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Stellar occultations by Saturn's rings observed with the Visual and InfraredMapping Spectrometer (VIMS) onboard the Cassini spacecraft reveal that dustyfeatures such as the F ring and the ringlets in the Encke and the Laplace Gapshave distinctive infrared transmission spectra. These spectra show a narrowoptical depth minimum at wavelengths around 2.87 microns. This minimum islikely due to the Christiansen Effect, a reduction in the extinction of smallparticles when their (complex) refractive index is close to that of thesurrounding medium. Simple Mie-scattering models demonstrate that the strengthof this opacity dip is sensitive to the size distribution of particles between1 and 100 microns across. Furthermore, the spatial resolution of theoccultation data is sufficient to reveal variations in the transmission spectrawithin and among these rings. For example, in both the Encke Gap ringlets and Fring, the opacity dip weakens with increasing local optical depth, which isconsistent with the larger particles being concentrated near the cores of theserings. The strength of the opacity dip varies most dramatically within the Fring; certain compact regions of enhanced optical depth lack an opacity dip andtherefore appear to have a greatly reduced fraction of grains in the few-micronsize range.Such spectrally-identifiable structures probably represent a subsetof the compact optically-thick clumps observed by other Cassini instruments.These variations in the ring's particle size distribution can provide newinsights into the processes of grain aggregation, disruption and transportwithin dusty rings. For example, the unusual spectral properties of the F-ringclumps could perhaps be ascribed to small grains adhering onto the surface oflarger particles in regions of anomalously low velocity dispersion.

The Christiansen Effect in Saturn's narrow dusty rings and the spectral identification of clumps in the F ring