Visions of the Cosmos

Planetary Science

The Solar System

Saturn

Saturn and the Rings of Saturn

 

Saturn is the second largest planet in the Solar System.  Like Jupiter it is a Gas Giant Planet.  It has a diameter of 120,536 kilometres, approximately 9.4 times that of Earth.   It has a very low bulk density of about 0.7 times that of water – it is often said that if it were placed in a large enough ocean it would float.  It is 94.3 times as massive as the Earth.

The illustration below of the beautiful ringed planet compares the 
size of Saturn with that of the Earth.  Illustration Courtesy ESA/NASA 
It rotation rate is about 10 hours 40 minutes.  It has a year of 29.46 
Earth Years.
It’s mean average distance from the Sun of 1,427 billion 
kilometres or 9.53 astronomical units.  An astronomical unit is the 
average distance from the centre of the Earth to the centre of the 
Sun:  = 149,597,870 kilometres.

Saturn appears to give out more energy than it receives from the Sun and the temperature at the top of the clouds is around –139 degrees Celsius

The surface of the atmosphere of Saturn bears many similarities with the surface of Jupiter, but the colour contrast is generally less.

There are extremely high velocity winds in the atmosphere of Saturn. 
The wind speeds in the atmosphere of Saturn have been are as
 high as 1800 km/hr 

 

The Interior of Saturn

 Like Jupiter, Saturn is largely 'liquid'. The slightly higher concentration of helium relative to hydrogen in the atmosphere is thought to be due to the colder temperature of Saturn. Under these colder conditions, liquid helium does not dissolve in liquid hydrogen and drops of helium sink to the centre, depleting the outer regions in helium.  Beneath the outer atmosphere liquid molecular hydrogen forms a thick layer.  Deep in the interior liquid metallic hydrogen takes the place of molecular hydrogen.

                                                        False colour image of Saturn Courtesy ESA/NASA

The magnetic field of Saturn is similar to that of Jupiter, but weaker. Electrical currents in liquid metallic hydrogen deep in the interior are assumed to be the cause of the magnetic field. The Cassini spacecraft is carrying sensitive magnetometer instruments and it is hoped that the results of the readings will provide information not only on the magnetosphere but also on the planets interior

At the very centre it is thought that there are molten rocks under very high temperatures and pressures.

The surface of Saturn bears many similarities with the surface of Jupiter, but the colour contrast is generally less.

There are large anticyclonic cells on the surface, apparently driven by the planet's internal heat source, but none are as large as the Great Red Spot on Jupiter, and they are not as abundant as on Jupiter.

For those brought up on simple A Level Chemistry only three states of matter are taught – solid, liquid and gas.  The ‘liquid’ molecular hydrogen in Saturn is above its critical temperature.  However it is under enormous pressures - it is strictly speaking a supercritical fluid.  Similarly under huge pressures hydrogen can become a 'liquid' metal.

.At the very centre it is thought that there are molten rocks under very high temperatures and pressures.

 

A few vital statistics are shown in the table below

 

 

Property

 

Mass

95 times the Earth

Mean Distance from Earth

1,200,000,000 kilometres

Minimum Distance from Sun

1,350,000,000 kilometres

Mean Distance from Sun

1.429,400,000 kilometres   9.54 AUs

Maximum Distance from Sun

1,500,000,000 kilometres

Revolution Period about Sun (‘Year’)

29.46 Earth Years

Rotational Period about Axis  (‘Day’)

10.67 hours (0.44 Earth Days)

Bulk Density

687 kgs/metre3 (Earth 5.51 kgs/metre3)

Inclination of Axis

26o73” (Earth 23o45”)

The Rings of Saturn

Photograph looking down on the rings from the Cassini Spacecraft.     Courtesy NASA/JPL/Space Science Laboratory

Apart from the Sun and the Moon, Saturn is the most well known sight in the heavens because of its resplendent ring system.  For several years now we have known that all the four large planets of the Outer Solar System have ring systems but those of Jupiter, Uranus and Neptune are very faint and pale into insignificant when compared with Saturn.  Saturn's rings were first discovered by Galileo but the image was so unclear that his telescope was unable to distinguish them clearly from the planet itself.  It was left to J.D.Cassini a few years later to distinguish them from the planet itself and to even see that there were at least two sets of rings separated by a gap which we now call the Cassini Division.  It was left to Cassini and to Huygens to study the rings as closely as possible with the telescopes of the times they lived in. In the late twentieth century the rings were found to be a complex system of an extremely large number of many rings.  Now that the Cassini- Huygens Mission has reached the Saturn System more and more is being discovered every day about Saturn and its magnificent rings. 

In the photograph, the large 'Cassini Division' through which the spacecraft plunged during the Saturn Orbital Insertion can be clearly seen.   Many other divisions are clear in the picture showing a large number of separate rings

Galileo Galilei    15 February 1564-8 January 1642 G.D.Cassini 8 June 1625 14 September 1712 Christiaan Huygens 14 April 1629  8 July 1695
 

Image Courtesy NASA/JPL/Space Science Laboratory

Saturn's magnificent rings show some of their intricate structure in this image taken on 11 May 2004, by the Cassini-Huygens narrow-angle camera. Although they appear to be solid structures, the rings are composed of millions of individual particles, each one orbiting the planet on its own path.

Satellites visible in this image: Janus (181 kilometres across) above the rings, and icy Enceladus (499 kilometres across) below the rings. The F ring shepherd moons Prometheus and Pandora can be seen along Saturn's outermost F ring if the image is further contrast enhanced. The image was taken in visible light from a distance of 26.3 million kilometres from Saturn. The image scale is 158 kilometres (98 miles) per pixel. Contrast in the image was enhanced to aid visibility.

 

Illustration by David Seal NASA/ JPL/ Caltech

 

This is a computer-rendered image of Cassini-Huygens during the Saturn Orbit Insertion (SOI) manoeuvre, just after the main engine has begun firing. The spacecraft is moving out of the plane of the page and to the right (firing to reduce its spacecraft velocity with respect to Saturn) and has just crossed the ring plane.
The SOI manoeuvre, which is approximately 90 minutes long, will allow Cassini to be captured by Saturn's gravity into a five-month orbit. Cassini-Huygens's close proximity to the planet after the manoeuvre offers a unique opportunity to observe Saturn and its rings at extremely high resolution.
 

The particles in Saturn's rings are composed primarily of water ice and range from microns to metres in size. The rings show a tremendous amount of structure on all scales; features like spokes and irregular ripples, are puzzling.

Some of them might be signs of asteroids plunging through the ring system, or others might be caused by tiny moonlets, as yet undiscovered, very close to Saturn's rings.

Some of this structure is related to gravitational perturbations by Saturn's many moons, but much of it remains unexplained. Hopefully, Cassini-Huygens mission will throw light on these mysteries.

Solar System