Spectral characteristics of a spoke on the Saturn Rings

On 2008, July, the Cassini/VIMS spectrometer detected spokes on the Saturns B ring for the first time. These are the first measurements of the complete reflectance spectrum of spokes in a wide spectral range (0.35-0.51 mu m). Until now, only one spoke, imaged by VIMS on July 9th, has been studied in some details. The spectrum is consistent with a population of spheroidal water ice particles with a quite wide size distribution centered at about 1.90 mu m (modal radius), i.e. substantially greater than previously thought. This result is obtained, after accurate spatial analyses, by means of radiative transfer modeling of the reflectance spectral contrast of the spoke, making no assumptions about the B ring microphysics. A number density of about 0.01-0.1 grains/cm3 is also deduced. If confirmed, the unexpected abundance of micron-sized grains may have implications for the spoke physical models, since the amount of energy required for the spoke formation increases by about one order of magnitude. This kind of observations may also constrain the size selection effects thought to be produced by the forces governing the spokes evolution.