Radiative transfer modeling of Saturn's Outer B ring

A Study of Saturn's Ring Phase Curves from HST Observations

“…An alternate explanation for the OE was proposed by Mishchenko and Dlugach (1992) who showed that the OE could be consistent with coherent backscattering if the effective radius of regolith grains was about 0.1-1 µm. However, this effective grain size is much smaller than that derived from spectroscopic observations (Pollack et al 1973, Clark and McCord 1980, Doyle et al 1989, Alix 1998. The OE could be also explained in terms of the shadow-hiding effect, in which regolith grains occult their own shadows (Hapke 1986), but this has not been investigated to date.…”

Cassini Imaging Science: Initial Results on Saturn's Rings and Small Satellites

“…Because the rings_ scattering function is insensitive to the physical thickness of the rings in this intermediate-phase viewing geometry, our spectra provide evidence that most small-scale variations in ring brightness result from compositional differences (17,18). If the rings began as nearly pure water ice, regions of smaller optical depth are expected to become more polluted by interplanetary dust, resulting in reduced particle albedos (19,20). Spokes, the transient, quasi-radial ghostly features observed by Voyager in Saturn_s B ring (21) and seen also in Hubble Space Telescope (HST) images (22), were not seen in any Cassini image taken on approach to Saturn, during which the solar incidence angle was È65-and the emission angle was È73-.…”

The behavior of spokes in Saturn’s B ring

“…For small phase angles, numerical values of the phase function for spherical and irregular particles obtained by Doyle et al (1989) are similar for the phase angle range of the early southern movies, so our previous modeling of those images is unaffected by the particles' shapes.…”

“…Additional Voyager observations at high phase angles showed that while both the spokes and the spoke free regions of the ring had darkened, the spokes had reversed contrast, becoming bright features on the ring. This indicated that the spokes were composed of forward-scattering, micron-sized particles (Smith et al, 1981;Doyle et al, 1989;Doyle and Grün, 1990). A later analysis of data taken by the Hubble Space Telescope (McGhee et al, 2005) modeled the spoke particle size distribution and determined the effective size of a spoke particle to be r eff = 0.57 ± 0.05 µm with an effective variance of b = 0.09 ± 0.03 (see Doyle and Grün (1990), Eq.…”

“…The most straightforward way to account for this discrepancy is to assume that the spoke particles have "irregular" (i.e., non-spherical) shapes, as such particles have phase functions which scatter far more light than Mie scatterers at the intermediate phase angles we observe in the northern observations. Doyle et al (1989) had previously considered the effects of irregular particles in Voyager data, comparing the phase function of spherical particles with those of "cube-like" and "plate-like" particles, using the semi-empirical theory of Pollack and Cuzzi (1980) for the irregular particles.…”