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Science and Engineering Research Council

Royal Greenwich Observatory

Information Leaflet No. 42: 'Saturn'.


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 that of 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 kilometres. 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 ten per cent 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 1,000 km. deep in which are the cloud structures that look like the surface of the planet.

The Atmosphere:

Saturn is composed of about 94 percent hydrogen and 6 percent 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:

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.


Saturn has 8 satellites with diameters greater than 200 km. Of these, Titan is, by far, the largest; with a diameter of 5,150 km. it is the second largest satellite in the Solar System. It is probably the only satellite that has an atmosphere; this atmosphere is denser than the Earth's, but is composed almost entirely of methane.

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 1,050 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 that is 100 km wide and 4 to 5 km deep.

Dione is 1,120 km in diameter. It shows many craters and large plains.

Rhea has a diameter of 1,530 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.


Saturn can be easily 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.

A general view of Saturn and its satellites:


ARVAL's Note:
Saturn and two of its moons, Tethys (above) and Dione, were photographed by Voyager 1 on November 3, 1980, from a distance of 13 million kilometers (8 million miles). The shadows of Saturn's three bright rings and Tethys are cast onto the cloud tops. The limb of the planet can easily be seen through the 3,500-kilometer-wide (2,170 mile) Cassini Division, which separates ring A from ring B.

Approximately every 15 years the Earth and the Sun pass through the plane of the rings, making their observation difficult, but letting us observe them edge-on.
Sep 04, 2009 (Earth-crossing)
Aug 10, 2009 (Sun-crossing)
Mar 23, 2025 (Earth-crossing)
May 06, 2025 (Sun-crossing)

Produced by the Information Services Department of the Royal Greenwich Observatory.

PJA Thu Nov 25 10:45:09 GMT 1993

ARVAL's Note:

Is it Snowing Microbes on Enceladus?

"There's a tiny moon orbiting beyond Saturn's rings that's full of promise, and maybe -- just maybe -- microbes."

"NASA's Cassini spacecraft has revealed watery jets erupting from what may be a vast underground sea. These jets, which spew through cracks in the moon's icy shell, could lead back to a habitable zone that is uniquely accessible in all the solar system."

"More than 90 jets of all sizes near Enceladus's south pole are spraying water vapor, icy particles, and organic compounds all over the place", says Carolyn Porco.

"Cassini has flown several times now through this spray and has tasted it. And we have found that aside from water and organic material, there is salt in the icy particles. The salinity is the same as that of Earth's oceans."

"She believes the small moon, with its sub-surface liquid sea, organics, and an energy source, may host the same type of life we find in similar environments on Earth."

See Is it Snowing Microbes on Enceladus? (NASA Science, March 27, 2012)

Updated: June 26 '97, March 29 '12, June 25 '14

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See About the Web Pages of Observatorio ARVAL.

For some illustrative images and excellent texts, link to: Saturn in Calvin J. Hamilton's Views of the Solar System

You can also link to ARVAL's Gallery, to see Hubble Space Telescope: Saturn Storms.

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