Saturn 土星(英)

Saturn¹ 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

　　Hardcopy The New Solar System

　　Summarizes what we have learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.

　　Encyclopedia² of the Solar System

　　A more scholarly introduction the planetary science for those who want to dig a little deeper.

　　The Compact NASA Atlas³ of the Solar System

　　This road map of the solar system contains lots of maps and data as well as photos. In Roman mythology⁴, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus⁵ and Gaia and the father of Zeus (Jupiter). Saturn is the root of the English word "Saturday".

　　Saturn has been known since prehistoric⁶ times. Galileo was the first to observe it with a telescope in 1610; he noted⁷ 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 joint⁸ NASA / ESA project) arrived on July 1, 2004 and will orbit Saturn for at least four years.

　　Saturn is visibly flattened⁹ (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 rotation¹⁰ and fluid state. The other gas planets are also oblate, but not so much so.

　　Saturn is the least dense¹¹ 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, methane¹², ammonia and "rock", similar to the composition of the primordial¹³ Solar Nebula¹⁴ from which the solar system was formed.

　　Saturn's interior is similar to Jupiter's consisting of a rocky core, a liquid metallic¹⁵ hydrogen layer and a molecular¹⁶ 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 mechanism¹⁷ 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 atmospheric¹⁸ 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.

　　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 misnomer¹⁹ 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 extraordinarily²⁰ 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 "spokes²¹" which were first reported by amateur astronomers²². Their nature remains²³ a mystery, but may have something to do with Saturn's magnetic field.

　　Saturn's outermost²⁴ 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 clumps²⁵ of ring material, or mini moons. The strange braided appearance visible in the Voyager 1 images is not seen in the Voyager 2 images perhaps because Voyager 2 imaged regions where the component²⁶ rings are roughly parallel. They are prominent in the Cassini images which also show some as yet unexplained wispy²⁷ spiral structures.

　　There are complex tidal resonances²⁸ 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 paucity³⁰ of material in the Cassini division, which seems to be similar to the Kirkwood gaps in the asteroid³¹ 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 regenerated³² by ongoing³³ 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 astronomical³⁴ telescope. There are several Web sites that show the current position of Saturn (and the other planets) in the sky. More detailed³⁵ and customized charts can be created with a planetarium³⁶ program.

　　Saturn's Satellites

　　Saturn has 34 named satellites:

　　Of those moons for which rotation rates are known, all but Phoebe and Hyperion rotate synchronously³⁷.

　　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 resonance²⁹; Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance.

　　Distance Radius³⁸ Mass

　　Satellite (000 km) (km) (kg) Discoverer Date

　　--------- -------- ------ ------- ---------- -----

　　Pan 134 10 ? Showalter 1990

　　Atlas 138 14 ? Terrile 1980

　　Prometheus 139 46 2.70e17 Collins 1980

　　Pandora 142 46 2.20e17 Collins 1980

　　Epimetheus 151 57 5.60e17 Walker 1980

　　Janus 151 89 2.01e18 Dollfus 1966

　　Mimas 186 196 3.80e19 Herschel 1789

　　Enceladus 238 260 8.40e19 Herschel 1789

　　Tethys 295 530 7.55e20 Cassini 1684

　　Telesto 295 15 ? Reitsema 1980

　　Calypso 295 13 ? Pascu 1980

　　Dione 377 560 1.05e21 Cassini 1684

　　Helene 377 16 ? Laques 1980

　　Rhea 527 765 2.49e21 Cassini 1672

　　Titan 1222 2575 1.35e23 Huygens 1655

　　Hyperion 1481 143 1.77e19 Bond 1848

　　Iapetus 3561 730 1.88e21 Cassini 1671

　　Phoebe 12952 110 4.00e18 Pickering 1898

　　Saturn's Rings

　　Radius Radius approx. approx.

　　Name inner outer width position mass (kg)

　　---- ------ ------ ----- -------- --------

　　D-Ring 67,000 74,500 7,500 (ring)

　　Guerin Division

　　C-Ring 74,500 92,000 17,500 (ring) 1.1e18

　　Maxwell Division 87,500 88,000 500 (divide)

　　B-Ring 92,000 117,500 25,500 (ring) 2.8e19

　　Cassini Division 115,800 120,600 4,800 (divide)

　　Huygens Gap 117,680 (n/a) 285-440 (subdiv)

　　A-Ring 122,200 136,800 14,600 (ring) 6.2e18

　　Encke Minima 126,430 129,940 3,500 29%-53%

　　Encke Division 133,410 133,740

　　Keeler Gap 136,510 136,550

　　F-Ring 140,210 30-500 (ring)

　　G-Ring 165,800 173,800 8,000 (ring) 1e7?

　　E-Ring 180,000 480,000 300,000 (ring)

　　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 density³⁹ 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 entirely⁴⁰ empty; the rings are not perfectly⁴¹ circular.