Sub-centimeter particles in Saturn's A ring

Abstract:

We characterize the particle size distribution in Saturn’s A ring by measuring and modeling diffracted light observed in stellar occultation data from the Cassini Ultraviolet Imaging Spectrograph (UVIS). The UVIS High Speed Photometer measures starlight in a bandpass centered at 0.15µm. At this short wavelength the diffraction angle for particles in Saturn’s main rings is small, and the signal from forward-scattered light can only be distinguished from the stellar signal when the star is observed to be outside, but very near to, a sharp ring edge. Observations of bright stars (with a high signal-to-noise ratio) show diffraction signals (photon counts above the unocculted stellar photon flux) extending up to ~20 km from the ring edges that define the Encke Gap, the Keeler Gap, and the outer edge of the A ring indicating the presence of millimeter-sized particles. We model the diffraction signal and compare the results with the observed signal in order to constrain the size distribution of the population of particles in Saturn’s A ring. We model the particle size distribution as a power-law distribution. The shape of the diffraction signal constrains the slope of the power-law and the size of the smallest particles. We find a decrease in the radii of the smallest particles and a steepening of the power-law size distribution in the outermost parts of the A ring. This finding suggests that more energetic interparticle collisions due to satellite perturbations in that region cause larger ring particles to shed smaller particles from their outer layers or otherwise fragment.