Saturn is the sixth planet from the Sun and the second largest.
orbit: 1,429,400,000 km (9.54 AU) from Sun
diameter: 120,536 km (equatorial)
mass: 5.68e26 kg
History of Saturn
In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn is the root of the English word “Saturday” (see Appendix 5).
Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturn’s rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings. Saturn’s rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus (and shortly thereafter around Jupiter and Neptune).
Saturn was first visited by NASA’s Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini (a joint NASA / ESA project) arrived on July 1, 2004 and will orbit Saturn for at least four years.
Saturn is visibly flattened (oblate) when viewed through a small telescope; its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so.
Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water.
Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and “rock”, similar to the composition of the primordial Solar Nebula from which the solar system was formed.
Saturn’s interior is similar to Jupiter’s consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.
Saturn’s interior is hot (12000 K at the core) and Saturn radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism as in Jupiter. But this may not be sufficient to explain Saturn’s luminosity; some additional mechanism may be at work, perhaps the “raining out” of helium deep in Saturn’s interior.
The bands so prominent on Jupiter are much fainter on Saturn. They are also much wider near the equator. Details in the cloud tops are invisible from Earth so it was not until the Voyager encounters that any detail of Saturn’s atmospheric circulation could be studied. Saturn also exhibits long-lived ovals (red spot at center of image at right) and other features common on Jupiter. In 1990, HST observed an enormous white cloud near Saturn’s equator which was not present during the Voyager encounters; in 1994 another, smaller storm was observed (left).
Two prominent rings (A and B) and one faint ring (C) can be seen from the Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Division (but this is somewhat of a misnomer since it was very likely never seen by Encke). The Voyager pictures show four additional faint rings. Saturn’s rings, unlike the rings of the other planets, are very bright (albedo 0.2 – 0.6).
Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely.
Saturn’s rings are extraordinarily thin: though they’re 250,000 km or more in diameter they’re less than one kilometer thick. Despite their impressive appearance, there’s really very little material in the rings — if the rings were compressed into a single body it would be no more than 100 km across.
The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings.
Voyager confirmed the existence of puzzling radial inhomogeneities in the rings called “spokes” which were first reported by amateur astronomers (left). Their nature remains a mystery, but may have something to do with Saturn’s magnetic field.
Saturn’s outermost ring, the F-ring, is a complex structure made up of several smaller rings along which “knots” are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. The strange braided appearance visible in the Voyager 1 images (right) is not seen in the Voyager 2 images perhaps because Voyager 2 imaged regions where the component rings are roughly parallel. They are prominent in the Cassini images which also show some as yet unexplained wispy spiral structures.
There are complex tidal resonances between some of Saturn’s moons and the ring system: some of the moons, the so-called “shepherding satellites” (i.e. Atlas, Prometheus and Pandora) are clearly important in keeping the rings in place; Mimas seems to be responsible for the paucity of material in the Cassini division, which seems to be similar to the Kirkwood gaps in the asteroid belt; Pan is located inside the Encke Division and S/2005 S1 is in the center of the Keeler Gap. The whole system is very complex and as yet poorly understood.
The origin of the rings of Saturn (and the other jovian planets) is unknown. Though they may have had rings since their formation, the ring systems are not stable and must be regenerated by ongoing processes, perhaps the breakup of larger satellites. The current set of rings may be only a few hundred million years old.
Like the other jovian planets, Saturn has a significant magnetic field.
When it is in the nighttime sky, Saturn is easily visible to the unaided eye. Though it is not nearly as bright as Jupiter, it is easy to identify as a planet because it doesn’t “twinkle” like the stars do. The rings and the larger satellites are visible with a small astronomical telescope. There are several Web sites that show the current position of Saturn (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.
Saturn has 62 named satellites (as of spring 2010):
- The three pairs Mimas-Tethys, Enceladus-Dione and Titan-Hyperion interact gravitationally in such a way as to maintain stable relationships between their orbits: the period of Mimas’ orbit is exactly half that of Tethys, they are thus said to be in a 1:2 resonance; Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance.
- See Scott Sheppard’s site for the latest about recently discovered moons (there are lots).
- There are 9 more that have been discovered but as yet not named.
