Phantoms in the Dark: Azimuthally and Radially Limited Regions of Translucence in Opaque Regions of Saturn’s B Ring

Abstract:

Two of the brightest stars observed by Cassini’s Ultraviolet Imaging Spectrograph (UVIS) stellar occultation experiment of Saturn’s rings, Beta Centauri and Alpha Crucis, are also at the highest elevation angles out of the ring plane. They provide the highest signal-to-noise of these stellar occultations to measure large optical depths. Put another way, they set the upper limit on measurable optical depth by UVIS at spatial scales of ~10 m in the rings. We have identified 36 regions in the B2 region of the B ring where the signal is indistinguishable from the background signal with a minimum optical depth of 5.36 at 6 m resolution. In most opaque regions we see occasional, but not repeatable, radially limited detections of the star shining through the rings. While not fully transparent, like the previously identified ghosts seen in the C ring and Cassini Division (Baillié et al. 2013, Astron J. 145, 171), these features are narrow in the radial dimension, sometimes at the limit of the resolution of the occultation. Transparencies range from a few per cent to > 20 per cent. The distribution of these translucent features, nicknamed phantoms for their similarities to the ghosts, varies between opaque regions. They are either transient, azimuthally limited, or both. We present the distribution of phantoms and opaque regions in the B ring. While it is possible that the phantoms are under-resolved completely transparent openings in the rings, like the ghosts, many phantoms span several points with a peak transparency of several per cent, suggesting instead that the phantoms have a high optical depth but are qualitatively different than the surrounding opaque region.