Friday, 17 February 2012

Saturn – Lord of the Rings

Saturn Data
Mass: 5.69 x 1026 kg or 95.2 times Earth’s
Equatorial diameter: 120 536 km or 9.5 times Earth’s
Surface gravity: 1.16 gees
Axial tilt: 26.7°
Mean surface temperature: -80 Celsius
Rotation period: 10.23 hours
Orbital period: 26.5 years
Inclination of orbit to ecliptic: 2.5°
Orbital eccentricity: 0.056
Distance from the Sun: 9.00–10.07 AU
Sunlight strength: 0.0099–0.012 of Earth’s
Satellites: >30
Largest satellite: Titan, diameter 5150 km

Now we are truly entering the depths of interplanetary space. Twice as far from the Sun as Jupiter we encounter the second largest planet. This is Saturn, Lord of the Rings. Saturn is like Jupiter in many ways. The two planets have similar sizes, they share almost identical chemical compositions and their interiors are made of the same layers, though in different proportions. And, like Jupiter, Saturn spins very quickly, its clouds stretched into bands parallel to the planet’s equator. But it is the incredible rings that make Saturn so special. They are very bright, made of countless chunks of tumbling ice, and span a volume of space that would stretch more than a third the distance from the Earth to the Moon. Saturn also has a large number of its own moons, including the second-biggest satellite in the Solar System, Titan.

Physical Overview
The second largest planet is in many ways a small, low-contrast version of Jupiter, with about one-third of its larger cousin’s mass and 84 per cent of its diameter. Like Jupiter, Saturn is a gas giant: a ball of hydrogen and helium in a liquid state. But, because of the comparatively low overall mass of the planet, Saturn’s materials are not so compressed. Inside, Saturn hides a dense core of ice and rock, similar to but less massive than Jupiter’s. It also has a stock of liquid metallic hydrogen in its interior. But, again because of the relatively low pressure, this region is not as extensive as it is in Jupiter. A thick shell of normal liquid hydrogen tops the liquid metallic hydrogen. And, as on Jupiter, this liquid interior gradually blends into Saturn’s gaseous atmosphere.

This hydrogen–helium atmosphere is in some way like Jupiter’s. It has dark belts of sinking, low-pressure gas, and brighter zones of rising highpressure regions. As on Jupiter these belts and zones are stretched around the planet because of the rapid rotation. Moreover, enhanced images of Saturn show that its atmosphere shares some of the other features found on Jupiter, such as the oval storm systems, though on smaller scales. And yet, despite these similarities, Saturn’s face is very bland compared with Jupiter’s vibrant variegation. This is partly because of Saturn’s placement in the Solar System. Saturn is twice as far from the Sun, so its atmosphere receives about one-quarter of the solar energy per unit area that Jupiter does. Its gases cool quickly as they rise up through the frigid atmosphere, and cloud-forming condensation thus occurs relatively low down where the clouds are shielded from sunlight. In addition, Saturn’s outermost atmosphere contains a layer of methane haze. This obscures the clouds below even more and makes the planet’s atmosphere paler still. Overall, Saturn is a pallid Jupiter. But what it lacks in atmospheric colouration it more than makes up for in its ring system.

Saturn’s Rings
Like the rings of Jupiter, Saturn’s are made up of billions of fragments on independent orbits. But Saturn’s rings are very bright and extensive compared with Jupiter’s, and this is because the ring particles are different. Saturn’s rings are bright because the fragments are icy. The particles are also much larger than the dust-sized, rocky motes in Jupiter’s rings, ranging in size from sand-grains up to kilometre-scale, flying mountains.

Saturn’s rings are much more complex than was originally thought. Earth-bound observers identified three rings and labelled them A, B and C. Astronomers expected that these individual rings would appear smooth in close-up photographs taken by the Voyager 1 probe. But these close-ups instead revealed that the large rings are themselves made up of thousands of thinner ringlets. Evidently, therefore, the particles are not uniformly distributed. Some of the rings are relatively dark. They no doubt contain darker or smaller particles or are less dense. And in some places there are gaps where virtually no rings are seen at all. The most celebrated of these is the broad Cassini division, roughly 4700 kilometres wide. The Cassini division and other gaps in Saturn’s rings are somewhat like the Kirkwood gaps. Recall that these are those regions in the asteroid belt where there are far fewer asteroids than usual. In the same way that Jupiter’s gravity sweeps these gaps in the asteroid belt, so the gravity of some of Saturn’s moons are responsible for the Cassini and other divisions. Similarly, the gravitational effects of small moons called shepherd satellites are responsible for keeping the individual ringlets so narrow. And Saturn has many moons to do this job.

