The A Ring of Saturn

A close up of the A Ring. The Cassini Division lies between the A Ring and the B Ring, the Encke Gap (though the image says division) is within the A Ring, and the Roche Division separates the A Ring from the F Ring

Image Credit:

 Sky and Telescope

 

The A Ring of Saturn is the farthest of the main rings and the first one discovered. It is composed of material similar to that of the B Ring and therefore is bright. It is separated from the B Ring by the Cassini Division, and has similar structure as the B ring.

One of the main features of the A Ring is the Encke Gap, discovered by James Keeler when he was working at Lick Observatory near San Jose, California. (Keeler was working at Allegheny Observatory when he discovered the rings were not solid). The gap was named in honor of Johann Encke who had discovered that the A Ring was not uniformly bright. The gap is about 325 km wide and centered at 133,590 km from Saturn's center. It is kept clear by the orbit of a small moon, Pan, and contains at least three thin ringlets which are knotted due to the gravitational influence of passing moons.

Encke Gap (PIA06534)

Image Credit:

NASA/JPL/Space Science Institute

The Keeler Gap, named for James Keeler, was discovered by the Voyager probe and is about 42 km wide and 250 km from the outer edge of the A Ring. Daphnis orbits with the gap and keeps it clear, much like Pan with the Encke Gap. Daphnis actually is inclined with respect to the rings and actually causes waves at the edges of the gap.

Keeler Gap with Daphnis within. Notice the waves at the edges of the gap.

Image Credit:

NASA/JPL/Space Science Institute

 

Besides the gaps, there are many moonlets that orbit within the ring and helps create waves and spokes within the ring structure. They were first discovered in Cassini images and by 2008, over 150 moonlets have been identified. They were only discovered because of the influence their gravity has on the A ring. Based on evidence, there are possibly thousands of these small moonlets, no more than several kilometers in diameter. They orbit in a path that is only 3000 km wide at a distance of 130,000 km.

As you may have noticed, the rings themselves contain both divisions and gaps. The IAU (International Astronomical Union) defines a division as a separation between two distinct rings and a gap as a small opening in a ring itself. Hence the Cassini division divides the B Ring from the A Ring and the Encke Gap and Keeler Gap are gaps in the structure of the A Ring.