Sunday, September 6, 2009

Planet 6. Saturn

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

This road map of the solar system contains lots of maps and data as well as photos. 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".

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

Saturn's Satellites and Rings

Saturn has 34 named satellites:

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

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.

Distance Radius 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)

Few big Satellites of Saturn. 1. Titan

Titan is the fifteenth of Saturn's known satellites and the largest:

orbit: 1,221,830 km from Saturn
diameter: 5150 km
mass: 1.35e23 kg

In Greek mythology the Titans were a family of giants, the children of Uranus and Gaia, who sought to rule the heavens but were overthrown and supplanted by the family of Zeus.

Discovered by Christiaan Huygens in 1655.

It was long thought that Titan was the largest satellite in the solar system but recent observations have shown that Titan's atmosphere is so thick that its solid surface is slightly smaller than Ganymede's. Titan is nevertheless larger in diameter than Mercury and larger and more massive than Pluto.

Surface view One of the principal objectives of the Voyager 1 mission was the study of Titan. Voyager 1 came within 4000 km of the surface. We learned more in the few minutes of that fly-by than in the previous 300 years. Then in late 2004, the Cassini orbiter began a series of close encounters with Titan, taking data with many instruments. And in January 2005, the Huygens probe actually landed on the surface of Titan and sent back images from the surface.
Titan is similar in bulk composition to Ganymede, Callisto, Triton and (probably) Pluto, ie about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser because it is so large that its gravity slightly compresses its interior.

Smoggy Titan Alone of all the satellites in the solar system, Titan has a significant atmosphere. At the surface, its pressure is more than 1.5 bar (50% higher than Earth's). It is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. The organics are formed as methane, which dominates in Titan's upper atmosphere, is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker. In many ways, this is similar to the conditions on Earth early in its history when life was first getting started. But it is this thick hazy atmosphere that makes it so hard to see Titan's surface.

Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere. It is therefore directly exposed to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere. It may also drive some of Titan's peculiar chemistry.

At the surface, Titan's temperature is about 94 K (-290 F). At this temperature water ice does not sublimate and thus there is little water vapor in the atmosphere. Nevertheless, there appears to be a lot of chemistry going on; the end result seems to be a lot like a very thick smog.

There are scattered variable clouds in Titan's atmosphere in addition to the overall deep haze. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must be responsible for the orange color as seen from space. Analysis of the Huygens data will tell us a great deal about the details of the atmospheric chemistry.

Erosion features Prior to Cassini's arrival, it seemed likely that the clouds would produce a rain of ethane or methane onto the surface perhaps producing an "ocean" up to 1000 meters deep. However, this seems not to be the case at least at the present time. There is little doubt that some active processes are occuring on Titan; there are few if any craters visible indicating that the surface must be very young. But it may be that the "lakes" are more slushy than liquid or that the basins are not filled with liquid at all times. Preliminary results from Huygens indicate that while Titan's rivers and lakes appear dry at the moment, rain may have occurred not long ago. There is clear evidence for "precipitation, erosion, mechanical abrasion and other fluvial activity". In addition, Cassini has found evidence of a peculiar kind of volcano on Titan that may account for some of the unusual features of Titan's atmosphere.

We are beginning to get some understanding of Titan's surface by combining the data from all the sources available. Large ground based observatories operating in the infra-red can see some details as can the Hubble Space Telescope. These show a huge bright "continent" (preliminarily called "Xanadu") on the hemisphere of Titan that faces forward in its orbit and some darker regions that are suggestive of oceans or lakes. Cassini's much higher resolution infrared images (below right, click for animation) show the same structures in more detail. And the close-ups from Huygens (left) show what appear to be drainage channels and shorelines.

These observations also confirm that Titan's rotation is in fact synchronous like most of Saturn's other moons.

Cassini's IR camera has detected a strange and as yet unexplained bright spot on Titan's surface.

Titan is a difficult object to study. The Cassini instruments are specifically designed to penetrate the haze, its radar mapper can see right thru it and the Huygens images show the surface clearly. But the orbiter images are still frustratingly vague and the Huygens images are few in number and cover only a tiny area. Analysis of this data will take some time; Titan is a very strange place.

Few big Satellites of Saturn. 2. Rhea

Rhea ("REE a") is the fourteenth of Saturn's known satellites and the second largest:

orbit: 527,040 km from Saturn
diameter: 1530 km
mass: 2.49e21 kg
In Greek mythology Rhea was the sister and wife of Cronus (Saturn) and the mother of Demeter, Hades (Pluto), Hera, Hestia, Poseidon (Neptune), and Zeus (Jupiter).

Discovered by Cassini in 1672.

Though somewhat larger, Rhea is otherwise very similar to Dione. They both have similar compositions, albedo features and varied terrain. Both rotate synchronously and have dissimilar leading and trailing hemispheres.

Rhea is composed primarily of water ice with rock making up less than 1/3 of its mass.

The leading hemisphere is heavily cratered and uniformly bright. Like Callisto, the craters lack the high relief features seen on the Moon and Mercury.

On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters.

Few big Satellites of Saturn. 3. Iapetus

Iapetus ("eye AP i tus" ) is the seventeenth of Saturn's known satellites and the third largest:

orbit: 3,561,300 km from Saturn
diameter: 1460 km
mass: 1.88e21 kg

In Greek mythology Iapetus was a Titan, the son of Uranus, the father of Prometheus and Atlas and an ancestor of the human race.

Discovered by Cassini in 1671.

With a density of only 1.1, Iapetus must be composed almost entirely of water ice.

Trailing side of Iapetus from Voyager 2 The leading and trailing hemispheres of Iapetus are radically different. The albedo of most of the leading hemisphere is about .04, as dark as lampblack, whereas the trailing hemisphere's albedo is .6, as bright as Europa. This difference is so striking that Cassini noted that he could see Iapetus only on one side of Saturn and not on the other.

One explanation of this is that the leading hemisphere is dusted with a coating of material knocked off of Phoebe or some other Saturnian body. However, the color of the leading half of Iapetus and that of Phoebe don't quite match. Another possibility is that some active process within Iapetus is responsible. The puzzle is compounded by the fact that the dividing line between the two sides is inexplicably sharp.

Near the edge of the dark overlaying material On the last day of 2004, Cassini made its first close encounter with Iapetus. The images show that the dark material overlays the topography, indicating that it is relatively young. And as in the image to the left, along the edge of the dark area there are many craters where only one side is covered by the dark material; the boundary between the two regions isn't so sharp after all. So far the Cassini's data do not resolve the puzzle of the origin of the dark material but there's more to come!

Iapetus's giant equatorial ridge Cassini's first encounter with Iapetus also revealed another striking feature not seen before: a ridge 13 kilometers higher than the surrounding terrain that extends at least 1300 km almost almost exactly parallel with Iapetus's equator (click the image at right for more).

All of Saturn's moons except for Iapetus and Phoebe are very nearly in the plane of Saturn's equator. Iapetus is inclined almost 15 degrees

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