RADIATIVE-TRANSFER MODELING CONSTRAINTS ON THE SIZE OF THE SPOKE PARTICLES IN SATURNS RINGS

Color Voyager imaging data of the outer B ring of Saturn have been studied with the idea of putting constraints on the spokes' particle sizes, an important parameter for spoke formation and evolution theories. Colors of seven spokes have been measured (in back-scatter geometry) and in each case the spoke's apparent optical depth increases with wavelength. We found that the outer B ring has a significantly higher albedo at longer visible wavelengths, and correct for this effect. We also derive a relationship for the contribution of the spokes' (small particle) optical depth, taking into account the contributions from multiple scattering and a slightly flatter large-particle phase function in the spoke regions when compared to the B-ring regions outside spokes. The spokes' optical depths still appear, in general, to increase or remain constant with increasing wavelength, indicating that the extinction efficiency Qe is decreasing or constant with wavelength here and therefore that the effective spoke particle radii are about 0.4-0.5 μm or greater. Accounting for the steeper color slopes of some of the spokes, taking into account the effect of various size distributions, we conclude that the residual reddening in these could be due to their having a somewhat narrow size distribution. This would indicate that these spokes' effective particle radii are narrowly constrained to about r = 0.6 ± 0.2 μm. Consequences of these particle sizes for spoke theories are discussed. © 1990.