Notes: * distance is kilometers from Saturn’s center * the “Encke Minima” is a slang term used by amateur astronomers, not an official IAU designation
This categorization is actually somewhat misleading as the density of particles varies in a complex way not indicated by a division into neat regions: there are variations within the rings; the gaps are not entirely empty; the rings are not perfectly circular.
Interesting Facts about Saturn
- Saturn is 890,424,928 miles (1,433,000,000 kilometres) away from the sun. That makes it the sixth planet from the sun falling behind Jupiter.
- In Roman mythology, Saturn is the father of Jupiter, king of the gods. This is where Saturn got its name as it is similar to the planet Jupiter in many ways.
- Saturn has the nickname “the Ringed Planet”. This nickname is because of the large bright ring system that goes around the planet. These rings are made of ice and dust and are 20 metres thick. There are three main ones, and these can be viewed from Earth using a telescope. Smaller rings were identified by spacecraft later. They go out as far as 7,891 miles (12,700km) away from the planet itself.
- Saturn orbits the sun in 29.4 Earth years. That is much slower than Jupiter’s 11.86 years.
- Saturn has a diameter of 75,098 miles (142,750 kilometres). It is 95 times the mass of Earth.
- Saturn is the planet furthest away that can still be viewed with the naked eye. This makes it the last planet people were aware of in ancient times before technology allowed us to look further into the solar system.
- Saturn has at least 150 moons. 53 of these have been given formal names. The largest of the moons is called Titan and is the second largest of the solar system. Jupiter’s moon Ganymede is larger. Titan has its atmosphere made up of mostly nitrogen. It is composed of water ice and rock.
- Saturn is on an axial tilt of 27 degrees. That is very similar to Earth and so Saturn experiences seasons like we do.
- Saturn has the lowest density of all the planets. Its density is so low that if there were a body of water big enough, Saturn would float in it!
- A day on Saturn takes 10 hours and 34 minutes. This is the time it takes to turn on its axis.
- Saturn is not a perfect sphere but is slightly flattened. Its equator measurement is 10% less than its polar measurements. This is thought to be because of its quick rotation and its density.
- Saturn’s atmosphere is mostly Hydrogen. As you get further into the planet, the layers of hydrogen get denser until they become metallic leaving a very hot core at the centre.
- Saturn appears yellow. This is due to an outer layer of ammonia crystals in its atmosphere. Underneath these crystals is a layer of clouds mostly made up of water ice. Below that are layers of sulfur ice and cold hydrogen mixtures.
- Saturn has a much weaker magnetic field than Jupiter. It is even slightly weaker than that found on Earth.
- Saturn experiences regular storms called “Great White Spots”. These storms were first spotted back in 1876 and had appeared to happen every 20 to 30 years and can cover the entire planet.
- Saturn has the strongest winds in the solar system. These winds can reach a staggering speed of 1,118 miles (1,800 kilometres) per hour. Hurricane winds on Earth only reach about 246 miles (396 kilometres) per hour!
- Saturn is one of the Gas Giants. Whilst Jupiter is made up entirely of gas; scientists believe Saturn may have a solid core of rock that is roughly the same size as planet Earth.
- Four spacecraft missions have made it to Saturn. They have all hugely widened our knowledge of the planet and one craft, called Cassini, is still orbiting Saturn today.
More about Saturn and its Satellites
- more Saturn images
- facts about Saturn
- from NSSDC
- Cassini Home Page at JPL
- Saturn Events from TAMU
- Saturn’s Ring System
- the cause of the ring gaps, from Phil Plait’s excellent Bitsize Astronomy site
- Saturn Ring Plane Crossings
- Voyager Saturn Science Summary
- Saturn Ring Plane Crossing info from JPL, info and images from STScI
- Sun Crossing of Saturn’s Ring Plane from WIYN Observatory
- Historical Background of Saturn’s Rings and satellites (from JPL)
- Saturnian System Nomenclature Tables
- Saturn’s rings in natural color from Cassini
- How does Saturn generate its internal heat?
- What are the “spokes” in the rings?
- What is the origin of the rings? What does that tell us about the origin of the solar system as a whole? Why are Saturn’s rings so much more dramatic than the others?
- The NASA/ESACassini orbiter entered into orbit around Saturn on July 1st, 2004. In addition to an extensive survey of Saturn and its major moons, it will drop a probe (called Huygens, built by the European Space Agency) onto the surface of Titan.