Time evolution of the total mass of debris satisfying q < a_Roche (blue curves) and a_eq < a_Roche (red curves) in the direct N-body simulations for the three representative initial conditions. The present mass of Saturn’s rings is shown by a black line. Here, q = a(1 − e) is the periapsis distance and a_eq is the equivalent circular radius. At t = 0, the mass satisfying a_eq < a_Roche is sufficiently large to account for the present-day mass of Saturn’s rings, as assumed in the equivalent-circular-orbit interpretation. However, once the actual collisional evolution is followed, this mass rapidly decreases by orders of magnitude. The mass satisfying q < a_Roche also decreases with time, but less drastically.