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David

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Saturn

Saturn

Saturn as seen by the approaching Cassini spacecraft.


Saturn and its rings close to equinox

This mosaic of Saturn and its rings was taken close to equinox, when the Sun crosses the planet's equator. The images were captured on August 12, 2009 by the Cassini spacecraft. Source: NASA/JPL/Space Science Institute).


Saturn's rings and their shadow

Saturn's rings and their shadow, imaged by the Cassini spacecraft.


Saturn's giant infrared ring

Artist's concept of the giant infrared ring around Saturn.


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 km/h (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

model of the interior of Saturn

 

At Saturn's center is believed to lie a core of rocky material about the size of Earth, but more dense. Around this is a metallic hydrogen shell some 30,000 km deep, surmounted, in turn, by a region composed of liquid hydrogen and helium with a gaseous atmosphere some 1,000 km 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 82 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 km 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  

 


Saturn's rings
The main rings and divisions
Saturn's rings artwork
Artwork of the rings

 


In Oct 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 km (3.7 million miles) away from the planet and extends outward roughly another 12 million km (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.