discuss these recent CIRS rings observations and their implications in the context of such work. (B ring region definitions are taken from Colwell et al. (2009).)This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. Copyright 2018 California Institute of Technology. Government sponsorship acknowledged.