Observing Saturn's rings with Cassini

Azimuthal variations in the reflectivity (at both visual and radar wavelengths), brightness temperature and optical depth of Saturn's A ring have been widely observed and are generally attributed to what are now referred to as `self-gravity wakes'. Similar structures are observed in numerical models of marginally-stable shearing disks (Julian & Toomre 1966; Salo 1995). Comparisons of the rings' transmission profile from selected Cassini stellar occultations now suggest (Colwell etal., 2007) that self-gravity wakes are as ubiquitous in the B ring as they are in the A ring. Fits to a simple model suggest that much of the fine-scale structure in the B ring is attributable to variations in the spacing of essentially opaque wakes, while the optical depth in the interwake gaps remains relatively constant. Tentatively we attribute the latter to a 'haze' of somewhat smaller particles which envelops a thinner layer of meter-size, self-gravitating particles (Salo & Karjalainen 2003).

An inspection of the optical depth profile in the central B ring suggests that there may be a critical value near 2.0, at which the rings' radial structure becomes irregular and above which the rings are essentially opaque. This threshhold also corresponds to a marked change in ice band depths, and may represent a transition between a closely-packed monolayer and a much more opaque multilayer, or even a local `crystallization' in the rings (Tremaine 2003).

I will also touch on several other curiosities, including the plethora of satellite-driven waves and wakes in the A ring; a set of as-yet unidentified waves in the C ring; and the multiple strands and clumps of the ever-puzzling F ring.

Phil Nicholson