"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. "There are truly remarkable processes at work on Titan's surface and in the atmosphere of Titan which are very very similar to those occurring on Earth."

"We now have the key to understanding what shapes Titan's landscape," said Dr Martin Tomasko, Principal Investigator for the Descent Imager-Spectral Radiometer (DISR).

"We landed right on the boundary of some very rugged terrain, near the top of a ridge system about 100 metres high where some dark material - probably photochemical smog - was washing off the ridges"

"Geological evidence for precipitation, erosion, mechanical abrasion and other fluvial activity says that the physical processes shaping Titan are much the same as those shaping Earth."

The same processes of precipitation, erosion, and abrasion that shape the Earth are also occurring on Titan, although the ingredients are very different.

Toby Owen, Cassini Interdisciplinary Scientist, investigating the atmospheres of both Titan and Saturn, says the data shows the atmosphere is, as expected, mostly made of nitrogen. As the probe descended, the levels of methane detected rose steadily. There are also traces of argon, but no primordial argon, unlike Earth. This absence, he says, is a clue to the formation of the moon.

The data also confirms that the liquid on the surface of Titan is methane, not ethane. This discovery raises the question of where all the CH
4 is coming from. The researchers explained that it can't be left over from Titan's primordial atmosphere, or it would all have been used up. Photochemical reactions occurring in the upper atmosphere break the methane down, and other organic compounds form as a result.

"Whenever you see a planet with a dominant nitrogen atmosphere, you have to ask yourself, Where is the carbon?' because carbon is more abundant than nitrogen."

Owen says that on Earth, the carbon became carbon dioxide (CO
2), which then became carbonate rocks. If the CO2 from carbonate rocks were put back into the atmosphere, the Earth's surface pressure would increase 70 times its present level, making our planet more like Venus.

"Venus and Mars, which have predominately CO
2 atmospheres, represent normal conditions," says Owen. "Earth is kind of odd, because the CO2 is not there."

Titan is odd, too, because its methane has not turned into CO
2 over time. The reason for that, says Owen, is because there is no oxygen on Titan to facilitate the transition.

"Water, which is the dominant oxygen reservoir on these planets, is frozen out on the surface," says Owen. "There's no source of free oxygen available, which is a good thing, or Titan would have exploded a long time ago."

Another puzzle about Titan's atmosphere involves the lack of noble gases like krypton and xenon.

"We find these gases in our atmosphere, we're all breathing them right now," says Owen. "We find them on Venus, on Mars, on Jupiter, but they're not present on Titan at limits that we can detect with our sensitivity - which is equal to one thousand times less than what we see on the Earth. Surely there's an interesting clue there as to how Titan formed, which we're going to be working on over the next few months."

The vast amount of methane on Titan makes the whole moon flammable. But where is all the methane coming from? If methane has been producing the moon's signature photochemical smog for millions or even billions of years, why hasn't it been used up by now?

Heat from the probe's batteries warmed the soil beneath it and the probe got a whiff of the methane that forms the clouds, rain, rivers and lakes of Titan.

"The methane was right there under the surface, just like the desert after it has rained," said David Southwood, ESA head of science. "Just like sand on the beach, the top layer may be dry but underneath it is liquid.

"That tells you that here is regular rainfall of liquid methane. You have clouds, you have storms, you have convection systems that take the methane out of the surface and lift it up," said Dr Southwood.

As Huygens descended it sent back images of river systems and deltas, protrusions of ice cut through by channels, dried out pools where liquid had drained or evaporated away and stones rounded by erosion.

As for what it would feel like to walk on Titan, it's possible that a human laden with gear would quickly crunch through a thin crust of frozen chemicals or ice and then sink several centimeters into a mud-like substance. That's what seemed to happen to the 351 kg Huygens when it finally plopped down on the moon's surface after a two-and-a-half hour parachute ride through its atmosphere.

Professor John Zarnecki of Britain's Open University, who led the Huygens design team, said Titan would some day be able to sustain human life.

"I don't want to scare you but in about 4 billion years our sun will expand. It will become a red giant," he said. "Long after life has become impossible on Earth, temperatures will rise on Titan.

"The water ice will melt, oxygen will be released and because we have the basic building blocks of life in the form of organic molecules, then for a brief time Titan might be a rather good place for life."

"This is a tantalising glimpse of the processes at work on Titan and shows how the weather systems operate with methane-forming clouds and raining down on to the surface, producing the drainage channels, river beds and other features that we see in the images," Professor Zarnecki said.

"We cannot say there is absolutely no chance for life on Titan," said Dr Francois Raulin, one of three interdisciplinary scientists on the Huygens mission.

"There is no chance for life on the surface because it is too cold and there is no liquid water.

"However, models of Titan's interior show there should be an ocean about 100 km deep at around 300 km below the surface."

If the models are correct, this ocean would be composed mostly of liquid water with about 15% ammonia at a temperature of about -80
oC, said Dr Raulin.

"We have liquid water, organics not so far away; we have everything on Titan to make life.

The European Space Agency and the American National Aeronautics and Space Administration (Nasa) jointly ran the $ 3 billion Cassini-Huygens mission. It took the Cassini-Huygens spacecraft seven years to cover the 3 billion kilometres of the circuitous journey to Saturn - which is about 1 billion kilometres away. Huygens separated from Cassini in Christmas Day and landed on Titan on 14 January 2005.


First impressions - January 2005

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