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Last Updated: Saturday, 14 February, 2004, 23:07 GMT
Moon probe set for white-knuckle descent
By Richard Black
BBC science correspondent, in Seattle

Titan may have pitch-black oily lakes

Scientists have been giving details of the Huygens space probe, which is due to land on Saturn's moon Titan in just under a year's time.

The probe, a joint mission between the US and European space agencies, will focus on the oily oceans which researchers believe cover much of the surface.

Huygens marks a new stage in man's quest to explore the Solar System.

It will be the first time that a craft has landed on a moon other than our own.

Depending on where it touches down, it may also be the first time that something made by the hands of humans has entered an ocean anywhere else than on Earth.

But Titan's oceans are completely unlike Earth's - they are dark and oily, made of liquid methane and ethane.

Piggyback ride

At least, that is what Huygens scientists like Dr Ralph Lorenz, from the University of Arizona, believe - though no one will know for sure until the probe arrives.

Second largest moon in the Solar System; only Ganymede is larger
Only moon in the Solar System with a thick atmosphere
Keeps its same face toward Saturn as it orbits the planet
Lakes of liquid ethane and methane may cover moon's surface
"Titan remains the largest single piece of unexplored real estate in the Solar System," he told the annual meeting of the American Association for the Advancement of Science here in Washington State.

"Last year, there were observations with the Arecibo radio telescope which showed Titan glints, suggesting that there are relatively dark, very smooth areas on its surface.

"The simplest explanation is that there are lakes and seas of liquid hydrocarbons."

Huygens is currently hurtling through space attached to another spacecraft, Cassini.

Launched in 1997, they will arrive at Saturn on 1 July this year. Cassini will spend four years in orbit around the giant planet, and when it approaches Titan next January, it will release the little Huygens for its trip down to the surface.

Like all landings, it will be a white-knuckle ride.

Like Earth

"Huygens will scream into Titan's atmosphere at 6km/s encased in a heatshield and then will throw out a parachute and slowly drift down to the surface," Dr Lorenz said.

Lord of the Rings - Cassini approaches Saturn

"I think what we will see is a rugged but muted landscape; we'll see a lot of impact craters and a lot of those will be filled with liquid to form circular lakes, ring-shaped lakes and bull's-eye lakes.

"I think we will see something maybe a little like Sweden or northern Canada."

Ralph Lorenz says that what we learn from Titan's lakes and oceans could be relevant here on Earth. For the first time, scientists will be able to study oceans made of a different substance, pulled by a different gravity.

How will waves behave? Will the coasts of these oily oceans be eroded? And how do they interact with Titan's atmosphere - the only other in the solar system made largely of nitrogen?

Biochemical clues

"Many important oceanographical processes, like the transport of heat from low to high latitudes by ocean currents, or the generation of waves by wind, are known only empirically on Earth," Dr Lorenz said.

"If you want to know how big waves get for a given windspeed, you just go out and measure both of them, get a lot of datapoints, and fit a line through them.

"But that's not the same as understanding the underlying physics and being able to predict how things will be different if circumstances change. By giving us a whole new set of parameters, Titan will really open our understanding of how oceans and climates work."

The environment on Titan is hostile - the temperature is a frigid minus 179 degrees Celsius. But scientists say it is remarkably like Earth in some ways and it may hold clues about the early chemistry of life on our planet.

Dr Lorenz is a member of both the Cassini spacecraft's radar mapping team and a co-investigator of the Surface Science Package on the Huygens probe.

The BBC's Richard Black
"It will be the first time that a craft has landed on a moon other than our own"

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