By Dr David Whitehouse
BBC News Online science editor
Astronomers at the Paranal Observatory in Chile have obtained the best images yet of Titan, the major moon of Saturn.
Clouds near Titan's south pole
They show what may be clouds in its thick and hazy atmosphere of nitrogen, methane and other hydrocarbons.
The Chandra X-ray telescope has also studied Titan's atmosphere as the moon passed in front of the glowing wreckage of an exploded star.
It has discovered new information about the size of the atmosphere which may require the entry of the Huygens probe in January 2005 to be adjusted.
Although it was imaged in some detail by the Voyager missions of the 1970s, there is much about Titan we do not know.
Astronomers are aware that it is one of the most significant objects in the Solar System - the second largest moon and the only one with a thick atmosphere.
TITAN - MOON OF SATURN
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
Detailed observations of Titan are essential to ensure the success of the European Huygens spacecraft when it plunges into Titan's atmosphere next year.
The Paranal astronomers used an adaptive optics package on the 8.2m Yepun telescope to obtain their images.
The package uses a flexible mirror to compensate for the distortions in images caused by the Earth's turbulent atmosphere.
The sharpened pictures from Yepun show a formation near the moon's south pole which is apparently a cloud feature of some sort. Further observations are planned later this month.
Nasa's Chandra telescope, which is positioned in Earth-orbit, observed Titan as the moon passed in front of the X-ray-glowing gas in the Crab Nebula - the hot debris of a star that destroyed itself in a supernova explosion seen in 1054.
Titan's silhouette against this gas enables the extent of its atmosphere to be probed - and it seems slightly larger than previous estimates have suggested.
Titan appears dark against the X-ray glow
"Saturn was about 5% closer to the Sun in 2003, so increased solar heating of Titan may account for some of the atmospheric expansion," says astronomer Hiroshi Tsunemi, of Osaka University, Japan.
The expansion of the atmosphere may have implications for the Huygens probe.
"If Titan's atmosphere has really expanded, the trajectory (of Huygens) may have to be changed," says Tsenemi.