The team said its model compared well with recent near-infrared spectroscopy and optical images of recent cloud observations of Saturn's largest moon at its south pole and relatively temperate regions near 40� South.

The team, led by Pascal Rannou of the Service d'Aeronomie at the Universite de Versailles-St-Quentin in France, developed a general circulation model that can predict how Titan's major cloud features are produced. It can even reproduce cloud formation for the complete Titan year - equivalent to 30 Earth years - and they are confident the model can predict cloud patterns in the next few years of Cassini's observations.

Scientists have known about condensate clouds on Titan since the Voyager missions of the 1980s. Because of the extremely cold temperatures in the moon's atmosphere, scientists assumed that most organic compounds forming in the upper atmosphere by photochemistry would condense into clouds and sink. Likewise, they hypothesized that methane would condense at high altitudes after rising from the surface.

Eventually, researchers created models of Titan's atmosphere that predicted how drops of ethane and methane formed. Other researchers studied the methane cycle separately in a circulation model, but without cloud dynamics.

Their studies of the models generally found that methane clouds formed as air parcels cooled while moving upward and from equator to pole, but the existing models could not capture fine details of the processes.

Rannou's team combined a cloud model with a general circulation model, and they said it allows them to identify and explain the formation of several types of ethane and methane clouds, including the south polar and sporadic clouds in the temperate regions, particularly those that form at 40 degrees south in the summer hemisphere.

When they ran the simulation, they found that the predicted physical properties of the clouds matched well with recent observational data. Methane clouds appear in locations where the model predicts ascending air motions. Observed south polar clouds also appear exactly where predicted, at an altitude of around 20 to 30 kilometers.

"Clouds in our circulation model are necessarily simplified relative to the real clouds, however the main cloud features predicted find a counterpart in reality," Rannou said. "Consistently, our model produces clouds at places where clouds are actually observed, but it also predicts clouds that have not, or not yet, been observed."

It turns out Titan's cloud patterns are similar to the main cloud patterns on Earth and Mars. The puzzling clouds at 40� S are produced exactly like tropical clouds are in the intertropical convergence zones on Earth and Mars.

In addition, Titan's polar clouds seem to be similar to those produced at mid-latitudes on Earth, although clouds on the giant moon appear only at certain longitudes - something scientists suspect may be due to a tidal effect from Saturn.

The team said they will compare their cloudiness predictions for the coming years with further observations by Cassini and ground-based telescopes, and they will focus on the role of circulation on cloud distribution.

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