The spectrum of a Saturn ring spoke from Cassini/VIMS

On 2008, July, the Cassini/VIMS spectrometer detected spokes on the Saturn's B ring for the first time. These are the first measurements of the complete reflectance spectrum of the spokes in a wide spectral range (0.35-0.51 m). Here we will focus on a single broad-shaped spoke, imaged by VIMS on July, 9. Radiative transfer modeling supports a pure water ice composition for the spoke's grains, but their size distribution is found to be wider than previously thought: preliminary results yields a modal value of about 1.90 m (reff = 3.5 m, veff = 0.3) and a number density of about 0.01-0.1 grains/cm³. The unexpected abundance of micron-sized grains in the spoke may have implications for the formation models since the energy requirement increases by at least one order of magnitude. These kind of observations may also constrain the size selection effects thought to be produced by the forces governing the spokes' evolution.