The rings of Saturn
Sen—Saturn's rings are its most distinctive and enchanting feature. First spotted in 1610 by Italian astronomer Galileo Galilei using an early telescope, this delicate, 282,000 kilometer-diameter structure, which is only ten to 1,000 meters thick in places, has bewitched astronomers ever since.
The countless particles and debris that comprise the rings range in size from ten meters down to thousandths of a millimeter; each in its own individual orbit around Saturn. Amazingly, despite the fact that they are comprised of 99.9 per cent water-ice, they’re stratified into many distinct, coloured bands. This colouring is the result of trace amounts of rocky material, as well as organic compounds exposed to ultraviolet rays from the Sun.
Spacecraft such as NASA’s Voyager and Cassini probes have shown us, in exquisite detail, that Saturn’s rings are a dynamic place exhibiting dark-coloured spokes, waves and other perturbations, and even fragile, vertical structures up to two and a half kilometers high.
Different sections of Saturn’s ring system are labelled from ‘A’ to ‘G’, in the order that they were first discovered. The broad, bright-coloured A and B bands—14,600 and 25,500 kilometers wide, respectively—are the widest and most obvious sections.
Then going inwards towards the planet are the C and D rings—17,500 and 7,500 kilometers wide, respectively. These are darker and trickier to see from Earth.
Fine vertical structures on the outer edge of the B ring casting shadows, as captured by the Cassini probe. Scientists think that passing moonlets throw material upwards above the plane of the rings. Image credit: NASA, JPL-Caltech, Space Science Institute.
Italian astronomer Giovanni Cassini saw in 1675 that the A and B rings appear separated by a gap, a feature now known as the Cassini Division. However, flybys by the Voyager probes in the 1980s showed that there are actually dark rings within the 4700 kilometer-wide division—very difficult to spot from Earth. There are, however, many true gaps within the rings in the Cassini Division.
There are two main reasons why gaps, some only being a few tens of kilometers wide, appear in the rings. The first is because of a phenomenon known as orbital resonance. The gravitational action of some of Saturn's moons, orbiting around the planet, create unstable orbital regions in certain places. This serves to clear material away in those areas, thus opening them up. The second reason is that moons and small ‘moonlets’ embedded in the ring themselves can clear material away directly.
Going away from Saturn, beyond the ‘A’ band are thinner and more diffuse rings. The ‘F’ ring is 30—500 kilometers wide, whilst the ‘G’ ring is 8,000 kilometers in width and much fainter than the main rings.
The ‘E’ ring is very diffuse and ‘foggy’ and created by geyser eruptions from the moon Enceladus. Unlike macroscopic main-ring particles, these water-ice crystals are microscopic in size. And where Saturn’s main rings span a diameter of 282,000 kilometres, The ‘E’ ring is 960,000 kilometers in diameter and 300,000 kilometers in width. It’s also a lot thicker than the main ring—increasing up to 30,000 kilometers away from Saturn.
Saturn's largest ring, its outermost, is the Phoebe ring. Discovered in 2009, it is at least ten times bigger than the next-largest ring, stretching out to more than 15 million kilometers from the planet. It’s thought to be the largest ring in the Solar System.
The age and origin of all of Saturn’s rings are still a mystery. It has much yet to teach us.
This stunning mosaic made up of images taken by the Cassini probe shows the full extent of the main ring system; click here for the full-size, annotated image. Image credit: NASA, JPL, Space Science Institute.