A magical mystery tour of Titan

Saturn's biggest moon will reveal its true nature to scientists when the Huygens probe finally lands this week. Steve Connor reports

This Friday marks the start of one of the most ambitious unmanned space missions in history. Early in the morning, London-time, a series of alarm clocks is scheduled to wake up the Huygens space probe, which for 20 days has been hurtling silently towards the thick atmosphere of Titan, the largest moon of Saturn and the second largest in the solar system. Once awake, the probe should spend the next few hours performing an astounding act of scientific discovery as it makes its descent to the mysterious surface of Titan.

This Friday marks the start of one of the most ambitious unmanned space missions in history. Early in the morning, London-time, a series of alarm clocks is scheduled to wake up the Huygens space probe, which for 20 days has been hurtling silently towards the thick atmosphere of Titan, the largest moon of Saturn and the second largest in the solar system. Once awake, the probe should spend the next few hours performing an astounding act of scientific discovery as it makes its descent to the mysterious surface of Titan.

Huygens - named after Christiaan Huygens, the 17th-century Dutch astronomer who discovered Titan - separated from its mother spacecraft on Christmas Eve after a journey of seven years and two billion miles. As the Cassini spacecraft continues to orbit Saturn, it will act as a communications beacon to beam back to Earth the scientific data gathered by Huygens in the two hours or so it takes to plunge through Titan's atmosphere.

By any measure, Titan is a weird place. Its orange glow is caused by a noxious cocktail of gases in its atmosphere, which obscures what lies beneath from the gaze of telescopes. Scientists have no idea if Huygens will land on an ocean of hydrocarbon liquids, a swampy landscape of tar-like mud, or solid ground.

Scientifically, the most exciting outcome would be for it to land in a lake or ocean of liquid, says John Zarnecki of the Open University, a leading scientist on the Huygens mission. "It's a distinct possibility that I could be the very first scientist to carry out oceanography on an outer planet of the solar system," he says. "But equally, the probe could land with a thud on hard ground or a squelch into a morass of extraterrestrial slime - no one knows for sure... The instruments on board have been designed to handle a range of possibilities."

If the probe lands on a hard surface, it has a 70 per cent chance of surviving the impact. If the surface is covered in sludge or liquid, that figure goes up. "It would be a softer landing," says the scientist. "We're more likely to survive the initial impact, but we don't know how our instruments would react to contact with such chemicals. One theory is that Huygens will splash down into a lake or ocean of liquid methane." But before it lands, Huygens has to traverse the moon's thick atmosphere. It will be travelling at 2,450kmph (1,522mph) as it makes its initial entry into Titan's atmosphere, at an altitude of 1,270km. Within the next couple of hours, it will decelerate to just five metres per second by the time it hits the surface - equivalent to the impact of jumping from a chair to the ground.

A heat shield will protect the probe - which is about the size of a washing machine - from the initial friction, which could raise its temperatureto 8000C (14432F). When the probe slows down to 400 metres per second, a pilot parachute is scheduled to be deployed, pulling out a larger main parachute as the probe's velocity slows from 1,500kmph to 300kmph in one minute.

When the main parachute is fully deployed, the heat shield should fall away from the base of the probe to reveal the scientific instruments that will gather the all-important data. About 15 minutes later, the main chute will be jettisoned as it is replaced by a smaller parachute designed to allow a steady descent at a speed of 20kmph.

Huygens will rotate as it drops and its cameras will scan the surrounding scene over a full 360 degrees. As well as taking pictures of Titan's clouds, the probe's spectrometers will analyse the dust and molecules in the atmosphere. As it breaks through the haze, the probe should get a spectacular glimpse of the moon's surface. Its cameras are capable of taking up to 1,100 picturesand its radar altimeter - designed to measure the probe's height above ground - will be used to gather valuable information about the structure and texture of Titan's surface.

Ian Halliday, chief executive of the Particle Physics and Astronomy Research Council, says the importance of the £2bn Cassini-Huygens mission - funded by Nasa and the European Space Agency - cannot be over-estimated. "Superlatives can come easy when talking about space missions but this particular voyage of scientific discovery is truly awesome," Professor Halliday says. "Titan is a mysterious place and raises many scientific questions. Its thick atmosphere is mostly nitrogen, but there are also methane and many other organic compounds. Some of them would be signs of life if they were on our planet. Organic compounds form when sunlight destroys methane. If sunlight is continuously destroying methane on Titan, how is methane getting into the atmosphere?"

The surface temperature of Titan hovers around minus 180C. This makes it impossible for any water to exist in liquid form, and highly improbable that life exists on the moon. Scientists hope that the peculiar chemistry of Titan might provide information about the nature of our own planet about four billion years ago, when life was on the verge of existence. As Monica Grady, a meteorite specialist at the Natural History Museum in London, says: "Because of the similarities in the atmospheres, it will give us a fascinating analogue of the type of reactions that would have been happening on Earth."

Professor Carl Murray of Queen Mary, University of London, says Titan may well turn out to be quite unique in the solar system. "There's no other natural satellite with such a dense atmosphere and we know the surface is unusual because there is some sort of constant resurfacing going on that we don't know about. It really is an enigma," he says.

Titan is often described as a monster moon; with a diameter of 5,150km, it is bigger than Mercury. Titan was discovered by Huygens in 1655 and, with modern telescopes that are sensitive to the near infra-red part of the spectrum, its surface appears through the orange haze to have light and dark features, which some astronomers believe may be tantalising evidence of lakes or seas of hydrocarbons. Whatever the exact nature of the moon, it has a weather system based on organic compounds rather than water. It is also very cold, which explains why compounds such as methane and ethane exist as liquids rather than gases.

If the landing is soft enough for the probe to survive, its surface instruments are designed to continue working for a couple of hours longer, until it loses contact with the Cassini mothercraft as it flies below Titan's horizon. If Huygens floats on a liquid ocean, a sonar will attempt to measure its depth. If it lands in sludge or on soft ground, a probe will try to gauge how hard it is and if the surface is level.

This weekend, scientists should finally discover Titan's true nature. John Zarnecki is hoping for a splashdown, but will be happy as long as Huygens manages to reach one of the most mysterious places in the solar system. "Let's just say that after a seven-year voyage and 20 years of planning, design and build, I will be extremely pleased to land, whatever the surface," he says.