Saturn

Saturn as seen by the Hubble Space Telescope Saturn as seen by Hubble ©NASA and The Hubble Heritage Team (STScI/AURA) Saturn is probably the best known and most beautiful planet in the solar system. While its possession of a ring system it not unique, it has a set of rings which are far more extensive and more easily seen than any other planet. It is this ring system that makes Saturn so beautiful.

Saturn is the second largest planet in the solar system with a diameter of 120,000 km. It orbits the Sun every 30 years at a distance about ten times that of the Earth. The shape of the planet is a markedly oblate spheroid with a polar diameter some 10% smaller than that at the equator. Saturn is the least dense of all the planets its mean density being only 0.7 times that of water.

The visits by the Voyager spacecraft revised almost all that we know about Saturn, its rings and its satellites.

The planetary interior

Like Jupiter, Saturn is composed mainly of the light elements hydrogen and helium. At its centre there is believed to be a core of rocky material about the size of the Earth, but more dense. Around this is a metallic hydrogen shell some 30,000 km deep. Above this is a region composed of liquid hydrogen and helium with a gaseous atmosphere some 1000 km deep in which are the cloud structures that look like the surface of the planet.

The atmosphere

Saturn is composed of about 94% hydrogen and 6% helium. The clouds are composed of very small amounts of other chemical elements combined with hydrogen to give such compounds as ammonia, methane and phosphine. Because Saturn is colder than Jupiter the more colourful chemicals occur lower in its atmosphere and are not seen; this results in much less dramatic markings but they are similar to those seen on Jupiter, taking the form of bands with some smallish spots.

The rings

An image of Saturn from NASA's Cassini spacecraftAn image of Saturn from NASA's Cassini spacecraft Image: NASA/JPL/Space Science Institute Saturn's rings were first seen by Galileo but were identified as a ring system by Huygens in 1656. For many years Saturn was thought to be unique in having a ring system but we know now that all the major gaseous-planets have ring systems although none is so prominent as that of Saturn.

The rings are divided up into several distinct rings with gaps between them. The largest gap was discovered by Cassini in 1675 but we now know that there is a very complex structure to the ring system.

The rings are composed of many, many small particles up to about 10 metres across. These are thought to have originated in a satellite which collided with a minor planet and/or that they are made of matter which was present when the planets were formed. Saturn's rings are very reflective and could be composed of ices such as make up comets.

Appearance

Saturn can easily be seen with the naked-eye. With good binoculars it can be seen to have a non-circular shape and the rings can be seen with a small telescope which will also show the largest satellite, Titan. About every 15 years the Earth passes through the plane of the rings which are then seen edge-on (or rather, not seen!) although their shadow onto the disk of Saturn may still be visible.

Satellites

Saturn's satellites and ring structureSaturn's satellites and ring structure. ©NASA Saturn has 8 satellites with diameters greater than 200 km:

  • Titan is by far the largest with a dimeter of 5150 km and is the second largest satellite in the solar system. It is probably the only satellite which has an atmosphere. The atmosphere is denser than the Earth's but is 80% nitrogen, 6% argon, a few per cent methane, a little water and small amounts of organic materials such as ethane, carbon dioxide and hydrogen cyanide. Apart from the very low temperatures - 180 °C - conditions are similar to those found on the early Earth.
  • Mimas has a diameter of 390 km. Its surface is very cratered and the Voyager pictures show one giant crater with a diameter almost equal to one third of that of the satellite.
  • Enceladus has a diameter of 500 km. It shows cratering and also complex geological structures indicating large crustal movements.
  • Tethys has a diameter of 1050 km. It appears to be made of ice and is heavily cratered. There is a huge trench-like structure extending a quarter of the way around the satellite which is 100 km wide and 4 to 5 km deep.
  • Dione is 1120 km in diameter. It shows many craters and large plains.
  • Rhea has a diameter of 1530 km and is heavily cratered.

There are several small satellites some of which are believed to be responsible for 'shepherding' some of the features seen in the structure of the rings.

Results from the Hubble Space Telescope

Saturn's aurora as seen by HubbleThis is the first image of Saturn’s ultraviolet aurora, taken by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope in October 1997, when Saturn was a distance of 810 million miles (1.3 billion km) from Earth. © J.T. Trauger (Jet Propulsion Laboratory) and NASA The Hubble Space Telescope (HST) has allowed astronomers to obtain images of Saturn which show details not seen since the Voyager encounters of the early 1980s.

In 1997 HST observed the auroral displays on Saturn (right). These are caused by the interaction between the solar wind and Saturn's magnetic field and are similar to the displays of 'northern lights' seen on Earth although on a much larger scale. However Saturn’s aurora can only be seen in ultraviolet light, which is invisible from the Earth's surface but clearly visible from HST. The Hubble images reveal ripples and patterns that evolve slowly independently of the rotation of the planet.

Storm on Saturn as seen by HubbleThis NASA Hubble Space Telescope image of the ringed planet Saturn shows a rare storm that appears as a white arrowhead-shaped feature near the planet's equator. The storm is generated by an upwelling of warmer air, similar to a terrestrial thunderhead. The east-west extent of this storm is equal to the diameter of the Earth, about 7900 miles. © Reta Beebe (New Mexico State University), D. Gilmore, L. Bergeron (STScI), and NASA

The NASA-HST image below of Saturn shows a rare storm that appears as a white arrowhead-shaped feature near the planet's equator. The storm is generated by an upwelling of warmer air, similar to a terrestrial thunderhead. The east-west extent of this storm is equal to the diameter of the Earth, about 7900 miles.

Cassini Huygens

On the 1 July 2004, Cassini–Huygens entered Saturn’s atmosphere. During the Saturn tour, Cassini will complete 74 orbits of the ringed planet, 44 close flybys of the mysterious moon Titan, and numerous flybys of Saturn's other icy moons. The probe will have sampled the atmosphere during its parachute assisted descent (see picture left).

After being strapped to Cassini for 7 years, Huygens separated in December 2004 to make its journey to Titan – released to travel at 30cm per second to be captured by Titan's gravitational pull. The probe cruised towards Titan over the next three weeks, before making a soft landing on its surface on 14 January 2005.

Cassini Huygens probe on TitanAn artist's conception of the moment before impact of the Cassini-Huygens probe on the icy surface of Titan. Courtesy of NASA

Titan's nitrogen-rich atmosphere extends 10 times further into space than Earth's atmosphere. This means the outer fringes of Titan's atmosphere reach almost 600 kilometres (373 miles) into space. Huygens is able to detect this region of Titan's atmosphere, awakening the probe's science instruments on entry.

Huygens is equipped with six science instruments designed to study the content and dynamics of Titan's atmosphere and collect data and images on the surface.