Astronomers were given a chance to an important discovery on Titan - one of Saturn's moons - when a pair of rare celestial alignments occurred in November 2003.

The phenomenon validated the atmospheric model employed for planning the entry trajectory for ESA's Huygens probe. The results gave clues on placing the descent of Huygens and to make observations about the upper layers of Titan's atmosphere.

The alignment occurs when Titan passes directly in front of a distant star, blocking the light coming from the star. That's because - due to Titan's thick atmosphere - the light does not 'turn off' straight away but decreases gradually as the atmosphere's layers slide in front of the star. The behavior of the light during the process gives clues about Titan's atmosphere.

In November 2003, 14 months before Huygens' passing through Titan's atmosphere, the moon passed in front of two stars, just seven and a half hours apart. The first occultation, as these phenomena are known, occurred after midnight from the Indian Ocean and the southern half of Africa; the second one was watched from Western Europe, the Atlantic Ocean, Northern and Central Americas.

Teams of astronomers set up along the occultation tracks. "Titan's atmosphere acts like a lens, so at the very middle of the occultation, a bright flash occurs," explained Bruno Sicardy, Observatoire de Paris, France.

If Titan's atmosphere had presented a uniform structure, the central flash would have been a light point, visible only at the center of the moon. But in fact, the central flash fell across the Earth in a triangular shape. "It is like the light falling through a glass of water and making bright patterns on the table. The focused light is not perfectly round because the glass is not a perfect lens," said Sicardy.

The flash's analysis pointed that Titan's atmosphere was flattened at the North Pole, because at the time of the occultation, Titan's South Pole was tilted towards the Sun. This tilting increased the atmosphere's temperature there, provoking a movement towards the north of the moon, where it lost warmth and fell to the surface.

Another unknown issue was solved by this occultation: the speed of a fast moving, high altitude wind (above 200 kilometers (125 miles)) blowing around Titan at 50 degrees north. It was estimated at 200 meters per second (720 kilometers (450 miles) per hour), making a full moon circuit in less than 24 h. "It is like the jet stream on Earth," said Sicardy. "Furthermore, we told the Huygens team to expect some bumps near 510 kilometres altitude, due to a narrow and sudden temperature variation."

This proved true: Huygens was shaken by such a layer during its 14 January 2005 entry. "A temperature inversion was indeed detected by the accelerometers during entry at this very altitude" says Jean-Pierre Lebreton, Huygens project scientist.

Photo credit: ESA