Titan and Other Satellites
So far, astronomers have found 30 moons around Saturn – more than any other planet – the latest 12 added as recently as 2000 and 2001. Roughly speaking, the satellites fall into three size classes. Titan, easily the largest, is out there by itself, 5150 kilometres across. Much smaller than Titan, meanwhile, are the next six moons, Iapetus, Rhea, Dione, Tethys, Enceladus and Mimas, in order of decreasing size. These form the second class of satellites, are essentially spherical, and have diameters of 398–1440 kilometres. Lastly, the smallest moons are all irregularly shaped and between 13 kilometres and 370 kilometres across. Virtually all of these
moons, even the very small ones, contain significant quantities of ice. Titan is half ice and half rock, but for the others the ratio is about 60 : 40 or 70 : 30 ice to rock.

Titan’s name is very fitting, for it is a monster satellite. It is the second largest known moon, after Jupiter’s Ganymede, and like that world is larger than the planet Mercury. Titan has a remarkably thick atmosphere – even denser than Earth’s – which like our atmosphere consists mainly of nitrogen. But Titan is nothing like the Earth. For a start it is exceptionally cold, with a surface temperature of around -180 Celsius. In fact it is because of this frigidity that Titan has an atmosphere at all. At these temperatures, gases move so slowly that they are unable to escape Titan’s relatively feeble gravity – around 14 per cent as strong as ours. Instead, the gases cling to the surface in a thick shroud 1.6 times denser than Earth’s atmosphere at sea level. That surface, meanwhile, is unlike any other in the Solar System. Planetary scientists have detected various hydrocarbons, and they suspect that parts of Titan are covered in liquid seas or oceans. But they are not oceans of water – it’s far too cold for that. Instead, Titan’s oceans are made of liquid ethane, methane and nitrogen, possibly enveloping the surface – up to 1 kilometre deep in parts – in a tarry goo. Sadly, nobody has yet glimpsed Titan’s surface because its clouds are as impenetrable as Venus’. We shall have to wait until the Huygens probe arrives at this world in 2004 and descends through its dark, orange skies.

History of the Saturnian System
We have seen already that Saturn formed in the same way that Jupiter did. A large icy planetesimal appeared first, which drew in gas from the Solar Nebula to create a disc around itself. Saturn grew at the centre of the disc. Since its birth, like Jupiter, it has been cooling down and contracting, and even now has a hot interior. However, the rings could not have been formed alongside the planet itself, for the heat would have evaporated them. And in any case, dynamical studies show that the rings are expected to last, at most, only one-tenth the age of the Solar System. Instead, Saturn’s icy rings are the remains of large comets or moons that were broken up by the planet’s gravity a few hundred million years ago – different in origin to Jupiter’s rings.

Further out in the Saturn disc the material presumably lumped together to form many of the satellites. Most of the moons orbit the planet close to its equatorial plane and in the same direction – facts consistent with their formation in a disc. Yet there are irregularities with the Saturnian system that do not conform to a discal birth. The most important discrepancy is that the moons do not seem to be related to each other. Recall that the Galilean satellites have densities that decrease away from the planet Jupiter. This is what we expect: rocky worlds emerge close in where it is hot, and lightweight icy bodies lump together further out. The trouble is that all of the Saturnian satellites are very icy, and they do not show this pattern of decreasing density with distance from the planet. Also, only one of Saturn’s satellites, Titan, is anywhere as near as large as Jupiter’s big four. The rest are puny. The answer could be that many of the moons have suffered major impacts since their formation – impacts so devastating that the moons’ surfaces now bear little testament to the past. The other difference compared with Jupiter is that even Saturn’s smallest moons are icy. They are captured icy planetesimals, rather than the asteroids that Jupiter – being close to the asteroid belt – found it easier to net.

Source :
Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.



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