2004, a new space odyssey

The unexplored world of Saturn's largest moon may yield vital clues to the origins of life. But what precisely will be revealed when the Huygens space probe arrives in Titan's icy realm in four years' time?

Its target is a range of gleaming ice mountains that rise from a dark, oily sea. This new ocean - a mixture of methane and ethane in a rich, gloopy concoction - laps at the base of towering cliffs discolouring the ice. The waves, which to human eyes seem strangely out of proportion, but which are natural for a hydrocarbon sea in a world of low gravity, have eroded the base of one ice cliff. You can make out the scars of recent icefalls. Icebergs, riding strangely high above the waves, are scattered around the shores of the unexplored and alien continent. Welcome to Titan, the largest moon of Saturn.

Its target is a range of gleaming ice mountains that rise from a dark, oily sea. This new ocean - a mixture of methane and ethane in a rich, gloopy concoction - laps at the base of towering cliffs discolouring the ice. The waves, which to human eyes seem strangely out of proportion, but which are natural for a hydrocarbon sea in a world of low gravity, have eroded the base of one ice cliff. You can make out the scars of recent icefalls. Icebergs, riding strangely high above the waves, are scattered around the shores of the unexplored and alien continent. Welcome to Titan, the largest moon of Saturn.

The ground beneath your feet has the reddish colour that dominates the lower altitudes of this strange world. Your feet sink a few centimetres into the slushy ooze, but your tracks are covered up in a matter of days by the rain. Because of the rain you have to clean your spacesuit's faceplate often. It's always raining here, but not the water rain of Earth or even the sulphuric acid rain of Venus. Looking towards the horizon you can see undulating hills with dark red and yellow peaks and rivulets of ochre on their flanks as the methane rain finds its way back to the ocean. Like veins, the methane covers the foothills, but somehow the high peaks are still bright, fresh ice.

Then something out of the ordinary in this alien world catches your eye. It first becomes visible as a bright pinpoint of light high in the dirty red clouds that cover much of Titan and make glimpses of space rare. Looking hard you can just see a white parachute trailing behind. As it swings gently to and fro' during its decent you notice it is spinning. As it nears the ground the pool of light from the searchlight becomes smaller and brighter. Just before touchdown in the mushy snow, the methane snowflakes make it look as if the probe is descending through a pillar of light.

Through a break in the clouds you can see one of the great sights of the solar system. The thick crescent of Saturn is rising above the clouds and ahead of it, pointing straight up into the sky, are its brilliant rings. They emerge from darkness and with a sunlight sharp-edge that forms a thin, banded ellipse preceding the planet.

We can only imagine what it would be like to walk on the surface of Titan. Our ignorance of it compares with what we knew about Mars before the space age. Yet Titan could be the most important object for scientific scrutiny in the entire solar system. More significant than barren and long-dead Mars and possibly more important than all the promise of an ocean beneath Europa's ice crust as it circles Jupiter some 400 million miles closer than Titan to the Sun, this chemically-rich world may hold in its molecules precious clues to the origin of life in our solar system.

In the late 1970s two Voyager probes flew past Saturn and gave us a few tantalising glimpses of Titan. Since then astronomers have been desperate to get back for a more prolonged look. In four years time, a spaceprobe is due to land on the moon and give astronomers their first real insight into this 4.5bn year old world.

The probe, called Cassini/Huygens, is named after two 17th century astronomers who observed Titan with early telescopes. Later this year it will pass Jupiter, the half-way point on its journey and scientists are already planning its observations as it encounters Jupiter's so-called "bow-shock", its interface with the wind from the Sun.

The mother probe, Cassini, will make a two-year survey of the gas giant Saturn, but before this, as it approaches orbital capture and crosses the ring system, it will release the Huygens probe which will make one of the most exciting and scientifically important journeys - back in time to the mystery of biogenesis.

After seven years dormant in space, Huygen's onboard timer will wake it up just 15 minutes before it encounters the first whisps of Titan's upper-atmosphere. The first three minutes are the worst. The shock-front, just in front of the heat shield, reaches a temperature of 12,000C.

When the onboard accelerometers detect a velocity of Mach 1.5 a command will be sent to deploy the first parachute. Suddenly the probes back cover falls and the main chute deploys. The probe's velocity falls to subsonic and the glowing heat-shield is jettisoned.

Then follows 15 minutes of drifting and making measurements of the physical and chemical properties of the atmosphere. Just under two hours later the probe's downward pointing searchlight is switched on and Huygens crashes into the methane snow. Its first task, if it survives, is to take a picture and relay it back to Earth via the mother probe.

Radar contact with Titan was made in 1989. Earth's biggest radio telescopes sent energetic radio pulses across the breadth of the solar system and picked up a barely detectable echo. Faint, but enough to tell us something. The echoes were different on different sides of the moon. It was speculated that there were lakes and possibly shallow oceans of ethane with continents of frozen methane ice.

After the first Voyager images, scientists had to wait until 1994 for new visual images. that revealed fresh detail. The Hubble Space Telescope discovered a mysterious, bright feature the size of Australia near its equator. Was it a range of ice mountains continually eroding under methane rain? Titan receives only one per cent of the Earth's sunlight, and daylight here is like twilight on Earth. It is tidally locked to Saturn and the moon's day lasts 15.9 Earth-days - the time it takes to circle Saturn.

There is undoubtedly a lot of water ice on Titan. At 90 degrees above absolute zero ice is as strong as granite and can make mountains. The wind that blows constantly from the methane ocean freezes out on the mountains' upper slopes, and clouds form on their tops. The methane rain that erodes these hills and exposes bright fresh ice creates Titan's gleaming spires.

If there is methane rain then it implies that there has to be a source of methane, otherwise all the methane in the atmosphere would be destroyed in about 10,000 years. It cannot be solid methane ice, because it would not give off much methane gas. It is therefore probably some source of liquid methane, which would be stable on the surface only if it is mixed with ethane, which also exists on Titan. The boiling point of the combination of the two is very close to the surface temperature.

The methane-ethane ocean is probably mushy, gunky and very dark. Hubble images do indeed show a very large, dark feature on the opposite side to the bright landmark on Titan.

Titan would have an interesting and significant methane cycle. Methane would rise from the ground and diffuse in the atmosphere. There the Sun's ultraviolet radiation would photochemically destroy it, creating the organic haze that envelopes the planet. In the atmosphere, big organic molecules would stick together like tar and slowly sink back to the surface, continually raining down from the atmosphere.

The best Earth-based images ever taken of Titan were captured in infrared light using the world's largest telescope, the Keck II in Hawaii. The striking image was among the first Keck images to be taken with adaptive optics technology that uses rapid mirror adjustments to remove the Earth's atmospheric turbulence from the images. Better even than pictures taken by the Hubble telescope, they revealed features that could be frozen landmasses separated by chilly hydrocarbon seas and lakes.

A theoretical analysis of Titan's atmosphere reveals that it may occasionally rain. Methane rain with larger drops than rain on Earth that fall more slowly. They may lead to streams, rivers and oceans with rolling waves larger than on Earth.

Titan beckons as a world of wonders and significance. If you were there to witness the landing of Huygens in 2004 you would be able to kneel down and push your gloved hand into the mushy snow, lift it and watch it trickle down your arm while contemplating biogenesis.

Dr David Whitehouse is the Science Editor of BBC News Online