Titan and Other Satellites

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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.

Image above: Saturn's atmosphere, like Jupiter, is banded - the product of rapid rotation. But the extreme cold means that the clouds form lower down where they are less distinct, and a methane haze dulls the colours and makes the atmosphere less vibrant than Jupiter's. This close-up image, taken by Voyager 1 in 1980, reveals two dark oval storms (right), each about 10 000 kilometres across. Courtesy of NASA/JPL/Caltech.

Image above: Saturn's atmosphere, like Jupiter, is banded - the product of rapid rotation. But the extreme cold means that the clouds form lower down where they are less distinct, and a methane haze dulls the colours and makes the atmosphere less vibrant than Jupiter's. This close-up image, taken by Voyager 1 in 1980, reveals two dark oval storms (right), each about 10 000 kilometres across. Courtesy of NASA/JPL/Caltech.

Image above: Like Jupiter, Saturn is surrounded by a large system of varied satellites. This composite shows the seven largest on the same scale, from left: Titan, Rhea, Iapetus, Dione, Tethys, Enceladus and Mimas. The numbers indicate the moons' positions relative to Saturn, with 1 (Iapetus) being the furthest out. For comparison, our Moon is about 67 per cent the size of Titan. Courtesy of NASA/JPL/ Caltech/National Space Science Data Center.

Image opposite: In late 2004, the hardy little Huygens probe will descend through the thick atmosphere of Titan and provide astronomers with their first ever glimpse beneath this world's permanent cloud blanket. Titan, largest moon of Saturn, is one of only two or three places in the Solar System where astronomers think life might, just might, exist when the Sun grows warmer.

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