Saturn is the sixth planet from the Sun and the second largest of our 8 planets. Saturn is one of the four gas giants, made primarily of hydrogen and helium. Saturn orbits the Sun at a distance of 1.4 billion kilometres. A year on Saturn is 29.5 Earth years.
Saturn is known best for its rings of ice that orbit the planet. With its rings and moons orbiting the planet, Saturn is sometimes thought of as a mini solar system and understanding Saturn may therefore result in a greater understanding of the entire solar system.
Saturn's rings were first observed through a telescope by Galileo in 1610 although he thought the rings were large moons on either side of the planet. What Galileo had actually observed through his telescope were the rings; it was to be another 45 years before Dutch astronomer Christopher Huygens discovered that what Galileo had observed were in fact flat rings. In 1675 Italian astronomer Giovanni Cassini discovered more about the nature of the rings including the gap now known as the Cassini division.
The rings are made up of billions of pieces of water ice. The ice pieces often collide with one another, breaking up and ensuring fresh surfaces continue to reflect sunlight and appear bright to distant observers. The ice pieces range in size, many small pieces a centimetre in size as well as larger pieces that are like orbiting icebergs.
The main rings on closer examination are themselves made up of ringlets. Saturn's moons play an important part in the structure of the rings, for example, creating gaps in the rings and, in the case of the moon Enceladus, feeding the rings with fresh ice particles.
The rings orbit Saturn at a fast speed, the rings closest to Saturn travel at 80,000 kmph.
The rings are very wide but not deep. The ring span is approximately 100,000 kilometres wide but less than 1 kilometre thick. Some of the rings are only a few metres in thickness.
NASA's space exploration of Saturn and its moons have been named after the astronomers who discovered the details of the rings, Cassini and Huygens. The Cassini spacecraft, launched by NASA in 1997, reached Saturn's orbit in 2004 and has provided much information and many spectacular images of Saturn, its rings and its moons.
Saturn has 62 moons. Titan is Saturn's largest moon and the second largest moon in the solar system (the largest is Jupiter's moon, Ganymede). Other moons include Enceladus which is geologically active (see below); Mimas which is known as the 'death star' moon because its large crater marking makes it look like a rocky version of the death star from the Star Wars films; Iapetus, nicknamed the Yin and Yang moon because it is light (clean ice) on one side and dark (covered in carbon) on the other. Iapetus also has a vast mountainous ridge running round its equator; Hyperion, this moon is not round was probably a comet that became permanently snared by Saturn's gravity.
Titan is the second largest moon in the solar system and Saturn's largest moon. Unusually for a small planetary body, Titan has a dense atmosphere made primarily of nitrogen and methane.
The images of Titan sent back by Cassini show this atmosphere as a thick blue line surrounding the moon. Titan has kept hold of its atmosphere because it is far enough from the Sun that the atmospheric molecules move slowly enough for Titan's gravity to stop them escaping into space. The lower temperatures of the outer solar system mean gas molecules move slower than those with a higher temperature nearer the Sun.
The outer space position of Titan means its very cold - the surface temperature of Titan is -180 degrees Celsius.
The Cassini and Huygens exploration of Saturn discovered that Titan has lakes of liquid methane. Methane exists on Titan in solid, liquid and gaseous states - just like water on Earth that exists as ice, liquid and vapour.
The Cassini space probe carried a landing probe, Huygens, design to land on the surface of Titan. Huygens landed on the surface of Titan in January 2005 taking pictures as it descended through the Titan atmosphere. The pictures taken by Huygens and Cassini revealed a landscape very similar to one that you can find on Earth, with mountains, lakes and river beds covered in rounded stones and pebbles that had clearly had their surfaces smoothed by flowing liquid.
Titan is therefore an amazing world, with lakes, rain, mountains and an atmosphere.
Enceladus is a very small icy moon - only about 500 kilometres (300 miles) across. It appears as a very bright moon because its icy surface is highly reflective of the sunlight that reaches it. Large parts of its surface are smooth with no evidence of impact craters - this is because its surface is young and constantly re-generating because it is geologically active.
Enceladus has geysers that spray the surface with fresh layers of ice which over time have filled in the pot holes, constantly giving the surface a fresh layer of ice. These geysers were first photographed by Cassini in 2005.
Enceladus sits in the E ring and the ice particles thrown out from its geysers help form the E ring in which it sits.
Enceladus's internal heat source is thought to be generated by its elliptical orbit, resulting in different gravitational forces being exerted on the moon as it moves closer and then further away from Saturn during its orbit. The different gravitational forces stretch and squash Enceladus during its orbit, creating internal friction and therefore heat. The internal heat melts ice underground into water which is then shot out of the geysers. The geysers erupt on a large scale, sending out water thousands of kilometres into space where it freezes to ice particles. Some land back on the surface and some join the E ring.
On the southern side of Enceladus are its "tiger stripes" - four parallel claw marks scratched into the smooth icy surface. The tiger stripes are about 120 kilometres in length.