A probe to Titan has found that liquid methane rains lash Saturn's largest moon, a freezing, primitive but active world of ridges, peaks, river beds and deserts scoured by the same forces of erosion as Earth, scientists said Friday.

Methane (search) is a highly flammable gas on Earth, but on Titan, it is liquid because of the intense pressure and cold.

"There is liquid that is flowing on the surface of Titan. It is not water — it is much too cold — it's liquid methane, and this methane really plays the same big role on Titan as water does on Earth," said mission manager Jean-Pierre Lebreton at a news conference.

Titan's rains appear to be liquid methane, not water, and black-and-white photos from the probe showed a rugged terrain of ridges, peaks and dark vein-like channels, suggesting the moon 744 million miles away is scoured by the same erosion forces that shape Earth.

Titan's appearance has long intrigued scientists — and Europe's Huygens probe (search) landed Jan. 14, making it the first moon other than the Earth's to be explored.

Scientists believe methane gas breaks up in Titan's atmosphere, forming smog clouds that then rain methane down to the surface.

"We've got a flammable world, and it's quite extraordinary," said Toby Owen, a scientist from Honolulu's Institute for Astronomy.

But unlike Earth, where water constantly circulates back into the atmosphere, Titan's methane never evaporates back into airborne smog.

"There must be some source of methane inside Titan which is releasing the gas into the atmosphere. It has to be continually renewed, otherwise it would have all disappeared," Owen said.

Titan has river systems and deltas, protrusions of frozen water ice cut through by channels, apparent dried out pools where liquid has perhaps drained away, and stones — probably ice pebbles — that appear to have been rounded by erosion, the scientists said.

The bottoms of the dried-out river channels are coated with what seem to be particles of smog that fall out of Titan's atmosphere, coating the whole terrain. The dirt apparently gets washed off the ridges to collect in the river beds.

It did not appear to be raining when Hyugens descended through Titan's haze on parachutes, "but it has been raining not long ago," Lebreton said.

"Does it rain only once a year? Is there a wet season once a year? Does it rain more frequently? We don't know," said another team member Martin Tomasko of the University of Arizona.

The area where the probe landed is "more like Arizona, or someplace like that, where the river beds are dry most if the time," he said. "Right after the rain you might have open flowing liquids, then there are pools, the pools gradually dry out."

Huygens was spun off from the Cassini mother ship on Dec. 24. The $3.3 billion Cassini-Huygens mission to explore Saturn and its moons was launched in 1997 from Cape Canaveral, Fla. — a joint effort between NASA (search), the European Space Agency, and the Italian space agency.

Scientists think Titan's atmosphere is similar to that of the early Earth and studying it could provide clues to how life arose here.