Particle sizes in planetary rings from forward-scattering and spectral-extinction observations

Abstract

The forward scattering geometry is favored for observing the size distributions of particles in planetary rings. One reason is the dominance of the collective diffraction lobe of the particles in the near-forward scattering. For tenuous rings, like the Jovian ring, Saturn's ring C, ring F, and Cassini division, observation of the shape of this lobe as a function of scattering angles decouples the size information from the exact particle shape, surface structure, and material. For more opaque rings, such as Saturn's ring A and possibly some of the Uranian rings, the shape of the forward lobe can be contaminated by multiple scattering which must be deconvolved before size information can be recovered. Knowledge of the opacity of the rings is required to carry out the deconvolution.with the degree of success largely dependent on the apparent ring opening. Here, the forward geometry provides a convenient means for determining the “true” opacity by measurements of the extinction of the direct ray between the spacecraft source and receiver. At the present time, only the radio occultation technique has the capability of separating this direct signal component from the forward-scattered signal. This separation is based on the distinct spectral signatures of the direct and scattered components, and permits simultaneous and independent estimates of the opacity and of the strength and shape of the forward-scattering phase function. Because opacity is an area-weighted measure of the particle distribution , while the diffraction lobe is an area-squared weighted measure of the distribution these two observables complement each other by binding the size distribution over different size ranges.