Small-Scale Structure in the Core of Saturn’s B Ring

Saturn’s most massive ring, the B ring, has been divided into five regions based on the radial structure of its optical depth (Marouf et al. 2006, BAAS 38, 552). Regions B2 and B3 contain the locations with the highest optical depths. The Cassini UVIS HSP observed 15 occultations of the bright UV star Beta Centauri (Hadar). Several regions in the B ring core are opaque to Hadar with transparencies of less than 0.002 at the 10-m radial resolution of the occultations. These opaque regions are separated by irregular structures with embedded narrow (< 100 m) features with transparencies on the order of 0.1-0.2. The boundaries of the opaque regions are circular, but within each of the opaque regions there are multiple non-axisymmetric windows where the transparency jumps to as much as 0.2 on spatial scales of less than 50 m. The two brightest stars in the Hadar system contribute comparable amounts to the observed HSP signal. The projected separation of these stars in the ring plane is typically tens of meters in both the radial and azimuthal directions allowing us to measure or constrain the dimensions of these narrow openings we dub “phantoms” due to their similarity to the completely transparent “ghosts” identified in the C ring plateaus (Baillie et al. 2013, Astron. J. 145, 171). High resolution images of these regions reveal non-axisymmetric structure on larger spatial scales. The number and dimensions of the phantoms varies between the opaque regions. Phantoms do not appear at the same location in different occultations, suggesting that they are due to local particle dynamics rather than any large-scale instability or perturbation. Embedded boulders or miniature moonlets, like the propeller objects seen in the A ring (Tiscareno et al. 2006, Nature 440, 648; Tiscareno et al. 2019, Science 364, 1054) have been proposed to explain the C ring ghosts. Further work will be needed to determine if similar objects in the dense B ring can explain phantoms as well as the source of the larger inhomogeneities seen in Cassini images.