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A European probe plunged into the murky atmosphere of Saturn's moon Titan and landed safely on its surface yesterday, transmitting pictures of a strange world apparently shaped by flowing liquid.

The mission represents one of the most ambitious ventures in the history of space exploration and could provide clues to the building blocks of life.

No spacecraft has ever landed on a body so far away — more than 1.2 billion kilometres from Earth.

"This is an historic event," David Southwood, director of science for the European Space Agency, told a news conference at mission control in Darmstadt, Germany.

In fact, the probe, named Huygens, survived at least 11/2 hours on the frigid surface — much longer than expected. Onboard microphones might have even picked up the sounds of howling wind blowing across the surface of the moon.

The probe has sent back close-up pictures showing what appears to be a plain strewn with rounded boulders or ice blocks. Aerial shots taken during the descent reveal a hilly terrain apparently cut with channels or riverbeds.

"Clearly there is liquid matter flowing on the surface of Titan," said Marty Tomasko, who is in charge of one of the imaging devices on the spacecraft.

The liquid is unlikely to be water because the surface temperature plummets to -180 Celsius — cold enough to freeze water as solid as concrete.

Instead, the channels could have been cut by liquid methane that rained down from the orange-coloured sky, Prof. Tomasko speculated.

With the exception of Earth, very few places in the solar system show evidence of surface liquid. (Images of Mars indicate liquid once flowed on its surface, but that could have happened in the distant past.)

It will likely take a while for scientists to interpret the Huygens data, which some say could shed light on the evolution of life itself.

"It [the data] has got to be unravelled, put together, and then scientists, being scientists, are going to argue about what it means as we piece together our place in the universe and how we came to be," Prof. Southwood said.

Titan, bigger than the planets Mercury and Pluto, is one of the most mysterious bodies in the solar system. It is wrapped in a thick orange haze, making its surface impossible to see from orbit.

Scientists believe Titian's atmosphere, made up of nitrogen, methane and other hydrocarbons, resembles Earth's atmosphere as it was about four billion years ago. They think Titan could contain the complex organic compounds considered the building blocks of life.

"Going to explore Titan is a bit like making a trip back in time to see our own past," said Claudio Sollazzo, head of spacecraft operations for the European Space Agency.

Because it is so cold, no one expects Titan to contain living organisms. But it could act like a deep freeze, preserving the recipe of life left over from the dawn of the solar system.

Yesterday's landing on Titan represents the culmination of a seven-year journey across the solar system. Titan rode piggyback aboard the U.S. spacecraft Cassini, which blasted off from Cape Canaveral, Fla., in 1997. In June of 2004, the pair went into orbit around Saturn. And, on Christmas Eve, the two craft separated and Huygens began the final leg of its journey to the surface of Titan.

The probe , named after the 17th-century Dutch astronomer Christiaan Huygens, who discovered Titan, was built like a shellfish. It had a super-hard exterior to protect the delicate instruments inside during the long journey through space.

The blunt front end of the spacecraft acted as a brake and a heat shield as the speeding probe slammed into Titan's upper atmosphere. Then, a series of parachutes further slowed descent and the hard outer shell fell away so the probe's instruments could begin taking readings of the moon's harsh environment.

The probe was equipped with cameras to snap panoramic images during its descent — and its onboard microphone could record the wind, and possibly thunder.

"We should be able to hear a foreign world," Michael McKay, ESA's flight operations director, said in a telephone interview. The sound of thunder would indicate the presence of lightning — which is considered crucial for fusing complex organic molecules.

During its 21/2-hour parachute descent, Huygens transmitted its data to the Cassini spacecraft orbiting overhead. With its large antenna, Cassini then relayed this wealth of information to the anxious scientists waiting on Earth. It took 67 minutes for the data, moving at the speed of light, to travel the vast distance between Saturn and Earth.

One of the great mysteries still to be settled is the source of methane in Titan's upper atmosphere, noted Philip Stooke, a professor of astronomy and geography at the University of Western Ontario.

Ultraviolet light from the sun breaks down methane and produces smog — just like on Earth. Over time, all the methane should have disappeared. Scientists have speculated that atmospheric methane gas is constantly replenished by evaporation from large lakes of liquid methane on the surface.

So far, there is no clear sign of large methane lakes. Nonetheless, "the images transmitted by Huygens show a landscape that looks as if it has been shaped by liquid," Prof. Stooke said. "Perhaps it soaks into the ground and breaks out and flows across the surface but we don't see large open pools."

With a report from Associated Press