The smallest particles in Saturn’s A and C Rings

Abstract

Radio occultations of Saturn’s main rings by spacecraft suggest a power law particle size-distribution down to sizes of the order of 1cm (Marouf, E.A., Tyler, G.L., Zebker, H.A., Simpson, R.A., Eshleman, V.R. [1983]. Icarus 54, 189–211; Zebker, H.A., Marouf, E.A., Tyler, G.L. [1985]. Icarus 64, 531–548.). The lack of optical depth variations between ultraviolet and near-IR wavelengths indicate a lack of micron-sized particles. Between these two regimes, the particle-size distribution is largely unknown. A cutoff where the particle-size distribution turns over must exist, but the position and shape of it is not clear from existing studies.Using a series of solar occultations performed by the VIMS instrument on-board Cassini in the near-infrared, we are able to measure light forward scattered by particles in the A and C Rings. With a model of diffraction by ring particles, and the previous radio work as a constraint on the slope of the particle size distribution, we estimate the minimum particle size using a truncated power-law size distribution. The C Ring shows a minimum particle size of 4.1-1.3+3.8mm, with an assumed power law index of q=3.1 and a maximum particle size of 10m.The A Ring signal shows a similar level of scattered flux, but modeling is complicated by the presence of self-gravity wakes, which violate the assumption of a homogeneous ring, and higher optical depths, which require multiple-order scattering. If q<3, our A Ring model requires a minimum particle size below one millimeter (<0.34mm for an assumed q=2.75, or 0.56-0.16+0.35mm for a steeper q=2.9) to be consistent with VIMS observations. These results might seem to contradict previous optical (Dones, L., Cuzzi, J.N., Showalter, M.R. [1993]. Icarus 105, 184–215) and infrared (French, R.G., Nicholson, P.D. [2000]. Icarus 145, 502–523) work, which implied that there were few particles in the A Ring smaller than 1cm. But, because of the shallow power law, relatively little optical depth (between 0.03 and 0.16 in extinction, or 0.015–0.08 in absorption) is provided by these particles.