  The hypothesis of a recent origin of Saturn's rings and its mid-sized moons
is actively debated. It was suggested that a proto-Rhea and a proto-Dione might
have collided recently, giving birth to the modern system of mid-sized moons.
It is also suggested that the rapid viscous spreading of the debris may have
implanted mass inside Saturn's Roche limit, giving birth to the modern Saturn's
ring system. However, this scenario has been only investigated in very
simplified way for the moment. This paper investigates it in detail to assess
its plausibility by using $N$-body simulations and analytical arguments. When
the debris disk is dominated by its largest remnant, $N$-body simulations show
that the system quickly re-accrete into a single satellite without significant
spreading. On the other hand, if the disk is composed of small particles,
analytical arguments suggest that the disk experiences dynamical evolutions in
three steps. The disk starts significantly excited after the impact and
collisional damping dominates over the viscous spreading. After the system
flattens, the system can become gravitationally unstable when particles are
smaller than $\sim$ 100 m. However, the particles grow faster than spreading.
Then, the system becomes gravitationally stable again and accretion continues
at a slower pace, but spreading is inhibited. Therefore, the debris is expected
to re-accrete into several large bodies. In conclusion, our results show that
such a scenario may not form the today's ring system. In contrast, our results
suggest that today's mid-sized moons are likely re-accreted from such a
catastrophic event.
