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The Solar System | Mercury | Venus | Earth | Mars | Ceres | Jupiter
Saturn | Chiron | Uranus | Neptune | Pluto
The Kuiper Belt and the Oort Cloud | Quaoar | Eris | Sedna

The ringed planet Saturn.

Saturn is the sixth planet from the Sun and the furthest known to classical observers (the Greeks and the Romans). It's about 95 times more massive than the Earth, and about ten times as far away from the Sun. It has a low density, so low that it could float in a bath of water if one could be found that was big enough.

The most famous feature of Saturn, though, is its incredibly beautiful ring system.

The Facts

  • Mass: 568.8×1024kg
  • Equatorial Radius: 60,268km
  • Mean Density: 690kg/m3
  • Length of Day: 10.233 hours
  • Period of Revolution about Sun: 10,760.5 days
  • Acceleration due to Gravity: 9.05m/s2
  • Mean Orbital Velocity: 9.64km/s
  • Inclination of Axis: 26.73°
  • Mean Distance from the Sun: 9.5388 AU

Internal Structure

Saturn has a rocky core 25,000km across that's composed of silicates, minerals and various ices under tremendous pressure and temperature (around 10 million times Earth's atmospheric pressure and 14,000K).

Beyond the core is a shell of metallic hydrogen1. This stretches up to almost 11,460km, which makes it less than a tenth of the thickness of Jupiter's metallic hydrogen zone.

Above this lies a layer of helium droplets that's 4,200km thick. This is thought to be responsible for the heat production of the planet: Saturn radiates almost twice as much heat as it receives from the Sun, thought to be due to condensation of the helium droplets and their fall in towards the core. Beyond the helium droplets lies the remainder of the planet's material. It reaches out to 29,000km and is composed of 93% hydrogen and 7% helium, making it light in weight. This is the outermost layer of Saturn and is what is visible from Earth.

A strange, hexagon-shaped feature in Saturn's north polar region was first photographed by Voyager 1 in the 1980s. The latest probe, Cassini, imaged it again in March 2007, proving it to be an enduring element. It looks like a huge honeycomb with other hexagon shapes enclosed - like a set of neatly stacked pastry-cutters. And it is huge: with a radius of 15,000 miles (25,000km), it could swallow at least three Earths. This hexagon-shaped figure - the six sides are very nearly equal - extends 60 miles (100km) deep into the atmosphere. Further information is provided in the separate h2g2 Entry.

Storm Clouds Brewing

The clouds of Saturn are similar to those of Jupiter, consisting of fast moving jet-streams causing, as on the biggest of the planets, eddies and even a red spot, albeit one that is much smaller than that on Jupiter, being only 6000km across compared to Jupiter's 26,000km whopper. Saturn also possesses many other ovals, mainly in the north, but unlike on Jupiter, they tend to move around each other when they meet - on Jupiter they tend to coalesce.

Little is known about how Saturn's cloud layer behaves. It is now known, however, that close to the equator the cloud bands reach high speeds of 480m/s, but at the poles the speeds are zero. This indicates that the clouds merely 'keep pace' with the rotation of the planet around its axis.

Above the cloud layer lies the atmosphere of the giant. Extending outwards to a height of 30km, this atmosphere is made up of ammonia clouds. The Voyager craft have shown that the upper atmosphere contains some coloured clouds too, but these are at an unknown height.

The Rings

The rings of Saturn are the most spectacular of any planet in the known solar system. They were first spotted by Galileo Galilei in July 1610, but his telescope wasn't quite up to scratch, and even though he saw the rings in the most prominent way (with the full face of the rings towards him), he thought Saturn was a triple planet. When he looked again seven years later, the rings were edge on and, being only 1km thick, had seemingly vanished. He declared: 'Saturn has swallowed her children.'

We now know that Saturn's rings are visible in different aspects depending on the relative position of it and Earth. The cycle follows a 14.5 year period from fully open, through edge-on, and back to open. This mystery was finally cleared up by the Dutch astronomer Huygens, who managed to observe the ring system in 1655.

However, it wasn't until 1856 that Clerk Maxwell managed to explain what the rings actually were. His calculations showed that a solid ring would be torn to tatters by gravitational effects, so he predicted that the rings were made up of many small, electrostatic particles. This has since been borne out by further observations and two satellite flybys.

Until the Voyager spacecraft passed Saturn within the past three decades, no one realised just how many rings there are, and and just how thin they are. Previously, upper estimates put the number at three. There are, however, at least 10,000 rings arranged into bands, with 'shepherd moons' keeping them on the straight and narrow. It is also known that electrostatic influences help keep them in line, with the outer three ringlets being braided. Lightning has been detected from the rings.

In October 2009, scientists confirmed the existence of another ring around Saturn. This new ring is much further out than the more familiar rings, 13 million km compared with 240,000 km. It is formed by debris from the moon Phoebe, which is hit by the remnants of the formation of the Solar System. The moon Iapetus picks up some of this ring dust, leading to the moon's previously unexplained two-tone surface. The ring was properly identifed using the Spitzer space telescope, which works in the infrared spectrum, where the ring is most visible. If the ring was bright enough to be seen from Earth in the night sky, it would appear much larger than the diameter of a full moon.

The Satellites

It is perhaps in the nature of this odd planet to have odd moons. The following list is by no means complete, as there are over 60 confirmed satellites, but it highlights some of the oddest features of the Saturnian system.

Titan - The largest moon of Saturn is covered by a dense orange fog that is difficult to penetrate. It consists of 90% nitrogen, but has complex organic molecules present, including some amino acids. While doubting the existence of life at such low temperatures (liquid nitrogen is thought to form lakes, rivers and seas on its surface), scientists are using Titan as a model for what the earliest stages of life look like.

Tethys - This moon shows evidence of geothermal activity and has a rift valley along its surface.

Mimas - This moon has an impact crater covering a quarter of its surface, making it look like a giant eyeball. The impact is thought to have had a force just short of that required to shatter the poor little ball of ice and rock into smithereens.

Janus and Epimethius - These two moons' orbits are so close to each other that they should have collided long ago. What has prevented that is that when they approach each other, a peculiar effect of gravity makes them flip positions so the faster becomes the slower, and vice versa. Very weird indeed...

The Great White Spot

A 'brilliant white spot' on Saturn was discovered by comedian Will Hay (also an amateur astronomer) in 1933. It resulted in him gaining a fellowship to the Royal Astronomical Society. What became known as the Great White Spot were really huge storms big enough to be viewed periodically from Earth by telescope. The Royal Astronomical Society wrote an obituary for Hay.

The h2g2 Tour of the Solar System

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1A state of hydrogen that has never been achieved on Earth.

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