The first scientific observations of the planet Saturn were made by Galileo Galilei in 1610, who, having made the astounding discovery that Jupiter has moons (and therefore that not everything revolves around the Earth), turned his attention to this object. In addition to seeing the extended disk of the object, he made out some strange appendages sticking out from the planet, which he described as “ears.” It was not until 1655 that Christian Huygens was able to properly resolve this structure to see that Saturn is surrounded by a large ring. In 1675, Giovanni Cassini found that the ring was divided into two by a gap that now bears his name. As observations have improved over past few hundred years, it has become clear that Saturn is surrounded by a beautifully structured array of rings and ringlets, with an increasingly complex nomenclature needed to describe them
Portrait of Saturn obtained by the Cassini spacecraft. A series of blue-grey storms are visible in the southern hemisphere. The upper atmosphere in the northern hemisphere is fairly cloud-free in this image, so scattering of sunlight produces a blue sky by the same process that it does on Earth. (Image from NASA/ JPL/Space Science Institute.)
Our understanding of Saturn was further revolutionized when we began to obtain close-up views from passing space probes. The early work of Pioneer 11 and the Voyager spacecraft laid the groundwork for the most recent Cassini probe, which entered into orbit around Saturn in 2004, with an initial three-year mission to study the planet in great detail, as well as dropping a probe onto the mysterious moon Titan.
From this wealth of observations, we now know a great deal about the planet Saturn. With a radius of 120,000 km, it is the second largest planet in the Solar System. Actually, Saturn is not very spherical, with a polar radius some 10% smaller than this value. This flattening arises from the combination of the planet’s rapid rotation rate (a day on Saturn is only ten hours long) and the low pull of gravity holding it together (Saturn’s average density is very low – given a large enough bucket of water, the planet would float!).
The low density arises because Saturn is a gas giant, consisting mostly of hydrogen gas, with molecules like methane causing the system of subtly-coloured cloud bands that we see as the outermost parts of the planet. We cannot observe the interior directly, but it is believed that the density increases to such an extent that hydrogen near the centre is compressed into a metallic liquid state. The metallic hydrogen layer is probably responsible for Saturn’s strong magnetic field (about 8000 times as strong as the Earth’s). At its centre, Saturn is thought to contain a rocky core, but its size is still very uncertain.
The rings of Saturn, as viewed by the Cassini spececraft. This mosaic of six images covers a distance of approximately 62,000 kilometres along the ring plane, from a radius of 74,565 kilometres to 136,780 kilometres from the planet’s center. The rings are made primarily of ice, but the different colours are believed to indicate varying degrees of contamination by rock and carbon compounds. (Original image from NASA/JPL/Space Science Institute.)
By far the most stunning feature of Saturn is its rings. We now know that other giant planets have rings, but Saturn’s are by far the most impressive. Since material at different radii orbits at different speeds, the rings cannot be solid objects; in fact, they consist of a multitude of lumps of icy material ranging in size from dust grains to boulders.
Although we still do not understand the detailed origins of the wealth of structure in the rings, the concept of resonances must play a key role. Any material in the Cassini Division turns out to orbit Saturn at exactly twice the rate of the large moon Mimas, so it gets a regular gravitational pull from this moon. Just as a child’s swing pushed at the right frequency will move progressively further from its original position, so any material in this region will be swept away, creating the wide “Cassini Division” visible in the above image. This kind of large-scale resonant effect is clearly important, but many subtler phenomena are also at work. For example, the very faint outer “F Ring” in this picture is “shepherded” by the moons Prometheus and Pandora that orbit on each side of it.
Putting all of this information together has enabled Crystal Nebulae to produce a detailed three-dimensional model of this most beautiful of celestial objects.