Phantoms in the Dark: Mesoscale Openings in the Core of Saturn’s B Ring

The Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) observed 15 occultation traces of the bright star Hadar (Beta Centauri) in 2008-2009 at an elevation above the ring plane of 66.7 degrees. The combination of the high signal from this star (up to 600 counts per msec integration period) and its high elevation above the rings means these occultations provide the strongest constraints on the transparency of high optical depth regions as observed by UVIS.

 

We use the excess variance that is introduced into the UVIS stellar occultation counting rates by the presence of ring particles to determine when the observed signal is purely background with no transmitted starlight. When the observed variance is equal to the mean, we identify the region as opaque. We have identified several regions that are opaque in the UVIS occultations in the B2 and B3 regions (Marouf et al. 2006). When comparing these regions between occultations of the same star, we discovered non-repeating, narrow regions where the transparency jumps from less than 1% up to as much as 20%. These regions are typically less than 100 m in radial extent and frequently less than 50 m. These regions do not repeat at the same location between occultations. In addition, we are able to take advantage of the fact that Hadar is a binary star and both components produce measurable signals in the HSP data. These partially transparent features produce two distinct signals in the occultation data if the azimuthal extent of the feature is at least as large as the projected separation of the two stars onto Saturn’s rings. In many cases we find that there is only one feature observed, indicating that the features are not only narrow in the radial direction but also of limited (< 100 m) extent in the azimuthal direction. We refer to these features as “phantoms” due to their similarity to the completely transparent regions identified in the C ring plateaus dubbed “ghosts” (Baillie et al. 2013).

 

Figure 1 shows an example of an opaque region in the B2 region of the B ring in all 15 occultations of Hadar. In all but two of the 15 profiles there is a narrow spike with a transparency of 5% up to nearly 20% in the region between 100,090 km and 100,130 km. If a transparent region is observed in all occultations at the same location, we identify that as a boundary between two opaque regions rather than as a phantom. The region between ~100,140 – 100,160 km in Figure 1 has complicated structure that is not exactly repeated between the occultations but marks the boundary between two opaque regions. The phantoms, observed between 100,090 km and 100,130 km, do not repeat and are generally very narrow. The first 15 km of the next opaque region, seen from 100,165-100,180 km in Figure 1 are virtually free of phantom features.

 

The projected separation of the two components of Hadar is less than 50 m in the azimuthal direction, and frequently phantoms are only seen to pass behind one of the two stars. The actual transparency of narrow phantoms is thus approximately twice as high as that shown in Figure 1, because the transparency is based on the combined brightness of the two components. Baillie et al. (2013) suggested that ghosts are the nearly empty openings in the background ring carved out by small moonlets akin to the propeller features imaged in the A ring. The situation may be similar for the phantoms, but the dynamics of neighboring ring particles resulting from embedded moonlets in the dense B ring may not produce the same orbital perturbations that give rise to propeller features. We will describe the distribution and characteristics of opaque regions and phantoms in the B2 and B3 regions.

Figure 1: Transparency of 100 km of the B2 region of Saturn’s B ring showing an opaque region extending from approximately 100084 km to 100140 km, and part of another beginning at about 100162 km. Each curve is offset by 5% for clarity.

Bibliography:

Baillié, K., J. E. Colwell, L. W. Esposito, and M. C. Lewis 2013. Meter-sized Moonlet Population in Saturn’s C Ring and Cassini Division. Astron J.145, 171, doi:10.1088/0004-6256/145/6/171.

 

Marouf, E. A., French, R. G., Rappaport, N. J., McGhee, C. A., Wong, K., Thomson, F. S., Anabtawi, A. 2006. Structure and properties of Saturn’s Ring B from Cassini radio occultations. Bull. Am. Astron. Soc.  38, 552.