Irregular Structure in the Core of Saturns B Ring

Saturns broad and optically thick B ring has been divided into regions based on structure in optical depth (Marouf et al. 2006 Bull. Am. Astron. Soc. 38, 552; Colwell et al. 2009, Saturn from Cassini-Huygens, Springer, 375). Regions B2 and B3 have sub-regions where the transparency in stellar occultations is indistinguishable from zero. Within these regions, however, there are narrow features with transparency up to about 0.2 that are non-axisymmetric. We surveyed the B2 and B3 regions using the most sensitive stellar occultations for high-optical-depth regions made by the Cassini Ultraviolet Imaging Spectrograph (UVIS). In the B2 region, these narrow phantoms are generally more isolated and appear relatively infrequently within their respective opaque sub-regions. In the B3 region, where the average optical depth is more uniformly high than in B2, the structure is qualitatively different. There, the phantoms are numerous and seen in all occultations, but with lower transparencies. They are non-axisymmetric structures in both B2 and B3. The opaque sub-regions are separated by circular features of low transparency and irregular structure characterized by narrow spikes in transparency at the resolution limit of the Cassini UVIS stellar occultation data (~ 10 m). Cassini images show mainly featureless texture in the azimuthal direction in the opaque regions in B2 with more streaky texture seen in the optically thick regions of B3. We present characteristics of the phantoms and irregular structure and place them in the larger context of the B ring regions revealed by imaging.