Moonlets in Saturn’s A Ring

Figure 1: An image of Saturn’s main rings taken by the Cassini spacecraft on 21 June 2004, a few days before the spacecraft entered orbit around Saturn. Credit: NASA/JPL/Space Science Institute.

Figure 2: An annotated version of an image of Saturn’s A Ring. This image was taken by the Cassini spacecraft on 9 May 2007, at a distance of approximately 1.1 million km from Saturn. Credit: NASA/JPL/Space Science Institute.

Saturn’s main rings are comprised primarily of water-ice particles ranging between ~1 cm and ~10 m in radius. In 2006, four ‘propeller’-shaped features were discovered in Saturn’s A Ring from images previous taken by the Cassini spacecraft. Simulations have show that these ‘propeller’-shaped features are formed by gravitational perturbations of ring particles from moonlets measuring tens to hundreds of metres in size.

Figure 3: Images of 4 ‘propeller’-shaped features discovered in Saturn’s A Ring. These images were taken by the Cassini spacecraft in 2004 and the nominal image resolution is 52 m per pixel. (M. S. Tiscareno et al., 2004)

These moonlets are embedded within the A Ring and are too small to be seen directly by the Cassini spacecraft. Instead, the Cassini spacecraft sees the ‘propeller’-shaped disturbances created by these moonlets. By 2008, ~150 features have been found in Saturn’s A Ring that are associated with moonlets embedded within the ring. About half of these features are sufficiently well resolved to reveal a characteristic ‘propeller’ shape.

As the moonlets orbit Saturn within the A Ring, they partially sweep up the ring particles around them to create the observed ‘propeller’-shaped features. However, they are not large enough to sweep clean their entire orbit around Saturn, unlike the moons Pan and Daphnis. Both these moons reside within gaps in Saturn’s A Ring. Pan orbits inside the 325 km wide Encke Gap, while Daphnis orbits inside the 42 km wide Keeler Gap.

The ‘propeller’-shaped features associated with the moonlets lie primarily in three ~1000 km wide belts in the middle section of Saturn’s A Ring, between 126,750 km and 132,000 km from Saturn’s centre. Also, relatively larger moonlets do exist and they can create bigger ‘propeller’-shaped features that can be tracked for years. On such feature is even nicknamed Bleriot, after a French aviator named Louis Bleriot. Bleriot is believed to be associated with a moonlet measuring ~1 km in size, significantly larger than many other moonlets.

Figure 4: In this image taken by the Cassini spacecraft, the central ‘propeller’ structure of Bleriot is estimated to be ~100 km in length. Credit: NASA/JPL-Caltech/Space Science Institute.

References:

- M. S. Tiscareno et al., “100-metre-diameter moonlets in Saturn’s A ring from observations of ‘propeller’ structures”, Nature 440, 648-650 (30 March 2006)

- M. Seiß, F. Spahn, M. Sremčević and H. Salo, “Structures induced by small moonlets in Saturn’s rings: Implications for the Cassini Mission”, Geophysical Research Letters Vol. 32 L11205 (2005)

- M. Sremčević et al., “A belt of moonlets in Saturn’s A ring”, Nature 449, 1019-1021 (25 October 2007)

- M. S. Tiscareno et al., “The Population of Propellers in Saturn’s A Ring”, Astronomical Journal, Volume 135, Page 1083-1091 (2008)