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Last Updated: Friday, 17 December, 2004, 11:30 GMT
Titan clouds seen to come and go
Cassini saw layering in Titan's photochemical haze

Scientists now have their first direct evidence of changing weather patterns on Titan, Saturn's largest moon.

When the Cassini spacecraft flew past the satellite on Monday it spied clouds at mid-latitudes that were not present on its last flyby in October.

The observations will allow researchers to investigate atmospheric dynamics on Titan, the only moon in the Solar System with a thick covering of gas.

The results came out at the American Geophysical Union's (AGU) Fall Meeting.

This week has been the tale of two moons
Dr Carolyn Porco
"We see for the first time discrete cloud features at mid-latitudes, which means we see direct evidence of weather, and we can get wind speeds and atmospheric circulation over a region we hadn't been able to measure before," said Dr Kevin Baines, from the US space agency's (Nasa) Jet Propulsion Laboratory.

Cassini also saw banding and layering in the moon's upper atmosphere.

Moons apart

In addition, mission scientists took the opportunity at the meeting to share images of Dione, one of Saturn's smaller icy moons, which Cassini passed at a distance of 81,400km (50,600 miles) on Tuesday.

Cassini inspected Dione and threw up a surprise

These pictures are the most detailed since the Voyager 1 probe returned pictures of the 1,120km-wide (695 miles) satellite in 1980.

They revealed large surface cracks and bright ice cliffs where scientists thought they would see thick ice deposits.

"This is one of the most surprising results so far. It just wasn't what we expected," said the Cassini imaging team leader, Dr Carolyn Porco.

But if Dione has been a revelation, Titan remains an enigma.

New clouds appear on Titan

With its thick nitrogen-rich atmosphere and photochemical smog, surface features are obscured from visible inspection.

Cassini has to use its multi-wavelength instruments to pierce the shroud, but, even so, scientists are still struggling to interpret the dark and bright areas detected.

Because of its chemistry and temperature, many suspect the satellite harbours vast regions of oily sludge that overlie ice-rock. It is possible, also, that there are moving oceans of liquid hydrocarbons, such as ethane and methane.

Up and down

Whether the dark areas mark out the extent of such seas, no-one seems quite sure. The bright areas could conceivably be the hydrocarbon zones, scientists concede.

"This week has been the tale of two moons," said Dr Porco.

Cassini will get even closer to Dione in October next year

"One of them has been like out of a Lewis Carroll story and gets curiouser and curiouser the more we look at it and the closer we get, and the other turns out to be gloriously clear and has given us what I consider to be one of our most surprising results so far with very little effort," she told the AGU meeting.

The true nature of Titan's surface will only become apparent as Cassini conducts more flybys and obtains some radar altimetry data which will indicate heights.

"At the moment, we don't know up from down; we don't know if dark is low-lying or if bright is high," emphasised Dr Porco.

An important contribution, too, will come from the Huygens probe, currently attached to Cassini, but soon to be ejected to take direct measurements of Titan's environment.

Collision course

Mission scientists told the AGU that the engineering model of the moon's atmosphere chimed with recent observations.

"It all looked good. The conclusion is that we can proceed to the next step," said Jean-Pierre Lebreton, the Huygens project manager. "We got the green light."

Engineers are confident Huygen's three-parachute descent system will open at the correct moments to give a gentle drift down to the surface.

"They have also now corrected Cassini's trajectory through the Saturnian system to put it on a crash course with Titan," Dr Lebreton explained.

"This has to be so because when Huygens is released it will have no manoeuvring capability and, so, Cassini has to put it on a collision course with Titan for 14 January."

Cassini, though, will be in no danger. It will execute another course correction after releasing Huygens to make sure it passes Titan at a safe distance and is able to relay the small probe's scientific data back to Earth.

The Cassini-Huygens mission is a co-operative mission between the US space agency Nasa, the European Space Agency (Esa) and the Italian Space Agency (Asi).

Nature Cassini sees changing weather on Titan - 35 hrs ago
MSNBC Cassini Craft Spies Saturn Moon Dione - 38 hrs ago
ABCNEWS.com Cassini Craft Spies Saturn Moon Dione - 41 hrs ago
Washington PostCassini Craft Spies Saturn Moon Dione - 47 hrs ago
Herald Tribune Titan still puzzles, but Cassini unveils other Saturn moon - 50 hrs ago
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