The first light of dawn is reflected off a lake on Titan. This image, taken by NASA's Cassini spacecraft on 17 December 2009, confirmed the presence of liquid in the moon's northern hemisphere.
Credit: NASA
FOR THE PAST 25 years, John Zarnecki has been haunted by a double nightmare. The physicist - head of the space sciences department at Britain's Open University in Milton Keynes - led the team that designed, tested and manufactured key instruments for Europe's spaceprobe, Huygens. The craft took 10 years to construct and required a further eight years to reach its target: Titan, the giant moon of Saturn.
"All that time, I lived with the prospect of the project being cancelled and then, once Huygens was on the launch pad, that it would blow up or that there would be a major breakdown when it reached deep space," he recalls.
Hundreds of other engineers, physicists, chemists and administrators who had helped build and launch Huygens were in a similar nervous situation, of course. Hence the renewed tension when, on 14 January 2005, at the European Space Operations Centre in Darmstadt, Germany, the probe, which had been carried to Saturn on NASA's spacecraft Cassini, swept into Titan's thick orange atmosphere.
"If Huygens had failed, it would have been a disaster. We knew we wouldn't get another shot at Titan for 20 years," says Zarnecki.
That was bad enough. But Zarnecki was afflicted by another unpleasant dream. He and his colleagues had spent years persuading the European Space Agency (ESA) to fund Huygens by talking up Titan as if it was the Solar System's hottest piece of scientific real estate.
"To convince the administrators who handled the dosh, we told them this was a great place to visit. Yet we only had a few hunches that Titan might be interesting."
In 1980, the Voyager 1 spacecraft had flown past Titan and returned images of a world covered by an orange photochemical haze of nitrogen, methane and a few other hydrocarbon gases. The surface temperature was pinpointed at -179˚C. Not much to get excited about, you would have thought.
However, calculations suggested that, at this temperature, methane can behave like a gas, a liquid or a solid - much as water behaves on Earth: as ice, as vapour or as water. There could be seas of methane on Titan, it was argued. It was also possible that complex hydrocarbons were being made in the upper atmosphere where the Sun's ultraviolet radiation was interacting with methane. And on the ground, channels could be bursting with torrents of liquid methane.
"That's the picture we sold to the agency," says Zarnecki. "But I was never fully convinced. At the back of my mind, I couldn't help worrying that we would end up with egg on our faces, that Titan would turn out to be really dull and boring - just a lump of ice, with a smooth, bland icy surface with nothing going on there."
Then Huygens began its two-and-a-half hour descent by parachute through Titan's atmosphere, beaming back data from its sensors and cameras to Cassini, which was sweeping through space near the moon. Eventually, the probe - named after the 17th century Dutch astronomer Christiaan Huygens, who discovered Titan - settled gently on the moon's surface, still transmitting data.
Its signals were then relayed by Cassini across the void to ESA scientists and engineers. Even at the speed of light, they took a further 67 minutes to reach Earth.
This was, to put it mildly, a cliffhanger of a mission. I was in the Darmstadt control room that day, and I can vividly remember the atmosphere: it crackled with tension and nervous anticipation. Then, the first results came in. They showed the probe had worked perfectly and, more to the point, they revealed Titan to be one of the most unusual places ever visited by a spacecraft. Zarnecki's twin nightmares were unfounded.
"We could see river channels, lakes and seas, dunes, and possibly low volcanism, some sort of movement of the crust," he says. "There was weather. There was meteorology in the lower atmosphere. Everything that we had promised was there, and a whole lot more. But how it all fitted together - well, we are only just beginning to understand that now."
