Looking down on creation

IT is one of the most ambitious space missions ever attempted, and it could help solve the mystery of how life began, but John Zarnecki knows all he can do now is wait. But then, he's become very familiar with waiting.

As the key British scientist working on the Huygens probe, Professor Zarnecki is into the last few days of an undertaking which began more than 15 years ago. After eight years of preparation, and a seven year voyage through space, the probe separated from its mother ship on Christmas Day, and is due to touch down on Titan, one of Saturn's moons, on January 14.

But if something goes wrong, there is nothing more he can do.

"It is totally on its own. It sends its data back, but we are not able to send any commands to it. In one sense, we might as well go to bed and go to sleep," he says, before adding, "There is no chance that we will."

It was 1989 that Prof Zarnecki and his Open University team were chosen to take part in the Huygens mission, and October 1997 when Huygens left Earth, on board Nasa's Cassini spacecraft, on a 2.2 billion mile, $3.2bn journey to the second largest planet in the solar system.

Cassini has been orbiting Saturn since July last year, relaying information back to Earth, and last month it released the European Space Agency's Huygens probe, to begin the three-week descent to Titan, only the fourth world, after the Moon, Mars and Venus, on which humans have attempted to land a spacecraft.

"There are about 100 moons in the solar system, but Titan is unique in at least one way," says Prof Zarnecki. "All moons, except for Titan, are completely lacking any atmosphere. Titan has got a really thick atmosphere, thicker than the Earth's."

Titan has fascinated astronomers ever since it was discovered, by Christiaan Huygens, in 1655.

The second largest moon in the solar system, and larger than Mercury, Titan's hazy orange appearance led some to conclude that it was entirely composed of gas, but this is now known to be the thick atmosphere cloaking the surface. This atmosphere is largely nitrogen, with traces of other elements, and is so dense that even the Voyager probe was unable to see through the clouds.

But it was able to detect the presence of liquid methane, and the Hubble telescope has seen patterns beneath the clouds which may be continents of land, leading astronomers to conclude that its surface may be made up of land masses and vast oceans, not unlike the Earth's.

But it is what lies above the surface which has most intrigued scientists. The nitrogen-heavy atmosphere is believed to be similar to the Earth's atmosphere some 4.5 billion years ago, the moment when it is thought life began. Although at temperatures down to minus 179 C, Titan is far too cold to support life as we know it, but Prof Zarnecki says it could hold clues to one of our most enduring mysteries.

"If we want to understand how life evolved, then a critical part of that evolution is the atmosphere," he says.

"It was probably chemistry in the atmosphere which created the right sort of chemicals which then led to the beginning of life. By going to Titan, we're studying Earth's early history. It is like Earth's early atmosphere, but stuck in a deep freeze."

Although Titan's size means it has the gravity to keep an atmosphere, there are, as yet, only theories as to why it alone has one, while others of similar size do not.

"Was it born with an atmosphere, or did it acquire one?," says Prof Zarnecki. "One theory is that a great big comet crashed into the surface and a massive ball of ice vapourised, and then this gas was held by gravity and then became the atmosphere. But it is a mystery why it is the only one."

But while there may be much to learn about Titan, Huygens will have only a few hours to make its discoveries and report its information back via Cassini.

The 319kg probe is now coasting unguided and unpowered towards Titan, before being woken by a timer four hours before it enters the atmosphere. Once it reaches the top of the clouds, its descent will be slowed by three parachutes, and it will have around two-and-a-half hours before its batteries run out.

The British part of the mission will look at Titan's surface, although Prof Zarnecki and his team still don't know if they will touch down on land, liquid or a mixture of the two. But it does have the scope to banish memories of Beagle II, which aimed to explore the surface of Mars but has not been heard from since Christmas Day, 2003.

"The atmosphere is exciting enough, but it is possible that the surface will even more exotic," says Prof Zarnecki. "We have never been able to see directly to the surface, but there are theories that it might be partly covered by liquid lakes and seas.

'For the first time ever, if it ends up bobbing up and down on Titan, we would be able to do oceanography in a totally different environment."

Throughout the mission's long duration, even by the standards of space missions, Prof Zarnecki has had to accustom himself to waiting, but it is these last few days when the tension will be worst.

"To some extent, you steel yourself and you build up some sort of emotional barrier, but if it doesn't work it is going to be devastating," he says. "This work is very high risk, and there is a chance, we all know that, but I would not have missed this opportunity for anything. It has been an absolutely wonderful experience.

"It is arguably the world's most sophisticated robot and it has got to do everything on its own, and now we just have to let it get on with it."