A chilly outpost of the Solar System that mimics Earth

Titan could be an analogue for our own planet, new pictures suggest.

Saturn is centre stage in the sky this month, but for scientists the beautiful ringed world is increasingly being eclipsed in importance by its biggest moon, Titan.

When the Voyager I flew past Saturn in 1980, it found Titan is unique among all the moons of all the planets in the Solar System. It is the only moon with more than a trace of atmosphere. In fact, Titan's "air" is twice as dense as the Earth's, and - like our planet - the atmosphere is made up mainly of nitrogen. (Hydrogen, methane or carbon dioxide predominate in other planetary atmospheres.)

Voyager was programmed to photograph Titan's surface in intimate detail, but all the images showed was unrelieved orange cloud. These clouds held their own fascination, because they are made of organic "gunge". Ultraviolet radiation from the Sun has broken up molecules of methane and ethane in the atmosphere and welded them together as complex molecules.

Drops of sticky rain may fall from the clouds to mingle with pools of liquid ethane on Titan's hidden, chilly surface. At one time, astronomers even thought Titan might be covered entirely by a deep ocean of liquid ethane.

But now Athena Coustenis, an astronomer at the Meudon Observatory in Paris, has disproved this theory by taking pictures of the surface of Titan with a telescope on the Earth. She had to employ two new tricks. First, Coustenis observed not light but infra-red radiation, which can penetrate the organic drops in the clouds. Second, she used a giant European telescope at La Silla in Chile that is fitted with "adaptive optics" - a small flexible mirror that continually bends to compensate for blurring caused by the Earth's atmosphere, and so keeps the tiny features on Titan in crisp focus.

Coustenis found several bright regions, indicating that the surface of Titan is mainly solid, though there may be some seas of liquid ethane. There is a polar cap at the north pole, which may consist of frozen ethane or methane "snow". Most exciting is a very bright spot near Titan's equator. It is probably a huge mountain, perhaps three times the height of Mount Everest, and capped with ethane or methane snow.

What has raised such a giant mountain on a world much smaller than the Earth? One strong contender is a vast volcano. A highly volcanic surface would also account for Titan's dense atmosphere, with its rich cocktail of gases. We'll know better when the international Cassini-Huygens spacecraft arrives in 2004. While Cassini orbits Saturn, Huygens will descend through Titan's clouds and land on the frozen surface.

Many scientists see in Titan an analogue for the early Earth, where organic compounds forged by ultraviolet radiation dissolved in the oceans and gave rise to life. Until now, Titan has been regarded as "an early Earth in deep freeze", where the primitive molecules just accumulate on the chilly surface. But if Titan has active volcanoes, which warm their surroundings enough to melt ice into water, then maybe the reactions that formed the first cells on Earth (and possibly Mars) might have occurred on this chilly outpost of the Solar System as well.

What's Up?

At its closest to Earth this month, Saturn is visible all night in the south. Although not as brilliant as the brightest stars, it is the most prominent object in a barren region of sky. Another giveaway is its yellow glow: unlike stars, planets don't twinkle. A small telescope will reveal its famous rings and its biggest moon, cloud-covered Titan.

The full moon lies near Saturn on 26-27 September. Watch through the early hours of the morning and you'll see a stunning sight. The moon gradually enters the Earth's shadow, and by 3.19am is in total eclipse. As the moon fades, Saturn will appear increasingly brilliant.

It's the second total lunar eclipse this year. In April, the eclipsed moon appeared a reddish colour, because sunlight was bent around into the Earth's shadow by our planet's atmosphere. No one can predict how bright this month's eclipse will be: check it out for yourself.

Other sights include Jupiter, still shining brightly in the south- west after sunset, and Venus and Mars in the early morning.

National Astronomy Week

From 21 to 28 September, Britain celebrates its fifth National Astronomy Week. It commemorates the 150th anniversary of the discovery of Neptune, and also coincides with the total lunar eclipse and the best view of Saturn this year.

Observatories throughout the UK will be opening their doors to the public, and schools will be coordinating astronomy-based projects. For further information, contact National Astronomy Week, Jodrell Bank Science Centre, Macclesfield, Cheshire, SK11 9DL; phone 01477 571874; fax 01477 571875; World Wide Web http: //www.ast.cam.ac.uk/naw96/

Diary (all times BST)

4 Sept: 8.07pm moon at last quarter

13 Sept: 0.08am new moon

17 Sept: Mercury at inferior conjunction

20 Sept: 12.23pm moon at first quarter

22 Sept: 7pm autumn equinox

26 Sept: Saturn at opposition

27 Sept: 3.51am full moon; 3.19am-4.29am total lunar eclipse

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