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Last Updated: Tuesday, 14 December, 2004, 11:42 GMT
Cassini returns Titan pass images
Cassini can discern bright and dark surface features

The Cassini spacecraft has made one last flyby of Titan before it despatches the Huygens probe for a rendezvous with the Saturnian moon.

It has given engineers a final chance to obtain detailed information on the behaviour of the satellite's atmosphere

This data is essential if Huygens is to have a fighting chance of surviving its 6km/s entry into Titan's thick "air" on 14 January next year.

The flyby has also given scientists another peek at the mysterious moon.

Because of the smoggy conditions on Titan, the two previous Cassini flybys in July and October failed to give any clear indication of the surface conditions Huygens is likely to meet - if it makes it that far.

Mud bath

Speculation remains that vast areas of Titan are covered in organic (carbon-based) compounds, possibly ethane and methane.

At temperatures down to -180C (-290F), these could take the form of an oily "mud"; it is not beyond credibility that there are even seas and rivers of liquid hydrocarbons on the surface.

The images returned by Cassini from Monday's flyby once again reveal light and dark patches on the surface. It is not yet apparent, however, what precociously these variations in brightness indicate.

And one intriguing infrared image obtained with the spacecraft's wide-angle camera shows pronounced banding in the Titan atmosphere.

What does seem clear, though, from all the Cassini's data so far is that the moon has very few craters, suggesting its surface is being reworked, either by weathering or some kind or volcanism.

Titan, Nasa/Space Science Institute/JPL
Banding is visible in the Titan atmosphere in the infrared
Cassini will execute more than 40 flybys of the moon (which is bigger than the planets Mercury and Pluto) in the course of its four-year mission, and scientists say they will need all of them to build up a full picture of the unusual satellite.

The closest approach on the latest pass took place at 1246GMT on Monday at an altitude of 2,358km (1,465 miles).

Cassini scientists are expected to give more detailed information on what the spacecraft's instruments saw at a news conference at the American Geophysical Union's Fall Meeting later in the week.

The 2.7m-wide 319kg Huygens probe will be sprung off the side of Cassini on 25 December. The robotic lab will then cruise for 20 days before entering Titan's atmosphere at high speed.

If engineers have the atmospheric model correct, a three-parachute descent system should open at the correct moments to give Huygens a gentle drift down to the surface.

The probe's observations will be relayed back to Cassini for onward transmission to Earth.

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