Size distribution of particles in Saturn's rings, missed moonlets and   misinterpretation of Chariklo rings
  Brilliantov et al. (PNAS, 2015) propose a model for the size distribution ~R^-3 for small particles with radius R and ~exp(-(R/Rc)^3) for large particles, where Rc=5.5 m. In 1989 Longaretti found analytically ~R^-3 for small particles and R^-6 for large ones (2). The law R^-6 also describes moonlets with size ~ 0.1-1 km. Cut-off law from Brilliantov et al. model does not describe moonlets and requires new mechanism for the origin of ~ 1 km size bodies. This model does not take into account the key effects of self-gravitation of large particles and differential rotation of the rings. Longaretti used a more accurate model of destruction of particles: "A relative increase of the erosion/destruction rate of the large particles must take place, because these particles have relative velocities of collision larger than the dispersion velocity, due to the differential Keplerian motion". The new model does not explain the difference between the rings and the satellites and the authors suggest that their calculations are applicable to the rings of the Chariklo and Chiron. In fact, the Chariklo rings are not examples of planetary rings, but proto-satellite disks. Simplified physical model that does not describe moonlets and the outer boundary of the rings makes the new model not a step forward but a step backwards in comparison with the papers of 20-30 years old. 
Nick Gorkavyi
astro-ph.EP