Saturn is the sixth planet from the Sun. It is a gas giant, second only in size to Jupiter with a diameter more than 9 times that of Earth. It has a spectacular system of rings and a large collection of moons. Saturn also has the most flattened shape of any of the major planets. Its average density is so low that if placed in a big enough tub of water, it would easily float.

 

mean distance from Sun	1,427 million km (886.9 million mi, 9.54 AU)
perihelion distance	1,349 million km (838.6 million mi)
aphelion distance	1,512 million km (940 million mi)
diameter: equatorial/polar	120,536 km (74,913 mi) / 108,728 km (67,574 mi)
equatorial diameter (Earth = 1)	9.449
mass (Earth = 1)	95.2
mean density	0.69 g/cm3
axial period	10.2 hours
axial inclination	26.7°
orbital period	29.46 years
orbital inclination	2.5°
orbital eccentricity	0.056
gravity at cloud tops (Earth = 1)	0.92
escape velocity	35.47 km/s (127,690 km/h, 79,360 mph)
atmospheric composition	96.3% hydrogen, 3.25% helium, traces methane, ammonia, etc
mean temperature (cloud tops)	-180 °C (-292 °F)
albedo	0.61
number of moons	23

 

History of observations

Saturn is the most distant of the five planets known to ancient stargazers. At its brightest (magnitude –1.9), it outshines every star in the sky, including brilliant Sirius. In 1610, Galileo Galilei became the first astronomer to gaze at Saturn through a telescope. To his surprise, he saw a pair of objects on either side of the planet, which he later drew as "cup handles" attached to the planet on each side. In 1659, Christiaan Huygens announced that this was a ring encircling the planet. In 1675, Giovanni Cassini discovered a gap between what are now called the A and B rings.

 

Atmosphere

Saturn's atmosphere consists of 96.3% hydrogen and 3.25% helium, with traces of methane, ammonia, ethane, ethylene, and phosphine. Because Saturn is colder than Jupiter, the more colorful chemicals sink lower in its atmosphere and can't seen. The result is markings that are much less dramatic than those on Jupiter, although bands and some small spots are still visible.

 

Alternate jet streams of east-west and west-east circulation, with speeds of up to about 1,800 kilometers per hour (1,100 mph), have been traced in the motion of the cloud tops and are responsible for the banded appearance of the planet. Electrical processes and heat from within Saturn enrich the layered chemical mixture of the atmosphere which probably transitions from superheated water near the core to ammonia ice at the observed cloud tops.

 

Interior

 

At Saturn's center is believed to lie a core of rocky material abiloout the size of Earth, but more dense. Around this is a metallic hydrogen shell some 30,000 kilometers deep, surmounted, in turn, by a region composed of liquid hydrogen and helium with a gaseous atmosphere some 1,000 kilometers deep.

 

Magnetic field

Saturn has a modest magnetic field similar in strength to Earth's. In 1997 the Hubble Space Telescope observed auroral displays caused by the interaction between the solar wind and the planetary magnetic field.

 

Moons

Saturn has 83 known moons.

 

moon	semimajor axis (km)	orbital period (days)	orbital eccentricity	orbital inclination (°)	diameter (km)
Pan	133,584	0.575	0.0000	0.001	35 × 35 × 23
Daphnis	136,505	0.594	~0.0	~0.0	6-8
Atlas	137,670	0.602	0.0012	0.003	46 × 38 × 19
Prometheus	139,380	0.613	0.0022	0.008	119 × 87 × 61
Pandora	141,720	0.629	0.0042	0.050	103 × 80 × 64
Epimetheus	151,422	0.694	0.0098	0.335	135 × 108 × 105
Janus	151,460	0.695	0.0068	0.163	193 × 173 × 137
Mimas	185,404	0.942	0.0202	1.566	415 × 394 × 381
Methone	194,440	1.010	0.000	0.007	~3
Anthe	197,700	1.04	0.001	0.1	~2
Pallene	212,280	1.15	0.004	0.18	~4
Enceladus	237,948	1.37	0.005	0.02	513 × 503 × 497
Tethys	294,619	1.888	0.000	1.12	1081 × 1062 × 1055
Telesto	294,619	1.888	0.000	1.19	29 × 22 × 20
Calypso	294,629	1.888	0.000	1.56	30 × 23 × 14
Dione	377,396	2.737	0.002	0.02	1123
Helene	377,396	2.737	0.002	0.20	36 × 32 × 30
Polydeuces	377,396	2.737	0.019	0.18	3.5
Rhea	527,108	4.518	0.001	0.35	1529
Titan	1,221,930	15.95	0.029	0.35	5151
Hyperion	1,481,010	21.28	0.123	0.43	360 × 280 × 225
Iapetus	3,560,820	79.33	0.029	15	1460 × 1498 × 1425
Kiviuq	11,273,100	446.87	0.1551	49.458	~16
Ijiraq	11,340,600	450.88	0.3476	50.409	~12
Phoebe	12,921,800	548.40	0.1809	172.992	230 × 220 × 210
Paaliaq	15,112,900	693.64	0.4409	46.896	~22
Skathi	15,609,100	728.08	0.2614	148.792	~8
Albiorix	16,194,300	769.41	0.6110	36.424	~32
S/2007 S2	16,523,200	792.96	0.2181	176.68	~6
Bebhionn	16,821,800	814.56	0.3574	42.099	~6
Skoll	17,576,900	870.02	0.4294	155.551	6
Erriapo	17,749,000	882.83	0.4615	37.887	~10
Siarnaq	17,808,100	887.24	0.4779	44.914	~40
Tarqeq (LII)	17,880,100	892.55	0.1066	49.864	~7
S/2004 S13	18,056,300	905.85	0.2610	167.379	~6
Hyrrokkin	18,347,400	927.85	0.3552	153.342	~8
Tarvos	18,562,800	944.23	0.5438	33.679	~15
Mundilfari	18,589,900	946.30	0.1844	169.187	~7
Greip (LI)	18,654,100	951.20	0.3170	172.851	~6
S/2006 S1	18,930,200	972.41	0.1303	154.232	~6
Jarnsaxa (L)	19,039,700	980.85	0.1942	163.173	~6
Bergelmir	19,061,300	982.52	0.1730	157.421	~6
S/2004 S17	19,099,200	985.45	0.2259	166.881	~4
Narvi	19,239,900	996.36	0.2814	138.231	~7
Suttungr	19,359,500	1005.67	0.1278	174.243	~7
Hati	19,709,300	1033.05	0.3080	163.131	~6
S/2004 S12	19,905,900	1048.54	0.3963	164.042	~5
Farbauti	19,950,000	1052.03	0.1859	158.435	~5
Bestla	20,300,100	1079.85	0.7309	146.965	~7
S/2007 S3	20,462,800	1092.85	0.1296	177.220	~5
Aegir	20,482,900	1094.46	0.2253	167.425	~6
S/2004 S7	20,576,700	1101.99	0.5541	165.596	~6
Thrymr	20,598,100	1003.71	0.3935	174.577	~7
S/2006 S3	21,076,300	1142.37	0.4710	150.818	~6
Kari	21,981,200	1216.72	0.3871	147.971	~7
Fenrir	22,598,700	1268.35	0.1257	162.796	~4
Loge	23,206,500	1319.86	0.1789	166.687	~6
Sutur	23,316,600	1329.27	0.4016	166.354	~6
Ymir	23,639,600	1356.98	0.2665	172.656	~18
Fornjot	25,770,000	1544.49	0.1561	167.779	~6

