The four largest planets, Saturn, Jupiter, Uranus and Neptune, known collectively as the gas giants, all have spectacular ring systems. Their rings lie a plane around the planet and are variously composed of cosmic dust, water ice and organic materials.
The spectacular rings of Saturn have attracted the attention of some of the finest minds on Earth. These rings can be clearly seen using an amateur telescope on Earth. Galileo first observed these rings in 1610. Christiaan Huygens first identified them as a disk. Robert Hook observed them. The astronomer Giovanni Domenico Cassini saw that they were composed of many smaller rings. He identified a large gap between the main rings which called the Cassini Division to this day. The great mathematician Pierre-Simon Laplace thought that the rings might contain many solid ringlets. In 1859 the physics genius James Clerk Maxwell came to the correct answer. The rings are not be solid because they would be pulled apart by gravitational forces. They are comprised of numerous small particles in orbit around the planet.
Several satellite probes have passed close to the rings of Saturn and extended our knowledge. The Voyager 1 passed by in 1980. The Cassini probe passed by in 2009.
The main rings of Saturn extend from 7,000km to 80,000km above Saturn’s equator. They are made almost entirely of water ice with a tiny proportion of organic and silicate material. The particles within the ring range in size from 1 centimetre to 10 metres in diameter. Edge on, the rings are surprisingly sharp, with a width of perhaps just 10 metres. The fine structure within the rings is not entirely understood. It is thought to arise from the gravitational pull of Saturn’s many moons. The moons sweep material from their own orbits and concentrate it in the rings. Minas, for instance is responsible for the Cassini gap. The natural moons Prometheus and Pandora shepherd the F ring. Scientists believe that as yet undiscovered moons may be responsible for unexplained gaps between the rings. The Phoebe ring lies far out from the main ring. The natural satellite Phoebe and its ring have a retrograde orbit which is tilted at 27 degrees to the main.
The origin of the rings of Saturn is an open mystery. There are two competing theories. In the 19th century Édouard Roche proposed that they were formed by the disintegration of as long lost moon by the gravitational forces of the giant planet. It was probably knocked out of a stable orbit some 4 billion years ago. An alternative theory proposes that they are debris left over from the formation of Saturn.
The rings of Jupiter were discovered by the Voyager I space probe in 1979. They are very faint and can only be seen by the largest telescopes on Earth. Most of our knowledge comes from a flypast made by the Galileo orbiter space probe during the 1990s.
The Jovian ring system has four features. There is an inner ring known as the “halo ring”, there is a brighter main ring and two indistinct outer rings known as the gossamer rings. impacts. When the “New Horizons” spacecraft passed by in February/March 2007 it produced images showing that the main ring has a detailed fine structure. Unlike Saturn whose rings are made of water ice the rings of Jupiter are made from fine dust. Analysis of the dust suggests that the rings are made of the same material as the Jovian moons, Amalthea, Thebe and Metis. At some time in their history the moons must have been shattered by high velocity impacts. The rings are thought to be unstable. The halo is produced by dust particles spiralling in towards Jupiter. It becomes much thicker as Jupiter is approached. The rings are maintained by new dust produced by the collision of objects within the main ring. The rings are relatively thin, being no more than 30km in width. Intriguingly ripples on the surface of the disc are believed to date from disturbances set up when comet Shoemaker-Levy 9 collided with Jupiter in 1995.
In March 1977 James Elliot and colleagues at the MIT Wallace Observatory made a chance discovery. They discovered the first nine Rings of Uranus while using the Kuiper Airborne Observatory to investigate the planetary atmosphere. When Voyager 2 mission flew past in 1986 two more rings were discovered. Two more were discovered using the Hubble Space Telescope between 2003 and 2005. Thirteen distinct rings are now known. The rings are very dark, faint and lie close to the planet. The inner rings are 38,000 kilometres from the planet: the outer rings 98,000 kilometres. Edge-on, the rings are only a few kilometres wide. They are thought to be made of water ice with a crust of organic materials.
Scientists are currently puzzled by the stability of the Rings of Uranus. As in the case of Saturn, astronomers believe that the rings are held in place by small shepherding moons or moonlets. The brightest ring is held in place by the moons Cordelia and Orphelia. Studies of this ring suggest that it is quite young in astronomical terms with an age of about 600 million years. The young rings are thought to be being replenished by collisions between moonlets and ring particles. The rings are thought to be made of relatively large particles ranging from 0.2 to 20 metres in diameter. Scientists are still searching for moons or moonlets that held the rings together.
In the early 1980s when Neptune passes in front of the rich background of stars known as the Milky Way scientists suspected that some of the starlight was being obscured by a system of rings around Neptune. In 1989 the Voyager 2 spacecraft produced clear images of the rings. There are five rings. In places the rings are distinct. For the most part they are faint and indistinct. In places they form incomplete resembling arcs. The Adams ring is the most distinct. It is dynamically stable because of the presence of Galatea, the inner moon of Neptune.
The rings of Neptune are made up of very fine particles of water ice. As in the rings of Uranus the ice particles are thought to be coated with organic materials which make them appear dark. The rings are thought to be young compared to the age of the Solar System. They are believed to be made up of material ejected from collisions with the moons and moonlets that are in orbit around Neptune.
Astronomers speculate that there might be rings around Pluto. The moons of Pluto are so configured that a temporary ring system might be possible. Advocates recomment that the New Horizons probe keeps clear of a certain area of space just in case of collision during its approach to Pluto in 2015.
In recent years scientists have made good progress in their understanding and observation of the rings of the outer planets. The ring systems are similar in that all lie in a plane in a razor thin disc. In composition and appearance they vary widely from the spectacular dust rings of Saturn to the faint water ice and organic rings of Neptune. The observation and understanding of these rings is a flourishing rich research area.