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Dynamics of Saturn’s Ring (II. Propeller Structure)

The Cassini spacecraft has found various unexpected structures in Saturn’s ring. One such type of structure is a propeller. This is a symmetric, elongated structure like an airplane propeller. Many propeller-shaped structures have now been found. It is thought that moonlets about 100 m in diameter hidden within the ring create propellers. Here we introduce a computer simulation of the formation process of a propeller.

	Saturn’s ring consists of numerous small, icy particles. If we observe the ring from a distance, it looks like a uniform thin disk.
	If we approach the ring, complicated structures appear. These structures are formed due to the effects of the self-gravity. These are called self-gravity wakes, which form the basic structure of Saturn’s ring.
	Many small structures called propellers have been found through Cassini’s observations. They are symmetric elongated structures, typically hundreds of meters to a few kilometers in size.
	In order to investigate the formation process of a propeller, we performed computer simulations. We focused on a small region in Saturn’s ring whose size is a few kilometers and performed simulations within it.
	A small moonlet with a 100 m diameter is located at the center of the computational box. The self-gravity wakes can be seen around the moonlet. The elongated structure stretching out from the moonlet is a propeller.
	Because the effects of the moonlet’s gravity are strong, the particles are attracted to it and accumulate on the surface of the moonlet.
	The particles that do not collide with the moonlet are dispersed by the moonlet’s gravity. Thus, the particles are cleared away and the dark elongated regions form.
	In the inner region, the ring material rotates quickly while in the outer region the ring material rotates slowly. The difference in the rotational velocity elongates the structure. This is the reason why a propeller-shaped structure forms.

Details of Numerical Simulation

Purpose	The environment and processes of propeller formation
Calculation Model	Method: N-body simulation with inelastic collisions
Calculation Model	Number of Particles: ~ 750,000 particles
Computer	GRAPE-DR (CfCA，NAOJ)
Time Scale	A few days
Spatial Scale	~ 10 km
Researchers	Shugo Michikoshi, Eiichiro Kokubo (NAOJ)
Reference	Michikoshi & Kokubo, 2011, ApJ, 732, L23

Details of Visualization

The simulation is done with periodic boundary conditions.When an object passes out through one side of the unit cell, it re-appears on the opposite side. In this way, a large system can be approximated by a small simulation box. In the video, multiple copies of the simulation box are distributed around the central one when looking out over the wide expanse of the ring.
In the previous video, Dynamics of Saturn’s Ring (I. Wake Structure), the scene that shows the large-scale structure of the ring and the scene that shows the detailed simulation are connected by cross-fading. This time, these scenes are connected seamlessly, allowing comparison of the scales of the ring and a propeller.