 

Rings

Saturn has the largest and most spectacular ring system in the Solar System. Composed of swarms of ice-rock particles ranging in size from a centimeter to several meters across (and possibly even as large as a kilometer), Saturn's rings are much brighter (with an albedo of up to 0.6) than any other known planetary rings. Though some 170,000 kilometers wide (more if the tenuous outer portion of the E-ring is included), the rings are only about a km or so thick and have a total mass of about 0.01 that of the Moon. When Earth occasionally moves through the same angle as the rings, which are slanted at 27°, they almost completely disappear from view. The rest of the time, only the outer A-ring, the brighter B-ring and the bluish inner C-ring are clearly visible through telescopes on Earth, together with several dark gaps, including the Cassini Division and the Encke Division, in which ring material is much sparser.

 

Four additional faint rings together with a wealth of other complex and puzzling features were discovered or confirmed by the Voyager probes. Among these features are puzzling radial inhomogeneities called spokes, first reported by amateur astronomers, that may be an effect caused by Saturn's magnetic field. The F-ring, a narrow, wavy structure just outside the A ring, is confined by two small satellites and consists of at least five individual strands with embedded knots that may be clumps of ring material or mini moons. Voyager 1 images (but not those of Voyager 2) also showed the F ring to have strange braided appearance. Complex tidal resonances exist between a number of Saturn's moons and the ring system. Some of the moons – the so-called shepherd moons, Atlas, Pandora, and Prometheus – are important in keeping the rings in place; Mimas seems to be responsible for the paucity of material in the Cassini division; while Pandora is located inside the Encke Division.

 

Saturn's rings and divisions
name	inner radius (km)	outer radius (km)	width (km)	notes
D-ring	67,000	74,500	7,500	Very tenuous. Discovered in 1969
Guerin division
C-ring (Crepe ring)	74,500	92,000	17,500	Gauzy appearance. Discovered by Bond in 1850
Lyot (Maxwell) division	87,500	88,000	500
B-ring	92,000	117,500	25,500	The brightest ring
Cassini division	115,800	120,600	4,800	Discovered by Cassini in 1675
Huygens gap	117,680	118,000	285-440
A-ring	122,200	136,800	14,600
Encke minimum	126,430	129,940	3,500
Encke division	133,580	133,910	330
F-ring	140,210		30-500
G-ring	165,800	173,800	8,000
E-ring	180,000	480,000	300,000

 

The main rings and divisions

Artwork of the rings

 

In October 2009 the discovery was announced of a colossal new ring around Saturn. The ring only shows up in the infrared part of the spectrum and was found using NASA's Spitzer Space Telescope. The new ring, or belt, is so large that if it were visible in the night sky, it would span the width of two full Moons. The bulk of its material starts about 6 million kilometers (3.7 million miles) away from the planet and extends outward roughly another 12 million kilometers (7.4 million miles); its orbit is tilted 27° from the main ring plane. One of Saturn's farthest moons, Phoebe, circles within the ring, and is likely the source of its material.

 

The ring is tenuous, consisting of widely-dispersed particles of ice and dust. Spitzer's infrared eyes were able to spot the glow of the cool dust, which has a temperature of only about 80 K (–193°C, –316°F).

 

The discovery may help solve an age-old riddle of one of Saturn's moons. Iapetus has a strange appearance – one side is bright and the other is really dark, in a pattern that resembles the yin-yang symbol. Giovanni Cassini first spotted the moon in 1671, and years later figured out it has a dark side, now named Cassini Regio in his honor.

 

Saturn's supersized ring could explain how Cassini Regio came to be so dark. The ring is circling in the same direction as Phoebe, while Iapetus, the other rings and most of Saturn's moons are all going the opposite way. According to the scientists, some of the dark and dusty material from the outer ring moves inward toward Iapetus, slamming the icy moon like bugs on a windshield.