National Astronomical Observatory of Japan



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


This week, the Cassini spacecraft will finish its 20-year operation and dive into Saturn as a “Grand Finale” mission. This spacecraft captured many wondrous images of Saturn. One of them shows a “propeller structure” in one of Saturn’s rings. Saturn’s rings are composed of countless ice particles with sizes ranging from a few centimeters to a few meters. It is thought that the reason for this propeller structure is the gravity of moonlets with diameters of approximately several hundred meters, buried in the rings. This video visualizes how the propeller structures are formed based on calculations by GRAPE (GRAvity PipE), which is dedicated to gravitational N-body problems.

The World the Ice Particles Depicted

From the video, in addition to the propeller structure, you can see the characteristic structure formed by the ice particles of Saturn’s rings. In the first half of the video, you can see how numerous ice particles create patterns like water ripples. This structure, called a “wake structure,” is a result of the effects of the ice particles gathering together through mutual gravitational attraction and the effects of the fact that the clumps of particle tend to be stretched because the orbital speed around Saturn varies depending on the position. See “Dynamics of the Ring of Saturn (I. Wake Structure Formation).

Propeller Structure Unmasked

In the second half of the video, we approach the propeller structure. The gravity of a moonlet buried in Saturn’s ring causes the surrounding ice particles to gravitate towards the satellite and accumulate on the surface. Ice that does not fall and pile up on the satellite surface has its movement disturbed by the gravity of the satellite. Due to this influence, drop-shaped holes extend before and behind the satellite. The propeller structures are thought to be formed in this way. The typical size of the observed propeller structures is from several hundred meters to several kilometers.

Video Data

Computer UsedGRAPE-DR
Time ScaleA few days
Spatial ScaleApproximately 10 kilometers
Calculations byShugo Michikoshi and Eiichiro Kokubo
CreditsShugo Michikoshi, Eiichiro Kokubo, Takaaki Takeda, Four-Dimensional Digital Universe Project, NAOJ

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