No other body in the solar system more closely resembles Earth than Saturn's moon, Titan. It has methane lakes and seas and scientists now believe that an underground ocean could even harbour life, says Robin McKie

Five years ago, several hundred scientists gathered at the European Space Operations Centre in Darmstadt, Germany, to witness a remarkable event: the reception of the first signals to be sent from the surface of Saturn's largest moon, Titan.

It had taken engineers a decade to plan and construct Huygens, Europe's unmanned mission to this mysterious world. In addition, the probe's 2-billion-mile journey there, on board its Nasa-built mother craft, Cassini, took a further seven years. A great many careers depended on the mission's success. Hence the tension in the control room on 14 January, 2005.

"If Huygens had failed, it would have been a disaster. We knew we wouldn't get another shot at Titan for 20 years," says Professor John Zarnecki. His Open University team designed key instruments for the probe, which is named after the 17th-century Dutch astronomer Christopher Huygens who discovered Titan.

"There was another nagging worry. We had talked up Titan as an incredibly exciting place in order to get the space agencies to fund Huygens. Yet we only had the word of the theoreticians that this orange fuzzball, as it appeared in our best images, was interesting. If it turned out to be dull and boring, we would have egg all over our faces."

Then the data arrived and from the very first results, greeted with jubilation at Darmstadt, it was clear that the European Space Agency's probe had performed superbly. More to the point, Titan appeared to be a very exciting place indeed. Photographs revealed shorelines bounding dark lakes while the surface was found to have a crunchy constituency, likened by one researcher to creme brulee.

Since then, scientists have been studying signals sent back by Huygens during its two-and-a-half-hour parachute descent through Titan's thick, orange-coloured, nitrogen-rich atmosphere. These provided enough data to fill no more than a simple memory stick, making Huygens's results some of the most expensive and precious to be collected by scientists.

At the same time, its mother ship, Cassini, has continued its camera and radar sweeps of the haze-covered moon to uncover key surface features. In combination, these two sets of results – only recently fully analysed – have uncovered a world far more extraordinary than ever suspected.

"There are lakes and seas that make Titan the only other place, other than the Earth, in the solar system with large, stable bodies of liquid on its surface," says Zarnecki. "There are also river channels; great stretches of dunes; weather and meteorology; complex hydrocarbons; and – most excitingly – powerful signs that Titan has a subsurface ocean that could provide a home for primitive life. Titan turns out to be an incredible place. It's a moon that would be a planet."

In fact, Titan looks like Earth in many ways – with one critical exception. It is extraordinarily cold, with atmospheric temperatures 200 degrees below that of our planet. Ten times further from the Sun than Earth, Titan consequently receives a hundredth of the solar heat that bathes our world. There may be complex organic material littering the place, but conditions are simply too cold on Titan for life to evolve from this material on the surface.

It turns out that those rivers, lakes and seas on the surface are not watery affairs but are made of methane which plays the same meteorological role on Titan as water does on Earth. It evaporates from great seas of liquid methane, like the giant Kraken Mare. Then it condenses and falls as methane rain, sometimes setting off flash floods that carve out riverbeds like the ones picked out by the cameras on Huygens.

"Water exists in three forms on Earth: liquid, vapour and ice," says Zarnecki. "The same is true for methane on Titan. Hence its role in driving the moon's weather systems."

As for the existence of Titan's complex hydrocarbons, these are formed in its upper atmosphere where the Sun's weak, ultra-violet radiation breaks down methane into molecules that re-form into more complex, petrol-like hydrocarbons. These are responsible for much of the haze that has hidden Titan's surface like a smog over Los Angeles. Devices on board Huygens have detected ethane, acetylene and other complex hydrocarbon molecules – an oilman's dream. These hydrocarbons are then swept to the ground by the moon's rain, though gravity on Titan is so weak that the resulting oily droplets would be far larger than raindrops on Earth.

These seas and lakes don't tell the whole story, however. As Huygens settled on the moon's surface, it photographed a landscape of pebbles that turn out to be made not of stone, but of ice, evidence that water exists at least in one form on Titan.

The fact is that if Titan were not so cold, it would probably be bursting with life, so plentiful are its supplies of organic raw materials, scientists suggest. The moon is, in effect, a chilled leftover from the formation of the early solar system. It is, therefore, of enormous scientific importance, according to Al Diaz, science associate administrator of Nasa, which collaborated with Europe on Huygens. "Titan is a time machine that gives us a chance to look at conditions that existed on early Earth," he said after Huygens's results were received.

In any case, this moon may yet have its day as a home of complex lifeforms. "In a couple of billion years, our Sun will expand to become a type of star called a red giant and will envelop Earth in superhot plasma," says Zarnecki. "Our oceans will boil off and the Earth will become a very unpleasant place to live. By contrast, temperatures will go up nicely by a couple of hundred degrees on Titan. This will be the new Eden."

Crucial to this scenario are Cassini's radar observations which reveal that Titan has a highly irregular rotation. "All planets and moons have slight spin irregularities, including Earth," explains Zarnecki. "This lengthens or shortens our day by a microsecond or two. But on Titan the effect is much greater, suggesting the existence of an underground ocean which separates the moon's crust from its core. This layer acts as the fluid in a giant ballbearing which allows Titan's crust and core to spin at different rates, hence those irregularities."

As for the make-up of that liquid layer, evidence points to water as the prime candidate. And that, in turn, has exciting consequences, say scientists. Titan probably has a hot core which is keeping that layer of water in a warm liquid state. Thus, we have the prospect of a rich soup of hydrocarbons filtering through Titan's crust to a subterranean ocean.

"These discoveries make Titan very interesting biologically. We have got loads of organics on the surface and liquid water down below. Can the two mix? Have they been mixing for billions of years? In other words, are there thriving colonies of bugs down there, crawling about and living very happily below Titan's surface?"

Not surprisingly, such a prospect is fuelling scientists' appetites for a return mission to Titan. Several probes are being planned including a joint European-American mission that would carry airships and balloons. These would take advantage of the moon's thick atmosphere, which is denser than Earth's, and its low gravity, which is a seventh of ours.

"Flying on Titan should be easy and by following its winds, we should be able to sail round it in a couple of weeks, looking for promising places to land and investigate," says Zarnecki. "We could also sail robot ships on its methane seas and become the first extraterrestrial mariners. Most important, however, we could try and find signs of simple biology on Titan. That would be pure gold."